effect of restorative materials on cuspal flexure
TRANSCRIPT
Effect of restorative materials on cuspal flexure
John MedigeVYuru Deng**/XJnyi Yu***/Elaine L. Davis****/Robert B. Joynt****
The purposes of this .study were {1) to establish a methodology for determining surface strains in two locations ofthc same tooth under intact, prepared, and restored conditions and(l) to compare the effects on stiffness of different restorative materials in a tooth subjected to cuspal loading. Two linear strain gauges were tnounted on each of 30 extracted ma.\iltary premolar teeth. Teeth were mounted in poly(methyl methacrylate) resin and randomly ussigned to one of three study groups according to the restorative material and application technique to be used. Statistical analysis indicated a statistically significant interaction between restorative material and tooth condition at both proximal and buccal sites and a statistically significant difference in stiffness between teeth restored with Tenure/Marathon V and those restored with either amalgam or Seotchbond 2/P-50 at the proximal site. Results suggest that the methods employed provide a useful, nondestruetive means of testing the same tooth under various conditions. (Quintessence inl 1995:26:571-576.)
Introduction
Preparation reduces the stmctnral integrity of teeth. Because resin composite can be bonded to enamel with the acid-etching technique, a more conservative cavity preparation is possible than for amalgam restorations. The use of dentinal bonding systems can fiirther expand the area of attachment between tooth structure and resin composite, increasing both mechanical stiffness and resistance to mieroleakage.
* Associate Professor, Deparlmcnl of Mechanical and Aerospace Engineering, State Universily of New York at Buffalo, School of
Engineering and Applied Sciences. Buffalo, New Yoflt
' * Graduate Student, Department of Biomaterials, Stale University of New York at Buffalo, School of Dental Medicine, Buffalo, New York,
**' Assistant Professor, Department of Restorative Dentistrj', State University of New York at Buffalo, School of Dental Medicine, Buffalo, New York,
**** Associate Professor. Depanment of Restorative Dentistry, State University of New York at Buffalo, School of Dental Medicine, Buffalo New York.
Reprint requests: Dr E. L, Davis, Associate Professor, Departmenl of Restorative Dentislry, State University of New York at Buffalo, Schoolof Denial Medicine, 335 Squire Hall, Soulh Campus, Buffalo, New York Í42I4,
The use of resin composites to restore posterior teeth has been a controversial subject since resin composites came into widespread use in the late 1960s, Studies investigating the purported reinforce- ment effect of resin composites on weakened tooth structure have produced varied results.'""
Fracture tests by Gelb et al' indicated that resin- bonded restorations restore tooth strength lost after cavity preparation, Morin et aF found that tooth rigidlt>' is increased when an acid-etching technique is used to treat enamel before placement of resin composites. Eakle* found that the use of dentinal bonding agents further increases resistance to fracmre of teeth restored with resin.
In most previous studies of cuspal flexure and fracture, a load was applied in an axial direction through a hard spherical or cylindrical testing device to the occlusal surface of the tooth. This procedure results in high stress concentrations on the triatigular ridges of the facial and lingual cusp inclines, rather than distribution of stress over the occlusal surface.
The purposes of the present study were (I) to establish a methodology for determining the defor- mation of teeth caused by cuspai loading, using a method that distributes force over a substantial portion
Ouintessejje*- 571
Dental Research
Fig 1 Loading device against cusloni-made casting.
ofthe occlusal surface and (2) to compare the pattern of structural deformation under load for intact, pre- pared (with mesio-occlusodistal [MOD] prepara- tions), and restored teeth. A maximal load of 300 N was used, which is within the range of normal chewing forces.''^ Restorations were either bonded (resin composite) or unbonded (amalgam).
Method and materials
Extracted maxillary premolar teeth were collected and placed in a i% hydrogen peroxide solution imme- diately following extraction. Teeth were examined visually with the aid of a transilluminating fiberoptic light. Only those teeth with no visible defects were retained. Maximal mesiodistal and buccolingual di- mensions were measured with a Boley gauge (Buffalo Dental). These two dimensions were totalled, and only those teeth within a specified range ( 16.6 to 19.3 mm) were included, to minimize differences in tooth size.
Thirty teeth were selected, cleaned with pumice, scaled, and placed in deionized water They were removed from water only long enough to complete necessary procedures. Tiie teeth were randomly as- signed to one of three groups, according to the restorative materials to be used: dental amalgam or one of two posterior resin composite restorative systems.
