1. Discuss the principle of adhesion, the current concept and its clinical application

Adherend – any substance bonded to another by an adhesive

Adherent – sticking, clinging

Adhesive – a substance that causes something to adhere

From Bayne (2005):

Adhesion involves an adhesive that is placed on an adherend, which forms an interface.

In most instances, depends on micromechanical interlocking of the adhesive with the

substrate irregularities.

5 prerequisites to form good adhesion:

1) Clean adherend: surface of adherend must be free of any surface debris, films of oil or

dirt that could prevent an adhesive from coming into good contact with the adherend.

Surface debris is in the form of smear layer. Smear layer is the organic debris (collagen,

water, hidroxyapatite) from tooth preparation that remains on the dentine surface.

Surface can be cleaned with water, solvent or acid. Smear layer can only be removed

totally with acid etching (37% phosphoric acid) or partially with acidic monomers.

2) Good wetting : the adhesive must spread onto the adherend with a low contact angle so

that it wets the surface and develops good intimate contact. This is provided by primer

which usually contained a high level of solvents like acetone or water (up to 95%).

These solvents help in dissolving components, mix with water present in dentine and

provide low contact angle for wetting. Unfortunately, some substrates are hydrophilic,

and some are hydrophobics. Enamel and dentine are hydrophilic, while most composites

are hydrophobic. Hydrophilic materials do not wet hydrophobic surfaces very well and

vice versa.

3) Intimate adaptation : if good wetting occurs and the material flows adequately, then it

can penetrate into all surface interstices and develop good adaptation. Poor adaptation

results in unfilled spaces which crack may stem and propagate, causing bonding failure.

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4) Bonding : The adhesive should interact in as many ways as possible with the adherend

by developing physical bonding, chemical bonding (only GIC) or micromechanical

bonding (most important).

How is the mechanism of bonding to enamel? During etching with acids, hydroxyapatite

crystals at the exposed ends and side surfaces will dissolve, creating surface relief on

enamels. The monomers from primers and bonding agents will penetrate into the

surfaces, become polymerized and create micromechanical interlocking.

How about mechanism of bonding to dentine? When the bonding system is applied, the

site of action is only the region between the tubules, not the tubules itself. Only the

microtags that form within the intertubular dentine, creating the hybrid layer, actually

produce the bond strength. In other words, it is a micromechanical interlocking of resin

around dentinal collagen fibrils that have been exposed by demineralization.

5) Good curing : Full curing must be achieved. Undercuring causes debonding.

The current concept:

Buonocore 1955

Found that etching enamel with phosphoric acid increased the duration of adhesion under

water.

Attempted to introduce dentine adhesive but was unsuccessful. Dentine was not etched in

these early bonding system, because of limited understanding of smear layer, resulting in

weak and unreliable bond strength.

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Fusayama 1979

Tried to simplify bonding to enamel and dentine by etching the preparation with 40%

phosphoric acid.

Unfortunately, it was not understood that this procedure overetched dentine and resulted in

the collapse of exposed collagen fibers.

Nakabayashi 1982

Was the first to investigate the mechanism of bonding to dentine. Identified hybrid layer

resulted from polymerized methacrylate and dentine.

Thus, the development of 4th generation bonding systems. The total-etch technique, permits

the etching of enamel and dentine simultaneously using phosphoric acid for 15-20 seconds.

The surface must be left moist, to prevent collagen collapse.

5th generation consists of two different types of adhesive materials: ‘one-bottle systems’ and

self-etching primer bonding systems.

One –bottle systems (two-steps): Popular in US & Europe. Combines primer and adhesives

into one solution. To be applied after etching enamel and dentine simultaneously with

phosphoric acid for 15-20 seconds. Shows high bond strength.

Self-etching primer (two-steps): Developed by the Japanese. Combines etching and priming

steps followed by bonding agent.

Yoshiyama 1996

Demonstrated no significant differences between one-bottle systems and self-etching primer

bonding systems in a lab study.

Landuyt 2006

Demonstrated that bonding effectiveness in Clearfil self-etch bond is improved by

additional etching with phosphoric acid limited to enamel only.

