Dent Mater8:208-210,May, 1992
The plane of fracture and shear bond strength of three composite inlay systems J.A.Scott ~, R.Strang 2 a n d W.P.Saunders I
Department of ConservativeDentistry, GlasgowDental Hospital and School,Glasgow, Scotland 2Departmentof Clinical Physics and Bioengineering, West of Scotland Health Boards, Scotland
Abstract. Composite inlays have been introduced to overcome some of the problems encountered with direct placement techniques. However, some doubt has been expressed about the strength of the bond between the inlay surface and the composite luting cement due to the decreased number of C=C bonds remaining following supercuring of the inlay. This study investigated the bond strength of three composite inlay systems to etched bovine enamel and recordedthe plane of fracture of the samples. The bond strength of one system was significantly greater than that of the other groups and was increased by the application of an unfilled resin enamel bonding agent. Unfilled resin applicationdid not significantly alter the other systems tested. In only one of the systems tested was the inlay/ composite cement bond found to be the weak link in the bonding procedure. In this group, the application of the manufacturer's specially formulated bond enhancer did not lead to an increase in bond strength.
INTRODUCTION Composite inlays have recently become available as an alternative restoration for posterior teeth. Although made from the same materials as direct posterior composite fillings, it has been suggested that the fabrication ofresin inlays extraorally, and their cementation to the tooth with only a thin layer of composite, overcomes some of the problems connected with direct restorations. Problems have been identified such as cuspal fracture (McCullock and Smith, 1986) and marginal leakage (Jensen and Chan, 1985) associated with polymerization shrinkage. The resin inlays are also subjected to a secondary curing process by heat and either pressure or light. This has been shown by Wendt (1987a; 1987b) to improve colour stability, physical properties and in vitro wear. The secondary curing process probably works in two ways: 1) it allows relaxation of stresses which build up within the composite during polymerization, in much the same way as annealing relieves the stresses within a work-hardened alloy (De Gee et al.,1990); and 2) it promotes increased conversion of double carbon bonds which increases the amount of crosslinking within the material leading to an improvement in strength. However, it has been suggested that this increased conversion of C=C means that there may be insufficient remaining reactive groups to bond to the composite cement (Rees and Jacobson, 1991). This study measured the shear bond strength of three
208 Scott et aL/Composite inlay, bond strength, restorative dentistry
commerciallyavailable composite inlay systems when bonded to enamel. The effect of unfilled bonding resins and, where appropriate, bond enhancers, was also studied. In addition, the plane of fracture of these restorations was recorded in an attempt to discover if the inlay/composite cement bond is indeed the weak link in the system.
MATERIALSAND METHODS Bovine incisors were used in this study, as they are readily available and have a large enamel surface area for bonding (Nakamichi et al.,1983). Each crown was embedded in autopolymerizing polymethylmethacrylateresin in an aluminium square such that between 3 and 5 mm of the labial surface protruded. This surface was ground flat using P300 and P600 grade silicon carbide paper. These samples were stored in water at 5°C until required. Composite inlays, with a surface area of 25 mm 2and 1 mm thick, were prepared using a specially made perspex mold which was placed on a base of dental stone (Jade Stone, Whipmix Corp., Louisville, KY, USA.). Both the mold and the stone were coated with the appropriate separating medium. The composite was packed into the mold and a glass slide placed onto the surface of the composite. The light curing materials (See Table) were cured for 60 s with a Translux, curing light (Kulzer GmbH, Werheim, Germany). After