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Effect of thermal cycling on fracture strength and microleakage in teeth restored with a bonded composite resin
Effect of thermal cycling on fracture strength and microleakage in teeth restored with a bonded composite resin
W. S. Eakle Department of Restorative Dentistry, University of California, USA
Eakle WS. Effect of thermal cycling on fracture strength and microleakage in teeth restored with a bonded composite resin Dent Mater 1986: 2: 114-117. Abstract - Temperature variations in the clinical range may adversely affect bonded composite resin restorations. The aim of this study was to determine the effect of thermal cycling on fracture strength and microleakage in posterior teeth restored with bonded composite resin. Fourteen matched pairs of maxillary premolars were prepared with an M O D slot of uniform dimensions and restored with a posterior composite resin bonded to enamel and dentin (Scotchbond and P-30). One member of each pair was randomly assigned to an uncycled control group; the other to a test group which was cycled between 5 ~ and 55~ After cycling the test group, both groups of teeth were stained for microleakage with silver nitrate solution, then fractured by occlusal loading in a universal testing machine. The fracture strength of teeth in the groups was compared by Student's matched pairs t test, and microleakage was compared by the Wilcoxon signed-rank test. A significant decrease in fracture strength (P = 0.02) of the thermal-cycled teeth was seen when compared with the control teeth. Extensive microleakage was found among both control and thermalcycled teeth. The results of this in vitro study suggest that variations in temperature in the clinical range may reduce the fracture strength gained with bonded posterior composite resins. Polymerization shrinkage may produce clinically significant microleakage even before thermal cycling of the teeth.
Recent laboratory studies have shown that MOD composite resin restorations bonded to enamel and dentin can increase the fracture strength of posterior teeth (1-8). However, microleakage has been noted around margins of bonded composite resin restorations, particularly those in dentin, and tends to increase if the restorations are cycled through temperature extremes, such as those that might occur in the mouth (913). If microleakage increases, then perhaps the bond to tooth structure is weakened. The purpose of this study was to examine the effect of thermal cycling on fracture strength and microleakage in posterior teeth restored with bonded composite resin.
Material and methods
Fourteen matched pairs of maxillary premolars (right and left from the same mouth) extracted for orthodontic purposes were used in the study. Each tooth was prepared with an M O D slot using a #56 bur in a high-speed handpiece with air-water spray. The preparation extended across the occlusal surface with parallel walls through the central groove and no proximal steps. The preparations were 1.5 mm wide and 3 mm deep. A metal gauge of the same dimensions served as a guide in preparation. After preparation, the cavity walls and floor were scrubbed for 15 s with 3% hydrogen peroxide on a cotton pellet to remove gross debris. The preparations were rinsed with water for 30 s
Key Words: thermal cycle, fracture strength, microleakage, composite resin, bonding agent. Dr. W. Stephan Eakle, Department of Restorative Dentistry, School of Dentistry, University of California, D-3212, 707 Parnassus Avenue, San Francisco, CA 94143, USA
ReceivedJuly 30, 1985; accepted November 15, 1985.