The apex of each tooth was centered on the base portion oía two-piece break-apart form (SampMCup, Buehler), so that the long axis of the tooth was perpendicular to the plane of the base. The root portion of the tooth was then embedded in poly- ( methyl methacrylate) (PMMA) tray resin (Tramix. Stratford-Cookson) to a point approximately 2 mm below the cementoenamel junction (CEJ), to approxi- mate the height of healthy alveolar bone. The base of each mounted specimen was trimmed to expose a cross section of the root, reducing the root length by approximately 3 mm. The exposed root allowed transmission of applied force entirely through tooth structure and prevented subsidence of tiie tooth in the resin during testing.
A custom-designed loading head was fabricated for testing specimens. The loading device was beveled based on the calculation of average occlusal cuspal inclines for nine previously selected maxillary pre- molars, 37 degrees on the buccal cuspal inciine and 33 degrees on the lingual cuspal inciine.
A casting, which adapted to both the cuspal inclines ofthe occiusai surface and the beveled faces ofthe loading device, was made for each specimen to distribute the load over the outer parts ofthe occlusal surface (Fig 1). A strip of 28-gauge relief wax was applied to the central portion ofthe occlusal surface of each specimen. An impression was then made of each mounted specimen and poured up with improved dental stone. The layer of wax provided a spacer to prevent contact of the finished casting with the restoration. The casts were removed from the impres- sion, trimmed, and allowed to set for at least 24 hours.
The loading head was mounted in a surveyor and positioned so that a wax pattern could be fabricated. Wax was applied to the occlusal surface ofthe cast and extended just over the buccal and litigual cusps. The mounted loading head was warmed and used to form the upper surface ofthe wax pattern. The thickness of the wax pattern was kept to about 1 mm.
The wax pattern was sprued and invested with Beauti Cast investment material (Whip Mix), placed in a water bath for 1 hour, and then cast with Williams Technique Metal 35 (lvoclar). The sprue was removed and the surface of the casting was finished and polished. The casring was cut into two pieces along the central groove to allow the buccal and lingual cusps to flex independently during the testing procedure.
A custom resin composite matrix was fabricated on the proximal surfaces of each intact tooth. Rosin
572 Quintessence International Volume 26, Number 8/1995
Dental Research
Fig 2 Proximai view of strain gauges on buccai anc proximai surfaces.
Fig 3 Dimensions of MOD cavity preparations.
A = 1/3 of B C = 1/3ofD
D
B
i A
composite (Opalux, ICI Pharmaceuticals) was adapt- ed to both proximal surfaces and activated with a high-intensity light source (Prisma Lite, LD Caulk) for 30 seconds, A thin layer of toil {Matrix Strip. Den-Mat) was cemented to the inner surface ofthe matrix to prevent adhesion ofthe restorative material,
A linear strain gauge (grid length 0,79 mm, nominal resistance 120 ohms; model EA-O6-O31DE-12O, Mea- surements Group) was mounted on the buccal surface of each tooth in a vertical orientation. An identical gauge was placed horizontally on enamel, at the proximal CEJ (Fig 2), The backing ofthe proximal gauge was trimmed to approximately 1 mm in width and placed immediately above the proximal CEJ. Both gauges were primed with a catalyst before attachment with an adhesive (M-bond 200, Measurements Group), Bondable terminais were attached to the side of the PMMA base, with the same adhesive, and wired to the gauges.
The installed gauges were then tested with a strain gauge installation tester (Model 1300, Measurements Group) to assure their functionality'. The strain gauge, solder, and connecting wires were covered with adhesive (Mirro 3, Kerr/Sybron) and sealant (737 RTV, Dow Corning) to prevent moisture contam- ination. After the coatings were set. each specimen was placed in a container of deionized water maintained at room temperature.
Three groups were established according to resto- rative material to be used: amalgam. Scotchbond 2 with P-50 (3M Dental) and Tenure with Marathon V (Den-Mat), Each mounted specimen was placed on the lower platen of an axial testing machine (T22K, MTS}, The beveled loading device was used to test each tooth at a crosshead speed of 1 mm/niin to a maximal load of 300 N,
Specimens in all three groups were prepared for MOD restorations after they had been tested intact, Cavit>' dimensions are shown in Fig 3, Hach cavity was cut with a No, 56 FG bur (SS White). All specimens were then retested.
Specimens in the first group were restored with Valiant-Ph.D. (LD Caulk}, a high-copper dental amalgam. The resin composite rnatrix was secured whh a special retainer to protect the gauges. After condensation, the matrix and retainer were removed and the restoration was carved to normal anatomic form.
