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Effect of thermal stress on dentin adhesives used individually and in combination
Clinical Materiaki6 (1990) 57-64
Effect of Thermal Stress on Dentin Adhesives Used Individually and in Combination
G. Sob” & L. J. Hendersonb n Department
of Operative University (Received
Dentistry, b Department of Prosthetic Dentistry, of Singapore, Kent Ridge, Singapore 0511
26 October
1989; accepted
20 November
National
1989)
ABSTRACT Polymerization shrinkage remains an inherent problem in composite restorations. Dentin adhesives generally improve bonding to dentin but do not consistently provide leak-free restorations. This study evaluated a dentin adhesive that bonds to dentinal calcium and another that bonds to dentinal collagen when used separately and in combination. The measurement of contraction gaps at the restoration-dentin interface was used as the means of testing the effectiveness of dentin adhesion. The effect of thermal stress on the dentin-polymer bond was also investigated. Results showed that the bisphenyl A glycidyl methacrylate (BZSGMA)-based dentin adhesive was more effective when compared with the hydroxyethyl methacrylate (HEMA)-based adhesive. When used in combination, the mean contraction gaps of cavities were not found to be significantly better than those treated with only the BZSGMA-based adhesive. Thermal stress reduced the effectiveness of dentin adhesives whether applied individually or in combination.
INTRODUCTION The acid etching technique enables adhesion of tooth-coloured composite restorative materials to dental enamel without the need for traditional cavity preparation. Etching of enamel with phosphoric acid of suitable concentration results in selective demineralization of inorganic content and a highly irregular surface which promotes mechanical retention of the composite restorative material and its interme 57 Chical England.
Materials 0267-660.5/90/$03.50 Printed in Northern Ireland
0
1990
Elsevier
Science
Publishers
ktd,
58
G. Soh, L. J. Henderson
resin.l New applications of bonding technique now include conservative treatment for discoloured teeth and enamel defects2 Despite vast improvements made to the physical properties of composite restorative materials, polymerization contraction remains a problem that causes the setting composite to draw away from the cavity walls. Although acid etching improves retention of composite restorative materials to enamel, bonding to dentin poses a greater challenge because of its lower inorganic content. Increased utilization of bonding technique in the conservative treatment of lesions into dentin caused by cervical erosions, toothbrush abrasions or carious cavities emphasizes the need for improving bonding to dentin.3 It has been suggested that inadequate bonding causes microleakage at the tooth-restoration interface, which leads to staining and breakdown at the margins of the restorations, secondary caries, post-operative sensitivity and even pulp pathology from bacteria ingress.4T5 Previous studies have shown that the width and frequency of marginal contraction gaps seen in composite-filled dentin cavities can be reduced by treating the cavities with dentin bonding agents.6’9 Dentin bonding systems can be classified into two major categories. The first category is based on halophosphorus esters of bisphenyl A glycidyl methacrylate (BISGMA) which unites chemically with the calcium in dentin-l’ The second category comprises aqueous solutions of glutaraldehyde and B hydroxyethyl methacrylate (HEMA) which react with the dentinal co11agen.lC12 The most efficacious ratio of the mixture (as determined by tensile bond strength and marginal gap width) was found to be 5/35/60 (5% w/v glutaraldehyde, 35% w/v HEMA and 60% w/v water). cl3 Attempts have been made to improve the bonding of dentin adhesives by pretreatment of dentin,14,15 by varying the types and ratios of aldehydes and monomers used,16 and also by combining dentin adhesives of similar basic formulations.3 The purpose of this in-vitro study was first to test the effectiveness of an experimental HEMA-based dentin adhesive (VP819), which reacts with dentinal collagen, when used in combination with a BISGMAbased adhesive that reacts with dentinal calcium. Secondly, the effect of thermal stress on the dentin adhesives (used individually or in combination) was tested. The measurement of gaps at the restoration-dentin interface was used as the means of testing the effectiveness of the dentin adhesives. MATERIALS
AND METHODS
The study was carried out using extracted human teeth. After extraction, the teeth were stored in isotonic saline solution and kept moist at
Effect of thermal stress on dentin adhesives