removal from the mold, the flash was removed from the samples. They were then subjected to the appropriate supercuring process, Coltene samples in the D.I.500 oven (Coltene AG, Liechtenstein, Altstatten, Switzerland) and Kulzer samples in the Unilux A.C. (Kulzer GmbH). The Ivoclar samples were cured in the mold entirely in the Ivomat (Vivadent Dental GmbH, Jagst, Liechtenstein) pressure / heat oven as they do not light cure. On removal from the oven, the fitting surface of the Ivoclar samples were sandblasted with 50 pm A1203. No surface treatments were carried out on the other samples prior to bonding. At this stage all inlay samples were stored dry for 1 week prior to bonding. The enamel was etched for 60 s with phosphoric acid gel, washed for 30 s and then dried with oil free air for 10 s. Immediately prior to bonding, all the inlay samples were washed with water for 30 s and then dried. Ten inlays were bonded for each of the following groups (See Table for details of materials used): 1) hybrid material 1, bonded using composite cement 1; 2) hybrid material 1, bonded using composite cement I and unfilled resin 1; 3) hybrid material 2, bonded
MATERIAL Hybrid 1
Composite Cement 1 Unfilled Resin 1 Hybrid 2
Composite Cement 2
Unfilled Resin 2 Microfilled Composite Composite Cement 3
Unfilled Resin 3 Bond Enhancer
TABLE: MATERIALSUSED MANUFACTURER TRADENAME BATCHNO. Coltene AG Coltene 9123-245 Brilliant AItstatten Switzerland Coltene AG ColteneDUO 9110-103 Cement Coltene AG Coltene DUO 9110.030 Bond Kulzer GmbH Estilux 94.6.30 40 Posterior CVS Bereich Dental, Werheim, Germany Kulzer GmbH Adhesive 31.12.92 032 Cement 31.12.92 032 AAdhesive 94.6.30 47 Kulzer GmbH Bond Ivoclar 144046 Vivadent Dental Inlay\Onlay GmbH Ellwagen, Liechtenstein Dual Cement 360029 Vivadent Dental GmbH HelioBond 340067 Vivadent Dental GmbH Special Bond 2240427 Vivadent Dental GmbH
using composite cement 2; 4) hybrid material 2, bonded using composite cement 2 and unfilled resin 2; 5) microfilled material, bonded using composite cement 3; 6) microfilled material, bonded using composite cement 3 and unfilled resin 3; 7) microfilledmaterial, bonded using composite cement 3, unfilled resin 3 and bond enhancer. All materials were used as recommended by the manufacturers. In all cases, the bonding agent/resin was applied to both the enamel and inlay surfaces and cured for 60 s. The bond enhancer (Specialbond II) was applied to the microfilled composite inlay fitting surface only. It was blown dry and the bonding resin was applied and cured for 60 s. Although all the cements and some of the unfilled resins are dual curing, it has been shown by Hasegawa et al., (1991) that dual curing materials obtain better physical properties when they are light cured. Consequently, every sample was cured for 2 min with a Translux light. Two minutes was chosen as the curing time, as there is decreased light penetration through these materials, resulting in a need for increased exposure time (Breeling et al., 1991). During bonding, a constant force of 10N was applied to each sample. Following bonding the excess resin was removed from around the inlay samples, and they were stored in water at 37°C for one week. Prior to testing, the samples were thermocycled between 5 °, 37 ° and 55°C for 500 cycles. Testing was carried out using a specially constructed jig mounted in a universal testing machine (Nene M3000: Nene Instruments, Wellingborough, Northampton, UK). A crosshead speed of 0.5mm/min was used. Shear bond strengths were recorded and debonded specimens examined under a dissecting microscope at 40x magnification to assess the gross plane of fracture. The data obtained for shear bond strengths was analyzed using a two-way ANOVA to determine if there was any difference between the groups. Comparison of groups was then carried out using a Student-Newman-Keuls test.
'°[
Shear Bond Strength (MPa)
30
.
.
.
.
.
.
.
.
20
10
0
C-c
C-c*b
K-C
K-c*b
I-c
t-c*b
I-c*b*s
Fig. Shear bond strength of inlay to etched enamel. Key to graph: C = hybrid 1, K = hybrid 2, I= microfilled composite, c = composite cement, u = unfilled resin, b = bond enhancer.
