wear of teeth

Wear of human teeth: a tribological perspectiveR Lewis� and R S Dwyer-Joyce

Department of Mechanical Engineering, The University of Sheffield, Sheffield, UK

The manuscript was received on 14 June 2004 and was accepted after revision for publication on 27 September 2004.

DOI: 10.1243/135065005X9655

Abstract: The four main types of wear in teeth are attrition (enamel-on-enamel contact),abrasion (wear due to abrasive particles in food or toothpaste), abfraction (cracking inenamel and subsequent material loss), and erosion (chemical decomposition of the tooth).They occur as a result of a number of mechanisms including thegosis (sliding of teeth intotheir lateral position), bruxism (tooth grinding), mastication (chewing), toothbrushing, toothflexure, and chemical effects. In this paper the current understanding of wear of enamel anddentine in teeth is reviewed in terms of these mechanisms and the major influencing factorsare examined. In vitro tooth wear simulation and in vivo wear measurement and ranking arealso discussed.

Keywords: tooth wear mechanisms, tooth wear testing

1 INTRODUCTION

Teeth are used in all the major functions of themouth including, speech, breathing, taste, and chew-ing. Initially humans have 20 primary (or baby) teeth.These are eventually replaced, during childhood,with 32 secondary (or adult) teeth. To allow chewing,the mouth is arranged into two opposing arches ofteeth. Within each arch are different types of tooth(as shown in Fig. 1), which each have a differentfunction. Incisor teeth are used for shearing, thecanines evolved for holding prey, and molars areused for chewing.

Figure 1 also illustrates the different surfaces ofteeth. The buccal side is the side nearest the lips orcheek; the lingual side is that closest to the tongue.The occlusal surface, which will be mentioned fre-quently in describing various wear processes, isthat which meets a tooth or teeth in the oppositejaw during chewing or biting.

Tooth wear is becoming more of an issue as lifeexpectancy is increasing and teeth are required tolast longer. Dental practitioners need to be aware ofthe underlying issues and influencing factors as

also do manufacturers of dental products such astoothpastes and brushes. It is important that tooth-pastes designed to give cosmetically appealingwhite teeth are not doing damage to teeth whileremoving stain and plaque. An understanding oftooth wear mechanisms will also help to developimproved restoration materials.

Manufacturers of food products and particularlysoft drinks, which contain acids, need to be awareof how levels of acidity will affect teeth, particularlyin children who will consume larger amounts oftheir products. An understanding of the mechanismsand controlling factors in tooth enamel wear istherefore critically important.

1.1 Tooth structure and orientation

Figure 2 shows the schematic diagram of a sectionthrough a healthy tooth. It illustrates the differentlayers of the tooth and internal structure as well ashow the tooth is positioned relative to the gum andsurrounding jaw bone.

The crown of the tooth is the part visible above thegum (gingiva). The neck region is that part at the gumline, between the crown and the root. The root isembedded in the gum. Some teeth have just oneroot, such as incisors, whereas each molar has fourroots per tooth.

�Corresponding author: Department of Mechanical Engineering,

The University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK.

REVIEW PAPER 1

J03404 # IMechE 2005 Proc. IMechE Vol. 219 Part J: J. Engineering Tribology

Wear of human teeth: a tribological perspectiveR Lewis� and R S Dwyer-Joyce

Department of Mechanical Engineering, The University of Sheffield, Sheffield, UK

The manuscript was received on 14 June 2004 and was accepted after revision for publication on 27 September 2004.

DOI: 10.1243/135065005X9655

Abstract: The four main types of wear in teeth are attrition (enamel-on-enamel contact),abrasion (wear due to abrasive particles in food or toothpaste), abfraction (cracking inenamel and subsequent material loss), and erosion (chemical decomposition of the tooth).They occur as a result of a number of mechanisms including thegosis (sliding of teeth intotheir lateral position), bruxism (tooth grinding), mastication (chewing), toothbrushing, toothflexure, and chemical effects. In this paper the current understanding of wear of enamel anddentine in teeth is reviewed in terms of these mechanisms and the major influencing factorsare examined. In vitro tooth wear simulation and in vivo wear measurement and ranking arealso discussed.

Keywords: tooth wear mechanisms, tooth wear testing

1 INTRODUCTION

Teeth are used in all the major functions of themouth including, speech, breathing, taste, and chew-ing. Initially humans have 20 primary (or baby) teeth.These are eventually replaced, during childhood,with 32 secondary (or adult) teeth. To allow chewing,the mouth is arranged into two opposing arches ofteeth. Within each arch are different types of tooth(as shown in Fig. 1), which each have a differentfunction. Incisor teeth are used for shearing, thecanines evolved for holding prey, and molars areused for chewing.

Figure 1 also illustrates the different surfaces ofteeth. The buccal side is the side nearest the lips orcheek; the lingual side is that closest to the tongue.The occlusal surface, which will be mentioned fre-quently in describing various wear processes, isthat which meets a tooth or teeth in the oppositejaw during chewing or biting.

Tooth wear is becoming more of an issue as lifeexpectancy is increasing and teeth are required tolast longer. Dental practitioners need to be aware ofthe underlying issues and influencing factors as

also do manufacturers of dental products such astoothpastes and brushes. It is important that tooth-pastes designed to give cosmetically appealingwhite teeth are not doing damage to teeth whileremoving stain and plaque. An understanding oftooth wear mechanisms will also help to developimproved restoration materials.

Manufacturers of food products and particularlysoft drinks, which contain acids, need to be awareof how levels of acidity will affect teeth, particularlyin children who will consume larger amounts oftheir products. An understanding of the mechanismsand controlling factors in tooth enamel wear istherefore critically important.

1.1 Tooth structure and orientation

Figure 2 shows the schematic diagram of a sectionthrough a healthy tooth. It illustrates the differentlayers of the tooth and internal structure as well ashow the tooth is positioned relative to the gum andsurrounding jaw bone.

The crown of the tooth is the part visible above thegum (gingiva). The neck region is that part at the gumline, between the crown and the root. The root isembedded in the gum. Some teeth have just oneroot, such as incisors, whereas each molar has fourroots per tooth.

�Corresponding author: Department of Mechanical Engineering,

The University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK.

REVIEW PAPER 1


The two most important elements of the tooth fromthe tribological perspective are the outer enamel layerand the dentine, which lies underneath. Initially theenamel is exposed to the loads and chemical environ-ment within the mouth as a result of chewing etc. Ifthis layer is breached due to tooth fracture or wear,then the underlying dentine is exposed.

Enamel is thickest at the tip of each tooth (2–3 mm)and tapers to its thinnest at the cemento-enameljunction.

Enamel and dentine have very different structuresand properties (the properties of enamel and den-tine are given in Table 1). Enamel, which consistsof about 97 per cent inorganic material, is the mosthighly calcified and hardest tissue in the humanbody. The inorganic component is hydroxyapatitein the form of large elongated hexagonal crystals.These crystals extend through the thickness of theenamel and are nearly perpendicular to the surface.They are approximately uniform in size and have athickness of about 30 nm and a width of 60 nmwith lengths ranging between 100 and 500 nm [2].

Dentine has a much higher organic content thanenamel. It is composed of 70 per cent inorganicand 18 per cent organic material, with the remainderbeing water. The inorganic component, as withenamel, is hydroxyapatite. The crystals, however,are much smaller than those found in enamel,being 3 nm across and about 20 nm in length.Dentine is much softer and more ductile than

enamel (see Table 1). This means that it acts as acushioning layer to reduce the effect of chewingloads on the enamel.

As reported by Rees [3], failure loads for enamelvary widely (Table 2). This variation is probably dueto the difficulties in producing pure enamel speci-mens with no inherent flaws. Theoretical calcu-lations suggest that failure stresses for manymaterials lie in the range from E/5 to E/30, where Eis the elastic modulus of the material. However, forbrittle materials, because of flaws, this may benearer E/1000 [10]. Using this approach and avalue of 80 GPa for the elastic modulus of enamel,Rees et al. [4] calculated a theoretical failure stressfor enamel of 80 MPa. This is similar to the valuemeasured by Tyldesley [9].

Enamel has often been viewed as a homogeneousmaterial (see, for example, references [11] and[12]). Other investigations have shown, however,that mechanical properties of enamel vary withlocation on a tooth, local chemistry, and prism orien-tation. Results of microhardness testing [13] andcompression tests [14] indicated that the elasticmodulus E and hardness H may be slightly higherfor surface enamel than for subsurface enamel. Vick-ers indentation measurements have shown thatthese parameters obtained for a occlusal section ofenamel are generally higher than those obtained foran axial section [15].

Cuy et al. [16] carried out a comprehensive map-ping of enamel hardness and elastic modulus usinga nanoindentation process. Results indicated thatvalues of H were much higher than those previously

Fig. 1 Tooth orientation

Table 1 Mechanical properties of and enamel dentine [1]

Property Dentine Enamel

Young’s modulus (GPa) 10.2–15.6 20.0–84.2Shear modulus (GPa) 6.4–9.7 29Bulk modulus (GPa) 3.11–4.38 45, 65Poisson’s ratio 20.11–0.07 0.23, 0.30Compressive strength (GPa) 0.249–0.315 0.095–0.386Tensile strength (GPa) 0.040–0.276 0.030–0.035Shear strength (GPa) 0.012–0.138 0.06Knoop hardness 57–71 250–500Density (kg/m3) 2900 2500

Table 2 Reported failure stresses for enamel (from

reference [4])

Reference Test typeFailurestress (MPa)

Stanford et al. [5] Compression 134–278Cooper and Smith [6] Punch shear 64–93Hannah [7] Tensile 30–35Bowen and Rodriguez [8] Tensile 10Tyldesley [9] Flexural (four-point

bending)76

Fig. 2 Tooth structure

2 R Lewis and R S Dwyer-Joyce

Proc. IMechE Vol. 219 Part J: J. Engineering Tribology J03404 # IMechE 2005

The two most important elements of the tooth fromthe tribological perspective are the outer enamel layerand the dentine, which lies underneath. Initially theenamel is exposed to the loads and chemical environ-ment within the mouth as a result of chewing etc. Ifthis layer is breached due to tooth fracture or wear,then the underlying dentine is exposed.

Enamel is thickest at the tip of each tooth (2–3 mm)and tapers to its thinnest at the cemento-enameljunction.

Enamel and dentine have very different structuresand properties (the properties of enamel and den-tine are given in Table 1). Enamel, which consistsof about 97 per cent inorganic material, is the mosthighly calcified and hardest tissue in the humanbody. The inorganic component is hydroxyapatitein the form of large elongated hexagonal crystals.These crystals extend through the thickness of theenamel and are nearly perpendicular to the surface.They are approximately uniform in size and have athickness of about 30 nm and a width of 60 nmwith lengths ranging between 100 and 500 nm [2].

Dentine has a much higher organic content thanenamel. It is composed of 70 per cent inorganicand 18 per cent organic material, with the remainderbeing water. The inorganic component, as withenamel, is hydroxyapatite. The crystals, however,are much smaller than those found in enamel,being 3 nm across and about 20 nm in length.Dentine is much softer and more ductile than

enamel (see Table 1). This means that it acts as acushioning layer to reduce the effect of chewingloads on the enamel.

As reported by Rees [3], failure loads for enamelvary widely (Table 2). This variation is probably dueto the difficulties in producing pure enamel speci-mens with no inherent flaws. Theoretical calcu-lations suggest that failure stresses for manymaterials lie in the range from E/5 to E/30, where Eis the elastic modulus of the material. However, forbrittle materials, because of flaws, this may benearer E/1000 [10]. Using this approach and avalue of 80 GPa for the elastic modulus of enamel,Rees et al. [4] calculated a theoretical failure stressfor enamel of 80 MPa. This is similar to the valuemeasured by Tyldesley [9].

Enamel has often been viewed as a homogeneousmaterial (see, for example, references [11] and[12]). Other investigations have shown, however,that mechanical properties of enamel vary withlocation on a tooth, local chemistry, and prism orien-tation. Results of microhardness testing [13] andcompression tests [14] indicated that the elasticmodulus E and hardness H may be slightly higherfor surface enamel than for subsurface enamel. Vick-ers indentation measurements have shown thatthese parameters obtained for a occlusal section ofenamel are generally higher than those obtained foran axial section [15].

Cuy et al. [16] carried out a comprehensive map-ping of enamel hardness and elastic modulus usinga nanoindentation process. Results indicated thatvalues of H were much higher than those previously

Fig. 1 Tooth orientation

Table 1 Mechanical properties of and enamel dentine [1]

Property Dentine Enamel

Young’s modulus (GPa) 10.2–15.6 20.0–84.2Shear modulus (GPa) 6.4–9.7 29Bulk modulus (GPa) 3.11–4.38 45, 65Poisson’s ratio 20.11–0.07 0.23, 0.30Compressive strength (GPa) 0.249–0.315 0.095–0.386Tensile strength (GPa) 0.040–0.276 0.030–0.035Shear strength (GPa) 0.012–0.138 0.06Knoop hardness 57–71 250–500Density (kg/m3) 2900 2500

Table 2 Reported failure stresses for enamel (from

reference [4])

Reference Test typeFailurestress (MPa)

Stanford et al. [5] Compression 134–278Cooper and Smith [6] Punch shear 64–93Hannah [7] Tensile 30–35Bowen and Rodriguez [8] Tensile 10Tyldesley [9] Flexural (four-point

bending)76

Fig. 2 Tooth structure



reported. At the enamel surface H . 6 GPa andE . 115 GPa, while enamel at the enamel–dentinejunction H , 3 GPa and E , 70 GPa. These variationswere shown to correspond to changes in the chem-istry, microstructure, and prism alignment. Mechan-ical properties of enamel were also shown to varybetween the lingual and buccal side of the molar.

Other studies have shown that enamel in thecervical region of a tooth (lower part near thecemento-enamel junction) has poorer propertiesthan enamel near the top of a tooth. Stanford et al.[5] showed that enamel in the cervical region has a30 per cent lower compressive strength. It has alsobeen shown that the crystal structure is barely defin-able in this region [17] and there are fewer areasof gnarled enamel [18], where the enamel rodsintertwine, which leads to greater fracture resistance.

1.2 Chewing parameters and chemistry ofthe mouth

The chemistry of the mouth is extremely complexand plays a critical role in the wear of teeth. One ofthe most important components of this chemistryis saliva. This has a number of functions in themouth, it acts as lubricant to make chewing andswallowing easier, it is a buffer to acids produced inplaque, and it supplies calcium and phosphateions, which act to remineralize enamel.