59
all times during surface and cavity preparations. The teeth were mechanically cleaned and flattened on the buccal and lingual surfaces using wet Carborundum paper to a final coarseness grade of 600. A total of 120 cylindrical cavities, each of approximately 2-O mm diameter and I.50 mm depth, were prepared entirely in dentin and then randomly assigned to four groups of 30 cavities each. All cavities were cleane with pumice and water prior to restoration. The control group (Heliobond/Heliomolar) was restored with an intermediate resin (Heliobond, Vivadent, Schaan, Liechtenstein) and composite restorative material (Heliomolar, Vivadent, Schaan, Liechtenstein) according to the manufacturer’s instructions. The experimental HEMA-based dentin adhesive (VP819, Vivadent, Schaan, Liechtenstein) was applied to the cavities in the second group for 5 s and dried for 30 s with an air syringe prior to restoring with the intermediate resin and composite restorative similar to those cavities material, in the control grou (VP819/Heliobond/Heliomolar). In the third group, the cavities were eated with the BISGMA-based dentin adhesive (Prisma Universal ond, L. D. Caulk Co., Milford, Delaware, USA) according to the manufacturer’s instructions and restored with the composite restorative material (Prisma Universal Bond/Heliomolar). In the last group, the experimental HEMA-based dentin adhesive (VP819) was applied to t cavities for 5 s then dried with air for 30 s followed by application of t BISGMA-based adhesive and restoration with the composite restorative material. In all the groups, a plastic instrument was used to improve adaptation of the composite restorative material to the cavity walls. The slightly overfilled cavities were held against a celluloid matrix strip on a thin glass slab. The composite restorative material was d through the thin glass slab for 20 s by means of an Elipar Visi system (ESPE, Seefeld/Oberbay, FRG). The slab was remove and the restorations cured for another 20 s through the celluloid matrix strip. After restoration, the specimens were placed back in normal saline and further preparation was not undertaken until at least 15 min after polymerization. Excess composite restorative material was removed by wet grindin on carborundum paper (grade 220) followed by six 20 cm strokes eat on carborundum paper grades 320, 400 and 600. Following this, tke specimens were left in isotonic saline solution for 24 h prior to measurement. Fifteen cavities in each treatment group were randomly selected to be subjected to 200 thermocycles between 5 “C and 55 * with a dwell time of 10 s prior to the measurement of the contraction gaps. A Nikon Measurescope Model II (Nikon K.K., Tokyo, Japan) was used to measure four equidistant diameters for each restored cavity. The resultant gaps between the cavity wall and the composite
G. Soh, L. J. Henderson
60
material were measured where each of the four cavity diameters were recorded. The contraction gaps were measured by means of a Nikon Microphot Series V microscope (Nikon K.K., Tokyo, Japan) connected to a video camera and Colorado Video Micrometer 305 (Colorado Video Inc., Boulder, Colorado, USA), with the resulting image projected onto a high-resolution television monitor. The gaps between the cavity wall and the composite material were expressed as a percentage of the cavity diameter, thus giving four (equally spaced) percentage contraction recordings for each diameter.
RESULTS Average measurements made from four different directions provided the mean contraction gap for each specimen. Correlation analysis showed that the measurements, although made at different locations, were significantly correlated (p I 0.001) with one another as well as with the average value of their mean contraction gaps. The correlation coefficients from correlating with their respective average mean contraction gaps were fairly high (r > 0.78). The average mean contraction gaps were derived by averaging the contraction gaps over the number of specimens from which measurements were taken for each treatment group. (For simplicity, this value shall be referred to as the mean contraction gap.) Specimens treated with dentin adhesives achieved lower mean contraction gaps than those in the control group (Table 1). Specimens treated with a combination of two dentin adhesives (VP819/Prisma Universal Bond/Heliomolar) registered the lowest mean contraction gaps when compared with those Comparison
of Contraction
Gaps
TABLE 1 in Thermocycled Groups
Specimens
among
Treatment
Treatment
N
Mean contraction gap
SD
significance”
Heliobond/Heliomolar VP819/Heliobond/Heliomolar Prisma Universal Bond/ Heliomolar VP819/Prisma Universal Bond/ Heliomolar