and dried by a stream of air. A 37% orthophosphoric acid gel was applied to the enamel of each preparation using a fine brush. After 60 s the preparations were rinsed for 45 s with water and dried with air. A bonding agent (Scotchbond*) was applied to the enamel and dentin of the preparations. A gentle stream of air was used to spread and thin the bonding agent and to evaporate volatile solvents. The bonding agent was polymerized for 30 s using a high intensity fiberoptic light in the visible spectrum. Next, a mylar strip held in a matrix retainer was applied to each tooth. A highly filled, posterior composite resin ( P - 30*) was placed into each preparation and condensed against the * 3M Company, St. Paul MN, USA
Temperature effects bonded composites restorations cavity walls. The composite resin was applied in 2 separate layers, each approximately 1.5 m m thick. Each layer was polymerized for a total of 60 s (20 s each from occlusal, mesial, and distal directions). A f t e r 30 min, excess con=posite resin was trimmed to the cavosurface margins using 12-bladed carbide finishing burs. The restored teeth were randomly distributed so that a m e m b e r of each pair was assigned to one of 2 groups. The teeth were stored in 100% humidity for 24 h~ and then, teeth in one group were cycled between 2 water baths for a total of 480 cycles. Each cycle consisted of 30 s at 5~ and 30 s at 55~ Teeth in the second group remained in humidors at r o o m temperature and were not thermal cycled; these teeth served as the control group. Before staining, teeth in both groups had small retrograde amalgams placed to seal the apices of the roots. Clear nail polish was painted o v e r the roots and the amalgam plug. The teeth were first i m m e r s e d in a silver nitrate solution (50% by weight) for 4 h, then in x-ray developing solution for 2 h to stain for microleakage under the restorations. All teeth were prepared for fracture testing by e m b e d d i n g them in dental stone contained in metal casting rings to within 2 m m of the c e m e n t o e n a m e l junction leaving the crowns exposed. The teeth were again stored in 100% humidity to p r e v e n t dehydration before fracture testing. The teeth were fractured in a universal testing machine*. A ball-bearing 3/16 inch in diameter held in a specially designed testing head contacted the occlusal inclines of the buccal and lingual cusps. T h e points of contact with the teeth were flattened with a fine d i a m o n d stone to prevent slipping of the ball bearing. A compressive load was applied at a crosshead speed of 5 mm/min until fracture occurred. The forces n e e d e d to fracture the teeth were recorded and compared using Student's matched pairs ttest. A f t e r fracture, the surfaces of the cavity preparations were examined under an optical stereomicroscope for signs of microleakage around the restorations. In teeth with the restorations still b o n d e d to one wall, the restoration was r e m o v e d to allow observations of the staining under it. The composite re? Table Model 1122, Instron Corporation, Canton, MA, USA
sin was cut 3/4 of the way through its buccolingual dimension with a bur and r e m o v e d by twisting a small screwdriver in the cut until the resin fractured loose. T h e underlying silver stain was left intact. The amount of microleakage as indicated by the stain was scored for each of the 3 cavity surfaces according to the scale found in Table 2. Microleakage for the 2 groups was c o m p a r e d using the Wilcoxon signedrank test.
115
both the control and thermal-cycled groups. Staining seen in both groups often involved most or all of one wall (facial or lingual) and part of the pulpal floor (Fig. 1). T h e most extensively stained wall was, for the most part, the same wall from which the restoration separated when the tooth was fractured (Fig. 2). W h e n the greatest stain penetration of the 3 cavity walls was scored for each tooth and used to compute the mean for each group, there was not a significant difference in microleakage between the 2 groups (P > 0.05).
Results The teeth in the control group required significantly m o r e force to induce fracture than the teeth that were thermal cycled (P = 0.02). The m e a n force required to fracture teeth in each group is found in Table. 1. The teeth in both groups fractured at the interracial zone between the restoration and the facial or lingual wall of the preparation; no attempt was m a d e in this study to determine if residual bonding agent r e m a i n e d attached to enamel and dentin. The most frequent type of fracture in both groups was a vertical splitting of the tooth extending down onto the root beneath the investing stone. The fractures frequently involved the line angles at the junction of the pulpal floor and the facial or lingual wall. Microleakage occurred extensively in
Discussion The results of this study indicate that thermal cycling significantly reduces the fracture strength of teeth restored with b o n d e d composite resin. These findings are in a g r e e m e n t with those previous studies reporting a decrease in the bond strength of unfilled resins to enamel or to dentin after thermal cycling (9, 14). T h e resin c o m p o n e n t of the bonding agent (Scotchbond) used in this study contains a mixture of phosphorus esters of B i s - G M A . Temperature increases in the clinical range have been found to decrease the yield strength, ultimate strength, and elastic modulus of similar B i s - G M A resins (15). Draughn (1976) demonstrated a decrease in retention of composite and unfilled resin systems with thermal cy-
Table 1. Comparison of forces (by matched pairs t-test*) required to fracture teeth restored with MOD composite resin restorations. The experimental group was cycled in 5~ and 55~ water baths; the control group was not. Group
n
Mean force (kg)
S.D.