Saliva is pH 7 (neutral). However, the mouth ofa person with a particularly acidic diet could be atpH 3 and regurgitated gastric acid is pH 1.2. Softdrinks contain a range of different acids, whichcan range from pH 1 to pH 6. The UKs soft-drinkconsumption has risen by 56 per cent in the10 years before 1996, when 106 l were consumed[19]. This averages at 0.5 l per person per day. Thisimplies that exposure of teeth to an acid environ-ment is increasing, which will obviously have aneffect on wear of teeth. Chemical effects on teethwear mechanisms will be discussed in a later section.

A number of different techniques have been usedto determine chewing loads, which have shownthat they can be of the order of several hundredNewtons (Table 3). Tooth-on-tooth sliding distanceshave been measured and are around 0.9–1.2 mm[20]. During chewing, these contacts last about

100 ms and these periods add up to 15–30 min ofactual contact loading each day [21].

The different magnitudes of load and pH as well asfood slurry, environmental factors, teeth cleaning,etc., mean that the mouth provides an extremelycomplex tribological system. The key to understand-ing teeth wear mechanisms lies in interpreting howthese factors interact.

1.3 Wear types and terminology

The four terms attrition, abrasion, abfraction, anderosion are used in the dental literature for describ-ing the wear of teeth and dental materials. Attritiondescribes wear at sites of tooth to tooth (occlusal)contact; abrasion is used for wear at non-contactsites; abfraction (a relatively new term) refers toloss of enamel and dentine as a result of cracksformed during tooth flexure in the cervical regionof the tooth (Fig. 2); erosion is used to describematerial loss attributed to chemical effects.

These terms have caused confusion, becausethey are very different from the terminology usedby tribologists to describe similar wear processesand because they describe ‘clinical manifestations’rather than the underlying wear mechanisms [26].Mair et al. [27] suggested that each case should beconsidered in terms of its aetiology (causes) ratherthan its nomenclature. The aetiology of tooth wearmay be considered in terms of site, timing, andmechanism. The mechanisms and their interactionsare shown in Figs 3 and 4.

Definitions for the mechanisms are as follows.

Thegosis is the action of sliding teeth into lateralpositions. It has been suggested that this is agenetically determined habit originally establishedto sharpen teeth [28] (see Fig. 4a).

Bruxism is the action of grinding of teeth without thepresence of food (see Fig. 4a).

Table 3 Chewing loads

ReferenceMeasurementtechnique

Chewingload (N)

Anderson [22, 23] Strain gauge inlay 70–145Proeschel and

Morneburg [24]Intra-oral transducers Average 220

Morneburg andProeschel [25]

Strain gauges inimplant abutments

Average 220,maximum 450

Fig. 3 Tooth wear mechanisms and their interactions

(Adapted from reference [27])

Wear of human teeth: a tribological perspective 3


reported. At the enamel surface H . 6 GPa andE . 115 GPa, while enamel at the enamel–dentinejunction H , 3 GPa and E , 70 GPa. These variationswere shown to correspond to changes in the chem-istry, microstructure, and prism alignment. Mechan-ical properties of enamel were also shown to varybetween the lingual and buccal side of the molar.

Other studies have shown that enamel in thecervical region of a tooth (lower part near thecemento-enamel junction) has poorer propertiesthan enamel near the top of a tooth. Stanford et al.[5] showed that enamel in the cervical region has a30 per cent lower compressive strength. It has alsobeen shown that the crystal structure is barely defin-able in this region [17] and there are fewer areasof gnarled enamel [18], where the enamel rodsintertwine, which leads to greater fracture resistance.

1.2 Chewing parameters and chemistry ofthe mouth

The chemistry of the mouth is extremely complexand plays a critical role in the wear of teeth. One ofthe most important components of this chemistryis saliva. This has a number of functions in themouth, it acts as lubricant to make chewing andswallowing easier, it is a buffer to acids produced inplaque, and it supplies calcium and phosphateions, which act to remineralize enamel.

Saliva is pH 7 (neutral). However, the mouth ofa person with a particularly acidic diet could be atpH 3 and regurgitated gastric acid is pH 1.2. Softdrinks contain a range of different acids, whichcan range from pH 1 to pH 6. The UKs soft-drinkconsumption has risen by 56 per cent in the10 years before 1996, when 106 l were consumed[19]. This averages at 0.5 l per person per day. Thisimplies that exposure of teeth to an acid environ-ment is increasing, which will obviously have aneffect on wear of teeth. Chemical effects on teethwear mechanisms will be discussed in a later section.

A number of different techniques have been usedto determine chewing loads, which have shownthat they can be of the order of several hundredNewtons (Table 3). Tooth-on-tooth sliding distanceshave been measured and are around 0.9–1.2 mm[20]. During chewing, these contacts last about

100 ms and these periods add up to 15–30 min ofactual contact loading each day [21].

The different magnitudes of load and pH as well asfood slurry, environmental factors, teeth cleaning,etc., mean that the mouth provides an extremelycomplex tribological system. The key to understand-ing teeth wear mechanisms lies in interpreting howthese factors interact.

1.3 Wear types and terminology

The four terms attrition, abrasion, abfraction, anderosion are used in the dental literature for describ-ing the wear of teeth and dental materials. Attritiondescribes wear at sites of tooth to tooth (occlusal)contact; abrasion is used for wear at non-contactsites; abfraction (a relatively new term) refers toloss of enamel and dentine as a result of cracksformed during tooth flexure in the cervical regionof the tooth (Fig. 2); erosion is used to describematerial loss attributed to chemical effects.

These terms have caused confusion, becausethey are very different from the terminology usedby tribologists to describe similar wear processesand because they describe ‘clinical manifestations’rather than the underlying wear mechanisms [26].Mair et al. [27] suggested that each case should beconsidered in terms of its aetiology (causes) ratherthan its nomenclature. The aetiology of tooth wearmay be considered in terms of site, timing, andmechanism. The mechanisms and their interactionsare shown in Figs 3 and 4.

Definitions for the mechanisms are as follows.

Thegosis is the action of sliding teeth into lateralpositions. It has been suggested that this is agenetically determined habit originally establishedto sharpen teeth [28] (see Fig. 4a).

Bruxism is the action of grinding of teeth without thepresence of food (see Fig. 4a).

Table 3 Chewing loads

ReferenceMeasurementtechnique

Chewingload (N)

Anderson [22, 23] Strain gauge inlay 70–145Proeschel and

Morneburg [24]Intra-oral transducers Average 220

Morneburg andProeschel [25]

Strain gauges inimplant abutments

Average 220,maximum 450

Fig. 3 Tooth wear mechanisms and their interactions

(Adapted from reference [27])



Mastication is the action of chewing food that may con-tain abrasive particles. As shown in Figs 4c and d, twostages occur; during the first the teeth are broughttogether to a position of near contact (open phase),and the abrasive particles are suspended and freeto move in the food, which therefore acts as aslurry; in the second stage (closed phase), load isapplied to the food so that the abrasive particlesbecome trapped between the two tooth surfaces.

In the following sections, in vitro and in vivo testmethods for assessing wear are discussed. Thecurrent understanding of the wear mechanisms,shown in Figs 3 and 4, that lead to wear of enameland dentine are then outlined as well as the majorinfluencing factors on wear and how the mechan-isms interact.

2 METHODS OF WEAR ASSESSMENT

2.1 In vitro wear testing

As mentioned in the introduction, one of the mainreasons for studying the causes of tooth wear is to

improve tooth restorative materials. Clearly thisrequires a machine that will closely simulate the oralenvironment as well as loading cycles that occurduring chewing, etc.

Many attempts have been made to achieve thisand, as observed by Roulet [29], ‘there were almostas many wear testing devices as there were scien-tists who are interested in wear’ (de Gee and Pallav[30] include a comprehensive list of previouslyused in vitro test rigs to illustrate this point).Indeed most of the work cited in this paper hasbeen carried out on different test rigs, using differentloads, speeds, lubricants, abrasives, etc., whichmakes comparison of results difficult. To a certainextent the same problem has existed in the field oftribology, although a large of number of standardtest rigs do now exist. The same cannot be said ofdental research.

At the simplest level, pin-on-disc sliding test rigshave been utilized [31]. More complex reciprocatingwear test machines have been utilized to give a moreaccurate simulation of the sliding action of theteeth (such as those used in references [32] to [36].Other rigs have incorporated a unidirectional sliding

Fig. 4 Tooth wear mechanisms



Mastication is the action of chewing food that may con-tain abrasive particles. As shown in Figs 4c and d, twostages occur; during the first the teeth are broughttogether to a position of near contact (open phase),and the abrasive particles are suspended and freeto move in the food, which therefore acts as aslurry; in the second stage (closed phase), load isapplied to the food so that the abrasive particlesbecome trapped between the two tooth surfaces.

In the following sections, in vitro and in vivo testmethods for assessing wear are discussed. Thecurrent understanding of the wear mechanisms,shown in Figs 3 and 4, that lead to wear of enameland dentine are then outlined as well as the majorinfluencing factors on wear and how the mechan-isms interact.

2 METHODS OF WEAR ASSESSMENT

2.1 In vitro wear testing

As mentioned in the introduction, one of the mainreasons for studying the causes of tooth wear is to

improve tooth restorative materials. Clearly thisrequires a machine that will closely simulate the oralenvironment as well as loading cycles that occurduring chewing, etc.

Many attempts have been made to achieve thisand, as observed by Roulet [29], ‘there were almostas many wear testing devices as there were scien-tists who are interested in wear’ (de Gee and Pallav[30] include a comprehensive list of previouslyused in vitro test rigs to illustrate this point).Indeed most of the work cited in this paper hasbeen carried out on different test rigs, using differentloads, speeds, lubricants, abrasives, etc., whichmakes comparison of results difficult. To a certainextent the same problem has existed in the field oftribology, although a large of number of standardtest rigs do now exist. The same cannot be said ofdental research.

At the simplest level, pin-on-disc sliding test rigshave been utilized [31]. More complex reciprocatingwear test machines have been utilized to give a moreaccurate simulation of the sliding action of theteeth (such as those used in references [32] to [36].Other rigs have incorporated a unidirectional sliding

Fig. 4 Tooth wear mechanisms



motion, in which the test specimens return to theiroriginal position at the end of cycle (see, for example,reference [37]).

Most of the devices mentioned above have beenused for two-body wear studies. Clearly throughduring the chewing cycle, abrasive material is pre-sent in the mouth and so rigs have been designedto incorporate abrasive discs or slurries to accountfor this, such as the twin-disc approach used by deGee and Pallav [30] and the ball cratering rig usedby Vale Antunes and Ramalho [38].

A test rig for the study of abrasion and attrition hasbeen developed by Condon and Ferracane [39]. Ituses an extension of the unidirectional sliding inthat an enamel-tipped stylus is loaded against anddriven across a test material in one direction in thepresence of an abrasive slurry. At the end of eachpass, however, the vertical load is increased andthen released. The results have been shown to pro-duce a strong correlation with clinical observationsfor both abrasion and attrition. De Long and Douglas[40] developed the artificial mouth concept, whichallows natural teeth to be loaded in a manner thatsimulates physiological movement. A summary ofthe different test geometries used in tooth weartesting is shown in Fig. 5.

As with any test simulation, a number of defi-ciencies exist with the rigs described above thatmay restrict how well they represent reality. Thismay be that flat specimens are used, or water is used

as a lubricant, or no abrasive slurry is incorporated.It must also be noted that, except in a few cases, verylittle correlation exists between test results and clini-cal measurements. However, in many cases theserigs are used to rank materials and it could beargued that these are not large drawbacks. It mustbe understood that the oral environment is verycomplex and has many variables; as long as themost influential have been identified and can beused and controlled in the test rig being used, thisshould be satisfactory. In vitro testing offersresearchers much more control over experimentalvariables and the opportunity to take far more accu-rate wear measurements than in vivo testing, as willbe further highlighted in the next section.

As with test rigs to study wear due to contactbetween teeth, there are many different in vitrotoothbrushing test rigs. Again, they offer greater con-trol over experimental variables than in vivo testingbut suffer the same deficiencies as their contactwear counterparts. In situ testing provides a partialcompromise between in vivo and in vitro testing.As will be further explained in the toothbrushingand chemical effects sections, dentine or enamelspecimens are mounted in devices worn in themouth by subjects and then removed for ex vivo test-ing. Specimens can therefore be exposed to thechemical environment within the mouth, withoutexperiencing the varying loads from chewing, toothgrinding, etc.

Fig. 5 Dental wear test configurations



motion, in which the test specimens return to theiroriginal position at the end of cycle (see, for example,reference [37]).

Most of the devices mentioned above have beenused for two-body wear studies. Clearly throughduring the chewing cycle, abrasive material is pre-sent in the mouth and so rigs have been designedto incorporate abrasive discs or slurries to accountfor this, such as the twin-disc approach used by deGee and Pallav [30] and the ball cratering rig usedby Vale Antunes and Ramalho [38].

A test rig for the study of abrasion and attrition hasbeen developed by Condon and Ferracane [39]. Ituses an extension of the unidirectional sliding inthat an enamel-tipped stylus is loaded against anddriven across a test material in one direction in thepresence of an abrasive slurry. At the end of eachpass, however, the vertical load is increased andthen released. The results have been shown to pro-duce a strong correlation with clinical observationsfor both abrasion and attrition. De Long and Douglas[40] developed the artificial mouth concept, whichallows natural teeth to be loaded in a manner thatsimulates physiological movement. A summary ofthe different test geometries used in tooth weartesting is shown in Fig. 5.

As with any test simulation, a number of defi-ciencies exist with the rigs described above thatmay restrict how well they represent reality. Thismay be that flat specimens are used, or water is used

as a lubricant, or no abrasive slurry is incorporated.It must also be noted that, except in a few cases, verylittle correlation exists between test results and clini-cal measurements. However, in many cases theserigs are used to rank materials and it could beargued that these are not large drawbacks. It mustbe understood that the oral environment is verycomplex and has many variables; as long as themost influential have been identified and can beused and controlled in the test rig being used, thisshould be satisfactory. In vitro testing offersresearchers much more control over experimentalvariables and the opportunity to take far more accu-rate wear measurements than in vivo testing, as willbe further highlighted in the next section.

As with test rigs to study wear due to contactbetween teeth, there are many different in vitrotoothbrushing test rigs. Again, they offer greater con-trol over experimental variables than in vivo testingbut suffer the same deficiencies as their contactwear counterparts. In situ testing provides a partialcompromise between in vivo and in vitro testing.As will be further explained in the toothbrushingand chemical effects sections, dentine or enamelspecimens are mounted in devices worn in themouth by subjects and then removed for ex vivo test-ing. Specimens can therefore be exposed to thechemical environment within the mouth, withoutexperiencing the varying loads from chewing, toothgrinding, etc.