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Clinical application:

Self-etch systems do not remove the entire smear layer and smear plugs. Thus, it tends to

keep the surface sealed and prevent any post-operative sensitivities. Suitable in known cases

of post-op sensitivities.

Self-etch systems combines etching and priming steps, thus reduces the working time,

eliminate the washing out step and eliminate the risk of collagen collapse. So it is very

suitable to use in young children.

2. Discuss the various components of adhesive and their composition

Bonding agents contain three major ingredients which include etchant, primer and adhesive.

2.1 Composition

2.1.1 Etchants

Etchants are relatively strong acid solutions that are mainly based on phosphoric

acid.

Buonocore in 1955 reported the use of 85% phosphoric acid to improve the

retention of acrylic resin on enamel. Selective dissolution of enamel surface

produces microporosities within and around enamel prism. Bonding to dentin was

unsuccessful not until the late 70’s. The idea of dentine adhesion was first

described by Nakabayashi in 1982. In dentine etching, acid is used to remove or

penetrate through the smear layer and demineralize the dentine surface.

Acid etchants are also called conditioners to disguise the fact that most are

relatively strong acids.

Organic, polymeric and mineral etchants have also been used to produce etching

patterns but phosphoric acid solutions and gels (37%, 35% and 10%) have been

shown to produce the most reliable result.

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2.1.2 Primers

Primers are hydrophilic monomers usually carried in a solvent.

Solvent improves the wetting characteristic and adaptation of bonding agents to

the etched dentinal surface.

The solvents used in primers are acetone, ethanol-water or primarily water.

Therefore, primers have different evaporation rates, drying patterns and

penetration characteristics which can influence the resulting bond strength.

Scotchbond Multipurpose Plus Ethanol and water solvent system

Prime & Bond Acetone solvent system

One-Step Acetone solvent system

Single Bond Ethanol and water solvent system

Solvent can easily volatilize into the environment during manipulation. This may

allow the other components of the bonding agents, such as the monomers to

escape as well. Dentin bonding monomers are hazardous and it is recommended

to use high volume evacuation to avoid from contacting or breathing these

materials.

Acidic primers containing carboxylic acid groups are used in self-etching bonding

agents.

Solvent Advantages Disadvantages

Acetone Dries quickly Evaporates quickly, sensitive to

wetness of dentine, multiple coats

may be required

Ethanol/water Evaporates less quickly, less

sensitive to wetness of dentine

Extra drying time

Water Slow evaporation, not sensitive

to wetness of dentine

Long drying time, water can

interfere with adhesive if not

removed

Solvent free No drying, single coat Higher film thickness

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2.1.3 Adhesives

Adhesives are generally hydrophobic, dimetacrylate oligomers that are

compatible with monomers used in the primer and composite.

2.1.4 Initiators and Accelerators

Most bonding agents are light cured and contain an activator such as

camphorquinone and an organic amine. Dual-cured bonding agents include a

catalyst to improve self-curing.

2.1.5 Fillers

Inorganic fillers in the bonding agents range from 0.5% to 40% by weight.

Filled bonding agents may be easier to place on the tooth and may produce

higher in vitro bond strengths.

Micro-fillers also called nanofillers and submicron glass.

2.2 Classification

Bonding agents are classified by the number of steps needed to complete the bonding

process. Some also classified the bonding agents according to its chronological

developments.

2.2.1 Bonding process

Bonding system available includes the conventional, two steps and ‘all in one’

system.

Conventional bonding process

This system has three separate steps; etching, priming and adhesive steps.

It provides reliable bonding.

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The setback of this system is possible contamination of the bonded surface

which has to go through various distinct steps before adhesion is

accomplished. Therefore, it is technique sensitive and became less popular

these days.

Two step systems

There are two subgroups in this system:

I. Separate etch with combined priming and bonding step

Although one step has been eliminated, good infiltration of the

priming-bond into the demineralized dentine is questionable

II. Combined self etch and priming with separate adhesive

Also known as self etching primer. Acidic resin etches and

infiltrates the dentine simultaneously. The self etching priming

does not have to be rinsed away. The dentine buffers the acidity of

self etching primer and it is neutralized after the polymerization.