Range (kg)
Control Thermal cycled
14 14
105.4 93.6
18.8 14.0
80.4-146.4 60.4-110.0
* Matched pairs t-test statistics: t = 2.77, Degrees of freedom = 13, difference between the means = -11.79, S. D. (of the difference) = 15.92, p = 0.016. Critical value of t = 2.65 when p - 0.02. Table 2. Comparison of staining scores from microleakage in 2 groups of teeth with MOD bonded composite resin restorations. The experimental group was cycled in 5~ and 55~ water baths; the control group was not. Only the worst score of the 3 cavity walls for each preparation was used to compute the mean for each group. Scoring of stain: 0 = no penetration 1 = penetration of enamel only 2 = penetration beyond dentinoenamel junction up to 1/4 of wall 3 = penetration along 1/4-- 1/2 of the wall 4, = penetration along 1/2-3/4 of the wall 5 = penetration along 3/4- entire the wall Group
n
Mean Score
S.D.
Control Thermal cycled
14 14
4.29 4.64
1.20 0.84
Wilcoxon signed rank test
T=6.5 p>0.05
N=6 (Critical value = 21 when p = 0.032)
116
Eakle
Fig. 1. Matched pairs of premolars were similarly prepared and restored with composite resin bonded to enamel and dentin (Scotchbond and P-30). The tooth on the right was thermal cycled; the control tooth on the left was not. The photograph shows the cavity preparation after fracture of each tooth and removal of the composite resin restorations. The dark areas on the preparations walls are stains produced by silver nitrate leaking under the restoration prior to fracture testing. Teeth from both groups exhibit extensive staining.
Fig. 2. Matched pairs of premolars similarly prepared and restored have been stained by silver nitrate for microleakage. After fracture of the cusps on a testing machine the bonded composite resin (Scotchbond and P-30) remains attached to one wall and the pulpal floor as seen on the right half of the 2 fractured teeth. Evidence of microleakage (as indicated by the dark areas along the cavity walls and under the restoration along the pulpal floor) can be seen in the thermal cycled tooth (on the right) and control tooth (on the left).
cling and concluded that the differences in thermal expansion of tooth and resin could induce stresses that weaken the bond of the restoration to tooth (16). His conclusions differed from those drawn by Lee and Swartz (17) and Asmussen (18) who concluded that polymerization shrinkage was more likely to weaken the bond to tooth. The results of this study found extensive staining from microleakage in both
control and thermal cycled groups. The microleakage in both groups involved not only the pulpal floor, where the width of the enamel was relatively small, but also the facial or lingual wall (or both), where the enamel was much thicker. The most likely cause of the microleakage was polymerization contraction of the composite resin disturbing the bonds to the tooth structure. Parallel walls without cavosurface bevels were used in the cavity preparation
in this study. Thus, the restoration was bonded primarily to enamel cut longitudinally or slightly diagonally. A previous study has shown that the tensile bond strength to enamel, cut longitudinally and etched, is slightly more than half that of enamel cut transversely and etched (19). Other studies have also shown that resin tags formed against longitudinally sectioned, etched enamel produce a wavy trough and ridge pattern and are not as long as the cup and cone patterns formed against transversely cut, etched enamel (20, 21). As the resin tags formed against longitudinally cut etched enamel are shorter and have weaker bonds than transversely cut enamel, it is possible that the stress from polymerization contraction of the resin can exceed the bond strength to the enamel and dentin and create a gap (22). One study showed that the amount of marginal leakage with acidetched restorations was positively correlated to the degree of resin tag formation against the etched enamel (23). ,ther study showed that polymeron contraction of some composite is could pull the bonding agent y from the cavity wall before the :ling agent could fully set (24). chbond was found by others (25) to Lce the wall-to-wall polymerization raction in cavities with a diameter mm (and 90 ~ cavosurface margins), 95% of the restorations still had ,~inal gaps. In that study, all of the :ies with a diameter > 3 mm had ;inal gaps. The adhesive effect of chbond decreased as the cavity diter increased. Other studies have shown reduction, but not eliminl, of contraction gaps at the margin dentin bonding agents (13, 26). The findings of these previous studies lend support and explanation to findings in this study. Placing of a bevel on the cavosurface margins might have reduced the amount of microleakage seen in this study, because previous studies have demonstrated a reduction in microleakage in bevelled preparations compared to butt-joint preparations (10, 27, 28). Authors of another study recommended bevelled enamel cavity margin when acid-etched composite resins are used, because the bevel helps to distribute the forces generated by polymerization contraction over a larger area. They suggested that tensile stresses produced during hardening of the resin may be capable of opening cracks or surface crazing (29). In the
Temperature effects b o n d e d c o m p o s i t e s restorations
present study, the composite resin was inserted and cured in 2 horizontal layers (pulpal, then occlusal) that contacted the facial and lingual walls at the same time. This insertion technique might have contributed to the extensive" microleakage, as the polymerization shrinkage would tend to pull the resin centrally away from the cavity walls. Perhaps an insertion technique that uses small increments of composite resin placed diagonally and not contacting both facial and lingual cavity walls at the same time would reduce shrinkage away from the walls. A similar concept for incremental placement has been suggested to reduce polymerization shrinkage away from the gingival floor in Class V cavities when the gingival margin is in c e m e n t u m (30).