RESULTS The Fig. shows the shear bond strengths of the samples tested. These results showed that the hybrid 1, composite cement/ unfilled resin, specimens had the highest shear bond strength of 27.1 _+12MPa (_+1S.D.). This was significantly greater than any of the other samples tested (p < 0.05 for hybrid 1, composite cement alone and p < 0.01 for all other samples tested). There were no significant differences between any of the other samples tested. The use of unfilled resin had no effect on bond strength of the other systems tested nor did the application of the bond enhancer to group 7. Plane of fracture. In both the light cured hybrid systems, the plane of fracture was between enamel and composite, with the composite resin remaining firmly attached to the inlay surface. In the microfilledpressure/heat cured system, however, the cement always remained attached to the enamel surface, with the inlay surface appearing cement free. DISCUSSION For the two hybrid, light cured systems (Kulzer and Coltene), the bond between the inlay and enamel did not break at the inlay/cement interface as anticipated. From this result, it would appear that the composite inlay to composite cement bond in these systems is not as low as was thought. The bond between a luting cement and a composite inlay is essentially the same as that to a fractured composite resin. It follows that a comparable bond strength would be expected. The bond strength of a composite repair to a fractured composite has been shown to be only in the region of 25% to 75% of that of the unbroken material(Boyer et a/.,1978; Chan and Boyer 1983; Crumpler et a/.,1989). This is nonetheless in the region of 15-30 MPa, which is comparable with the bond strength of composite to etched enamel (Causton,1975). If this is true, the bond strengths obtained with the Hybrid I system are similar to those which might be expected. In this case, the bond strength ofthe resin cement to the inlay surface exceeded that to the enamel surface, and the bond fractured at the resin enamel interface. Application of unfilled resin increased the bond strength to the etched enamel, resultingin a higher shear strength. However, this was still less than that of the inlay to composite cement and hence, despite the increase in bond strength, no alteration in fracture plane occurred. Dental Materials/May 1992 209
With the hybrid 2 system, the bond strengths were significantly lower than for hybrid 1, although the plane of fracture remained the same. The decrease in bond strength between these two inlay systems could well be explained by the granular and relatively viscous nature of the luting composite supplied with this system when compared to that supplied for use with hybrid 1. This luting composite, although of similar filler content, flowed much more easily. The granular and viscous nature of composite cement 2 may prevent adequate wetting of the enamel surface and lead to the relatively low bond strength between the two. The fact that the luting cement remained attached to the inlay at this shear strength was not unexpected as Crumpler et al. (1989) in studies using Estilux posterior (hybrid 2 in this study) showed that it can achieve up to 88% ofits strength following repair, with a shear strength of 27 MPa being recorded. The fact that both these systems show the fracture plane to occur at the enamel composite cement bond suggests that there is no problem in bonding to these inlays despite the decrease in C=C bonds available. It would also seem probable from these results that there must be other important factors allowing bonding to occur between the two composites. Surface roughness, porosity and wettability of the inlay surface with the cement, leading to micromechanical interlocking, may well be as important as purely chemical bonding. The microfilledinlays fractured in the manner which would be expected if there were insufficient bonding sites remaining due to supercuring. This may well be the case in this material, as it is cured in a different manner which may result in an increased degree of methacrylate conversion relative to the other systems. The sandblasting as recommended by the manufacturer will lead to a relatively rough surface which should encourage bonding. However, from the plane of fracture, it would seem that despite this treatment, the surface is less retentive than either of the hybrid materials. Pressure during curing may also lead to a more condensed, less porous inlay surface to which it is difficult to form a good micromechanical bond. A bond enhancer has been developed to increase the bond strength of inlay to luting resin; however, in this study, no improvement was observed. The intended mechanism of action of this material has not been disclosed by the manufacturer so no further comment on its efficacy is possible at present. There are two other possible explanations for the relatively low shear bond strengths of this system. First, the pressure cured system uses a microfilled composite. As such, it has a higher coefficient of thermal expansion than the other two hybrid materials. This means that it will be subjected to more stress during thermocycling and will hence be more liable to bond breakdown (Smith,1985). Peutzfeldt and Asmussen (1991) have shown a greater decrease in retention of Ivoclar inlays followingthermocyclingthan of the other two systems. Second, Zidan et al. (1980) found a positive correlation between tensile strength of a composite and its bond strength. Similar studies, yet to be reported, have shown that the Ivoclar luting resin has a lower diametral tensile strength, and this may well account for the lower bond strengths observed with this material.