15 15
0.369 o-293
0.114 O-146
A A
15
o-143
0.126
B
15
0.112
0.035
B
0 Mean contraction gaps of groups with the same letters significant at p 5 O-05 by Tukey’s comparison of means.
tested
not statistically
Effect
of thermal
stress on dentin
adhesives
41
TABLE 2 Comparison
of
Contraction Gaps between Thermocycled and Specimens in the Four Treatment Groups Thermocycled
Treatmenl
Heliobond/Heliomolar VP819/Heliobond/ Heliomolar Prisma Universal Bond/ Heliomolar VP819/Prisma Universal Bond/Heliomolar
Non-thermocycled
Non-thermocycled Percentage Change (%)
N
Mean
SD
Mean
SD
15
0.369
0.114
0.285
0.147
15
0.293
0.146
0.138
0,053
+111.9
15
0.143
0.126
0.112
0.099
+27*6
15
0.112
0.035
0.087
0.091
+29.2
* Mean contraction gaps between thermocycled cally significant at p 5 0.05 by Student’s t-test.
and non-thermocycled
Level of significance
+29*.5 *
specimens tested statisti-
treated with only a single adhesive. Between the two groups of cavities treated with a single adhesive, those treated with the BISGMA-based adhesive (Prisma Universal Bond/Heliomolar) generated significantly aller mean contraction gaps than those in the experimental HEM based adhesive group. Thermal stress from thermocycling the specimens increased the mean contraction gaps (Table 2). Comparing the mean contraction gaps between thermocycled and non-thermocycled specimens in all the groups, the increase ranged from 27.6% to 111.9%. The mean ccmtraction gap of cavities treated with two types of dentin adhesives increased by 29.2%. The increases were generally not statisti significant, and the relative rank order of the mean contraction among treatment groups remained the same for both thermocycled a non-thermocycled groups.
DISCUSSION As the size of marginal contraction gaps in composite restorations can be influenced by a multiplicity of factors, experimental conditions ha to be well controlled. As the volumel’ and viscosity” of the composite restorative material may be important variables, the size of cavities was standardized and cavities were restored with a single type of composite restorative material. Restored cavities were stored in isotonic saline for a period of at least 24 h to allow for hygroscopic expansion of the composite restorative material which reduces the size of marginal contraction gaps. 193”Among the thermocycling techniques available,
62
G. Soh, L. J. Henderson
the one chosen for this study had been found adequate for demonstrating the leakage-inhibiting potential of various restorative materials.‘l This study confirms the findings of previous studies that dentin adhesives reduce rather than eliminate marginal contraction gaps in composite restorations. However, the combined treatment of dentin cavities by an adhesive that bonds to dentinal calcium and one that binds to dentinal collagen did not offer a significant advantage in reducing marginal contraction gaps when compared with treatment by a single adhesive that bonds only to dentinal calcium. This outcome runs contrary to the expectation that more complete bonding to dentin will result in significant reduction of mean contraction gaps. One study reported greater success from combining two HEMA-based dentin adhesives.6 Previous studies concluded that the effectiveness of dentin adhesives was not affected by thermal stress.3,7 But this study found that dentin adhesives are vulnerable to thermal stress as indicated by the general increase in mean contraction gaps of thermocycled specimens across all treatment groups. Although the increase in the mean contraction gaps is not significant except in cavities treated with the HEMA-based adhesive, it does indicate that the dentin-polymer bond cannot withstand temperature changes commonly encountered in the oral environment .22Even concurrent treatment of cavities with two types of dentin adhesives that are expected to supplement each other failed to prevent a similar increase in the mean contraction gap. It has been established that the difference in coefficient of thermal expansion between tooth structure and restorative materials introduces tensile and compressive stresses of a magnitude that can disrupt the bond between restoration and tooth, resulting in the formation of marginal contraction gaps at the restoration-tooth interface.23 While dentin adhesives, either used singly or in combination, do reduce marginal contraction gaps, their effectiveness can be undermined by thermal stress. Changes in oral temperature can be expected to occur frequently and will remain a potent source of thermal stress for restorations in general, and composite restorations in particular. For better protection of vital tooth tissues, efforts must now be directed towards formulating dentin adhesives capable of remaining effective under temperature-induced stress. REFERENCES 1. Buonocore, M. G., A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J. Dent. Res., 34 (1955) 849-53.
Effect of thermal stress on dentin adhesives
63
2. Weinstein,
3.
4. 5. 6.
7.