A 1-mm-wide occlusal marginal bevei was placed on the cavity preparation of resin composite specimens, at an angle of approximately 45 degrees to the unpre- pared surface. The enamel portion of the cavity preparation and 1 mm beyond the cavosurface margin were etched with 31% phosphoric acid for 30 sec- onds,' Specimens were then rinsed with water and dried with a stream of oil-free compressed air.
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Fig 4 MGan (± SE) proximal strain-force curves as a function oi restorative material and toolh condition A higher mean (ie, closer to íero¡ indicates greater stiffness.
Fig 5 Mean (+ SE) buccal strain-force curves as a function of restorative material and tooth condition. A higher mean (ie, closer to zero) indicates grealer stiffness.
Scotchbond 2 dentinal bonding agent was applied to the second group of specimens. The matrix was placed and the preparation was restored incrementaily with P-50 light-activated resin composite.'° Each increment was light activated for 20 seconds, and the entire restoration was exposed to the light source for an additional 60 seconds.
The Tenure dentinal bonding system was applied to the third group of specimens. The matrix and retainer were placed and the MOD cavity was restored with Marathon V resin composite, a light- and chemically activated (ie. dual-cured) paste system, applied in a syringe. The restorative material was light activated for 60 seconds.
The occlusal surface of each specimen was checked with articulating paper to confirm that the casting did not touch the restoration. Specimens were stored for at least 24 hours in 100% relative humidity, to allow for complete set of the restorations, and then retested.
Buccal and proximal strains, force, and loading head displacement were recorded with a condilioner/ amplifier (2100 System, Measurements Group) and a computerized data-acquisition system (IBM PC XT, Data Translation DT 280ÍA) using a customized program and commercial software (ASYST), Strain- force curves were plotted, and linear regressions were obtained. Data were recorded for each tooth under the
three conditions: intact, prepared but unrestored, and restored.
Each test was repeated three times. The first run was used to ensure proper seating and alignment. The second was used for data acquisition. The third run was used for verification, Muitivariate repeated-measures analysis of variance (ANOVA) and multiple-com- parison procedures were used to analyze the data.
Results
Thiee specimens with MOD cavity preparations frac- tured during testing and were eliminated from the analyses. Several other specimens were eliminated because of failure ofthe strain gauge, resulting in a total of 23 specimens (nine amalgam, seven Scotchbond 2/P-5O and seven Tenure/Marathon V} for the proxi- mal site and 23 specimens (eight amalgam, eight Scotchbond 2/P-50 and seven Tenure/Marathon V) for the buccal site.
Means and standard errors ofthe strain-force slopes for the three material groups under intact, prepared, and restored conditions are shown in Figs 4 (proxi- mal) and 5 (buccal), A negative siope indicates compressive (as opposed to tensile) strain.
Repeated-measures ANOVA indicated a significant interaction between restorative material and tooth
574 Quintessence International Volume 26, Number
Dental Research
Source
Mean square = 3.70
• yl - total score, across conditions (used to lest nialerial main efTect); y2 = difference score, restored minus prepared; y3 - difference seore, intact minus prepared.
t Main effect for condition, P- .0005. Uniyariate results were significant for both y2 (F,,20 = T.56; P- .012) and y3 (F, 20 ' 31''O; P- 0002).
X Material by condition interaction, P ~ .020. Univariate results were significant for y2 IF-, m» 7.63; P = .003) but nol for y3 (F, ,0 " 0.01; ^=.993).
Test of significance*
- F2,2o=0.22 Mean square = 1.53
* yl - tolal score, aeross conditions (used to test material rnain effect]; y2 - difference seore, restored tninus prepared; y3 = difference seore, intact minus prepared.
t Main effect for eondition.P^.0001, Univariate results wtre significant for both y2(Fi,2o- 1 i. 16;/>= .0003) and y3 (Fi,i(,= 50.66;/>=.0001 !.
Í Material by condition interaction, P = .042. Univariate results were significant for y2 ¡Fi 20 = ^•^'^- P = 026) but not for y3 IF2 lo = 0-33; P^.797).
condition (intact, prepared, or restored) at both proximal (F4 38 = 3.30; P- .02) (Table 1) and buccal sites (Fijg = 2.76, P - .04) (Table 2). Univariate results indicated that this interaction was significant for prepared versus restored, but not for intact versus prepared, conditions at both sites.