Fig. 5 Dental wear test configurations



2.2 In vivo tooth wear measurementand ranking

In vivo clinical studies of tooth wear have been notedto be problematic [41]. A number of mechanisms cancontribute to tooth wear, as explained in the preced-ing sections, and there is no evidence to showwhether any dominate. It is also impossible to isolateand vary key parameters that may influence wear.Clearly wear cannot be accelerated in vivo andstudies are reliant on volunteer compliance [42].While measures can be taken to try to unify testingconditions amongst the subjects, e.g. dietary regimescan be set and checks made on living/workingenvironment and possible recent illness to removethe likelihood that unanticipated factors affect testresults [43], it is still evident that each subject isindeed a variable, which leads to problems in inter-preting results.

The sensitivity of the measurement techniqueis also an important consideration. Lambrechtset al. [44] quantified enamel wear over a 48 monthperiod using impressions of teeth that were com-pared using stereomicroscopy images and compu-terised image fitting. Savill et al. [45] used a similarapproach. Replicas were taken of teeth, whichwere then digitized using a coordinate-measuringmachine fitted with an infrared probe. Imagestaken at different times were overlaid to determineany morphological changes. These, however, areadvanced techniques using expensive equipment.Most studies have used much less sophisticatedqualitative approaches involving subjective compari-son of surface features.

There are a number of indices for classifying toothwear. Most are based on a quantification of wear lossby evaluation of changes in the occlusal surface. Thiscan be combined with estimations of the degree ofworn enamel and the amount of exposed dentin[46, 47]. In others a qualitative assessment is com-bined with an estimation of the need for treatment[48]. Both types, however, have elements that relyon a degree of subjective evaluation. This, as wellas the fact that they are hard to use, has restrictedtheir use in research studies [41].

3 WEAR AT SITES OF OCCLUSAL CONTACT

3.1 Thegosis and bruxism

Most work on wear of enamel and dentine due toocclusal contact with sliding (as a result of thegosisand bruxism) has been in the form of in vitro exper-iments carried out on simple wear test apparatus. Alarge variety of test methods have been used with dif-fering contact geometries, loads, sliding speeds,lubricants, etc., which makes comparison of results

quite difficult. It is, however, possible to identifycommon trends as will be evident in the materialpresented.

Kaidonis et al. [32] carried out enamel-against-enamel unidirectional sliding wear tests (3 mmstroke length; 1.33 Hz). Extracted teeth were used,with each tooth being halved and then used in anupright position (crown parts in contact) as the topand bottom specimens in the tests. Wear was quan-tified using mass loss measurements. Initial testsrevealed that wear of enamel appeared to beindependent of sliding velocity.

Figure 6 shows results for a test run with a load of3.2 kg in dry conditions. Two phases of wear areapparent, which were also observed at all otherloads at which tests were run. This pattern hasbeen observed in other in vitro experimentation[49] and is also consistent with the results of clinicaltrials [44]. During the clinical studies, primary andsecondary phases described as running-in wear andsteady-state wear were identified. The initial phaseappeared to last for a period of 2 years beforetransition to the slower secondary phase.

The running-in wear phenomenon is commonlyseen in engineering contacts. It is attributed to theprocess of asperity smoothing, which leads to amore conformal contact and a subsequent reductionin the wear rate. It is probably the same process seenwith the enamel testing.

Kaidonis et al. [32] also conducted tests over arange of different loads. The different wear ratesseen in each phase of wear are shown in Fig. 7 andclearly increase as the load is increased. The resultsof tests for dry conditions and a pH 7 lubricant(water) at varying loads are shown in Fig. 8. Enamelwear rates increased steadily with increasing load.Similar magnitudes of wear were observed by Shaba-nian and Richards [50] for lubricated conditions(pH 7). It was postulated that three-body abrasion

Fig. 6 Wear of enamel specimens versus number of

test cycles (enamel versus enamel dry sliding

tests at 3.2 kg) [32]



2.2 In vivo tooth wear measurementand ranking

In vivo clinical studies of tooth wear have been notedto be problematic [41]. A number of mechanisms cancontribute to tooth wear, as explained in the preced-ing sections, and there is no evidence to showwhether any dominate. It is also impossible to isolateand vary key parameters that may influence wear.Clearly wear cannot be accelerated in vivo andstudies are reliant on volunteer compliance [42].While measures can be taken to try to unify testingconditions amongst the subjects, e.g. dietary regimescan be set and checks made on living/workingenvironment and possible recent illness to removethe likelihood that unanticipated factors affect testresults [43], it is still evident that each subject isindeed a variable, which leads to problems in inter-preting results.

The sensitivity of the measurement techniqueis also an important consideration. Lambrechtset al. [44] quantified enamel wear over a 48 monthperiod using impressions of teeth that were com-pared using stereomicroscopy images and compu-terised image fitting. Savill et al. [45] used a similarapproach. Replicas were taken of teeth, whichwere then digitized using a coordinate-measuringmachine fitted with an infrared probe. Imagestaken at different times were overlaid to determineany morphological changes. These, however, areadvanced techniques using expensive equipment.Most studies have used much less sophisticatedqualitative approaches involving subjective compari-son of surface features.

There are a number of indices for classifying toothwear. Most are based on a quantification of wear lossby evaluation of changes in the occlusal surface. Thiscan be combined with estimations of the degree ofworn enamel and the amount of exposed dentin[46, 47]. In others a qualitative assessment is com-bined with an estimation of the need for treatment[48]. Both types, however, have elements that relyon a degree of subjective evaluation. This, as wellas the fact that they are hard to use, has restrictedtheir use in research studies [41].

3 WEAR AT SITES OF OCCLUSAL CONTACT

3.1 Thegosis and bruxism

Most work on wear of enamel and dentine due toocclusal contact with sliding (as a result of thegosisand bruxism) has been in the form of in vitro exper-iments carried out on simple wear test apparatus. Alarge variety of test methods have been used with dif-fering contact geometries, loads, sliding speeds,lubricants, etc., which makes comparison of results

quite difficult. It is, however, possible to identifycommon trends as will be evident in the materialpresented.

Kaidonis et al. [32] carried out enamel-against-enamel unidirectional sliding wear tests (3 mmstroke length; 1.33 Hz). Extracted teeth were used,with each tooth being halved and then used in anupright position (crown parts in contact) as the topand bottom specimens in the tests. Wear was quan-tified using mass loss measurements. Initial testsrevealed that wear of enamel appeared to beindependent of sliding velocity.

Figure 6 shows results for a test run with a load of3.2 kg in dry conditions. Two phases of wear areapparent, which were also observed at all otherloads at which tests were run. This pattern hasbeen observed in other in vitro experimentation[49] and is also consistent with the results of clinicaltrials [44]. During the clinical studies, primary andsecondary phases described as running-in wear andsteady-state wear were identified. The initial phaseappeared to last for a period of 2 years beforetransition to the slower secondary phase.

The running-in wear phenomenon is commonlyseen in engineering contacts. It is attributed to theprocess of asperity smoothing, which leads to amore conformal contact and a subsequent reductionin the wear rate. It is probably the same process seenwith the enamel testing.

Kaidonis et al. [32] also conducted tests over arange of different loads. The different wear ratesseen in each phase of wear are shown in Fig. 7 andclearly increase as the load is increased. The resultsof tests for dry conditions and a pH 7 lubricant(water) at varying loads are shown in Fig. 8. Enamelwear rates increased steadily with increasing load.Similar magnitudes of wear were observed by Shaba-nian and Richards [50] for lubricated conditions(pH 7). It was postulated that three-body abrasion

Fig. 6 Wear of enamel specimens versus number of

test cycles (enamel versus enamel dry sliding

tests at 3.2 kg) [32]



was occurring and that the enamel particles in thecontact acted as a dry lubricant, although no visualevidence was offered to back this up. The results ofthe lubricated tests, however, may add weight tothis argument. Whereas at light loads the lubricantreduced wear rates to below those of the dry tests,as the load was increased, a threshold was reachedabove which the wear rates increased substantially.It was observed that at these loads the water wasdisplaced from the contact, also washing away anyparticles that may have acted as a solid lubricant.

Results are also shown in Fig. 8 for wear of dentine(versus enamel) [51] from tests carried out on thesame test apparatus using the same test parametersas Kaidonis et al. [32]. The differences in wear ratesat low loads were explained by the different struc-tures of the two materials. At lower loads the highmineral content of enamel and its correspondinghardness result in relatively low wear rates comparedwith dentine, with its higher organic content andrelative softness. At higher loads, however, the brittle

nature of enamel was thought to contribute to itshigher wear rate. Dentine has a connective tissuematrix, which makes it less prone to fracture underthese conditions.

The load thresholds observed, above which wear ofenamel increases rapidly, are much lower than loadsassociated with bruxism [52].

Zheng et al. [34] used a reciprocating wear testapparatus with artificial saliva to study the wearrate of enamel and dentine (from extracted teeth)against titanium balls. Wear behaviour and changein hardness in directions parallel and perpendicularto the occlusal surface were investigated (Fig. 9).Tests were carried out with a force of 20 N, an ampli-tude of 500 mm and a frequency of 2 Hz.

Figure 10 shows hardness measurements takenfrom the occlusal surface down into a tooth. Wearscar depths and hardness decrease as test locationsmove through the enamel and dento-enamel junc-tion to the dentine. Hardness decreases by 17 percent from the outer surface of the enamel to thedento-enamel junction and by a further 77 per centfrom the dento-enamel junction to the dentine.Different occlusal layers of enamel in a tooth clearlyexhibit different properties. This ties in with theobservations of Cuy et al. [16], mentioned in thesection on tooth structure and orientation.

Significant differences were noted in the wearscar morphologies between the enamel and dentine.Many particles were observed on the surface ofthe worn enamel, whereas ploughing marks wereobserved in the surface of the dentine in the slidingdirection. Based on the fact that enamel is morebrittle than dentine (see Table 1) it was inferredthat the enamel particles were mainly the productof microcracking and the ploughs as a result ofplastic deformation.

Fig. 7 Mean wear rates of the primary and secondary

phases at different loads [32]

Fig. 9 Wear test orientations: (a) parallel to the

occlusal surface (DEJ, dento-enamel junction);

(b) perpendicular to the occlusal surface [34]

Fig. 8 Wear of enamel [32] and dentine [51] against

load (enamel or dentine versus enamel sliding

tests for 89 � 103 cycles)



was occurring and that the enamel particles in thecontact acted as a dry lubricant, although no visualevidence was offered to back this up. The results ofthe lubricated tests, however, may add weight tothis argument. Whereas at light loads the lubricantreduced wear rates to below those of the dry tests,as the load was increased, a threshold was reachedabove which the wear rates increased substantially.It was observed that at these loads the water wasdisplaced from the contact, also washing away anyparticles that may have acted as a solid lubricant.

Results are also shown in Fig. 8 for wear of dentine(versus enamel) [51] from tests carried out on thesame test apparatus using the same test parametersas Kaidonis et al. [32]. The differences in wear ratesat low loads were explained by the different struc-tures of the two materials. At lower loads the highmineral content of enamel and its correspondinghardness result in relatively low wear rates comparedwith dentine, with its higher organic content andrelative softness. At higher loads, however, the brittle

nature of enamel was thought to contribute to itshigher wear rate. Dentine has a connective tissuematrix, which makes it less prone to fracture underthese conditions.

The load thresholds observed, above which wear ofenamel increases rapidly, are much lower than loadsassociated with bruxism [52].

Zheng et al. [34] used a reciprocating wear testapparatus with artificial saliva to study the wearrate of enamel and dentine (from extracted teeth)against titanium balls. Wear behaviour and changein hardness in directions parallel and perpendicularto the occlusal surface were investigated (Fig. 9).Tests were carried out with a force of 20 N, an ampli-tude of 500 mm and a frequency of 2 Hz.

Figure 10 shows hardness measurements takenfrom the occlusal surface down into a tooth. Wearscar depths and hardness decrease as test locationsmove through the enamel and dento-enamel junc-tion to the dentine. Hardness decreases by 17 percent from the outer surface of the enamel to thedento-enamel junction and by a further 77 per centfrom the dento-enamel junction to the dentine.Different occlusal layers of enamel in a tooth clearlyexhibit different properties. This ties in with theobservations of Cuy et al. [16], mentioned in thesection on tooth structure and orientation.

Significant differences were noted in the wearscar morphologies between the enamel and dentine.Many particles were observed on the surface ofthe worn enamel, whereas ploughing marks wereobserved in the surface of the dentine in the slidingdirection. Based on the fact that enamel is morebrittle than dentine (see Table 1) it was inferredthat the enamel particles were mainly the productof microcracking and the ploughs as a result ofplastic deformation.

Fig. 7 Mean wear rates of the primary and secondary

phases at different loads [32]

Fig. 9 Wear test orientations: (a) parallel to the

occlusal surface (DEJ, dento-enamel junction);

(b) perpendicular to the occlusal surface [34]

Fig. 8 Wear of enamel [32] and dentine [51] against

load (enamel or dentine versus enamel sliding

tests for 89 � 103 cycles)



This inference was backed up by the results ofindentation work carried out by Xu et al. [15] onenamel and dentine. This indicated that, duringindentation of enamel, cracks formed but nonecould be seen in dentine. It led to the conclusionthat enamel wear results from a microfracture pro-cess and dentine wear as a result of a ductile chipformation.

Figure 11 shows how the depth of wear scar variedat the different test locations and with test orien-tation. This is consistent with observations made byXu et al. [15]. The differences in wear rates betweentests sliding parallel to and perpendicular to theocclusal surface were attributed to the orientationof the crystals making up the enamel structure.

3.2 Mastication (closed phase)

During the closed phase of mastication (see Fig. 4),food particles are trapped between the tooth surfaces

and can cause abrasion to the enamel surface. Theentrapment of particles is clearly influenced by thenature of the contacting surfaces. Rougher surfaces,for example, may trap more particles than asmooth surface, although this will depend on therelative size of the particles compared with theroughness. It has been shown that scratches inenamel as a result of thegosis may act as particletraps during mastication [28].

Most work studying abrasive wear due to mastica-tion has been carried out on restorative materialsand has focused simply on ranking wear perform-ance of different materials, rather than investigatingthe mechanism of wear [38, 53].