A. R., Anterior composite resins and veneers: treatment planning preparation, and finishing. J. Am. Dent. Assoc., 117 (19 38-45. Munksgaard, E. C., Itoh, K. & Jorgensen, K. D., Dentin-polymer bo in resin fillings tested in vitro by thermo- and load-cycling. J. Dent. 64 (1985) 144-6. Phillips, R. W., New concepts in materials used for restorative dentistry. J. Am. Dent. Assoc., 70 (1965) 652-61. Brannstrom, M., The cause of postoperative sensitivity and its prevention. J. Endod., 12 (1986) 475-81. Munksgaard, E. C., Itoh, K., Asmussen, E. & Jorgensen, K. D., Effect of combining dentin bonding agents. Stand. J. Dent. Res., 93 (1985) 370-80. bond established by Munksgaard, E. C. & Irie, M., Dentin-polymer Gluma and tested by thermal stress. Scund. J. Dent. Res., 95 (1987)
18.5-90. 8. Munksgaard,
E. C. & Asmussen, E., Bond strength between dentine and resins mediated by mixtures of hema and glutaraldehyde. J. Dent. Res., 63 (1984) 1087-9. Munksgaard, E. C., Hansen, E. K. & Asmussen, E., Effect of fi adhesives on adaptation of resin in dentin cavities. Scan. J. Dent. Res., (1984) 187-91. Setcos, J. C., Dentin bonding in perspective. Am. J. Dent., 1 (198 169-200. Robinson, P. B. & Moore, B. K., The effect of microleakage of interchanging dentine adhesives in two composite resin systems in vitro. Br. Dent. J., 164 (1988) 77-9. Asmussen, E., Clinical relevance of physical, chemical, and bonding properties of composite resins. Operative Dent., 10 (1985) 61-73. Chantler, P., Bishop, B. M. & Henderson, L., An in-vitro investigation into storage life and composition of a new dentine adhesive (Gluma). Aust. Dent. J., 30 (1985) 222-3. Asmussen, E. & Bowen, R. L., Adhesion to dentin mediated by Glu effect of pretreatment with various amino acids. Stand. J. Dent. Res., (1987) 521-5. Cooper, L. F., Myers, M. L., Nelson, D. G. & Mowery, A. S., Shear strength of composite bonded to laser-pretreated dentine. J. Prosthet. Dent., 60 (1986) 45-9. Asmussen, E. & Munksgaard, E. C., Bonding of restorative resins to dentine promoted by mixtures of aldehydes and active monomers. Inr. Dent. J., 35 (1985) 160-5. Hansen, E. K. & Asmussen, E., Cavity preparation for restorative resins used with dentine adhesives. Stand. J. Dent. Res., 93 (1985) 474-9. Hansen, E. K., Effect of three dentin adhesives on marginal adaptation of two light-cured composites. Stand. J. Dent. Res., 94 (1986) 82-6. Hansen, E. K. & Asmussen, E., Comparative study of dentin adhesives. restorative
9.
18. 11.
12. 13.
14.
15.
16.
17. 18. 19.
Stand. J. Dent. Res., 93 (1985) 280-7. 20. Dickson, G., Hardening shrinkage and hygroscopic ite resins. J. Dent. Res., 61 (1982) 654-8.
expansion of compos-
64
G. Soh, L. J. Henderson
21. Crim, G. A., Swartz, M. L. & Phillips, R. W., Comparison of four thermocycling techniques. J. Prosthet. Dent., 53 (1985) 50-3. 22. Brown, W. S., Jacobs, H. R. & Thompson, R. E., Thermal fatigue in teeth. J. Dent. Res., 51 (1972) 461-7. 23. Nelson, R. J., Wolcott, R. B. & Paffenbarger, G. C., Fluid exchange at the margins of dental restorations. J. Am. Dent. ASSOC., 44 (1952) 288-95.
Effect of Thermal Stress on Dentin Adhesives Used Individually and in Combination
G. Sob” & L. J. Hendersonb n Department
of Operative University (Received
Dentistry, b Department of Prosthetic Dentistry, of Singapore, Kent Ridge, Singapore 0511
26 October
1989; accepted
20 November
National
1989)
ABSTRACT Polymerization shrinkage remains an inherent problem in composite restorations. Dentin adhesives generally improve bonding to dentin but do not consistently provide leak-free restorations. This study evaluated a dentin adhesive that bonds to dentinal calcium and another that bonds to dentinal collagen when used separately and in combination. The measurement of contraction gaps at the restoration-dentin interface was used as the means of testing the effectiveness of dentin adhesion. The effect of thermal stress on the dentin-polymer bond was also investigated. Results showed that the bisphenyl A glycidyl methacrylate (BZSGMA)-based dentin adhesive was more effective when compared with the hydroxyethyl methacrylate (HEMA)-based adhesive. When used in combination, the mean contraction gaps of cavities were not found to be significantly better than those treated with only the BZSGMA-based adhesive. Thermal stress reduced the effectiveness of dentin adhesives whether applied individually or in combination.