Summary T h e r m a l cycling between 5 ~ and 55~ water baths significantly reduced the fracture strength in teeth restored with a posterior composite resin and dentin bonding agent (P-30 and Scotchbond) as c o m p a r e d to uncycled teeth. The extensive microleakage found in uncycled teeth prepared with parallel walls and no cavosurface bevel tends to reinforce the r e c o m m e n d a t i o n for placing a cavosurface bevel, although this study did not specifically test this factor. Although this was an in vitro study, resuits suggest that temperature variations within the clinical range may significantly reduce the fracture strength gained by restoring teeth with bonded posterior composite resins. Additionally, shrinkage of the resins during polymerization may play a greater role in the initial d e v e l o p m e n t of microleakage than variations in temperature.
References 1. Share J, Mishell Y, Nathanson D. Effect of restorative materials on resistance to fracture of tooth structure in vitro. J Dent Res 1982: 61: 247, Abstr. 622.
2. Simonsen RJ, Barouch E, Gelb M. Cusp fracture resistance from composite resin in Class II restorations. J Dent Res 1983: 62: 254, Abstr. 761. 3. Landy NA, Simonsen RJ. Cusp fracture strength in Class II composite resin restorations. J Dent Res 1984:63 (Special issue): 175, Abstr. 40. 4. Mishell Y, Share J, Nathanson D. Fracture resistance of Class II amalgam vs. light-activated composite restoratons in vitro. J Dent Res 1984:63 (Special issue): 293, Abstr. 1099. 5. Watts D e , El Mowafy O, Grant AA. Mechanical properties of composite-restored lower molars. J Dent Res 1984: 63: 496, Abstr. 57. 6. Newman SM, Pisko-Dubienski R. Effects of composite restorations on strength of posterior teeth. J Dent Res 1984: 63: 523, Abstr. 1. 7. Eakle WS. Fracture resistance of teeth with Class II bonded composite restorations. J Dent Res 1985:64 (Special issue): 178, Abstr. 28. 8. Bakke JC, Duke ES, Norling BK, Windier S, Mayhew RB. Fracture strength of Class II preparations with a posterior composite. J Dent Res I985: 64 (Special issue): 350, Abstr. 1578. 9. Ortiz RF, Phillips RW, Swartz MS, Osborne JW. Effect of composite resin bond agent on microleakage and bond strength. J Prosthet Dent 1979: 41: 5157. 10. Crim, GA, Mattingly SL. Microleakage and the Class V composite cavosurface. J Dent Child 1980: 333-336. 11. Rupp NW, Venz S, Cobb EN. Sealing the gingival margin of compostite restorations. J Dent Res 1983: 62: 254, Abstr. 765. 12. Taylor DF, Konishi RN, Leinfelder KF. Effects of dentinal bonding on the leakage of composite restorations. N Carolina Dent Gazette 1984: May-June: 1012. 13. Gross JD, Retief DH, Bradley EL. Microleakage of posterior composite restorations. Dent Mater 1985: 1: 7-10. 14. Eliades G, Caputo AA. Dentin adhesives: composition and bonding with dentin. J Dent Res 1985: 64: 717, Abstr. 124. 15. Draughn RA. Effects of temperature on mechanical properties of compostite dental restorative materials. J Biomed Mater Res 1981: 15: 489-495. 16. Draughn RA. The effect of thermal cycling on retention of composite restoratives. J Dent Res 1976: 55: (Spec. Issue B): B137, Abstr. 303.