Received December 2, 1991/Accepted January 29, 1992
210 Scott et aL/Composite inlay, bond strength, restorative dentistry
Address correspondence and reprint requests to: J. A. Scott Dall Research Fellow Department of Conservative Dentistry Glasgow Dental Hospital and School 378, Sauchiehall Street Glasgow G2 3JZ Scotland
REFERENCES Boyer DB, Chan KC, Torney DL (1978). The strength of multilayer and repaired composite resin. J Prosthet Dent 39:63-67. Breeling LC, Dixon DL, Caughman WF (1991). The curing potential of light-activated composite resin luting cement. J Prosthet Dent 65:512-518. Causton BE (1975).Repair of abraded composite fillings. Br Dent J 139:286-288. Chan KC, Boyer DB (1983).Repair of conventional and microfilled composite resins. J Prosthet Dent 50:345-350. Crumpler DC, Bayne SC, Sockwell S, Brunson D, Robertson TM (1989).Bonding to resurfaced posterior composites. Dent Mater 5:417-424. De Gee AJ, Pallav P, Werner A, Davidson CL (1990). Annealing as a mechanism of increasing wear resistance of composites. Dent Mater 6:266-270. Hasegawa EA, Boyer DB, Chan DCN (1991). Hardening of dual-cured cements under compositeresin inlays. JProsthet Dent 66:187-192. Jensen M.E. Chan D.C.N. (1985) Polymerisation shrinkage and microleakage. In: Vanherle G and Smith DC, Ed. Posterior Composite Resin Restorative Materials. Peter Szulc Publishing Co., Netherlands. McCullock AJ, Smith BGN (1986). In Vitro studies of cuspal movement produced by adhesive restorative materials. Br Dent J 161: 405-409. Nakamichi I, Iwaku U, Fusayama T (1983). Bovine teeth as possible substitutes in the adhesion test. JDent Res 62:10761081. Peutzfeldt A, Asmussen E (1991). Retention of composite inlays in enamel dentine cavities. Dent Mater 7:11-14. Rees JS, Jacobson PH (1991). The current status of composite materials and adhesive systems. Part 6: Techniques for placement. Restorative Dentistry 21-23. Smith DC (1985). Posterior composite dental restorative materials: materials development. In Vanherle G and Smith DC, Ed. Posterior Composite Resin Dental Restorative Materials. Peter Szulc Publishing Co., Netherlands, 46-60. Wendt SL (1987a). The effect ofheat as a secondary cure upon the physical properties of three composite resins: I. Quint Int 18:265-271. Wendt SL (1987b). The effect of heat as a secondary cure upon the physical properties of three composite resins: II. Quint Int 18:265-271. Zidan O, Asmussen E, Jorgensen KD (1980). Correlation between tensile and bond strength of composite resins. Scand J Dent Res 88:348-351.
The plane of fracture and shear bond strength of three composite inlay systems J.A.Scott ~, R.Strang 2 a n d W.P.Saunders I
Department of ConservativeDentistry, GlasgowDental Hospital and School,Glasgow, Scotland 2Departmentof Clinical Physics and Bioengineering, West of Scotland Health Boards, Scotland
Abstract. Composite inlays have been introduced to overcome some of the problems encountered with direct placement techniques. However, some doubt has been expressed about the strength of the bond between the inlay surface and the composite luting cement due to the decreased number of C=C bonds remaining following supercuring of the inlay. This study investigated the bond strength of three composite inlay systems to etched bovine enamel and recordedthe plane of fracture of the samples. The bond strength of one system was significantly greater than that of the other groups and was increased by the application of an unfilled resin enamel bonding agent. Unfilled resin applicationdid not significantly alter the other systems tested. In only one of the systems tested was the inlay/ composite cement bond found to be the weak link in the bonding procedure. In this group, the application of the manufacturer's specially formulated bond enhancer did not lead to an increase in bond strength.
INTRODUCTION Composite inlays have recently become available as an alternative restoration for posterior teeth. Although made from the same materials as direct posterior composite fillings, it has been suggested that the fabrication ofresin inlays extraorally, and their cementation to the tooth with only a thin layer of composite, overcomes some of the problems connected with direct restorations. Problems have been identified such as cuspal fracture (McCullock and Smith, 1986) and marginal leakage (Jensen and Chan, 1985) associated with polymerization shrinkage. The resin inlays are also subjected to a secondary curing process by heat and either pressure or light. This has been shown by Wendt (1987a; 1987b) to improve colour stability, physical properties and in vitro wear. The secondary curing process probably works in two ways: 1) it allows relaxation of stresses which build up within the composite during polymerization, in much the same way as annealing relieves the stresses within a work-hardened alloy (De Gee et al.,1990); and 2) it promotes increased conversion of double carbon bonds which increases the amount of crosslinking within the material leading to an improvement in strength. However, it has been suggested that this increased conversion of C=C means that there may be insufficient remaining reactive groups to bond to the composite cement (Rees and Jacobson, 1991). This study measured the shear bond strength of three
208 Scott et aL/Composite inlay, bond strength, restorative dentistry
commerciallyavailable composite inlay systems when bonded to enamel. The effect of unfilled bonding resins and, where appropriate, bond enhancers, was also studied. In addition, the plane of fracture of these restorations was recorded in an attempt to discover if the inlay/composite cement bond is indeed the weak link in the system.