Maas [54] carried out in vitro compression tests toinvestigate the microscopic wear features thatappear on the occlusal surfaces of teeth, as a resultof abrasion by food particles (typically categorizedas pits with length-to-width ratio less than 4 or stria-tions with length-to-width ratio greater than 4).Abrasive particles (silicon carbide) of various sizesand different loads were used to study the generationof these features in enamel. Two loads were used,namely 50 and 100 kg, and particle sizes of 14, 23,and 73 mm. It is not clear whether the silicon carbideor the particle sizes are representative of particlesfound in food slurry. In the tests, specimens ofequal size were covered in a single layer of theabrasive particles (hence far more small particleswere used than larger particles). Unsurprisingly,results showed that the larger particles producedfewer larger wear features than the small particles.Total wear was determined using an area approach,which showed that the total wear area increasedwith increasing particle size. This goes against thenormal abrasive wear expectation, in which smallerparticles would produce more wear simply becausethere are far more present. This issue, however, wasnot addressed. Interestingly, wear seemed to beindependent of load. The results were very differentfrom a study carried out with shearing, whichshould simulate more what actually occurs duringchewing [55].

4 WEAR AT CONTACT-FREE SITES

4.1 Mastication (open phase)

Very little work has been attempted to study abrasivewear due to food slurry with no tooth contact. Thisis probably because it is a minor problem com-pared with the other tooth wear mechanisms asloads are low. Mair et al. [27] noted, however, thatit is a problem with some composites used intooth restoration. Tests have also shown that itmay cause a problem in the cavities in the surface

Fig. 10 Variations in wear depth and hardness as

functions of distance from the occlusal

surface (versus titanium balls) [34]

Fig. 11 A comparison of wear depth between different

contact zones for two different orientations

(versus titanium balls) [34] (DEJ, dento-

enamel junction)



This inference was backed up by the results ofindentation work carried out by Xu et al. [15] onenamel and dentine. This indicated that, duringindentation of enamel, cracks formed but nonecould be seen in dentine. It led to the conclusionthat enamel wear results from a microfracture pro-cess and dentine wear as a result of a ductile chipformation.

Figure 11 shows how the depth of wear scar variedat the different test locations and with test orien-tation. This is consistent with observations made byXu et al. [15]. The differences in wear rates betweentests sliding parallel to and perpendicular to theocclusal surface were attributed to the orientationof the crystals making up the enamel structure.

3.2 Mastication (closed phase)

During the closed phase of mastication (see Fig. 4),food particles are trapped between the tooth surfaces

and can cause abrasion to the enamel surface. Theentrapment of particles is clearly influenced by thenature of the contacting surfaces. Rougher surfaces,for example, may trap more particles than asmooth surface, although this will depend on therelative size of the particles compared with theroughness. It has been shown that scratches inenamel as a result of thegosis may act as particletraps during mastication [28].

Most work studying abrasive wear due to mastica-tion has been carried out on restorative materialsand has focused simply on ranking wear perform-ance of different materials, rather than investigatingthe mechanism of wear [38, 53].

Maas [54] carried out in vitro compression tests toinvestigate the microscopic wear features thatappear on the occlusal surfaces of teeth, as a resultof abrasion by food particles (typically categorizedas pits with length-to-width ratio less than 4 or stria-tions with length-to-width ratio greater than 4).Abrasive particles (silicon carbide) of various sizesand different loads were used to study the generationof these features in enamel. Two loads were used,namely 50 and 100 kg, and particle sizes of 14, 23,and 73 mm. It is not clear whether the silicon carbideor the particle sizes are representative of particlesfound in food slurry. In the tests, specimens ofequal size were covered in a single layer of theabrasive particles (hence far more small particleswere used than larger particles). Unsurprisingly,results showed that the larger particles producedfewer larger wear features than the small particles.Total wear was determined using an area approach,which showed that the total wear area increasedwith increasing particle size. This goes against thenormal abrasive wear expectation, in which smallerparticles would produce more wear simply becausethere are far more present. This issue, however, wasnot addressed. Interestingly, wear seemed to beindependent of load. The results were very differentfrom a study carried out with shearing, whichshould simulate more what actually occurs duringchewing [55].

4 WEAR AT CONTACT-FREE SITES

4.1 Mastication (open phase)

Very little work has been attempted to study abrasivewear due to food slurry with no tooth contact. Thisis probably because it is a minor problem com-pared with the other tooth wear mechanisms asloads are low. Mair et al. [27] noted, however, thatit is a problem with some composites used intooth restoration. Tests have also shown that itmay cause a problem in the cavities in the surface

Fig. 10 Variations in wear depth and hardness as

functions of distance from the occlusal

surface (versus titanium balls) [34]

Fig. 11 A comparison of wear depth between different

contact zones for two different orientations

(versus titanium balls) [34] (DEJ, dento-

enamel junction)



of teeth that form part of the food-sheddingsystem [56].

4.2 Toothbrushing

A comprehensive study of the role that abrasive par-ticles in toothpastes and toothbrushing play in toothwear has been carried out in which the literaturefrom 1966 to 2002 was searched for data [41] relatingto abrasion of enamel and dentine. The main findingsof the work were that, while there was a small amountof concern regarding the potential for tooth wear, mostinvestigations concluded that the abrasives wouldhave a minimal effect on enamel, their effect beingmore pronounced on dentine and dental restoratives.As a result, most work on toothpaste abrasivity hasbeen conducted on these materials [57–62].

Most of the tests carried out have been in vitro, ona wide variety of testing machines. More recently,in situ tests have been carried out to study the effects

of abrasive wear of dentine [63] and enamel [64].In these tests, dentine or enamel specimens weremounted on removable appliances worn in themouth. The specimens were removed and brushedex vivo for a set time. Results for both materials indi-cated that wear was very low, as shown in Fig. 12, butmeasurable abrasion occurred and varying abrasionrates were found between different toothpastes.Wear rates for dentine were an order of magnitudehigher than those for enamel.

While initially studies dealt with the effect ofthe toothpaste, work has been carried out to estab-lish the effect of toothbrush design on abrasion[45, 58, 65] and other factors such as brushing timeand brushing force and speed [58, 60, 66]. The tooth-paste was, however, found to have the most signifi-cant effect on abrasion.

It is interesting to note that tests to simulateautomatic versus manual brushing showed thatautomatic gave lower abrasive wear for the sametoothpaste [58, 65], as shown in Fig. 13.

Fig. 12 In situ wear measurements: (a) enamel after 4 weeks with ex vivo cleaning twice a day for

30 s [64]; (b) dentine after 5 and 10 days with ex vivo cleaning five times a day for 60 s [65]

Fig. 13 Wear data for toothbrushing tests to simulate manual (M) and automatic (A) brushes [65]



of teeth that form part of the food-sheddingsystem [56].

4.2 Toothbrushing

A comprehensive study of the role that abrasive par-ticles in toothpastes and toothbrushing play in toothwear has been carried out in which the literaturefrom 1966 to 2002 was searched for data [41] relatingto abrasion of enamel and dentine. The main findingsof the work were that, while there was a small amountof concern regarding the potential for tooth wear, mostinvestigations concluded that the abrasives wouldhave a minimal effect on enamel, their effect beingmore pronounced on dentine and dental restoratives.As a result, most work on toothpaste abrasivity hasbeen conducted on these materials [57–62].

Most of the tests carried out have been in vitro, ona wide variety of testing machines. More recently,in situ tests have been carried out to study the effects

of abrasive wear of dentine [63] and enamel [64].In these tests, dentine or enamel specimens weremounted on removable appliances worn in themouth. The specimens were removed and brushedex vivo for a set time. Results for both materials indi-cated that wear was very low, as shown in Fig. 12, butmeasurable abrasion occurred and varying abrasionrates were found between different toothpastes.Wear rates for dentine were an order of magnitudehigher than those for enamel.

While initially studies dealt with the effect ofthe toothpaste, work has been carried out to estab-lish the effect of toothbrush design on abrasion[45, 58, 65] and other factors such as brushing timeand brushing force and speed [58, 60, 66]. The tooth-paste was, however, found to have the most signifi-cant effect on abrasion.

It is interesting to note that tests to simulateautomatic versus manual brushing showed thatautomatic gave lower abrasive wear for the sametoothpaste [58, 65], as shown in Fig. 13.

Fig. 12 In situ wear measurements: (a) enamel after 4 weeks with ex vivo cleaning twice a day for

30 s [64]; (b) dentine after 5 and 10 days with ex vivo cleaning five times a day for 60 s [65]

Fig. 13 Wear data for toothbrushing tests to simulate manual (M) and automatic (A) brushes [65]



Work carried out recently by Lewis et al. [67] hasstudied how abrasive particles interact with tooth-brush filaments, which has furthered understandingof what is happening in a tooth cleaning contact(Fig. 14).

4.3 Tooth flexure

Abfraction, a relatively new term coined by Grippo[68], relates to cracking and subsequent enamel lossin the cervical region of a tooth, around thecemento-enamel junction (see Fig. 2). This is thoughtto be caused by shear stresses resulting from toothflexure, which lead to disruption of the bondsbetween the hydroxyapatite crystals that make upthe structure of enamel [69, 70]. The enamel in thisregion of the tooth has been noted to be less resilient(see section on tooth orientation and structure).

Finite element studies [71] and strain gaugemeasurements [72] have been used to study thestresses in the cervical area of a tooth and have shownthat they exceed the failure stresses for enamel(shown in Table 2). Fracture of the enamel is mostlikely to occur along the boundaries of the hydroxy-apatite crystals [73]. In the cervical region the crystalsare approximately horizontal (perpendicular to thesurface), which is the same direction as the principalstresses determined by Rees [71].

Clinical observations had suggested that damagedue to abfraction was more prevalent in maxillary(upper jaw) incisors [68, 74]. A finite element studyof stresses induced in three different tooth types [4]indicated that stresses in maxillary incisors were sev-eral times larger than those in canines and molars,which explained these observations. The reason forthe difference in stresses between the tooth types islikely to be related to the area of periontologicalligament associated with each tooth and theirmobility under loading. Rudd et al. [75] examined

horizontal displacement of teeth under load andfound that the mobility of incisors was greater thanthat of canines and molars. Jepson [76] calculatedthat incisors also had a smaller ligament area. Thisindicates that incisors are less well adapted tomanage applied occlusal loads.

Damage due to abfraction has been reported tooccur more commonly amongst bruxists [77, 78],where occlusal loads and hence tooth excursionsare greater.

5 CHEMICAL EFFECTS

5.1 Influence on mechanical properties

Increasing acidity (decreasing pH) has been clearlyshown to decrease both the hardness and the elasticmodulus of enamel [79, 80]. Figure 15 shows how thehardness and reduced elastic modulus of enamelchanges with exposure to citric acid of various pHvalues [79]. Extracted teeth were used in the testsand immersed in 50 ml of the appropriate solution,which was being stirred. The exposure time was120 s. This was based on the clearance time for citricacid in vivo [81, 82]. The values of modulus andhardness were determined using nanoindentation.The reduction in hardness implies that wear rates ofenamel in an acidic environment will increase. To acertain extent this is a correct assumption as willbecome evident in the subsequent discussions.

Figure 16 shows enamel hardness against exposuretime for water, Bordeaux red wine, and Coca-ColaTM

[80]. Clearly hardness decreases with increasingexposure time; so consideration of this parameter isimportant in assessing and comparing results. Winetasters in the Bordeaux region appear to be unlikelyto suffer too many ill effects to their teeth fromtheir job, however!

Fig. 14 Particle trapping at toothbrush filament tips [67]



Work carried out recently by Lewis et al. [67] hasstudied how abrasive particles interact with tooth-brush filaments, which has furthered understandingof what is happening in a tooth cleaning contact(Fig. 14).

4.3 Tooth flexure

Abfraction, a relatively new term coined by Grippo[68], relates to cracking and subsequent enamel lossin the cervical region of a tooth, around thecemento-enamel junction (see Fig. 2). This is thoughtto be caused by shear stresses resulting from toothflexure, which lead to disruption of the bondsbetween the hydroxyapatite crystals that make upthe structure of enamel [69, 70]. The enamel in thisregion of the tooth has been noted to be less resilient(see section on tooth orientation and structure).

Finite element studies [71] and strain gaugemeasurements [72] have been used to study thestresses in the cervical area of a tooth and have shownthat they exceed the failure stresses for enamel(shown in Table 2). Fracture of the enamel is mostlikely to occur along the boundaries of the hydroxy-apatite crystals [73]. In the cervical region the crystalsare approximately horizontal (perpendicular to thesurface), which is the same direction as the principalstresses determined by Rees [71].

Clinical observations had suggested that damagedue to abfraction was more prevalent in maxillary(upper jaw) incisors [68, 74]. A finite element studyof stresses induced in three different tooth types [4]indicated that stresses in maxillary incisors were sev-eral times larger than those in canines and molars,which explained these observations. The reason forthe difference in stresses between the tooth types islikely to be related to the area of periontologicalligament associated with each tooth and theirmobility under loading. Rudd et al. [75] examined

horizontal displacement of teeth under load andfound that the mobility of incisors was greater thanthat of canines and molars. Jepson [76] calculatedthat incisors also had a smaller ligament area. Thisindicates that incisors are less well adapted tomanage applied occlusal loads.

Damage due to abfraction has been reported tooccur more commonly amongst bruxists [77, 78],where occlusal loads and hence tooth excursionsare greater.

5 CHEMICAL EFFECTS

5.1 Influence on mechanical properties

Increasing acidity (decreasing pH) has been clearlyshown to decrease both the hardness and the elasticmodulus of enamel [79, 80]. Figure 15 shows how thehardness and reduced elastic modulus of enamelchanges with exposure to citric acid of various pHvalues [79]. Extracted teeth were used in the testsand immersed in 50 ml of the appropriate solution,which was being stirred. The exposure time was120 s. This was based on the clearance time for citricacid in vivo [81, 82]. The values of modulus andhardness were determined using nanoindentation.The reduction in hardness implies that wear rates ofenamel in an acidic environment will increase. To acertain extent this is a correct assumption as willbecome evident in the subsequent discussions.

Figure 16 shows enamel hardness against exposuretime for water, Bordeaux red wine, and Coca-ColaTM

[80]. Clearly hardness decreases with increasingexposure time; so consideration of this parameter isimportant in assessing and comparing results. Winetasters in the Bordeaux region appear to be unlikelyto suffer too many ill effects to their teeth fromtheir job, however!

Fig. 14 Particle trapping at toothbrush filament tips [67]



5.2 Effect on material loss (with no loador sliding)

Figure 15a shows an approximately linear relation-ship between enamel hardness and solution pH.Tests to measure material loss have revealed differ-ent trends (extracted teeth were used in the testsand agitated in the appropriate solution, materialloss being determined using profilometry), however,as shown by the example in Fig. 17a [83]. This couldbe for a number of reasons; the exposure time wasmuch longer for these tests (30 min) and sampleswere agitated. This is a good example of the generallack of uniformity in dental test methods, whichmeans that only broad trends can be drawn whencomparing data. The results for dentine specimensin the same publication (Fig. 17b), for example,were only exposed for 10 min, meaning that directcomparison with the enamel is impossible! It isclear, however, that material loss increases with

decreasing pH. The acids used in the tests are typicalof those found in soft drinks. Clearly the citricacids (CANaOH–citric acid adjusted with sodiumhydroxide and CATC–citric acid with titratableacidity) give the highest material losses. The resultsare a good guide to drinks manufacturers onwhat acids may be detrimental to teeth and whatlevels of pH may be unacceptable.