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17. Lee HL, Swartz ML. Scanning electron microscope study of composite restorative materials. J Dent Res 1970: 49: 149158. 28. Asmussen E. The effect of temperature changes on adaptation of resin fillings. Acta Odontol Scand 1974: 32: 161-171. 19. Munechika T, Suzuki K, Nishiyama M, Ohashi M, Horie K. A comparison of the tensile bond strengths of composite resins to longitudinal and transverse sections of enamel prisms in human teeth. J Dent Res 1984: 63: 1079-1082. 20. Crawford PJM, Whittaker DK. Etching and bonding patterns in human sub-surface enamel. Br Dent J 1977: 143: 261266. 21. Voss JE, Charbeneau GT. A scanning electron microscope comparison of three methods of bonding resin to enamel rod ends and longitudinally cut enamel. J A m Dent Assoc 1979: 98: 384389. 22. Davidson eL, de Gee A J, Feilzer A. The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 1984: 63: 1396-1399. 23. Shortall AC, Powis DR, Prosser HJ, Wilson AD. Quantitative in vitro assessment of marginal leakage and cavity wall adaption of composite resin restorative materials. J Dent Res 1985: 64: 663, Abstr. 1. 24. Moulder GJ, Ogle KR, Hood JAA. Effect of a combined adhesive-polymer system in reducing cusp flexibility of restored teeth. J Dent Res 1985: 64: 651, Abstr. 17. 25. Hansen EK. Effect of Scotchbond dependent on cavity cleaning, cavity diameter and cavosurface angle. Scand J Dent Res 1984: 92: 141-147. 26. Munksgaard EC, Itoh K, Jrrgensen KD. Dentin-polymer bond in resin fillings tested in vitro by thermo- and loadcycling. J Dent Res 1985:64 144-146. 27. Porte A, Lutz F, Lund MR, Swartz ML, Cocharan MA. Cavity designs for composite resins. Oper Dent 1984: 9: 50-56. 28. Lutz F, Krejci I, Imfeld T. In vitro marginal adaptation of Class V Scotchbond restorations. J Dent Res 1985:64 (Spec. issue): 244, Abstr. 628. 29. Bowen RL, Nemoto K, Rapson JE. Adhesive bonding of various materials to hard tooth tissues: forces developing in composite materials during hardening. J A m Dent Assoc 1983: 106: 475477. 30. Donovan TE. Adhesive restorative dentistry: a conservative perspective. J Calif Dent Assoc 1985: February: 13-22.
W. S. Eakle Department of Restorative Dentistry, University of California, USA
Eakle WS. Effect of thermal cycling on fracture strength and microleakage in teeth restored with a bonded composite resin Dent Mater 1986: 2: 114-117. Abstract - Temperature variations in the clinical range may adversely affect bonded composite resin restorations. The aim of this study was to determine the effect of thermal cycling on fracture strength and microleakage in posterior teeth restored with bonded composite resin. Fourteen matched pairs of maxillary premolars were prepared with an M O D slot of uniform dimensions and restored with a posterior composite resin bonded to enamel and dentin (Scotchbond and P-30). One member of each pair was randomly assigned to an uncycled control group; the other to a test group which was cycled between 5 ~ and 55~ After cycling the test group, both groups of teeth were stained for microleakage with silver nitrate solution, then fractured by occlusal loading in a universal testing machine. The fracture strength of teeth in the groups was compared by Student's matched pairs t test, and microleakage was compared by the Wilcoxon signed-rank test. A significant decrease in fracture strength (P = 0.02) of the thermal-cycled teeth was seen when compared with the control teeth. Extensive microleakage was found among both control and thermalcycled teeth. The results of this in vitro study suggest that variations in temperature in the clinical range may reduce the fracture strength gained with bonded posterior composite resins. Polymerization shrinkage may produce clinically significant microleakage even before thermal cycling of the teeth.