MATERIALSAND METHODS Bovine incisors were used in this study, as they are readily available and have a large enamel surface area for bonding (Nakamichi et al.,1983). Each crown was embedded in autopolymerizing polymethylmethacrylateresin in an aluminium square such that between 3 and 5 mm of the labial surface protruded. This surface was ground flat using P300 and P600 grade silicon carbide paper. These samples were stored in water at 5°C until required. Composite inlays, with a surface area of 25 mm 2and 1 mm thick, were prepared using a specially made perspex mold which was placed on a base of dental stone (Jade Stone, Whipmix Corp., Louisville, KY, USA.). Both the mold and the stone were coated with the appropriate separating medium. The composite was packed into the mold and a glass slide placed onto the surface of the composite. The light curing materials (See Table) were cured for 60 s with a Translux, curing light (Kulzer GmbH, Werheim, Germany). After removal from the mold, the flash was removed from the samples. They were then subjected to the appropriate supercuring process, Coltene samples in the D.I.500 oven (Coltene AG, Liechtenstein, Altstatten, Switzerland) and Kulzer samples in the Unilux A.C. (Kulzer GmbH). The Ivoclar samples were cured in the mold entirely in the Ivomat (Vivadent Dental GmbH, Jagst, Liechtenstein) pressure / heat oven as they do not light cure. On removal from the oven, the fitting surface of the Ivoclar samples were sandblasted with 50 pm A1203. No surface treatments were carried out on the other samples prior to bonding. At this stage all inlay samples were stored dry for 1 week prior to bonding. The enamel was etched for 60 s with phosphoric acid gel, washed for 30 s and then dried with oil free air for 10 s. Immediately prior to bonding, all the inlay samples were washed with water for 30 s and then dried. Ten inlays were bonded for each of the following groups (See Table for details of materials used): 1) hybrid material 1, bonded using composite cement 1; 2) hybrid material 1, bonded using composite cement I and unfilled resin 1; 3) hybrid material 2, bonded
MATERIAL Hybrid 1
Composite Cement 1 Unfilled Resin 1 Hybrid 2
Composite Cement 2
Unfilled Resin 2 Microfilled Composite Composite Cement 3
Unfilled Resin 3 Bond Enhancer
TABLE: MATERIALSUSED MANUFACTURER TRADENAME BATCHNO. Coltene AG Coltene 9123-245 Brilliant AItstatten Switzerland Coltene AG ColteneDUO 9110-103 Cement Coltene AG Coltene DUO 9110.030 Bond Kulzer GmbH Estilux 94.6.30 40 Posterior CVS Bereich Dental, Werheim, Germany Kulzer GmbH Adhesive 31.12.92 032 Cement 31.12.92 032 AAdhesive 94.6.30 47 Kulzer GmbH Bond Ivoclar 144046 Vivadent Dental Inlay\Onlay GmbH Ellwagen, Liechtenstein Dual Cement 360029 Vivadent Dental GmbH HelioBond 340067 Vivadent Dental GmbH Special Bond 2240427 Vivadent Dental GmbH
using composite cement 2; 4) hybrid material 2, bonded using composite cement 2 and unfilled resin 2; 5) microfilled material, bonded using composite cement 3; 6) microfilled material, bonded using composite cement 3 and unfilled resin 3; 7) microfilledmaterial, bonded using composite cement 3, unfilled resin 3 and bond enhancer. All materials were used as recommended by the manufacturers. In all cases, the bonding agent/resin was applied to both the enamel and inlay surfaces and cured for 60 s. The bond enhancer (Specialbond II) was applied to the microfilled composite inlay fitting surface only. It was blown dry and the bonding resin was applied and cured for 60 s. Although all the cements and some of the unfilled resins are dual curing, it has been shown by Hasegawa et al., (1991) that dual curing materials obtain better physical properties when they are light cured. Consequently, every sample was cured for 2 min with a Translux light. Two minutes was chosen as the curing time, as there is decreased light penetration through these materials, resulting in a need for increased exposure time (Breeling et al., 1991). During bonding, a constant force of 10N was applied to each sample. Following bonding the excess resin was removed from around the inlay samples, and they were stored in water at 37°C for one week. Prior to testing, the samples were thermocycled between 5 °, 37 ° and 55°C for 500 cycles. Testing was carried out using a specially constructed jig mounted in a universal testing machine (Nene M3000: Nene Instruments, Wellingborough, Northampton, UK). A crosshead speed of 0.5mm/min was used. Shear bond strengths were recorded and debonded specimens examined under a dissecting microscope at 40x magnification to assess the gross plane of fracture. The data obtained for shear bond strengths was analyzed using a two-way ANOVA to determine if there was any difference between the groups. Comparison of groups was then carried out using a Student-Newman-Keuls test.