5.3 Material loss with load applied (no sliding)

Wear also increases when a static or cyclic load isapplied to enamel, while exposed to an acidicenvironment [84, 85]. Tests carried out to determinewear of enamel in different areas of a tooth (Fig. 18)as a result of no load application (for 28 h) and simu-lated occlusal cyclic loading (0–100 N at 2 Hz for200 000 cycles; 28 h) in lactic acid (pH 4.5) indicatedthat load application clearly increases the wear rate[85] (Fig. 19) (extracted teeth were used and eitherplaced in a bath of solution or mounted in a servo-hydraulic test machine and surrounded by theappropriate solution, while being cyclically loaded,material loss was determined using profilometry ofimpressions of the test teeth). Wear was found tobe higher in the cervical region of a tooth. This tiesin with the observations, noted in the section ontooth structure and orientation, that enamel in thisregion of the tooth is thinner, softer, and less resilientthan enamel nearer the top of the tooth.

5.4 Material loss with load and sliding

Wear rates with the application of load and slidinghave shown slightly different trends from thoseseen with load and no load in an acidic environment.Tests have indicated that wear at around pH 3 is

Fig. 16 Change in enamel hardness with time during

exposure to Coca Cola, water and Bordeaux

red wine [80]

Fig. 15 Median (a) hardness and (b) reduced elastic modulus of enamel samples exposed to citric

acid compared with an untreated sample [79]



5.2 Effect on material loss (with no loador sliding)

Figure 15a shows an approximately linear relation-ship between enamel hardness and solution pH.Tests to measure material loss have revealed differ-ent trends (extracted teeth were used in the testsand agitated in the appropriate solution, materialloss being determined using profilometry), however,as shown by the example in Fig. 17a [83]. This couldbe for a number of reasons; the exposure time wasmuch longer for these tests (30 min) and sampleswere agitated. This is a good example of the generallack of uniformity in dental test methods, whichmeans that only broad trends can be drawn whencomparing data. The results for dentine specimensin the same publication (Fig. 17b), for example,were only exposed for 10 min, meaning that directcomparison with the enamel is impossible! It isclear, however, that material loss increases with

decreasing pH. The acids used in the tests are typicalof those found in soft drinks. Clearly the citricacids (CANaOH–citric acid adjusted with sodiumhydroxide and CATC–citric acid with titratableacidity) give the highest material losses. The resultsare a good guide to drinks manufacturers onwhat acids may be detrimental to teeth and whatlevels of pH may be unacceptable.

5.3 Material loss with load applied (no sliding)

Wear also increases when a static or cyclic load isapplied to enamel, while exposed to an acidicenvironment [84, 85]. Tests carried out to determinewear of enamel in different areas of a tooth (Fig. 18)as a result of no load application (for 28 h) and simu-lated occlusal cyclic loading (0–100 N at 2 Hz for200 000 cycles; 28 h) in lactic acid (pH 4.5) indicatedthat load application clearly increases the wear rate[85] (Fig. 19) (extracted teeth were used and eitherplaced in a bath of solution or mounted in a servo-hydraulic test machine and surrounded by theappropriate solution, while being cyclically loaded,material loss was determined using profilometry ofimpressions of the test teeth). Wear was found tobe higher in the cervical region of a tooth. This tiesin with the observations, noted in the section ontooth structure and orientation, that enamel in thisregion of the tooth is thinner, softer, and less resilientthan enamel nearer the top of the tooth.

5.4 Material loss with load and sliding

Wear rates with the application of load and slidinghave shown slightly different trends from thoseseen with load and no load in an acidic environment.Tests have indicated that wear at around pH 3 is

Fig. 16 Change in enamel hardness with time during

exposure to Coca Cola, water and Bordeaux

red wine [80]

Fig. 15 Median (a) hardness and (b) reduced elastic modulus of enamel samples exposed to citric

acid compared with an untreated sample [79]



lower than that at pH 7 [32, 35, 36] but, for pH 1,wear increases to a level well above that at pH 7[32, 50]. Results of sliding tests carried out byKaidonis et al. [32], illustrating this effect, areshown in Fig. 20 (for test details see section onthegosis and bruxism).

With sliding, wear rates would be expected toincrease, as a synergistic effect is likely to occur.The outer layer of enamel will be softened by theeffect of the acidic solution and its hardness willdecrease. The application of load and sliding willthen break this layer down causing its removal.This will expose fresh enamel, which will be soft-ened, and the process is repeated. Increasing theacidity of the solution would therefore be expectedto increase wear as hardness decreases linearly withdecreasing pH.

Eisenburger and Addy [35, 36], in similar workcarried out to investigate closely the interaction ofchemical effects with enamel to enamel sliding

contact, offered a possible explanation for this appar-ent anomaly. Observation of the wear surfaces for thedifferent conditions revealed that the specimens inthe more acidic solutions had smoother surfaces. Itwas hypothesized that this would decrease the fric-tion at the specimen interface and thereby reducefatigue wear. Eisenburger and Addy [35, 36], how-ever, did not test up to pH 1 and therefore offeredno reason for any change at this level of acidity.

Tests run to study the interaction of wear due totoothbrushing [86] and mastication [87] with chemi-cal effects have shown that wear increases withdecreasing pH. In these situations, however, thewear is being caused by abrasive particles ratherthan by an opposing surface undergoing a similarwear process.

In situ testing, similar to that described for tooth-brushing (see section on toothbrushing), has beenused to evaluate erosive wear of enamel and dentine[88, 89]. These tests consisted of mounting enamelor dentine specimens on to an intra-oral device,which were then exposed to an acid environment(with orange juice for example). Tissue loss was thenrecorded using surface profilometry. These testsshowed lower material losses than in vitro tests. Onereason for this may be that erosion in vivo isinfluenced by factors including remineralizationand pellicle formation.

6 DISCUSSION

6.1 Wear mechanisms

The terms used in clinical dentistry for wearmechanisms differ from those used in conventionalengineering tribology (and in some cases aremisleading). Table 4 shows the correspondencebetween commonly used terms.

Fig. 18 Diagrammatic representation of the buccal

surface of a tooth divided vertically and

horizontally into thirds [85]

Fig. 17 The effect of pH on (a) enamel loss (30 min exposure) and (b) dentine loss (10 min

exposure) [83] (CANaOH, CATC, PA–phosphoric acid adjusted with sodium hydroxide)



lower than that at pH 7 [32, 35, 36] but, for pH 1,wear increases to a level well above that at pH 7[32, 50]. Results of sliding tests carried out byKaidonis et al. [32], illustrating this effect, areshown in Fig. 20 (for test details see section onthegosis and bruxism).

With sliding, wear rates would be expected toincrease, as a synergistic effect is likely to occur.The outer layer of enamel will be softened by theeffect of the acidic solution and its hardness willdecrease. The application of load and sliding willthen break this layer down causing its removal.This will expose fresh enamel, which will be soft-ened, and the process is repeated. Increasing theacidity of the solution would therefore be expectedto increase wear as hardness decreases linearly withdecreasing pH.

Eisenburger and Addy [35, 36], in similar workcarried out to investigate closely the interaction ofchemical effects with enamel to enamel sliding

contact, offered a possible explanation for this appar-ent anomaly. Observation of the wear surfaces for thedifferent conditions revealed that the specimens inthe more acidic solutions had smoother surfaces. Itwas hypothesized that this would decrease the fric-tion at the specimen interface and thereby reducefatigue wear. Eisenburger and Addy [35, 36], how-ever, did not test up to pH 1 and therefore offeredno reason for any change at this level of acidity.

Tests run to study the interaction of wear due totoothbrushing [86] and mastication [87] with chemi-cal effects have shown that wear increases withdecreasing pH. In these situations, however, thewear is being caused by abrasive particles ratherthan by an opposing surface undergoing a similarwear process.

In situ testing, similar to that described for tooth-brushing (see section on toothbrushing), has beenused to evaluate erosive wear of enamel and dentine[88, 89]. These tests consisted of mounting enamelor dentine specimens on to an intra-oral device,which were then exposed to an acid environment(with orange juice for example). Tissue loss was thenrecorded using surface profilometry. These testsshowed lower material losses than in vitro tests. Onereason for this may be that erosion in vivo isinfluenced by factors including remineralizationand pellicle formation.

6 DISCUSSION

6.1 Wear mechanisms

The terms used in clinical dentistry for wearmechanisms differ from those used in conventionalengineering tribology (and in some cases aremisleading). Table 4 shows the correspondencebetween commonly used terms.

Fig. 18 Diagrammatic representation of the buccal

surface of a tooth divided vertically and

horizontally into thirds [85]

Fig. 17 The effect of pH on (a) enamel loss (30 min exposure) and (b) dentine loss (10 min

exposure) [83] (CANaOH, CATC, PA–phosphoric acid adjusted with sodium hydroxide)



It is evident from the analysis of the mecha-nisms leading to wear in teeth that those associatedwith occlusal contact cause the most wear prob-lems. The non-contact wear mechanisms, such asopen-phase mastication and toothbrushing, have asmaller impact on enamel wear. It is not evident,however, which of the contact mechanisms is themost dominant as it is very difficult to study invitro how they interact (and few attempts havebeen made) and completely impossible in vivo. It ispossible that dominant wear mechanisms will differfrom person to person.

Dentine wear is a much greater concern thanenamel wear. This is not unexpected as it is softerand less resilient than enamel and it does not remi-neralize as enamel is able to do. It is difficult to saywhat can be done to alleviate problems encounteredwhen the enamel layer is breached as the toothcannot be redesigned. Information on the wear pro-cesses and mechanisms is extremely important,however, in developing and assessing the perform-ance of restorative materials used to repair damagedor decaying teeth.

Most examples of excessive teeth wear rates occurwhere exceptional conditions are encountered. Thismay be because of hyperfunction (such as bruxism),where higher contact loads exist between teeth andgreater sliding occurs leading to increased tooth flex-ure, or because of severe climate or environmentalconditions, such as dusty, dry surroundings, orexcessively acidic atmospheres, which may beexperienced at home or at work, or because of con-ditions such as anorexia, where teeth are frequentlyexposed to gastric acids.

The largest single influencing factor on the wear ofteeth is clearly that of the chemical environment towhich teeth may be exposed. All wear mechanismsare synergized, including those which in normal con-ditions would not lead to excessive wear. The mainreason for this is the reduction in hardness of enameland dentine and erosive material loss in acidic con-ditions, which further increase with load applica-tion and motion (apart from the slight anomaly withsliding wear material loss). This is a significant obser-vation for the manufacturers of soft drinks, whichcan contain high levels of citric acid.

Surprisingly the synergistic effects of chemical andmechanical tooth wear have not been investigated.Research in conventional engineering tribocorrosionis advanced [90] and there are several techniques fortesting and modelling the processes. It is possible

Fig. 20 Wear of enamel against load for lubricants of

various pH values [32] (enamel-on-enamel

sliding tests for 89 � 103 cycles)

Fig. 19 Comparisons of volume loss from acid dissolution for loaded and unloaded enamel

samples in (a) the middle third and (b) the cervical third [85]

Table 4 Comparison of dental and engineering

tribology terminology

Dental tribology Engineering tribology

Attrition AdhesionTwo-body abrasion

Abrasion Three body abrasionErosion

Abfraction Fatigue wearErosion Chemical wear/corrosion



It is evident from the analysis of the mecha-nisms leading to wear in teeth that those associatedwith occlusal contact cause the most wear prob-lems. The non-contact wear mechanisms, such asopen-phase mastication and toothbrushing, have asmaller impact on enamel wear. It is not evident,however, which of the contact mechanisms is themost dominant as it is very difficult to study invitro how they interact (and few attempts havebeen made) and completely impossible in vivo. It ispossible that dominant wear mechanisms will differfrom person to person.

Dentine wear is a much greater concern thanenamel wear. This is not unexpected as it is softerand less resilient than enamel and it does not remi-neralize as enamel is able to do. It is difficult to saywhat can be done to alleviate problems encounteredwhen the enamel layer is breached as the toothcannot be redesigned. Information on the wear pro-cesses and mechanisms is extremely important,however, in developing and assessing the perform-ance of restorative materials used to repair damagedor decaying teeth.

Most examples of excessive teeth wear rates occurwhere exceptional conditions are encountered. Thismay be because of hyperfunction (such as bruxism),where higher contact loads exist between teeth andgreater sliding occurs leading to increased tooth flex-ure, or because of severe climate or environmentalconditions, such as dusty, dry surroundings, orexcessively acidic atmospheres, which may beexperienced at home or at work, or because of con-ditions such as anorexia, where teeth are frequentlyexposed to gastric acids.

The largest single influencing factor on the wear ofteeth is clearly that of the chemical environment towhich teeth may be exposed. All wear mechanismsare synergized, including those which in normal con-ditions would not lead to excessive wear. The mainreason for this is the reduction in hardness of enameland dentine and erosive material loss in acidic con-ditions, which further increase with load applica-tion and motion (apart from the slight anomaly withsliding wear material loss). This is a significant obser-vation for the manufacturers of soft drinks, whichcan contain high levels of citric acid.

Surprisingly the synergistic effects of chemical andmechanical tooth wear have not been investigated.Research in conventional engineering tribocorrosionis advanced [90] and there are several techniques fortesting and modelling the processes. It is possible

Fig. 20 Wear of enamel against load for lubricants of

various pH values [32] (enamel-on-enamel

sliding tests for 89 � 103 cycles)

Fig. 19 Comparisons of volume loss from acid dissolution for loaded and unloaded enamel

samples in (a) the middle third and (b) the cervical third [85]

Table 4 Comparison of dental and engineering

tribology terminology

Dental tribology Engineering tribology

Attrition AdhesionTwo-body abrasion

Abrasion Three body abrasionErosion

Abfraction Fatigue wearErosion Chemical wear/corrosion



that some of these techniques might find merit in thedental world.

6.2 Wear testing

Figure 21 summarizes the range of tests used forstudying and assessing tooth wear mechanisms andthe effect of influencing factors.