Recent laboratory studies have shown that MOD composite resin restorations bonded to enamel and dentin can increase the fracture strength of posterior teeth (1-8). However, microleakage has been noted around margins of bonded composite resin restorations, particularly those in dentin, and tends to increase if the restorations are cycled through temperature extremes, such as those that might occur in the mouth (913). If microleakage increases, then perhaps the bond to tooth structure is weakened. The purpose of this study was to examine the effect of thermal cycling on fracture strength and microleakage in posterior teeth restored with bonded composite resin.
Material and methods
Fourteen matched pairs of maxillary premolars (right and left from the same mouth) extracted for orthodontic purposes were used in the study. Each tooth was prepared with an M O D slot using a #56 bur in a high-speed handpiece with air-water spray. The preparation extended across the occlusal surface with parallel walls through the central groove and no proximal steps. The preparations were 1.5 mm wide and 3 mm deep. A metal gauge of the same dimensions served as a guide in preparation. After preparation, the cavity walls and floor were scrubbed for 15 s with 3% hydrogen peroxide on a cotton pellet to remove gross debris. The preparations were rinsed with water for 30 s
Key Words: thermal cycle, fracture strength, microleakage, composite resin, bonding agent. Dr. W. Stephan Eakle, Department of Restorative Dentistry, School of Dentistry, University of California, D-3212, 707 Parnassus Avenue, San Francisco, CA 94143, USA
ReceivedJuly 30, 1985; accepted November 15, 1985.
and dried by a stream of air. A 37% orthophosphoric acid gel was applied to the enamel of each preparation using a fine brush. After 60 s the preparations were rinsed for 45 s with water and dried with air. A bonding agent (Scotchbond*) was applied to the enamel and dentin of the preparations. A gentle stream of air was used to spread and thin the bonding agent and to evaporate volatile solvents. The bonding agent was polymerized for 30 s using a high intensity fiberoptic light in the visible spectrum. Next, a mylar strip held in a matrix retainer was applied to each tooth. A highly filled, posterior composite resin ( P - 30*) was placed into each preparation and condensed against the * 3M Company, St. Paul MN, USA
Temperature effects bonded composites restorations cavity walls. The composite resin was applied in 2 separate layers, each approximately 1.5 m m thick. Each layer was polymerized for a total of 60 s (20 s each from occlusal, mesial, and distal directions). A f t e r 30 min, excess con=posite resin was trimmed to the cavosurface margins using 12-bladed carbide finishing burs. The restored teeth were randomly distributed so that a m e m b e r of each pair was assigned to one of 2 groups. The teeth were stored in 100% humidity for 24 h~ and then, teeth in one group were cycled between 2 water baths for a total of 480 cycles. Each cycle consisted of 30 s at 5~ and 30 s at 55~ Teeth in the second group remained in humidors at r o o m temperature and were not thermal cycled; these teeth served as the control group. Before staining, teeth in both groups had small retrograde amalgams placed to seal the apices of the roots. Clear nail polish was painted o v e r the roots and the amalgam plug. The teeth were first i m m e r s e d in a silver nitrate solution (50% by weight) for 4 h, then in x-ray developing solution for 2 h to stain for microleakage under the restorations. All teeth were prepared for fracture testing by e m b e d d i n g them in dental stone contained in metal casting rings to within 2 m m of the c e m e n t o e n a m e l junction leaving the crowns exposed. The teeth were again stored in 100% humidity to p r e v e n t dehydration before fracture testing. The teeth were fractured in a universal testing machine*. A ball-bearing 3/16 inch in diameter held in a specially designed testing head contacted the occlusal inclines of the buccal and lingual cusps. T h e points of contact with the teeth were flattened with a fine d i a m o n d stone to prevent slipping of the ball bearing. A compressive load was applied at a crosshead speed of 5 mm/min until fracture occurred. The forces n e e d e d to fracture the teeth were recorded and compared using Student's matched pairs ttest. A f t e r fracture, the surfaces of the cavity preparations were examined under an optical stereomicroscope for signs of microleakage around the restorations. In teeth with the restorations still b o n d e d to one wall, the restoration was r e m o v e d to allow observations of the staining under it. The composite re? Table Model 1122, Instron Corporation, Canton, MA, USA
sin was cut 3/4 of the way through its buccolingual dimension with a bur and r e m o v e d by twisting a small screwdriver in the cut until the resin fractured loose. T h e underlying silver stain was left intact. The amount of microleakage as indicated by the stain was scored for each of the 3 cavity surfaces according to the scale found in Table 2. Microleakage for the 2 groups was c o m p a r e d using the Wilcoxon signedrank test.