'°[
Shear Bond Strength (MPa)
30
.
.
.
.
.
.
.
.
20
10
0
C-c
C-c*b
K-C
K-c*b
I-c
t-c*b
I-c*b*s
Fig. Shear bond strength of inlay to etched enamel. Key to graph: C = hybrid 1, K = hybrid 2, I= microfilled composite, c = composite cement, u = unfilled resin, b = bond enhancer.
RESULTS The Fig. shows the shear bond strengths of the samples tested. These results showed that the hybrid 1, composite cement/ unfilled resin, specimens had the highest shear bond strength of 27.1 _+12MPa (_+1S.D.). This was significantly greater than any of the other samples tested (p < 0.05 for hybrid 1, composite cement alone and p < 0.01 for all other samples tested). There were no significant differences between any of the other samples tested. The use of unfilled resin had no effect on bond strength of the other systems tested nor did the application of the bond enhancer to group 7. Plane of fracture. In both the light cured hybrid systems, the plane of fracture was between enamel and composite, with the composite resin remaining firmly attached to the inlay surface. In the microfilledpressure/heat cured system, however, the cement always remained attached to the enamel surface, with the inlay surface appearing cement free. DISCUSSION For the two hybrid, light cured systems (Kulzer and Coltene), the bond between the inlay and enamel did not break at the inlay/cement interface as anticipated. From this result, it would appear that the composite inlay to composite cement bond in these systems is not as low as was thought. The bond between a luting cement and a composite inlay is essentially the same as that to a fractured composite resin. It follows that a comparable bond strength would be expected. The bond strength of a composite repair to a fractured composite has been shown to be only in the region of 25% to 75% of that of the unbroken material(Boyer et a/.,1978; Chan and Boyer 1983; Crumpler et a/.,1989). This is nonetheless in the region of 15-30 MPa, which is comparable with the bond strength of composite to etched enamel (Causton,1975). If this is true, the bond strengths obtained with the Hybrid I system are similar to those which might be expected. In this case, the bond strength ofthe resin cement to the inlay surface exceeded that to the enamel surface, and the bond fractured at the resin enamel interface. Application of unfilled resin increased the bond strength to the etched enamel, resultingin a higher shear strength. However, this was still less than that of the inlay to composite cement and hence, despite the increase in bond strength, no alteration in fracture plane occurred. Dental Materials/May 1992 209
With the hybrid 2 system, the bond strengths were significantly lower than for hybrid 1, although the plane of fracture remained the same. The decrease in bond strength between these two inlay systems could well be explained by the granular and relatively viscous nature of the luting composite supplied with this system when compared to that supplied for use with hybrid 1. This luting composite, although of similar filler content, flowed much more easily. The granular and viscous nature of composite cement 2 may prevent adequate wetting of the enamel surface and lead to the relatively low bond strength between the two. The fact that the luting cement remained attached to the inlay at this shear strength was not unexpected as Crumpler et al. (1989) in studies using Estilux posterior (hybrid 2 in this study) showed that it can achieve up to 88% ofits strength following repair, with a shear strength of 27 MPa being recorded. The fact that both these systems show the fracture plane to occur at the enamel composite cement bond suggests that there is no problem in bonding to these inlays despite the decrease in C=C bonds available. It would also seem probable from these results that there must be other important factors allowing bonding to occur between the two composites. Surface roughness, porosity and wettability of the inlay surface with the cement, leading to micromechanical interlocking, may well be as important as purely chemical bonding. The microfilledinlays fractured in the manner which would be expected if there were insufficient bonding sites remaining due to supercuring. This may well be the case in