Because the materiai-by-condition interaction was significant for the prepared versus restored conditions but not intact versus prepared conditions, follow-up multiple comparisons (Tukey HSD) were made be- tween the prepared and restored data for each restora- tive material. In addition, comparisons between mate- rials at each location (proximal and buccai ) were made for both the prepared and restored conditions. A significance level of .05 was used for ali comparisons. These tests indicated statistically significant diiferences in slope between prepared and restored conditions for Tenure/Marathon V only, at both the proximal and buccal sites. In addition, statistically significant dif- ferences under the restored condition were found between Tenure/Marathon V and both amalgam and Scotchbond 2/P-50 at the proximal site.
These results indicated that teeth restored with Tenure/Marathon V were stifier than teeth that were prepared but not restored. Results did not provide staüstical evidence that teeth restored with either Scotchbond 2/P-50 or amalgam were any stiffer than prepared, unrestored teeth. At the proximal site, teeth restored with Tenure/Marathon V were significantly
stiffer than were those restored with either amalgam or Scotchbond 2/P-50. Results did not provide statisticai evidence of a difference in stiffness among groups at the buccai site.
Discussion
This study describes a nondestructive method of determining locai tooth deformation, allowing re- peated ioading ofthe same tooth under varied condi- tions. In addition, the use of individually fabricated castings and the custom-made loading device provides a means of appiying force to the same part of the occlusal surface in all tests without ioading the restoration.
Specimens restored with amaigam showed little or no recovery of tooth stiifness at either the proximai or buccal site. This was anticipated, because amalgam does not bond to tooth structure. Restoration with resin composite provided substantial recovery of tooth stiffhess for Tenure/Marathon V specimens at both buccal and proximal sites. For Scotchbond 2/P-5O specimens, resuits did not provide evidence to suggest recovery of tooth stiffness at either site.
Two factors, the bonding system and the resin composite, may have contributed to the difference observed between the resin composite groups. The two bonding systems differ in basic formulation and curing method, and in the way in which the smear layer is
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treated. In the Tenure system, the conditioner (which contains 2,5% nitric acid) removes the smear layer and opens dentinal tubule orifices, ln Scotchbond 2, the primer (which contains 2,5% maleic acid) dissolves the smear layer, and smear layer tags remain and occlude the tubules. Tenure solutions A plus B (N-tolyglycine-glycidyl methacr^'late plus pyromellitic acid dimethacrylate) are chemically cured aqueous bonding materiais with low film thickness, whereas Scotchbond 2 Is a light-cured, resin-based bonding material with greater film thickness.
The resin composites in these two systems also difFer, P-SO is a light-cured resin composite, while Marathon is a dual-cui-ed resin. Determination of the potential contribution of each of these factors—smear layer treatment, bonding system formulation and curing method, and resin composite—requires turther study.
In this study, sutface strains were measured in two locations. It would be imprudent to assume that these measurements alone indicate the entire state of stress in the tooth and its likelihood to fail. Strain is indicative of the local deformation only, and isolated strain measurements are subject to misinterpretation, A tooth is a complicated structure with complicated supporting structures and, as such, is extremely difTicult to analyze. Detailed knowledge of the geo- metry and of the mechanical properties of each part of the tooth and its supporting structures is required. These properties vary with location and direction. Thus, in the absence of more accurate measurements and/or analyses, the strains at the location where the stresses are likely to be relatively high might serve as indicators of load severity. Furthermore, because each tooth is tested under several conditions and is thus its own internal control, the strains under the various conditions may reasonably be construed to indicate the relative resistance to deformation under those condi- tions.
Future research should focus on the effects of long-term cyclic loading on tooth stiffness, because durability of restorative systems is an important clinical consideration. Future studies should also include a model that more closely simulates conditions of the oral environment. In addition, although the present investigation provided information regarding strain measurement on the tooth surface, future work should determine internal strains through the use of computational methods, such as finite-element analysis.
References
1, Gelb MN, Barouch E. Simonsen RJ, Resistance to cusp fracture in Class II prepared and restored premolars. J Prosthet Dent I986;55;1S4-185.
2, Share J, Mishell Y, Nathanson D, Effect of restorative material on resistance to fracture of tooth structure in vitro labstraet 622|, J Dent Res 19S2;61:247.