115
both the control and thermal-cycled groups. Staining seen in both groups often involved most or all of one wall (facial or lingual) and part of the pulpal floor (Fig. 1). T h e most extensively stained wall was, for the most part, the same wall from which the restoration separated when the tooth was fractured (Fig. 2). W h e n the greatest stain penetration of the 3 cavity walls was scored for each tooth and used to compute the mean for each group, there was not a significant difference in microleakage between the 2 groups (P > 0.05).
Results The teeth in the control group required significantly m o r e force to induce fracture than the teeth that were thermal cycled (P = 0.02). The m e a n force required to fracture teeth in each group is found in Table. 1. The teeth in both groups fractured at the interracial zone between the restoration and the facial or lingual wall of the preparation; no attempt was m a d e in this study to determine if residual bonding agent r e m a i n e d attached to enamel and dentin. The most frequent type of fracture in both groups was a vertical splitting of the tooth extending down onto the root beneath the investing stone. The fractures frequently involved the line angles at the junction of the pulpal floor and the facial or lingual wall. Microleakage occurred extensively in
Discussion The results of this study indicate that thermal cycling significantly reduces the fracture strength of teeth restored with b o n d e d composite resin. These findings are in a g r e e m e n t with those previous studies reporting a decrease in the bond strength of unfilled resins to enamel or to dentin after thermal cycling (9, 14). T h e resin c o m p o n e n t of the bonding agent (Scotchbond) used in this study contains a mixture of phosphorus esters of B i s - G M A . Temperature increases in the clinical range have been found to decrease the yield strength, ultimate strength, and elastic modulus of similar B i s - G M A resins (15). Draughn (1976) demonstrated a decrease in retention of composite and unfilled resin systems with thermal cy-
Table 1. Comparison of forces (by matched pairs t-test*) required to fracture teeth restored with MOD composite resin restorations. The experimental group was cycled in 5~ and 55~ water baths; the control group was not. Group
n
Mean force (kg)
S.D.
Range (kg)
Control Thermal cycled
14 14
105.4 93.6
18.8 14.0
80.4-146.4 60.4-110.0
* Matched pairs t-test statistics: t = 2.77, Degrees of freedom = 13, difference between the means = -11.79, S. D. (of the difference) = 15.92, p = 0.016. Critical value of t = 2.65 when p - 0.02. Table 2. Comparison of staining scores from microleakage in 2 groups of teeth with MOD bonded composite resin restorations. The experimental group was cycled in 5~ and 55~ water baths; the control group was not. Only the worst score of the 3 cavity walls for each preparation was used to compute the mean for each group. Scoring of stain: 0 = no penetration 1 = penetration of enamel only 2 = penetration beyond dentinoenamel junction up to 1/4 of wall 3 = penetration along 1/4-- 1/2 of the wall 4, = penetration along 1/2-3/4 of the wall 5 = penetration along 3/4- entire the wall Group
n
Mean Score
S.D.
Control Thermal cycled
14 14
4.29 4.64
1.20 0.84
Wilcoxon signed rank test
T=6.5 p>0.05
N=6 (Critical value = 21 when p = 0.032)
116
Eakle
Fig. 1. Matched pairs of premolars were similarly prepared and restored with composite resin bonded to enamel and dentin (Scotchbond and P-30). The tooth on the right was thermal cycled; the control tooth on the left was not. The photograph shows the cavity preparation after fracture of each tooth and removal of the composite resin restorations. The dark areas on the preparations walls are stains produced by silver nitrate leaking under the restoration prior to fracture testing. Teeth from both groups exhibit extensive staining.