this material, as it is cured in a different manner which may result in an increased degree of methacrylate conversion relative to the other systems. The sandblasting as recommended by the manufacturer will lead to a relatively rough surface which should encourage bonding. However, from the plane of fracture, it would seem that despite this treatment, the surface is less retentive than either of the hybrid materials. Pressure during curing may also lead to a more condensed, less porous inlay surface to which it is difficult to form a good micromechanical bond. A bond enhancer has been developed to increase the bond strength of inlay to luting resin; however, in this study, no improvement was observed. The intended mechanism of action of this material has not been disclosed by the manufacturer so no further comment on its efficacy is possible at present. There are two other possible explanations for the relatively low shear bond strengths of this system. First, the pressure cured system uses a microfilled composite. As such, it has a higher coefficient of thermal expansion than the other two hybrid materials. This means that it will be subjected to more stress during thermocycling and will hence be more liable to bond breakdown (Smith,1985). Peutzfeldt and Asmussen (1991) have shown a greater decrease in retention of Ivoclar inlays followingthermocyclingthan of the other two systems. Second, Zidan et al. (1980) found a positive correlation between tensile strength of a composite and its bond strength. Similar studies, yet to be reported, have shown that the Ivoclar luting resin has a lower diametral tensile strength, and this may well account for the lower bond strengths observed with this material.
Received December 2, 1991/Accepted January 29, 1992
210 Scott et aL/Composite inlay, bond strength, restorative dentistry
Address correspondence and reprint requests to: J. A. Scott Dall Research Fellow Department of Conservative Dentistry Glasgow Dental Hospital and School 378, Sauchiehall Street Glasgow G2 3JZ Scotland
REFERENCES Boyer DB, Chan KC, Torney DL (1978). The strength of multilayer and repaired composite resin. J Prosthet Dent 39:63-67. Breeling LC, Dixon DL, Caughman WF (1991). The curing potential of light-activated composite resin luting cement. J Prosthet Dent 65:512-518. Causton BE (1975).Repair of abraded composite fillings. Br Dent J 139:286-288. Chan KC, Boyer DB (1983).Repair of conventional and microfilled composite resins. J Prosthet Dent 50:345-350. Crumpler DC, Bayne SC, Sockwell S, Brunson D, Robertson TM (1989).Bonding to resurfaced posterior composites. Dent Mater 5:417-424. De Gee AJ, Pallav P, Werner A, Davidson CL (1990). Annealing as a mechanism of increasing wear resistance of composites. Dent Mater 6:266-270. Hasegawa EA, Boyer DB, Chan DCN (1991). Hardening of dual-cured cements under compositeresin inlays. JProsthet Dent 66:187-192. Jensen M.E. Chan D.C.N. (1985) Polymerisation shrinkage and microleakage. In: Vanherle G and Smith DC, Ed. Posterior Composite Resin Restorative Materials. Peter Szulc Publishing Co., Netherlands. McCullock AJ, Smith BGN (1986). In Vitro studies of cuspal movement produced by adhesive restorative materials. Br Dent J 161: 405-409. Nakamichi I, Iwaku U, Fusayama T (1983). Bovine teeth as possible substitutes in the adhesion test. JDent Res 62:10761081. Peutzfeldt A, Asmussen E (1991). Retention of composite inlays in enamel dentine cavities. Dent Mater 7:11-14. Rees JS, Jacobson PH (1991). The current status of composite materials and adhesive systems. Part 6: Techniques for placement. Restorative Dentistry 21-23. Smith DC (1985). Posterior composite dental restorative materials: materials development. In Vanherle G and Smith DC, Ed. Posterior Composite Resin Dental Restorative Materials. Peter Szulc Publishing Co., Netherlands, 46-60. Wendt SL (1987a). The effect ofheat as a secondary cure upon the physical properties of three composite resins: I. Quint Int 18:265-271. Wendt SL (1987b). The effect of heat as a secondary cure upon the physical properties of three composite resins: II. Quint Int 18:265-271. Zidan O, Asmussen E, Jorgensen KD (1980). Correlation between tensile and bond strength of composite resins. Scand J Dent Res 88:348-351.