3, Joynt RB, Wieczkowski G Jr, laockowski K, Davis EL, Effects of composite restorations on resistance to cuspal fracture m posterior teeth, J Prosthet Dent 1987;57;431-435,
4, Stampalia LL, Nicholls JL Bnidvik JS, Jones DW, Fraclure resistance of teeth with resin-bonded cestoratiotis, J Prosthet Dent i986;55;694-69a,
5, Morin D, DeLong R, Douglas WH, Cusp reinforcement by the acid-elch technique. J Denl Res 1984^63:1075-1078,
6, Eakle W, Fracture resistance of teetb restored with Class 11 bonded composite resin, J Dent Res I9K6,65:149-153,
7, Vïldmalm S-E, Ericsson SG. Maximal bite force with centric and eccentric load. J Oral Rehabil 1982;9:445-450,
S. Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Occlusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981:46:443-449.
9. Gilpatrick RO, Ross JA, Simonsen RJ, Resin to enamel bond strengths with variable etching limes [abstract 13081, J Dent Res
10. Weczkowski G Jr, Joynt RB, Kiockowski R, Davis EL, Effects of incremental versus bulk fill techniqtie on resistance to cuspal fracture of teeth restored with posterior composites, J Prosthet Dent 1988;60:283-287, n
576 Quintessence International Volume 26, Number 8/ .935
John MedigeVYuru Deng**/XJnyi Yu***/Elaine L. Davis****/Robert B. Joynt****
The purposes of this .study were {1) to establish a methodology for determining surface strains in two locations ofthc same tooth under intact, prepared, and restored conditions and(l) to compare the effects on stiffness of different restorative materials in a tooth subjected to cuspal loading. Two linear strain gauges were tnounted on each of 30 extracted ma.\iltary premolar teeth. Teeth were mounted in poly(methyl methacrylate) resin and randomly ussigned to one of three study groups according to the restorative material and application technique to be used. Statistical analysis indicated a statistically significant interaction between restorative material and tooth condition at both proximal and buccal sites and a statistically significant difference in stiffness between teeth restored with Tenure/Marathon V and those restored with either amalgam or Seotchbond 2/P-50 at the proximal site. Results suggest that the methods employed provide a useful, nondestruetive means of testing the same tooth under various conditions. (Quintessence inl 1995:26:571-576.)
Introduction
Preparation reduces the stmctnral integrity of teeth. Because resin composite can be bonded to enamel with the acid-etching technique, a more conservative cavity preparation is possible than for amalgam restorations. The use of dentinal bonding systems can fiirther expand the area of attachment between tooth structure and resin composite, increasing both mechanical stiffness and resistance to mieroleakage.
* Associate Professor, Deparlmcnl of Mechanical and Aerospace Engineering, State Universily of New York at Buffalo, School of
Engineering and Applied Sciences. Buffalo, New Yoflt
' * Graduate Student, Department of Biomaterials, Stale University of New York at Buffalo, School of Dental Medicine, Buffalo, New York,
**' Assistant Professor, Department of Restorative Dentistrj', State University of New York at Buffalo, School of Dental Medicine, Buffalo, New York,
**** Associate Professor. Depanment of Restorative Dentistry, State University of New York at Buffalo, School of Dental Medicine, Buffalo New York.
Reprint requests: Dr E. L, Davis, Associate Professor, Departmenl of Restorative Dentislry, State University of New York at Buffalo, Schoolof Denial Medicine, 335 Squire Hall, Soulh Campus, Buffalo, New York Í42I4,
The use of resin composites to restore posterior teeth has been a controversial subject since resin composites came into widespread use in the late 1960s, Studies investigating the purported reinforce- ment effect of resin composites on weakened tooth structure have produced varied results.'""
Fracture tests by Gelb et al' indicated that resin- bonded restorations restore tooth strength lost after cavity preparation, Morin et aF found that tooth rigidlt>' is increased when an acid-etching technique is used to treat enamel before placement of resin composites. Eakle* found that the use of dentinal bonding agents further increases resistance to fracmre of teeth restored with resin.
In most previous studies of cuspal flexure and fracture, a load was applied in an axial direction through a hard spherical or cylindrical testing device to the occlusal surface of the tooth. This procedure results in high stress concentrations on the triatigular ridges of the facial and lingual cusp inclines, rather than distribution of stress over the occlusal surface.
The purposes of the present study were (I) to establish a methodology for determining the defor- mation of teeth caused by cuspai loading, using a method that distributes force over a substantial portion
Ouintessejje*- 571
Dental Research
Fig 1 Loading device against cusloni-made casting.
ofthe occlusal surface and (2) to compare the pattern of structural deformation under load for intact, pre- pared (with mesio-occlusodistal [MOD] prepara- tions), and restored teeth. A maximal load of 300 N was used, which is within the range of normal chewing forces.''^ Restorations were either bonded (resin composite) or unbonded (amalgam).