May not be able to produce good seal due to insufficient etch.

Produce less sensitivity compared to conventional and single bottle

systems – Sano et al., 1998

Less technique sensitive

All in one systems

Also known as one-bottle systems

Combined all steps into one process

Incorporate the bonding resin to the self-etching priming material

May not etch the enamel as effectively as phosphoric acid

2.2.2 Adhesive generations

The ‘generation’ of a dentine adhesive follows the order in which they are

developed. Each generation utilizes different bonding procedures.

The two distinguishing features of classification by generations are the

removal of smear layer and the effective formation of a hybrid layer.

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The chronology of development for bonding of direct composites to tooth

structure:

Generatio

n

Time

period

Development

1950-1970 Experimentation with mineral acids for

bonding acrylic to enamel

Early

1970s

Acid-etching of enamel,enamel bonding agents

self cured

Late 1970s Hydrophobic enamel bonding agents,

hydrophilic dentine bonding agents,light cured

components

4 Mid to late

1980s

Removal of dentine smear layer,acidic

monomers,reduction of steps in bonding

technique.

Etch+primer+adhesive

5 Early

1990s

Etching to achieve hybrid layer in

dentine,hydrophilic agents for both enamel and

dentine,bonding to moist tooth structure,single

bottle primer-adhesive

Etch and rinse+primer/adhesive

6 Mid to late

1990s

Self-etching primers and primer adhesives,light

and dual cured options

Self etched primer+adhesive

7 Early

2000s

No-mix,self etching adhesives

Single bottle system

Etch/primer/adhesive

3.The biocompatibility concept of adhesive materials:

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Williams has defined biocompability as the ability of material to elicit an appropriate host

response in specific application.

The biocompability of dentin bonding agents is important since they are placed on etched dentin

near to pulp where tubular density and diameter are greatest. Un polymerized monomer in

bonding agent is a potential biological harm.

The risk of biological harm from degraded or un polymerized monomers is depended on several

factors:

1. The component must be free of polymer to diffuse in to the pulpal tissues.

2. The component must have solubility properties that encourage its diffusion into the pulp.

3. The time and dose of the pulpal exposure must be sufficient to cause a biologic reaction.

(Hanks et al., 1991) reported the toxic concentrations of 11 components of dental resins on

mouse fibroblasts.

(Ratanasathien et al., 1995) reported the additive cytotoxic effects produced by HEMA when

used as a solvent for BisGMA.

(Bouillaguet et al., 1996) demonstrated that HEMA is able to diffuse rapidly through dentin in

vitro to cause cytotoxicity.

(Rakich et al., 1999) demonstrated that resin monomers are hazard to inflammatory cells that are

common in the pulpal tissue.

(Noda et al., 2003) have shown that resins alter the secretion of inflammatory mediators from

human macrophages.

Factors that alter the diffusion and influence the toxicity of resins:

1. Remaining dentin thickness.

2. Dentin permeability.

3. Dentin location.

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Shallow cavities located in superficial or sclerotic dentin have less biological risk, because the

permeability of dentin is low and the thickness of the remaining dentin is adequate to prevent any

adverse effects from diffusing materials.

Gwinnett and Tay 1998 reported that application of total- etching adhesive to deep cavities cause

a persistent inflammation and granulomatous reaction in human pulp. They also reported that the

presence of resin globules displaced into the dentinal tubules and penetrating the pulp.

In deep dentin, the etching will make the substrate more permeable and hydrophilic. Increased

hydrophilicity limits the wetting of tubule wall by the resin. So, the self-etching adhesive

systems is indicated for young, deep, permeable dentin, because self etching adhesives leaves

some residual smear plug material in the tubules which limits the diffusion of uncured monomers

toward the pulp (Tay et al.,2000).

4. Discuss various methods of bond strength evaluation and its relationship to the dentine substrates.

Bond Strength

is the force per unit area required to break a bonded assembly with failure occurring in or near the adhesive/adherened interface (Hasium 1993).