INTRODUCTION The acid etching technique enables adhesion of tooth-coloured composite restorative materials to dental enamel without the need for traditional cavity preparation. Etching of enamel with phosphoric acid of suitable concentration results in selective demineralization of inorganic content and a highly irregular surface which promotes mechanical retention of the composite restorative material and its interme 57 Chical England.
Materials 0267-660.5/90/$03.50 Printed in Northern Ireland
0
1990
Elsevier
Science
Publishers
ktd,
58
G. Soh, L. J. Henderson
resin.l New applications of bonding technique now include conservative treatment for discoloured teeth and enamel defects2 Despite vast improvements made to the physical properties of composite restorative materials, polymerization contraction remains a problem that causes the setting composite to draw away from the cavity walls. Although acid etching improves retention of composite restorative materials to enamel, bonding to dentin poses a greater challenge because of its lower inorganic content. Increased utilization of bonding technique in the conservative treatment of lesions into dentin caused by cervical erosions, toothbrush abrasions or carious cavities emphasizes the need for improving bonding to dentin.3 It has been suggested that inadequate bonding causes microleakage at the tooth-restoration interface, which leads to staining and breakdown at the margins of the restorations, secondary caries, post-operative sensitivity and even pulp pathology from bacteria ingress.4T5 Previous studies have shown that the width and frequency of marginal contraction gaps seen in composite-filled dentin cavities can be reduced by treating the cavities with dentin bonding agents.6’9 Dentin bonding systems can be classified into two major categories. The first category is based on halophosphorus esters of bisphenyl A glycidyl methacrylate (BISGMA) which unites chemically with the calcium in dentin-l’ The second category comprises aqueous solutions of glutaraldehyde and B hydroxyethyl methacrylate (HEMA) which react with the dentinal co11agen.lC12 The most efficacious ratio of the mixture (as determined by tensile bond strength and marginal gap width) was found to be 5/35/60 (5% w/v glutaraldehyde, 35% w/v HEMA and 60% w/v water). cl3 Attempts have been made to improve the bonding of dentin adhesives by pretreatment of dentin,14,15 by varying the types and ratios of aldehydes and monomers used,16 and also by combining dentin adhesives of similar basic formulations.3 The purpose of this in-vitro study was first to test the effectiveness of an experimental HEMA-based dentin adhesive (VP819), which reacts with dentinal collagen, when used in combination with a BISGMAbased adhesive that reacts with dentinal calcium. Secondly, the effect of thermal stress on the dentin adhesives (used individually or in combination) was tested. The measurement of gaps at the restoration-dentin interface was used as the means of testing the effectiveness of the dentin adhesives. MATERIALS
AND METHODS
The study was carried out using extracted human teeth. After extraction, the teeth were stored in isotonic saline solution and kept moist at
Effect of thermal stress on dentin adhesives
59
all times during surface and cavity preparations. The teeth were mechanically cleaned and flattened on the buccal and lingual surfaces using wet Carborundum paper to a final coarseness grade of 600. A total of 120 cylindrical cavities, each of approximately 2-O mm diameter and I.50 mm depth, were prepared entirely in dentin and then randomly assigned to four groups of 30 cavities each. All cavities were cleane with pumice and water prior to restoration. The control group (Heliobond/Heliomolar) was restored with an intermediate resin (Heliobond, Vivadent, Schaan, Liechtenstein) and composite restorative material (Heliomolar, Vivadent, Schaan, Liechtenstein) according to the manufacturer’s instructions. The experimental HEMA-based dentin adhesive (VP819, Vivadent, Schaan, Liechtenstein) was applied to the cavities in the second group for 5 s and dried for 30 s with an air syringe prior to restoring with the intermediate resin and composite restorative similar to those cavities material, in the control grou (VP819/Heliobond/Heliomolar). In the third group, the cavities were eated with the BISGMA-based dentin adhesive (Prisma Universal ond, L. D. Caulk Co., Milford, Delaware, USA) according to the manufacturer’s instructions and restored with the composite restorative material (Prisma Universal Bond/Heliomolar). In the last group, the experimental HEMA-based dentin adhesive (VP819) was applied to t cavities for 5 s then dried with air for 30 s followed by application of t BISGMA-based adhesive and restoration with the composite restorative material. In all the groups, a plastic instrument was used to improve adaptation of the composite restorative material to the cavity walls. The slightly overfilled cavities were held against a celluloid matrix strip on a thin glass slab. The composite restorative material was d through the thin glass slab for 20 s by means of an Elipar Visi system (ESPE, Seefeld/Oberbay, FRG). The slab was remove and the restorations cured for another 20 s through the celluloid matrix strip. After restoration, the specimens were placed back in normal saline and further preparation was not undertaken until at least 15 min after polymerization. Excess composite restorative material was removed by wet grindin on carborundum paper (grade 220) followed by six 20 cm strokes eat on carborundum paper grades 320, 400 and 600. Following this, tke specimens were left in isotonic saline solution for 24 h prior to measurement. Fifteen cavities in each treatment group were randomly selected to be subjected to 200 thermocycles between 5 “C and 55 * with a dwell time of 10 s prior to the measurement of the contraction gaps. A Nikon Measurescope Model II (Nikon K.K., Tokyo, Japan) was used to measure four equidistant diameters for each restored cavity. The resultant gaps between the cavity wall and the composite
G. Soh, L. J. Henderson
60
material were measured where each of the four cavity diameters were recorded. The contraction gaps were measured by means of a Nikon Microphot Series V microscope (Nikon K.K., Tokyo, Japan) connected to a video camera and Colorado Video Micrometer 305 (Colorado Video Inc., Boulder, Colorado, USA), with the resulting image projected onto a high-resolution television monitor. The gaps between the cavity wall and the composite material were expressed as a percentage of the cavity diameter, thus giving four (equally spaced) percentage contraction recordings for each diameter.
RESULTS Average measurements made from four different directions provided the mean contraction gap for each specimen. Correlation analysis showed that the measurements, although made at different locations, were significantly correlated (p I 0.001) with one another as well as with the average value of their mean contraction gaps. The correlation coefficients from correlating with their respective average mean contraction gaps were fairly high (r > 0.78). The average mean contraction gaps were derived by averaging the contraction gaps over the number of specimens from which measurements were taken for each treatment group. (For simplicity, this value shall be referred to as the mean contraction gap.) Specimens treated with dentin adhesives achieved lower mean contraction gaps than those in the control group (Table 1). Specimens treated with a combination of two dentin adhesives (VP819/Prisma Universal Bond/Heliomolar) registered the lowest mean contraction gaps when compared with those Comparison
of Contraction
Gaps
TABLE 1 in Thermocycled Groups
Specimens
among
Treatment
Treatment
N
Mean contraction gap
SD
significance”
Heliobond/Heliomolar VP819/Heliobond/Heliomolar Prisma Universal Bond/ Heliomolar VP819/Prisma Universal Bond/ Heliomolar
15 15
0.369 o-293
0.114 O-146
A A
15
o-143
0.126
B
15
0.112
0.035
B
0 Mean contraction gaps of groups with the same letters significant at p 5 O-05 by Tukey’s comparison of means.
tested
not statistically
Effect
of thermal
stress on dentin
adhesives
41
TABLE 2 Comparison
of
Contraction Gaps between Thermocycled and Specimens in the Four Treatment Groups Thermocycled
Treatmenl
Heliobond/Heliomolar VP819/Heliobond/ Heliomolar Prisma Universal Bond/ Heliomolar VP819/Prisma Universal Bond/Heliomolar
Non-thermocycled
Non-thermocycled Percentage Change (%)
N
Mean
SD
Mean
SD
15
0.369
0.114
0.285
0.147
15
0.293
0.146
0.138
0,053
+111.9
15
0.143
0.126
0.112
0.099
+27*6
15
0.112
0.035
0.087
0.091
+29.2
* Mean contraction gaps between thermocycled cally significant at p 5 0.05 by Student’s t-test.