Method and materials
Extracted maxillary premolar teeth were collected and placed in a i% hydrogen peroxide solution imme- diately following extraction. Teeth were examined visually with the aid of a transilluminating fiberoptic light. Only those teeth with no visible defects were retained. Maximal mesiodistal and buccolingual di- mensions were measured with a Boley gauge (Buffalo Dental). These two dimensions were totalled, and only those teeth within a specified range ( 16.6 to 19.3 mm) were included, to minimize differences in tooth size.
Thirty teeth were selected, cleaned with pumice, scaled, and placed in deionized water They were removed from water only long enough to complete necessary procedures. Tiie teeth were randomly as- signed to one of three groups, according to the restorative materials to be used: dental amalgam or one of two posterior resin composite restorative systems.
The apex of each tooth was centered on the base portion oía two-piece break-apart form (SampMCup, Buehler), so that the long axis of the tooth was perpendicular to the plane of the base. The root portion of the tooth was then embedded in poly- ( methyl methacrylate) (PMMA) tray resin (Tramix. Stratford-Cookson) to a point approximately 2 mm below the cementoenamel junction (CEJ), to approxi- mate the height of healthy alveolar bone. The base of each mounted specimen was trimmed to expose a cross section of the root, reducing the root length by approximately 3 mm. The exposed root allowed transmission of applied force entirely through tooth structure and prevented subsidence of tiie tooth in the resin during testing.
A custom-designed loading head was fabricated for testing specimens. The loading device was beveled based on the calculation of average occlusal cuspal inclines for nine previously selected maxillary pre- molars, 37 degrees on the buccal cuspal inciine and 33 degrees on the lingual cuspal inciine.
A casting, which adapted to both the cuspal inclines ofthe occiusai surface and the beveled faces ofthe loading device, was made for each specimen to distribute the load over the outer parts ofthe occlusal surface (Fig 1). A strip of 28-gauge relief wax was applied to the central portion ofthe occlusal surface of each specimen. An impression was then made of each mounted specimen and poured up with improved dental stone. The layer of wax provided a spacer to prevent contact of the finished casting with the restoration. The casts were removed from the impres- sion, trimmed, and allowed to set for at least 24 hours.
The loading head was mounted in a surveyor and positioned so that a wax pattern could be fabricated. Wax was applied to the occlusal surface ofthe cast and extended just over the buccal and litigual cusps. The mounted loading head was warmed and used to form the upper surface ofthe wax pattern. The thickness of the wax pattern was kept to about 1 mm.
The wax pattern was sprued and invested with Beauti Cast investment material (Whip Mix), placed in a water bath for 1 hour, and then cast with Williams Technique Metal 35 (lvoclar). The sprue was removed and the surface of the casting was finished and polished. The casring was cut into two pieces along the central groove to allow the buccal and lingual cusps to flex independently during the testing procedure.
A custom resin composite matrix was fabricated on the proximal surfaces of each intact tooth. Rosin
572 Quintessence International Volume 26, Number 8/1995
Dental Research
Fig 2 Proximai view of strain gauges on buccai anc proximai surfaces.
Fig 3 Dimensions of MOD cavity preparations.
A = 1/3 of B C = 1/3ofD
D
B
i A
composite (Opalux, ICI Pharmaceuticals) was adapt- ed to both proximal surfaces and activated with a high-intensity light source (Prisma Lite, LD Caulk) for 30 seconds, A thin layer of toil {Matrix Strip. Den-Mat) was cemented to the inner surface ofthe matrix to prevent adhesion ofthe restorative material,
A linear strain gauge (grid length 0,79 mm, nominal resistance 120 ohms; model EA-O6-O31DE-12O, Mea- surements Group) was mounted on the buccal surface of each tooth in a vertical orientation. An identical gauge was placed horizontally on enamel, at the proximal CEJ (Fig 2), The backing ofthe proximal gauge was trimmed to approximately 1 mm in width and placed immediately above the proximal CEJ. Both gauges were primed with a catalyst before attachment with an adhesive (M-bond 200, Measurements Group), Bondable terminais were attached to the side of the PMMA base, with the same adhesive, and wired to the gauges.
The installed gauges were then tested with a strain gauge installation tester (Model 1300, Measurements Group) to assure their functionality'. The strain gauge, solder, and connecting wires were covered with adhesive (Mirro 3, Kerr/Sybron) and sealant (737 RTV, Dow Corning) to prevent moisture contam- ination. After the coatings were set. each specimen was placed in a container of deionized water maintained at room temperature.