There seems to be a high level of suspicion levelledat the results and conclusions drawn from in vitrotesting among the dental world. Researchers pub-lishing results from such tests are at pains to pointout the drawbacks and the lack of correlation within vivo observations. This should not be the case asin vitro laboratory testing provides the only meansto vary and control satisfactorily the critical par-ameters influencing a wear process. Laboratorytests must, however, be as representative as possibleand simplistic approaches such as using a pin-on-disc test are not. The results of in vivo testing,however, are largely subjective, can often only pro-vide a qualitative assessment of wear, and shouldbe subject to much greater scrutiny given the largenumber of variables that cannot be assessed orcontrolled. As in conventional engineering, the bestapproach is probably to understand, as far as poss-ible, the wear mechanism before trying to simulateit. Indeed, as Mair et al. [27] noted, increased under-standing of tooth wear has been ‘hindered by theunnecessary inclination to classify and reproduceclinical manifestations rather than understand theirunderlying mechanisms’, so this is where effortswould be best focused using the best methods avail-able. It seems that the problems inherent in wearassessment of engineering components, and the dif-ficulties of finding a representative test, are commonwith tooth wear. The added complication is thatthere are no sound approaches for tooth wear testingin the real environment.

6.3 Wear modelling

No attempt has been made at modelling wear ofenamel or dentine. This is probably because of thecomplexity of the oral and loading environment towhich teeth are exposed. The same is true in conven-tional engineering. However, sometimes simplemodels, while they may not give a completely accu-rate representation, can be useful in comparing andcontrasting wear situations. One simple approachthat could be used is the Archard sliding wearmodel [91]

K ¼Vh

Pd(1)

where K is the dimensionless wear coefficient, V isthe wear volume, P is the normal load, d is the slidingdistance, and h is the material hardness.

Clearly knowledge of the dimensionless wear coef-ficient K is vital in order to apply equation (1). Thesecan be derived from, for example, pin-on-disc slidingtests. In Table 5, sliding wear coefficients for typicalengineering materials (from reference [92]) are com-pared with those calculated for enamel and dentine(from the results of Kaidonis et al. [32] and Buraket al. [51] respectively). The values of K for enameland dentine are of the same order of magnitude ofthose for the engineering materials.

Using the tooth load data, sliding distances andtimes during which teeth are in contact presentedin section 1.2 (see references [20] to [25] andTable 3), it was possible to calculate a tooth wearrate using equation (1). A summary of the calculationis shown in Table 6.

The final result of 96 mm wear depth per year com-pares well with the depth of 65 mm per yearmeasured by Lambrechts et al. [93]. To obtainthe same order of magnitude with a wear modelwith such a large number of assumptions is encoura-ging. Sufferers of bruxism have been observed to

Fig. 21 Types of test used in tooth and restorative

material wear testing

Table 5 Typical values of dimensionless wear coefficient

K in sliding (results for engineering materials are from

dry pin-on-disc tests against mild steel [91]; enamel results

were derived from the results of Kaidonis et al. [32] and the

dentine results from Burak et al. [51])

Material K

Mild steel (on mild steel) 7 � 1023

a–b brass 6 � 1024

Polytetrafluoroethylen 2.5 � 1025

Hard tool steel 1.3 � 1024

Ferritic stainless steel 1.7 � 1025

Polythene 1.3 � 1027

Poly(methyl methacrylate) 7 � 1026

Enamel (on enamel) 6.4 � 1024

Enamel (on enamel) with saliva lubrication 2.9 � 1025

Dentine (on enamel) 1.7 � 1023



that some of these techniques might find merit in thedental world.

6.2 Wear testing

Figure 21 summarizes the range of tests used forstudying and assessing tooth wear mechanisms andthe effect of influencing factors.

There seems to be a high level of suspicion levelledat the results and conclusions drawn from in vitrotesting among the dental world. Researchers pub-lishing results from such tests are at pains to pointout the drawbacks and the lack of correlation within vivo observations. This should not be the case asin vitro laboratory testing provides the only meansto vary and control satisfactorily the critical par-ameters influencing a wear process. Laboratorytests must, however, be as representative as possibleand simplistic approaches such as using a pin-on-disc test are not. The results of in vivo testing,however, are largely subjective, can often only pro-vide a qualitative assessment of wear, and shouldbe subject to much greater scrutiny given the largenumber of variables that cannot be assessed orcontrolled. As in conventional engineering, the bestapproach is probably to understand, as far as poss-ible, the wear mechanism before trying to simulateit. Indeed, as Mair et al. [27] noted, increased under-standing of tooth wear has been ‘hindered by theunnecessary inclination to classify and reproduceclinical manifestations rather than understand theirunderlying mechanisms’, so this is where effortswould be best focused using the best methods avail-able. It seems that the problems inherent in wearassessment of engineering components, and the dif-ficulties of finding a representative test, are commonwith tooth wear. The added complication is thatthere are no sound approaches for tooth wear testingin the real environment.

6.3 Wear modelling

No attempt has been made at modelling wear ofenamel or dentine. This is probably because of thecomplexity of the oral and loading environment towhich teeth are exposed. The same is true in conven-tional engineering. However, sometimes simplemodels, while they may not give a completely accu-rate representation, can be useful in comparing andcontrasting wear situations. One simple approachthat could be used is the Archard sliding wearmodel [91]

K ¼Vh

Pd(1)

where K is the dimensionless wear coefficient, V isthe wear volume, P is the normal load, d is the slidingdistance, and h is the material hardness.

Clearly knowledge of the dimensionless wear coef-ficient K is vital in order to apply equation (1). Thesecan be derived from, for example, pin-on-disc slidingtests. In Table 5, sliding wear coefficients for typicalengineering materials (from reference [92]) are com-pared with those calculated for enamel and dentine(from the results of Kaidonis et al. [32] and Buraket al. [51] respectively). The values of K for enameland dentine are of the same order of magnitude ofthose for the engineering materials.

Using the tooth load data, sliding distances andtimes during which teeth are in contact presentedin section 1.2 (see references [20] to [25] andTable 3), it was possible to calculate a tooth wearrate using equation (1). A summary of the calculationis shown in Table 6.

The final result of 96 mm wear depth per year com-pares well with the depth of 65 mm per yearmeasured by Lambrechts et al. [93]. To obtainthe same order of magnitude with a wear modelwith such a large number of assumptions is encoura-ging. Sufferers of bruxism have been observed to

Fig. 21 Types of test used in tooth and restorative

material wear testing

Table 5 Typical values of dimensionless wear coefficient

K in sliding (results for engineering materials are from

dry pin-on-disc tests against mild steel [91]; enamel results

were derived from the results of Kaidonis et al. [32] and the

dentine results from Burak et al. [51])

Material K

Mild steel (on mild steel) 7 � 1023

a–b brass 6 � 1024

Polytetrafluoroethylen 2.5 � 1025

Hard tool steel 1.3 � 1024

Ferritic stainless steel 1.7 � 1025

Polythene 1.3 � 1027

Poly(methyl methacrylate) 7 � 1026

Enamel (on enamel) 6.4 � 1024

Enamel (on enamel) with saliva lubrication 2.9 � 1025

Dentine (on enamel) 1.7 � 1023



subject teeth to loads of up to 11 N for several hoursper day [21]. If this additional loading is added intothe calculation outlined in Table 5, using a daily con-tact time of two hours as a result of the bruxism at11 N, the total wear becomes 181 mm. This ties inwith the measurements made by Xhonga [77], whonoted that sufferers of bruxism could experiencewear rates three to four times higher than the65 mm seen by Lambrechts et al. [93].

A good understanding clearly exists of the mech-anisms leading to the wear of teeth and of the criticalinfluencing factors. One of the main challengesto researchers is to improve testing techniques toenhance the applicability of results and to gain abetter understanding of how the mechanisms inter-act and assess which may be the most dominant.Perhaps some of the lessons learnt in engineeringtribology can help in this regard.

7 CONCLUSIONS

Many different tooth wear mechanisms have beenidentified. The main cause of enamel and dentinewear, however, is occlusal contact during chewingand thegosis and is particularly prevalent in sufferersof hyperfunction such as bruxism. The occlusal contactwear mechanism involves a running-in process, typicalof wear seen in engineering component contacts.This can actually be beneficial as it causes contactsto become more conformal and leads to reducedwear rates. Wear during toothbrushing due to abrasiveparticles in toothpaste appears to be only a problemwith dentine and some restorative materials.

The major influencing factor on all the different wearmechanisms is the acidity of the oral environment.Increasing acidity reduces the hardness and Young’smodulus of enamel and dentine, which leads toincreased wear rates. Despite the importance of theeffect of acidity, synergistic effects have not beenstudied. This is an important challenge to researchers.

There are a wide variety of test platforms usedin tooth wear studies. In most cases there has beengreat difficulty in relating results to in vivo toothwear. This is further complicated by the lack oftechniques for quantifying wear in living teeth.

Predictive wear models for teeth have not beendeveloped, probably due to the great complexity ofthe oral environment and teeth loading conditions.A simple wear model put forward in this paper hasbeen shown to produce reasonable estimates ofenamel wear and could be further developed topredict wear of restoratives.

Most work in this area has been carried out in thedental field, rather than by engineering tribologists.As noted previously, collaboration between the twoin the future will help to advance this field of study.

REFERENCES

1 Braden, M. Biophysics of the tooth. Frontiers of OralPhysiology, 1976, 2, 1–37.

2 Berkovitz, B. K. B., Holland, G. R., and Moxham, B. J.A Colour Atlas and Textbook of Oral Anatomy, 1977(Wolfe Medical Publications Limited, London).

3 Rees, J. S. The effect of variation in occlusal loading onthe development of abfraction lesions: a finite elementstudy. J. Oral Rehabil., 2002, 29, 188–193.

4 Rees, J. S., Hammadeh, M., and Jagger, D. C. Abfrac-tion lesion formation in maxillary incisors, caninesand pemolars: a finite element study. Eur. J. Oral Sci.,2003, 111, 149–154.

5 Stanford, J. W., Paffenbarger, G. C., and Kampula, J.W. Determination of some compressive properties ofhuman enamel and dentine. J. Am. Dent. Assoc., 1958,57, 487–492.

6 Cooper, W. E. G. and Smith, D. C. Determination of theshear strength of enamel and dentine. J. Dent. Res.,1968, 47, 997–998.

7 Hannah, C. Mechanical properties of human enameland dentine and of composite restorative materials.PhD thesis, University of Manchester, 1974.

8 Bowen, R. L. and Rodriguez, M. M. Tensile strength andmodulus of elasticity of tooth structure and several res-torative materials. J. Am. Dent. Assoc., 1962, 64, 378–387.

9 Tyldesley, W. R. The mechanical properties of enameland dentine. Br. Dent. J., 1959, 106, 269–278.

10 Anderson, J. C., Leaver, K. D., Rawlings, R. D., andAlexander, J. M. Materials Science, 4th edition, 1990(Chapman and Hall, London).

11 Remizov, S. M., Prujansky, L. Y., and Matveesky, R. M.Wear resistance and microhardness of human teeth.Proc. Instn Mech. Engrs, Part H: J. Engineering forMedicine, 1991, 205(3), 201.

12 Willems, G., Celis, J. P., Lambrechts, P., Braem, M., andVanherle, G. Hardness and Young’s modulus deter-mined by nano-indentation technique of filler particlesof dental restorative materials compared with humanenamel. J. Biomed. Mater. Res., 1993, 27, 747–755.

Table 6 Tooth wear rate calculation

Parameter Value

Tooth-to-tooth contact timeduring one chew

0.1 s

Sliding distance in one chew 1 mmTotal tooth-to-tooth contact

time in 1 day900 s (15 min)

Total sliding distance in 1 day (900/0.1) � 1 ¼ 9000 mmTotal sliding distance d in 1 year 3 285 000 mmDimensionless sliding wear

coefficient K2.9 � 1025

Enamel hardness h 6 � 109 N/m2

Load P 150 NWear volume V in 1 year [from

equation (1)]2.41 mm3

Tooth area 25 mm2

Tooth wear depth in 1 year 96 mm



subject teeth to loads of up to 11 N for several hoursper day [21]. If this additional loading is added intothe calculation outlined in Table 5, using a daily con-tact time of two hours as a result of the bruxism at11 N, the total wear becomes 181 mm. This ties inwith the measurements made by Xhonga [77], whonoted that sufferers of bruxism could experiencewear rates three to four times higher than the65 mm seen by Lambrechts et al. [93].

A good understanding clearly exists of the mech-anisms leading to the wear of teeth and of the criticalinfluencing factors. One of the main challengesto researchers is to improve testing techniques toenhance the applicability of results and to gain abetter understanding of how the mechanisms inter-act and assess which may be the most dominant.Perhaps some of the lessons learnt in engineeringtribology can help in this regard.

7 CONCLUSIONS

Many different tooth wear mechanisms have beenidentified. The main cause of enamel and dentinewear, however, is occlusal contact during chewingand thegosis and is particularly prevalent in sufferersof hyperfunction such as bruxism. The occlusal contactwear mechanism involves a running-in process, typicalof wear seen in engineering component contacts.This can actually be beneficial as it causes contactsto become more conformal and leads to reducedwear rates. Wear during toothbrushing due to abrasiveparticles in toothpaste appears to be only a problemwith dentine and some restorative materials.

The major influencing factor on all the different wearmechanisms is the acidity of the oral environment.Increasing acidity reduces the hardness and Young’smodulus of enamel and dentine, which leads toincreased wear rates. Despite the importance of theeffect of acidity, synergistic effects have not beenstudied. This is an important challenge to researchers.

There are a wide variety of test platforms usedin tooth wear studies. In most cases there has beengreat difficulty in relating results to in vivo toothwear. This is further complicated by the lack oftechniques for quantifying wear in living teeth.

Predictive wear models for teeth have not beendeveloped, probably due to the great complexity ofthe oral environment and teeth loading conditions.A simple wear model put forward in this paper hasbeen shown to produce reasonable estimates ofenamel wear and could be further developed topredict wear of restoratives.

Most work in this area has been carried out in thedental field, rather than by engineering tribologists.As noted previously, collaboration between the twoin the future will help to advance this field of study.

REFERENCES

1 Braden, M. Biophysics of the tooth. Frontiers of OralPhysiology, 1976, 2, 1–37.

2 Berkovitz, B. K. B., Holland, G. R., and Moxham, B. J.A Colour Atlas and Textbook of Oral Anatomy, 1977(Wolfe Medical Publications Limited, London).

3 Rees, J. S. The effect of variation in occlusal loading onthe development of abfraction lesions: a finite elementstudy. J. Oral Rehabil., 2002, 29, 188–193.

4 Rees, J. S., Hammadeh, M., and Jagger, D. C. Abfrac-tion lesion formation in maxillary incisors, caninesand pemolars: a finite element study. Eur. J. Oral Sci.,2003, 111, 149–154.

5 Stanford, J. W., Paffenbarger, G. C., and Kampula, J.W. Determination of some compressive properties ofhuman enamel and dentine. J. Am. Dent. Assoc., 1958,57, 487–492.