Fig. 2. Matched pairs of premolars similarly prepared and restored have been stained by silver nitrate for microleakage. After fracture of the cusps on a testing machine the bonded composite resin (Scotchbond and P-30) remains attached to one wall and the pulpal floor as seen on the right half of the 2 fractured teeth. Evidence of microleakage (as indicated by the dark areas along the cavity walls and under the restoration along the pulpal floor) can be seen in the thermal cycled tooth (on the right) and control tooth (on the left).
cling and concluded that the differences in thermal expansion of tooth and resin could induce stresses that weaken the bond of the restoration to tooth (16). His conclusions differed from those drawn by Lee and Swartz (17) and Asmussen (18) who concluded that polymerization shrinkage was more likely to weaken the bond to tooth. The results of this study found extensive staining from microleakage in both
control and thermal cycled groups. The microleakage in both groups involved not only the pulpal floor, where the width of the enamel was relatively small, but also the facial or lingual wall (or both), where the enamel was much thicker. The most likely cause of the microleakage was polymerization contraction of the composite resin disturbing the bonds to the tooth structure. Parallel walls without cavosurface bevels were used in the cavity preparation
in this study. Thus, the restoration was bonded primarily to enamel cut longitudinally or slightly diagonally. A previous study has shown that the tensile bond strength to enamel, cut longitudinally and etched, is slightly more than half that of enamel cut transversely and etched (19). Other studies have also shown that resin tags formed against longitudinally sectioned, etched enamel produce a wavy trough and ridge pattern and are not as long as the cup and cone patterns formed against transversely cut, etched enamel (20, 21). As the resin tags formed against longitudinally cut etched enamel are shorter and have weaker bonds than transversely cut enamel, it is possible that the stress from polymerization contraction of the resin can exceed the bond strength to the enamel and dentin and create a gap (22). One study showed that the amount of marginal leakage with acidetched restorations was positively correlated to the degree of resin tag formation against the etched enamel (23). ,ther study showed that polymeron contraction of some composite is could pull the bonding agent y from the cavity wall before the :ling agent could fully set (24). chbond was found by others (25) to Lce the wall-to-wall polymerization raction in cavities with a diameter mm (and 90 ~ cavosurface margins), 95% of the restorations still had ,~inal gaps. In that study, all of the :ies with a diameter > 3 mm had ;inal gaps. The adhesive effect of chbond decreased as the cavity diter increased. Other studies have shown reduction, but not eliminl, of contraction gaps at the margin dentin bonding agents (13, 26). The findings of these previous studies lend support and explanation to findings in this study. Placing of a bevel on the cavosurface margins might have reduced the amount of microleakage seen in this study, because previous studies have demonstrated a reduction in microleakage in bevelled preparations compared to butt-joint preparations (10, 27, 28). Authors of another study recommended bevelled enamel cavity margin when acid-etched composite resins are used, because the bevel helps to distribute the forces generated by polymerization contraction over a larger area. They suggested that tensile stresses produced during hardening of the resin may be capable of opening cracks or surface crazing (29). In the
Temperature effects b o n d e d c o m p o s i t e s restorations
present study, the composite resin was inserted and cured in 2 horizontal layers (pulpal, then occlusal) that contacted the facial and lingual walls at the same time. This insertion technique might have contributed to the extensive" microleakage, as the polymerization shrinkage would tend to pull the resin centrally away from the cavity walls. Perhaps an insertion technique that uses small increments of composite resin placed diagonally and not contacting both facial and lingual cavity walls at the same time would reduce shrinkage away from the walls. A similar concept for incremental placement has been suggested to reduce polymerization shrinkage away from the gingival floor in Class V cavities when the gingival margin is in c e m e n t u m (30).
Summary T h e r m a l cycling between 5 ~ and 55~ water baths significantly reduced the fracture strength in teeth restored with a posterior composite resin and dentin bonding agent (P-30 and Scotchbond) as c o m p a r e d to uncycled teeth. The extensive microleakage found in uncycled teeth prepared with parallel walls and no cavosurface bevel tends to reinforce the r e c o m m e n d a t i o n for placing a cavosurface bevel, although this study did not specifically test this factor. Although this was an in vitro study, resuits suggest that temperature variations within the clinical range may significantly reduce the fracture strength gained by restoring teeth with bonded posterior composite resins. Additionally, shrinkage of the resins during polymerization may play a greater role in the initial d e v e l o p m e n t of microleakage than variations in temperature.
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