Three groups were established according to resto- rative material to be used: amalgam. Scotchbond 2 with P-50 (3M Dental) and Tenure with Marathon V (Den-Mat), Each mounted specimen was placed on the lower platen of an axial testing machine (T22K, MTS}, The beveled loading device was used to test each tooth at a crosshead speed of 1 mm/niin to a maximal load of 300 N,
Specimens in all three groups were prepared for MOD restorations after they had been tested intact, Cavit>' dimensions are shown in Fig 3, Hach cavity was cut with a No, 56 FG bur (SS White). All specimens were then retested.
Specimens in the first group were restored with Valiant-Ph.D. (LD Caulk}, a high-copper dental amalgam. The resin composite rnatrix was secured whh a special retainer to protect the gauges. After condensation, the matrix and retainer were removed and the restoration was carved to normal anatomic form.
A 1-mm-wide occlusal marginal bevei was placed on the cavity preparation of resin composite specimens, at an angle of approximately 45 degrees to the unpre- pared surface. The enamel portion of the cavity preparation and 1 mm beyond the cavosurface margin were etched with 31% phosphoric acid for 30 sec- onds,' Specimens were then rinsed with water and dried with a stream of oil-free compressed air.
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Fig 4 MGan (± SE) proximal strain-force curves as a function oi restorative material and toolh condition A higher mean (ie, closer to íero¡ indicates greater stiffness.
Fig 5 Mean (+ SE) buccal strain-force curves as a function of restorative material and tooth condition. A higher mean (ie, closer to zero) indicates grealer stiffness.
Scotchbond 2 dentinal bonding agent was applied to the second group of specimens. The matrix was placed and the preparation was restored incrementaily with P-50 light-activated resin composite.'° Each increment was light activated for 20 seconds, and the entire restoration was exposed to the light source for an additional 60 seconds.
The Tenure dentinal bonding system was applied to the third group of specimens. The matrix and retainer were placed and the MOD cavity was restored with Marathon V resin composite, a light- and chemically activated (ie. dual-cured) paste system, applied in a syringe. The restorative material was light activated for 60 seconds.
The occlusal surface of each specimen was checked with articulating paper to confirm that the casting did not touch the restoration. Specimens were stored for at least 24 hours in 100% relative humidity, to allow for complete set of the restorations, and then retested.
Buccal and proximal strains, force, and loading head displacement were recorded with a condilioner/ amplifier (2100 System, Measurements Group) and a computerized data-acquisition system (IBM PC XT, Data Translation DT 280ÍA) using a customized program and commercial software (ASYST), Strain- force curves were plotted, and linear regressions were obtained. Data were recorded for each tooth under the
three conditions: intact, prepared but unrestored, and restored.
Each test was repeated three times. The first run was used to ensure proper seating and alignment. The second was used for data acquisition. The third run was used for verification, Muitivariate repeated-measures analysis of variance (ANOVA) and multiple-com- parison procedures were used to analyze the data.
Results
Thiee specimens with MOD cavity preparations frac- tured during testing and were eliminated from the analyses. Several other specimens were eliminated because of failure ofthe strain gauge, resulting in a total of 23 specimens (nine amalgam, seven Scotchbond 2/P-5O and seven Tenure/Marathon V} for the proxi- mal site and 23 specimens (eight amalgam, eight Scotchbond 2/P-50 and seven Tenure/Marathon V) for the buccal site.
Means and standard errors ofthe strain-force slopes for the three material groups under intact, prepared, and restored conditions are shown in Figs 4 (proxi- mal) and 5 (buccal), A negative siope indicates compressive (as opposed to tensile) strain.
Repeated-measures ANOVA indicated a significant interaction between restorative material and tooth
574 Quintessence International Volume 26, Number
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Source
Mean square = 3.70
• yl - total score, across conditions (used to lest nialerial main efTect); y2 = difference score, restored minus prepared; y3 - difference seore, intact minus prepared.
t Main effect for condition, P- .0005. Uniyariate results were significant for both y2 (F,,20 = T.56; P- .012) and y3 (F, 20 ' 31''O; P- 0002).
X Material by condition interaction, P ~ .020. Univariate results were significant for y2 IF-, m» 7.63; P = .003) but nol for y3 (F, ,0 " 0.01; ^=.993).
Test of significance*
- F2,2o=0.22 Mean square = 1.53
* yl - tolal score, aeross conditions (used to test material rnain effect]; y2 - difference seore, restored tninus prepared; y3 = difference seore, intact minus prepared.
t Main effect for eondition.P^.0001, Univariate results wtre significant for both y2(Fi,2o- 1 i. 16;/>= .0003) and y3 (Fi,i(,= 50.66;/>=.0001 !.