6 Cooper, W. E. G. and Smith, D. C. Determination of theshear strength of enamel and dentine. J. Dent. Res.,1968, 47, 997–998.

7 Hannah, C. Mechanical properties of human enameland dentine and of composite restorative materials.PhD thesis, University of Manchester, 1974.

8 Bowen, R. L. and Rodriguez, M. M. Tensile strength andmodulus of elasticity of tooth structure and several res-torative materials. J. Am. Dent. Assoc., 1962, 64, 378–387.

9 Tyldesley, W. R. The mechanical properties of enameland dentine. Br. Dent. J., 1959, 106, 269–278.

10 Anderson, J. C., Leaver, K. D., Rawlings, R. D., andAlexander, J. M. Materials Science, 4th edition, 1990(Chapman and Hall, London).

11 Remizov, S. M., Prujansky, L. Y., and Matveesky, R. M.Wear resistance and microhardness of human teeth.Proc. Instn Mech. Engrs, Part H: J. Engineering forMedicine, 1991, 205(3), 201.

12 Willems, G., Celis, J. P., Lambrechts, P., Braem, M., andVanherle, G. Hardness and Young’s modulus deter-mined by nano-indentation technique of filler particlesof dental restorative materials compared with humanenamel. J. Biomed. Mater. Res., 1993, 27, 747–755.

Table 6 Tooth wear rate calculation

Parameter Value

Tooth-to-tooth contact timeduring one chew

0.1 s

Sliding distance in one chew 1 mmTotal tooth-to-tooth contact

time in 1 day900 s (15 min)

Total sliding distance in 1 day (900/0.1) � 1 ¼ 9000 mmTotal sliding distance d in 1 year 3 285 000 mmDimensionless sliding wear

coefficient K2.9 � 1025

Enamel hardness h 6 � 109 N/m2

Load P 150 NWear volume V in 1 year [from

equation (1)]2.41 mm3

Tooth area 25 mm2

Tooth wear depth in 1 year 96 mm



13 Meredith, N., Sherriff, M., Setchell, D. J., andSwanson, S. A. V. Measurement of the microhardnessand Young’s modulus of human enamel and dentineusing an indentation technique. Arch. Oral Biol., 1996,41(6), 539–545.

14 Craig, R. G., Peyton, F. A., and Johnson, D. W. Com-pressive properties of enamel, dental cements andgold. J. Dent. Res., 1961, 40(5), 936–945.

15 Xu, H. H. K., Smith, D. T., Jahanmir, S., Romberg, E.,Kelly, J. R., Thompson, V. P., and Rekow, E. D. Inden-tation damage and mechanical properties of humanenamel and dentine. J. Dent. Res., 1998, 77(3), 472–480.

16 Cuy, J. L., Mann, A. B., Livi, K. J., Teaford, M. F., andWeihs, T. P. Nanoindentation mapping of the mechan-ical properties of human molar tooth enamel. Arch.Oral Biol., 2002, 47, 281–291.

17 Poole, D. F. G., Newman, H. N., and Dibdin, G. H.Structure and porosity of human cervical enamelstudied by polarising microscopy and transmissionelectron microscopy. Arch. Oral Biol., 1981, 26, 977–982.

18 Scott, J. H. and Symons, N. B. B. Introduction to DentalAnatomy, 9th edition, 1982 (Churchill Livingstone,Edinburgh).

19 The 1997 Sucralose Drinks Report, Zenith InternationalLimited, 1997.

20 Rilo, B., Fernandez, J., Da Silva, L., Insua, A. M., andSantana, U. Frontal-plane lateral border movementsand chewing cycle characteristics. J. Oral Rehabil.,2001, 28(10), 930–936.

21 Caputo, A. and Wylie, R. Force generation and reactionwithin the periodontium, UCLA Periodontics InformationCentre Course Notes, http://www.dent.ucla.edu/pic/.

22 Anderson, D. J. Measurement of stress in masticationI. J. Dent. Res., 1956, 35, 664.

23 Anderson, D. J. Measurement of stress in masticationII. J. Dent. Res., 1956, 35, 667.

24 Proeschel, P. A. and Morneburg, T. Task-dependenceof activity/bite-force relations and its impact on esti-mation of chewing force from EMG. J. Dent. Res.,2002, 81(7), 464–468.

25 Morneburg, T. R. and Proschel, P. A. Measurement ofmasticatory forces and implant loads: a methodologicalclinical study. Int. J. Prosthodontics, 2002, 15(1), 20–27.

26 Mair, L. H. Wear in dentistry – current terminology.J. Dentistry, 1992, 20, 140–144.

27 Mair, L. H., Stolarski, T. A., Vowles, R. W., andLloyd, C. H. Wear: mechanisms, manifestations andmeasurement. Report of a workshop. J. Dentistry,1996, 24(1–2), 141–148.

28 Every, R. G. and Kuhne, W. G. Biomodal wear of mam-malian teeth. In Early Mammals (Eds D. M. Kermackand K. A. Kermack) 1971, pp. 23–27 (Academic Press,London).

29 Roulet, J. F. Development of appropriate measuringdevices. In Degradation of Dental Polymers, 1987,pp. 92–113 (Karger, Basel).

30 de Gee, A. J. and Pallav, P. Occlusal wear simulationwith the ACTA wear machine. J. Dentistry, 1994,22(Suppl. 1), 21–27.

31 McKinney, J. E. and Wu, W. Relationship betweensubsurface damage and wear of dental restorativecomposites. J. Dent. Res., 1992, 61, 1083–1088.

32 Kaidonis, J. A., Richards, L. C., Townsend, G. C., andTansley, G. D. Wear of human enamel: a quantitativein vitro assessment. J. Dent. Res., 1998, 77(12),1983–1990.

33 Li, H. and Zhou, Z. R. Wear behaviour of human teethin dry and artificial saliva conditions. Wear, 2002, 249,980–984.

34 Zheng, J., Zhou, Z. R., Zhang, J., Li, H., and Yu, H. Y.On the friction and wear behaviour of human toothenamel and dentin. Wear, 2003, 255, 967–974.

35 Eisenburger, M. and Addy, M. Erosion and attritionof human enamel in vitro. Part I: interaction effects.J. Dentistry, 2002, 30, 341–347.

36 Eisenburger, M. and Addy, M. Erosion and attrition ofhuman enamel in vitro. Part II: influence of time andloading. J. Dentistry, 2002, 30, 349–352.

37 Vowles, R. W., Williams, D. F., and Lockwood, M. Weartesting of dental restorative materials. J. Dent. Res.,1988, 67, 659.

38 Vale Antunes, P. and Ramalho, A. Study of abrasiveresistances of composites for dental restoration byball cratering. Wear, 2003, 255, 990–998.

39 Condon, J. R. and Ferracane, J. L. A new multi-modeoral wear simulator. Dent. Mater., 1996, 12, 218–226.

40 De Long, R. and Douglas, W. H. Development of anartificial oral environment for testing of dental restora-tives: biaxial force and movement control. J. Dent. Res.,1983, 62, 92–113.

41 Hunter, M. L., Addy, M., Pickles, M. J., and Joiner, A.The role of toothpastes and toothbrushes in the aetiol-ogy of tooth wear. Int. Dent. J., 2002, 52, 399–405.

42 Saxton, C. A. and Cowell, C. R. Clinical investigation ofthe effects of dentifrices on dentine wear at the cemento-enamel junction. J. Am. Dent. Assoc., 1981, 102, 38–43.

43 Kim, S. K., Kim, K. N., Chang, I. T., and Heo, S. J. Astudy of the effects of chewing patterns on occlusalwear. J. Oral Rehabil., 2001, 28, 1048–1055.

44 Lambrechts, P., Braem, M., Vuylsteke-Wauters, M.,and Vanherle, G. Quantitative in vivo wear of humanenamel. J. Dent. Res., 1989, 68, 1752–1754.

45 Savill, G., Grigor, J., Huntingdon, E., Jackson, R., andLynch, E. Toothbrush design: adapting for the future.Int. Dent. J., 1998, 48(Suppl. 1), 519–525.

46 Smith, B. G. N. and Knight, J. K. An index for measur-ing the wear of teeth. Br. Dent. J., 1984, 156, 435–438.

47 Smith, B. G. N. and Knight, J. K. A comparison ofpatterns of tooth wear with aetiological factors. Br.Dent. J., 1984, 157, 16–19.

48 Øilo, G., Dahl, B. L., Hatle, G., and Gad, A.-L. An indexfor evaluating wear of teeth. Acta Odontolpgica Scandi-navica, 1987, 47, 205–210.

49 Monasky, G. E. and Taylor, D. F. Studies in the wear ofporcelain, enamel and gold. J. Prosthetic Dentistry,1971, 25, 299–306.

50 Shabanian, M. and Richards, L. C. In vitro wear ratesof materials under different loads and varying pH.J. Prosthetic Dentistry, 2002, 87(6), 650–656.

51 Burak, N., Kaidonis, J. A., Richards, L. C., andTownsend, G. C. Experimental studies of humandentine wear. Arch. Oral Biol., 1999, 44, 885–887.

52 Gibbs, C. H., Mahan, P. E., Lundeen, H. C., Brehnan,K., Walsh, E. K., and Holbrook, W. B. Occlusal forces



13 Meredith, N., Sherriff, M., Setchell, D. J., andSwanson, S. A. V. Measurement of the microhardnessand Young’s modulus of human enamel and dentineusing an indentation technique. Arch. Oral Biol., 1996,41(6), 539–545.

14 Craig, R. G., Peyton, F. A., and Johnson, D. W. Com-pressive properties of enamel, dental cements andgold. J. Dent. Res., 1961, 40(5), 936–945.

15 Xu, H. H. K., Smith, D. T., Jahanmir, S., Romberg, E.,Kelly, J. R., Thompson, V. P., and Rekow, E. D. Inden-tation damage and mechanical properties of humanenamel and dentine. J. Dent. Res., 1998, 77(3), 472–480.

16 Cuy, J. L., Mann, A. B., Livi, K. J., Teaford, M. F., andWeihs, T. P. Nanoindentation mapping of the mechan-ical properties of human molar tooth enamel. Arch.Oral Biol., 2002, 47, 281–291.

17 Poole, D. F. G., Newman, H. N., and Dibdin, G. H.Structure and porosity of human cervical enamelstudied by polarising microscopy and transmissionelectron microscopy. Arch. Oral Biol., 1981, 26, 977–982.

18 Scott, J. H. and Symons, N. B. B. Introduction to DentalAnatomy, 9th edition, 1982 (Churchill Livingstone,Edinburgh).

19 The 1997 Sucralose Drinks Report, Zenith InternationalLimited, 1997.

20 Rilo, B., Fernandez, J., Da Silva, L., Insua, A. M., andSantana, U. Frontal-plane lateral border movementsand chewing cycle characteristics. J. Oral Rehabil.,2001, 28(10), 930–936.

21 Caputo, A. and Wylie, R. Force generation and reactionwithin the periodontium, UCLA Periodontics InformationCentre Course Notes, http://www.dent.ucla.edu/pic/.

22 Anderson, D. J. Measurement of stress in masticationI. J. Dent. Res., 1956, 35, 664.

23 Anderson, D. J. Measurement of stress in masticationII. J. Dent. Res., 1956, 35, 667.

24 Proeschel, P. A. and Morneburg, T. Task-dependenceof activity/bite-force relations and its impact on esti-mation of chewing force from EMG. J. Dent. Res.,2002, 81(7), 464–468.

25 Morneburg, T. R. and Proschel, P. A. Measurement ofmasticatory forces and implant loads: a methodologicalclinical study. Int. J. Prosthodontics, 2002, 15(1), 20–27.

26 Mair, L. H. Wear in dentistry – current terminology.J. Dentistry, 1992, 20, 140–144.

27 Mair, L. H., Stolarski, T. A., Vowles, R. W., andLloyd, C. H. Wear: mechanisms, manifestations andmeasurement. Report of a workshop. J. Dentistry,1996, 24(1–2), 141–148.

28 Every, R. G. and Kuhne, W. G. Biomodal wear of mam-malian teeth. In Early Mammals (Eds D. M. Kermackand K. A. Kermack) 1971, pp. 23–27 (Academic Press,London).

29 Roulet, J. F. Development of appropriate measuringdevices. In Degradation of Dental Polymers, 1987,pp. 92–113 (Karger, Basel).

30 de Gee, A. J. and Pallav, P. Occlusal wear simulationwith the ACTA wear machine. J. Dentistry, 1994,22(Suppl. 1), 21–27.

31 McKinney, J. E. and Wu, W. Relationship betweensubsurface damage and wear of dental restorativecomposites. J. Dent. Res., 1992, 61, 1083–1088.

32 Kaidonis, J. A., Richards, L. C., Townsend, G. C., andTansley, G. D. Wear of human enamel: a quantitativein vitro assessment. J. Dent. Res., 1998, 77(12),1983–1990.

33 Li, H. and Zhou, Z. R. Wear behaviour of human teethin dry and artificial saliva conditions. Wear, 2002, 249,980–984.

34 Zheng, J., Zhou, Z. R., Zhang, J., Li, H., and Yu, H. Y.On the friction and wear behaviour of human toothenamel and dentin. Wear, 2003, 255, 967–974.

35 Eisenburger, M. and Addy, M. Erosion and attritionof human enamel in vitro. Part I: interaction effects.J. Dentistry, 2002, 30, 341–347.

36 Eisenburger, M. and Addy, M. Erosion and attrition ofhuman enamel in vitro. Part II: influence of time andloading. J. Dentistry, 2002, 30, 349–352.

37 Vowles, R. W., Williams, D. F., and Lockwood, M. Weartesting of dental restorative materials. J. Dent. Res.,1988, 67, 659.

38 Vale Antunes, P. and Ramalho, A. Study of abrasiveresistances of composites for dental restoration byball cratering. Wear, 2003, 255, 990–998.

39 Condon, J. R. and Ferracane, J. L. A new multi-modeoral wear simulator. Dent. Mater., 1996, 12, 218–226.

40 De Long, R. and Douglas, W. H. Development of anartificial oral environment for testing of dental restora-tives: biaxial force and movement control. J. Dent. Res.,1983, 62, 92–113.

41 Hunter, M. L., Addy, M., Pickles, M. J., and Joiner, A.The role of toothpastes and toothbrushes in the aetiol-ogy of tooth wear. Int. Dent. J., 2002, 52, 399–405.

42 Saxton, C. A. and Cowell, C. R. Clinical investigation ofthe effects of dentifrices on dentine wear at the cemento-enamel junction. J. Am. Dent. Assoc., 1981, 102, 38–43.