Bond strength rests could also be called ‘debonding tests’. The purpose of breaking apart a bonded assembly is to try to establish a number value

showing how strong this bond was. The resistance to debonding is measured in terms of bonding strength (MPa).

Type of bond strength tests (Bayne S. 2005)The magnitude of the measured strength is depending on the type of test being conducted.

2 types of tests;1. Shear Bond Strength Test

To destroy interfacial bonding by sliding the adherends parallel to the interface.2. Tensile Bond Strength Test

Destroy interfacial bonding by pulling the adhesive perpendicularly away from the surface.

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The tests can be described depends to the sample size; Large samples test are called macro tests. Small specimens of the sample tests are called micro tests. The most common tests are macro shear (SBS), macro tensile (TBS) and micro

tensile bond strength (µTBS). In macro tests, shear strength is often 50% to 100% greater in value than tensile

strength. Micro tensile bond strengths tend to be much higher (often 2x to 4x than macro TBS

values because the defect concentration in the small cross-sectional interfacial areas in much smaller.

5 factors affecting performance of bond strength. Operator

Related to the technical abilities of the operator, age and eyesight.

Design Including biomechanical design of the cavity preparation such as the presence of a

smear layer, the actual outline form and the depth of the cavity.

Materials Depend on the composition, product age, temperature effects and percent

humidity effects.

Intraoral location The position of the restoration within the oral environment includes anterior-

posterior location, maxillary-mandibular location, lingual-facial location, premolar-molar location, tooth-flexure and tooth rotation effects.

Patient Individual patients have different fluoride exposures and history, diets, oral

hygiene IQs, caries risks and epidemiological risk factors.

Bond Strength Testing and relationship to the dentine substrates.

De Munck et al; 2005 Bond strength tests frequently used to screen adhesives.

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The rational of the testing method is that the stronger the adhesion between tooth and biomaterial, the better it will resist stress imposed by resin polymerization and oral function.

The effectiveness of adhesives can be determined by using testing different types of adhesives bonded to enamel and dentine.

Buonocore 1955 showed that bonding to enamel requires only an acid- etch step followed by the application of a fluid resin, without intermediary primer step. Primers do not negatively influence bonding effectiveness and are mandatory when a ‘wet-bonding’ procedure is carried out.

In enamel, because of superficial interaction of self-etch adhesives, thus less potential for micro-mechanical interlocking than a phosphoric-acid treatment. Additional chemical bonding capacity to hydroxyapatite contributed to the actual bonding effectiveness of ‘mild’ self-etch adhesives.

Among all adhesives tested, one-step self-etch adhesives produced the lowest µTBS. In dentine, three-step etch and rinse adhesives still surpassed than other simplified

application procedures.Statistical analysis of µTBS data showed that three-step etch-and-rinse adhesives bonded significantly more strongly to dentine than two-step etch-and-rinse and two-step self-etch adhesives.

Van Landuyt K et al; 2006 Carried out a study to test hypothesis that the two-step self-etch adhesive Clearfil SE

Bond bonds equally effective to enamel/dentine either with or without prior etching with phosphoric acid.

The result had shown increased bonding effectiveness of C-SE to enamel and more micro-retentive surface after acid –etched. However in dentine, phosphoric-acid etching prior to C-SE application significantly decreased the microtensile bond strength (µTBS) to dentine and an increased number of adhesive failures were observed.

The decreased bond strength of C-SE when bonded to acid-etched dentine is related to incomplete infiltration of the demineralised collagen network by the bonding resin.C-SE etch group required a ‘dry bonding’ procedure which causes a collapse and shrinkage of the collagen network and this may this may hinder efficient resin infiltration, leading to porous zones, especially at the bottom of the hybrid layer.

Phosphoric-acid etching generally results in abrupt transition of exposed collagen to unaffected dentine. Demineralized zone at the bottom of hybrid layer will be formed induced by the self-etching functional monomers.

While the primer penetrates adequately, the filled bonding resin of C-SE still may have been hampered to penetrate the exposed collagen network completely.