and non-thermocycled
Level of significance
+29*.5 *
specimens tested statisti-
treated with only a single adhesive. Between the two groups of cavities treated with a single adhesive, those treated with the BISGMA-based adhesive (Prisma Universal Bond/Heliomolar) generated significantly aller mean contraction gaps than those in the experimental HEM based adhesive group. Thermal stress from thermocycling the specimens increased the mean contraction gaps (Table 2). Comparing the mean contraction gaps between thermocycled and non-thermocycled specimens in all the groups, the increase ranged from 27.6% to 111.9%. The mean ccmtraction gap of cavities treated with two types of dentin adhesives increased by 29.2%. The increases were generally not statisti significant, and the relative rank order of the mean contraction among treatment groups remained the same for both thermocycled a non-thermocycled groups.
DISCUSSION As the size of marginal contraction gaps in composite restorations can be influenced by a multiplicity of factors, experimental conditions ha to be well controlled. As the volumel’ and viscosity” of the composite restorative material may be important variables, the size of cavities was standardized and cavities were restored with a single type of composite restorative material. Restored cavities were stored in isotonic saline for a period of at least 24 h to allow for hygroscopic expansion of the composite restorative material which reduces the size of marginal contraction gaps. 193”Among the thermocycling techniques available,
62
G. Soh, L. J. Henderson
the one chosen for this study had been found adequate for demonstrating the leakage-inhibiting potential of various restorative materials.‘l This study confirms the findings of previous studies that dentin adhesives reduce rather than eliminate marginal contraction gaps in composite restorations. However, the combined treatment of dentin cavities by an adhesive that bonds to dentinal calcium and one that binds to dentinal collagen did not offer a significant advantage in reducing marginal contraction gaps when compared with treatment by a single adhesive that bonds only to dentinal calcium. This outcome runs contrary to the expectation that more complete bonding to dentin will result in significant reduction of mean contraction gaps. One study reported greater success from combining two HEMA-based dentin adhesives.6 Previous studies concluded that the effectiveness of dentin adhesives was not affected by thermal stress.3,7 But this study found that dentin adhesives are vulnerable to thermal stress as indicated by the general increase in mean contraction gaps of thermocycled specimens across all treatment groups. Although the increase in the mean contraction gaps is not significant except in cavities treated with the HEMA-based adhesive, it does indicate that the dentin-polymer bond cannot withstand temperature changes commonly encountered in the oral environment .22Even concurrent treatment of cavities with two types of dentin adhesives that are expected to supplement each other failed to prevent a similar increase in the mean contraction gap. It has been established that the difference in coefficient of thermal expansion between tooth structure and restorative materials introduces tensile and compressive stresses of a magnitude that can disrupt the bond between restoration and tooth, resulting in the formation of marginal contraction gaps at the restoration-tooth interface.23 While dentin adhesives, either used singly or in combination, do reduce marginal contraction gaps, their effectiveness can be undermined by thermal stress. Changes in oral temperature can be expected to occur frequently and will remain a potent source of thermal stress for restorations in general, and composite restorations in particular. For better protection of vital tooth tissues, efforts must now be directed towards formulating dentin adhesives capable of remaining effective under temperature-induced stress. REFERENCES 1. Buonocore, M. G., A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J. Dent. Res., 34 (1955) 849-53.
Effect of thermal stress on dentin adhesives
63
2. Weinstein,
3.
4. 5. 6.
7.
A. R., Anterior composite resins and veneers: treatment planning preparation, and finishing. J. Am. Dent. Assoc., 117 (19 38-45. Munksgaard, E. C., Itoh, K. & Jorgensen, K. D., Dentin-polymer bo in resin fillings tested in vitro by thermo- and load-cycling. J. Dent. 64 (1985) 144-6. Phillips, R. W., New concepts in materials used for restorative dentistry. J. Am. Dent. Assoc., 70 (1965) 652-61. Brannstrom, M., The cause of postoperative sensitivity and its prevention. J. Endod., 12 (1986) 475-81. Munksgaard, E. C., Itoh, K., Asmussen, E. & Jorgensen, K. D., Effect of combining dentin bonding agents. Stand. J. Dent. Res., 93 (1985) 370-80. bond established by Munksgaard, E. C. & Irie, M., Dentin-polymer Gluma and tested by thermal stress. Scund. J. Dent. Res., 95 (1987)
18.5-90. 8. Munksgaard,
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