Í Material by condition interaction, P = .042. Univariate results were significant for y2 ¡Fi 20 = ^•^'^- P = 026) but not for y3 IF2 lo = 0-33; P^.797).
condition (intact, prepared, or restored) at both proximal (F4 38 = 3.30; P- .02) (Table 1) and buccal sites (Fijg = 2.76, P - .04) (Table 2). Univariate results indicated that this interaction was significant for prepared versus restored, but not for intact versus prepared, conditions at both sites.
Because the materiai-by-condition interaction was significant for the prepared versus restored conditions but not intact versus prepared conditions, follow-up multiple comparisons (Tukey HSD) were made be- tween the prepared and restored data for each restora- tive material. In addition, comparisons between mate- rials at each location (proximal and buccai ) were made for both the prepared and restored conditions. A significance level of .05 was used for ali comparisons. These tests indicated statistically significant diiferences in slope between prepared and restored conditions for Tenure/Marathon V only, at both the proximal and buccal sites. In addition, statistically significant dif- ferences under the restored condition were found between Tenure/Marathon V and both amalgam and Scotchbond 2/P-50 at the proximal site.
These results indicated that teeth restored with Tenure/Marathon V were stifier than teeth that were prepared but not restored. Results did not provide staüstical evidence that teeth restored with either Scotchbond 2/P-50 or amalgam were any stiffer than prepared, unrestored teeth. At the proximal site, teeth restored with Tenure/Marathon V were significantly
stiffer than were those restored with either amalgam or Scotchbond 2/P-50. Results did not provide statisticai evidence of a difference in stiffness among groups at the buccai site.
Discussion
This study describes a nondestructive method of determining locai tooth deformation, allowing re- peated ioading ofthe same tooth under varied condi- tions. In addition, the use of individually fabricated castings and the custom-made loading device provides a means of appiying force to the same part of the occlusal surface in all tests without ioading the restoration.
Specimens restored with amaigam showed little or no recovery of tooth stiifness at either the proximai or buccal site. This was anticipated, because amalgam does not bond to tooth structure. Restoration with resin composite provided substantial recovery of tooth stiffhess for Tenure/Marathon V specimens at both buccal and proximal sites. For Scotchbond 2/P-5O specimens, resuits did not provide evidence to suggest recovery of tooth stiffness at either site.
Two factors, the bonding system and the resin composite, may have contributed to the difference observed between the resin composite groups. The two bonding systems differ in basic formulation and curing method, and in the way in which the smear layer is
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treated. In the Tenure system, the conditioner (which contains 2,5% nitric acid) removes the smear layer and opens dentinal tubule orifices, ln Scotchbond 2, the primer (which contains 2,5% maleic acid) dissolves the smear layer, and smear layer tags remain and occlude the tubules. Tenure solutions A plus B (N-tolyglycine-glycidyl methacr^'late plus pyromellitic acid dimethacrylate) are chemically cured aqueous bonding materiais with low film thickness, whereas Scotchbond 2 Is a light-cured, resin-based bonding material with greater film thickness.
The resin composites in these two systems also difFer, P-SO is a light-cured resin composite, while Marathon is a dual-cui-ed resin. Determination of the potential contribution of each of these factors—smear layer treatment, bonding system formulation and curing method, and resin composite—requires turther study.
In this study, sutface strains were measured in two locations. It would be imprudent to assume that these measurements alone indicate the entire state of stress in the tooth and its likelihood to fail. Strain is indicative of the local deformation only, and isolated strain measurements are subject to misinterpretation, A tooth is a complicated structure with complicated supporting structures and, as such, is extremely difTicult to analyze. Detailed knowledge of the geo- metry and of the mechanical properties of each part of the tooth and its supporting structures is required. These properties vary with location and direction. Thus, in the absence of more accurate measurements and/or analyses, the strains at the location where the stresses are likely to be relatively high might serve as indicators of load severity. Furthermore, because each tooth is tested under several conditions and is thus its own internal control, the strains under the various conditions may reasonably be construed to indicate the relative resistance to deformation under those condi- tions.
Future research should focus on the effects of long-term cyclic loading on tooth stiffness, because durability of restorative systems is an important clinical consideration. Future studies should also include a model that more closely simulates conditions of the oral environment. In addition, although the present investigation provided information regarding strain measurement on the tooth surface, future work should determine internal strains through the use of computational methods, such as finite-element analysis.
References
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