This study concluded that the bonding effectiveness of Clearfil SE Bond can be improved by selectively etching the enamel margins of a cavity with phosphoric acid.

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Etching should limited to enamel only because etching to dentine led to the formation of a low-quality hybrid layer prone to nanoleakage, and to impaired bond strengths to dentine.

5.Discuss the durability of adhesion to tooth substrates.

Durability of resin-tooth interface was assessed based on the in vivo and in vitro methodologies.

In vivo, all possible aging factors interact simultaneously. All possible artificial aging methods

were evaluated at the same time to produce the best image possible of the in vivo behaviour of

adhesive restorations. Artificial aging factors included water storage, cycling loading,

mechanical loading and thermo-cycling.

Etch and rinse adhesive

Since the primer solvent within the etch and rinse adhesive is a major factor affecting the

handling ( Tay et al.,1998 ) and performance ( Carvalho et al.,2003 ), the etch and rinse

adhesives were categorized according to their respective solvent :

1. Ethanol based

Optibond adhesive by Kerr are the most frequently tested representatives of this category.

In vitro, the shear bond strength of the two-step version, optibond solo is significantly

decreased by water storage as well as by thermocycling. The µTBS of the optibond solo

decreased after 4 years of water storage, when the dentin interface was directly exposed to

the storage medium.

The bonding effectiveness of the three-step version was not affected by water storage,

thermocycling and mechanical loading.

This reduced in vitro durability can be link to its reduced infiltration/hybridization capacity.

In vivo, both the two-step and the three-step version seem to function well.

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2. Acetone based

The bonding effectiveness of the two-step version (one-step BISCO )decreased following

thermocycling and water storage.

The three step ( all bond 2 ) showed decreased shear bond strength significantly after 6

months of water storage, whereas no increase in microleakege could be detected within the

same period.

Although excellent in vitro bonding effectiveness can be obtained with the acetone based

adhesives, its high technique-sensitivity may explain the less than optimal long term results.

3. Water based

The most studied representatives are the three-step ( Scotchbond MP ) and two-step

( Scotchbond 1 ) etch and rinse adhesives.

The bonds produced by the three step version resisted dynamic loading in a fatigue test. in

longer observation, it is prone to some degradation by thermocycling as well as by water

storage. After in vivo functioning, the µTBS decreaesd due to hydrolytic breakdown the

interface. It has been hypothesized that this reduced durability is related to the incorporation

of a high-molecular weight polyalkenoid acid co-polymer. Phase separation was shown to

occur, with the co-polymer being filterd out by the collagen network and deposited as a

distinct gel on the surface of the collagen network. In extreme case, the gel may hinder

adequate resin-interdiffusion.

As conclusion, three step ethanol-water-based etch and rinse adhesives are still regarded as the

gold standard in terms of bond durability.

Self etch adhesives

Self etch adhesives make use of acidic monomers that simultaneously etch and prime dentin.

Without the need for rinsing, the application time of self etch adhesives is shorter and the

technique sensitivity lower.

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The bond produced by mild two-step self-etch adhesives to dentin seems quite stable in vitro

; (1) no significant decreased in µTBS occurred after 1 year of function in vivo although

SEM fracture analysis revealed an increased in porosity within the adhesive resin (2) up to

30000 thermal cycles did not significantly decreased the shear bond strength (3) combined

thermal and occlusal loading resulted in a small but statistically insignificant decreased of

µTBS (4) long term water storage decreased the µTBS of two-step self etch adhesives.

The strong self etch adhesive (Adper prompt ) showed in vitro studies that the µTBS of the

water-stored specimen was so low that most specimens did not survive specimen processing.

As conclusions, the bonds obtained by mild two-step self etch adhesives seem quite durable, in

contrast to 'all-in-one' adhesives that produce less durable bonds in vitro.

Glass Ionomer

The only self adhesives restorative materials due to ionic bond formation between the carboxyl

groups of polyalkenoic acid with hydroxyapetite at the tooth surface. Though the initial bond

strength was quite favorable, the µTBS decreased over time as a result of water storage. This

degradation process must probably due to decreased material properties rather than decreased in

bonding potential.

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