43 Kim, S. K., Kim, K. N., Chang, I. T., and Heo, S. J. Astudy of the effects of chewing patterns on occlusalwear. J. Oral Rehabil., 2001, 28, 1048–1055.

44 Lambrechts, P., Braem, M., Vuylsteke-Wauters, M.,and Vanherle, G. Quantitative in vivo wear of humanenamel. J. Dent. Res., 1989, 68, 1752–1754.

45 Savill, G., Grigor, J., Huntingdon, E., Jackson, R., andLynch, E. Toothbrush design: adapting for the future.Int. Dent. J., 1998, 48(Suppl. 1), 519–525.

46 Smith, B. G. N. and Knight, J. K. An index for measur-ing the wear of teeth. Br. Dent. J., 1984, 156, 435–438.

47 Smith, B. G. N. and Knight, J. K. A comparison ofpatterns of tooth wear with aetiological factors. Br.Dent. J., 1984, 157, 16–19.

48 Øilo, G., Dahl, B. L., Hatle, G., and Gad, A.-L. An indexfor evaluating wear of teeth. Acta Odontolpgica Scandi-navica, 1987, 47, 205–210.

49 Monasky, G. E. and Taylor, D. F. Studies in the wear ofporcelain, enamel and gold. J. Prosthetic Dentistry,1971, 25, 299–306.

50 Shabanian, M. and Richards, L. C. In vitro wear ratesof materials under different loads and varying pH.J. Prosthetic Dentistry, 2002, 87(6), 650–656.

51 Burak, N., Kaidonis, J. A., Richards, L. C., andTownsend, G. C. Experimental studies of humandentine wear. Arch. Oral Biol., 1999, 44, 885–887.

52 Gibbs, C. H., Mahan, P. E., Lundeen, H. C., Brehnan,K., Walsh, E. K., and Holbrook, W. B. Occlusal forces



during chewing and swallowing as measured by soundtransmission. J. Prosthetic Dentistry, 1981, 46, 443–450.

53 Condon, J. R. and Ferracane, J. L. Factors effectingdental composite wear in vitro. J. Biomed. Mater. Res.,1997, 38, 303–313.

54 Maas, M. A scanning electron-microscope study ofin vitro abrasion of mammalian tooth enamel undercompressive loads. Arch. Oral Biol., 1994, 39(1), 1–11.

55 Maas, M. Enamel structure and microwear: an experi-mental study of the response of enamel to shearingforce. Am. J. Phys. Anthropology, 1991, 85, 31–49.

56 Mair, L. H., Vowles, R. W., Cunningham, J., andWilliams, D. F. The clinical wear of three posteriorcomposites. Br. Dent. J., 1990, 169, 355–360.

57 Manly, R. S. Factors influencing tests on the abrasion ofdentin by brushing with dentifrices. J. Dent. Res., 1944,23, 59–72.

58 Phaneuf, E. A., Harrington, J. H., Ashland, A. B.,Dale, P. P., and Shklar, G. Automatic toothbrush: a newreciprocating action. J. Am. Dent. Assoc., 1962, 65, 12–25.

59 Manly, R. S., Wiren, J., Manly, P., and Keene, R. C. Amethod for measurement of abrasion of dentinby toothbrush and dentifrice. J. Dent. Res., 1965, 44,533–540.

60 Bjorn, H. and Lindhe, J. Abrasion of dentine bytoothbrush and dentifrice – a methodological study.Odontologisk Revy, 1966, 17, 17–27.

61 Heath, J. R. and Wilson, H. J. Abrasion of restorativematerials by toothpaste. J. Oral Rehabil., 1976, 3,121–138.

62 Harrington, E., Jones, P. A., Fisher, S. E., andWilson, H. J. Toothbrush–dentifrice abrasion. Br.Dent. J., 1982, 153, 135–138.

63 Addy, M., Hughes, J., Pickles, M. J., Joiner, A., andHuntington, E. Development of a method in situ tostudy toothpaste abrasion of dentine. J. Clin. Periodon-tology, 2002, 29, 896–900.

64 Joiner, A., Pickles, M. J., Matheson, J. R., Weader, E.,Noblet, L., and Huntington, E. Whitening toothpastes:effects on tooth stain and enamel. Int. Dent. J., 2002, 52,424–434.

65 McConnell, D. and Conroy, C. W. Comparisons ofabrasion produced by a simulated manual versus amechanical toothbrush. J. Dent. Res., 1967, 46(5),1022–1027.

66 Bjorn, H. and Lindhe, J. On the mechanics of tooth-brushing. Odontologisk Revy, 1966, 17, 9–16.

67 Lewis, R., Dwyer-Joyce, R. S., and Pickles, M. J. Inter-action between toothbrushes and toothpaste abrasiveparticles in simulated tooth cleaning. Wear, 2004, 257(3–4), 368–376.

68 Grippo, J. O. Abfractions. A new classification of hardtissue lesions of teeth. J. Esthetic Restorative Dentistry,1991, 3, 14–19.

69 McCoy, G. The etiology of gingival erosion. J. OralImplantology, 1982, 10, 361.

70 Lee, W. C. and Eakle, W. S. Possible role of tensile stressin the etiology of cervical erosion legions of teeth.J. Prosthetic Dentistry, 1984, 52, 374–380.

71 Rees, J. S. The role of cuspal flexure in the developmentof abfraction lesions: a finite element study. Eur. J. OralSci., 1998, 106, 1028–1032.

72 Nohl, F. S. and Setchell, D. J. Surface strains induced bymeasured loads on teeth in vivo. Eur. J. Prosthodonticsand Restorative Dentistry, 2000, 8, 27–31.

73 Boyd, A. Enamel structure and cavity margins. Opera-tive Dentistry, 1976, 1, 12–28.

74 Radentz, W. H., Barnes, G. P., and Cutright, D. E. Asurvey of factors possibly associated with cervicalabrasion of tooth surfaces. J. Periodontology, 1976, 47,148–154.

75 Rudd, K. D., O’Leary, T. J., and Stumpf, A. J. Horizontaltooth mobility in carefully screened subjects. Periodon-tics, 1964, 2, 65–68.

76 Jepson, A. Root surface measurement and a methodfor X-ray determination of root surface area. ActaOdontologica. Scandinavica, 1963, 21, 35–46.

77 Xhonga, F. A. Bruxism and its effect on the teeth. J. OralRehabil., 1977, 4, 65–76.

78 Lambrechts, P., Braem, M., and Vanherle, G. Evalu-ation of clinical performance for posterior compositeresins and dentine adhesives. Operative Dentistry,1987, 12, 53.

79 Barbour, M. E., Parker, D. M., Allen, G. C., andJandt, K. D. Human enamel dissolution in citric acidas a function of pH in the range 2.30 � pH � 6.30 – ananoindentation study. Eur. J. Oral Sci., 2003, 111,258–262.

80 Lupi-Pegurier, L., Muller, M., Leforestier, E.,Bertrand, M. F., and Bolla, M. In vitro action ofBordeaux red wine on the microhardness of humandental enamel. Arch. Oral Biology, 2003, 48, 141–145.

81 Bashir, E., Ekberg, O., and Lagerlof, F. Salivaryclearance of citric acid after an oral rinse. Journal ofDentistry, 1995, 23, 209–212.

82 Millward, A., Shaw, L., Harrington, E., and Smith, A. J.Continuous monitoring of salivary flow rate and pH atthe surface of the dentition following consumption ofacidic beverages. Caries Res., 1997, 31, 44–49.

83 West, NX., Hughes, J. A., and Addy, M. The effect of pHon the erosion of dentine and enamel by dietary acidsin vitro. J. Oral Rehabil., 2001, 28, 860–864.

84 Grippo, J. O. and Masi, J. V. Role of biodental engineer-ing factors (BEF) in the etiology of root caries. J. EstheticRestorative Dentistry, 1991, 3, 71–76.

85 Palamara, D., Palamara, J. E. A., Tyas, M. J.,Pintado, M., and Messer, H. H. Effect of stress on aciddissolution of enamel. Dent. Mater., 2001, 17, 109–115.

86 Hooper, S., West, N. X., Pickles, M. J., Joiner, A.,Newcombe, R. G., and Addy, M. Investigation of ero-sion and abrasion on enamel and dentine: a modelin situ using toothpastes of different abrasivity. J. Clin.Periodontology, 2003, 30(9), 802–808.

87 Davis, W. B. and Winter, P. J. The effect of abrasionon enamel and dentine and exposure to dietary acid.Br. Dent. J., 1980, 148, 253–256.

88 West, N. X., Maxwell, A., Hughes, J. A., Parker, D. M.,Newcombe, R. G., and Addy, M. A method to measureclinical erosion: the effect of orange juice consump-tion on erosion of enamel. J. Dentistry, 1998, 26,329–335.

89 Amaechi, B. T., Higham, S. M., and Edgar, W. M.Development of an in-situ model to study dentalerosion. In Tooth Wear and Sensitivity (Eds M. Addy,



during chewing and swallowing as measured by soundtransmission. J. Prosthetic Dentistry, 1981, 46, 443–450.

53 Condon, J. R. and Ferracane, J. L. Factors effectingdental composite wear in vitro. J. Biomed. Mater. Res.,1997, 38, 303–313.

54 Maas, M. A scanning electron-microscope study ofin vitro abrasion of mammalian tooth enamel undercompressive loads. Arch. Oral Biol., 1994, 39(1), 1–11.

55 Maas, M. Enamel structure and microwear: an experi-mental study of the response of enamel to shearingforce. Am. J. Phys. Anthropology, 1991, 85, 31–49.

56 Mair, L. H., Vowles, R. W., Cunningham, J., andWilliams, D. F. The clinical wear of three posteriorcomposites. Br. Dent. J., 1990, 169, 355–360.

57 Manly, R. S. Factors influencing tests on the abrasion ofdentin by brushing with dentifrices. J. Dent. Res., 1944,23, 59–72.

58 Phaneuf, E. A., Harrington, J. H., Ashland, A. B.,Dale, P. P., and Shklar, G. Automatic toothbrush: a newreciprocating action. J. Am. Dent. Assoc., 1962, 65, 12–25.

59 Manly, R. S., Wiren, J., Manly, P., and Keene, R. C. Amethod for measurement of abrasion of dentinby toothbrush and dentifrice. J. Dent. Res., 1965, 44,533–540.

60 Bjorn, H. and Lindhe, J. Abrasion of dentine bytoothbrush and dentifrice – a methodological study.Odontologisk Revy, 1966, 17, 17–27.

61 Heath, J. R. and Wilson, H. J. Abrasion of restorativematerials by toothpaste. J. Oral Rehabil., 1976, 3,121–138.

62 Harrington, E., Jones, P. A., Fisher, S. E., andWilson, H. J. Toothbrush–dentifrice abrasion. Br.Dent. J., 1982, 153, 135–138.

63 Addy, M., Hughes, J., Pickles, M. J., Joiner, A., andHuntington, E. Development of a method in situ tostudy toothpaste abrasion of dentine. J. Clin. Periodon-tology, 2002, 29, 896–900.

64 Joiner, A., Pickles, M. J., Matheson, J. R., Weader, E.,Noblet, L., and Huntington, E. Whitening toothpastes:effects on tooth stain and enamel. Int. Dent. J., 2002, 52,424–434.

65 McConnell, D. and Conroy, C. W. Comparisons ofabrasion produced by a simulated manual versus amechanical toothbrush. J. Dent. Res., 1967, 46(5),1022–1027.

66 Bjorn, H. and Lindhe, J. On the mechanics of tooth-brushing. Odontologisk Revy, 1966, 17, 9–16.

67 Lewis, R., Dwyer-Joyce, R. S., and Pickles, M. J. Inter-action between toothbrushes and toothpaste abrasiveparticles in simulated tooth cleaning. Wear, 2004, 257(3–4), 368–376.

68 Grippo, J. O. Abfractions. A new classification of hardtissue lesions of teeth. J. Esthetic Restorative Dentistry,1991, 3, 14–19.

69 McCoy, G. The etiology of gingival erosion. J. OralImplantology, 1982, 10, 361.

70 Lee, W. C. and Eakle, W. S. Possible role of tensile stressin the etiology of cervical erosion legions of teeth.J. Prosthetic Dentistry, 1984, 52, 374–380.

71 Rees, J. S. The role of cuspal flexure in the developmentof abfraction lesions: a finite element study. Eur. J. OralSci., 1998, 106, 1028–1032.

72 Nohl, F. S. and Setchell, D. J. Surface strains induced bymeasured loads on teeth in vivo. Eur. J. Prosthodonticsand Restorative Dentistry, 2000, 8, 27–31.

73 Boyd, A. Enamel structure and cavity margins. Opera-tive Dentistry, 1976, 1, 12–28.

74 Radentz, W. H., Barnes, G. P., and Cutright, D. E. Asurvey of factors possibly associated with cervicalabrasion of tooth surfaces. J. Periodontology, 1976, 47,148–154.

75 Rudd, K. D., O’Leary, T. J., and Stumpf, A. J. Horizontaltooth mobility in carefully screened subjects. Periodon-tics, 1964, 2, 65–68.

76 Jepson, A. Root surface measurement and a methodfor X-ray determination of root surface area. ActaOdontologica. Scandinavica, 1963, 21, 35–46.

77 Xhonga, F. A. Bruxism and its effect on the teeth. J. OralRehabil., 1977, 4, 65–76.

78 Lambrechts, P., Braem, M., and Vanherle, G. Evalu-ation of clinical performance for posterior compositeresins and dentine adhesives. Operative Dentistry,1987, 12, 53.

79 Barbour, M. E., Parker, D. M., Allen, G. C., andJandt, K. D. Human enamel dissolution in citric acidas a function of pH in the range 2.30 � pH � 6.30 – ananoindentation study. Eur. J. Oral Sci., 2003, 111,258–262.

80 Lupi-Pegurier, L., Muller, M., Leforestier, E.,Bertrand, M. F., and Bolla, M. In vitro action ofBordeaux red wine on the microhardness of humandental enamel. Arch. Oral Biology, 2003, 48, 141–145.

81 Bashir, E., Ekberg, O., and Lagerlof, F. Salivaryclearance of citric acid after an oral rinse. Journal ofDentistry, 1995, 23, 209–212.

82 Millward, A., Shaw, L., Harrington, E., and Smith, A. J.Continuous monitoring of salivary flow rate and pH atthe surface of the dentition following consumption ofacidic beverages. Caries Res., 1997, 31, 44–49.

83 West, NX., Hughes, J. A., and Addy, M. The effect of pHon the erosion of dentine and enamel by dietary acidsin vitro. J. Oral Rehabil., 2001, 28, 860–864.

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91 Archard, J. F. Contact and rubbing of flat surfaces.J. Appl. Physics, 1953, 24, 981–988.

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