Journal of Dentistry, 5, No. 4, 1977, pp. 295-302.
Printed
in Great Britain
The effect of enamel bonding agents on in vitro composite bond strengths Marion Rider, BMet, PhD B. Kenny, BEng, PhD Department of Mechanical Engineering, University of Sheffield
A. N. Tanner, BDS, PhD Charles Clifford Dental Howital, Sheffield
ABSTRACT The role of the enamel bonding agents in improving composite retention to etched enamel was investigated by comparing the bond strengths of the composites alone, the composites with the bonding agents and the bonding agents alone. The bonding agents were not found to increase composite retention to etched enamel, but thermal cycling results suggested that a more durable bond may form when these agents are employed. For all the materials tested the same mechanism of adhesion and mode of failure operated. This involved the formation of an unfilled resin layer which penetrated the enamel irregularities and remained on the enamel surface after failure.
INTRODUCTION The mechanism of bonding by direct filling materials to etched enamel has been investigated (Gwinnett and Buonocore, 1965; Gwinnett and Matsui, 1967; Buonocore et al., 1968), with the conclusion that penetrating ‘tag’extensions of the resin into the enamel surface irregularities were formed. These tag extensions have been observed on demineralized sectioned interface regions by optical and electron microscopy (Gwinnett and Matsui, 1967; Buonocore et al., 1968), and on resin and composite surfaces, after demineralization to separate them from the enamel, by scanning electron microscopy (Jorgensen and Shimokobe, 1975; McLundie and Messler, 1975). Discussion has arisen as to whether the tag length and ability of a material to penetrate the etched enamel surface is a function of its viscosity. Longer tag lengths and better surface penetration by materials with a low viscosity have been suggested by Buonocore et al. (1968) and McLundie and Messler (1975). However, Jorgensen and Shimokobe (1975) found no such correlation when comparing the tag lengths of low viscosity non-composite resins with those of the more viscous composite resins. Meurman and Nevaste (1975) reported no change in tag lengths with viscosity but did find a significant improvement in bond strength for a composite system incorporating a low viscosity enamel sealant. By contrast, the use of these enamel sealants has been found to produce no such significant increase in bond strength to etched enamel (Rider et al., 1977). Further investigations into the role of the enamel sealants in bonding to etched enamel are described in this paper. The present work reports on the in vitro tensile bond strength and environmental testing of composite systems which include these enamel bonding agents.
296
Journal of Dentistry, Vol. ~/NO. 4
A
t
32
.,
r
I
x
ll-
r
Fig. 1. The adhesion test piece. r, Acrylic resin cylinders; f, tooth cylinders; a, adhesive layer. Dimensions: x = not less than 5 mm for all specimens. For tensile specimens; y = 1 mm for Concise with Enamel Bond, Adaptic with Enamel Bond and Adaptic alone; 1, O-5 or 0.1 mm for Concise alone; O-05 mm for the two bonding agents alone. For shear specimens y = 1.5 mm for Concise and Concise with Enamel Bond.
MATERIALS AND METHODS Two composite systems, Concise (3M Co.) and Adaptic (Johnson & Johnson Ltd), both employing enamel bonding agents, were used in this study. Equal proportions by weight of the pastes and liquids were used for the two materials. Mixing and application were performed as recommended by the manufacturers. Sound human molars and bicuspids were collected and stored in physiological saline immediately after extraction until required for specimen preparation. Specimen preparation Universal adhesion test pieces (Fig. I) were prepared from standard enamel specimens using the method described by Rider et al. (1977). The specimen dimensions are given in Fig. 1. A standard enamel diameter of 3.5 mm was used throughout, but the adhesive layer thickness varied according to the product and the test objective. Enamel surfaces were polished on wet 600grit silicon carbide paper and etched for 60 seconds with either 37 per cent orthophosphoric acid as supplied with Concise for the Concise system specimens, or 50 per cent orthophosphoric acid containing 7 per cent by weight of zinc oxide for the Adaptic system specimens. Completed specimens were stored in a waterbath at 37 “C for 24 hours before testing or thermal cycling. Tensile testing All the specimens were tested on a Dartec testing machine (Dartec Ltd, Stourbridge) at a stroke speed of 1 mm/mm with the load at failure recorded on an XY plotter. The specimen was held during testing in split collet grips which fitted around the acrylic cylinders. The split collet grips were attached to the machine grips via universal joints to minimize any axial misalignment.
Rider et al: Enamel bonding
agents
297
Fig. 2. Thermal cycling apparatus. The baskets are shown here in the top position. a, Hot waterbath with heating-stirrer units; 6, cold waterbath with cooling coils; c, baskets; d, motor; e, constant level device. f, control panel.
The two halves of the fractured specimens were kept together for subsequent examination to determine the mode of failure. Fracture surfaces of two specimens from each test were also studied in more detail by scanning electron microscopy (Cambridge Stereoscan, Cambridge Scientific Instruments Ltd). Shear testing Shear bond strengths were evaluated after thermal cycling using the apparatus described by Rider (1977). The specimens were aligned in the special holder and tested at a punch speed of 5 mm/min with the failure load recorded on an XY plotter. Thermal cycling Prepared tensile and shear test pieces were thermally cycled using the apparatus (Tanner, 1976) shown in Fig. 2. The specimens were contained in the stainless steel baskets which were driven up to the top position by compressed air. A small motor allowed the baskets to turn so that the specimens could be exchanged between the two waterbaths. The hot waterbath was kept at 55+1 “C and the cold waterbath at 15+2 “C. A cycle time of 30 seconds was used throughout and the specimens were cycled for 24 hours, giving a total of 2880 cycles. The specimens were then returned to the 37 “C waterbath for 2 hours before testing following the same procedures as previously described.
298
Journal of Dentistry, Vol. ~/NO. 4
Table 1. Tensile bond strengths for Concise and Adaptic composite systems to etched enamel
Material Concise* Concise with Enamel Bond Concise bonding agent
Adaptic* Adaptic with Enamel Bond Adaptic bonding agent
??
Adhesive layer thickness tmm)
Mean tensile failure stress *rd. (MN/m’)
No. of specimens tested
1-o 1 *o
19.02*4-89 1967*4-76
9 8
All adhesive All adhesive
0.05
20.26t3.06
8
1.0 1-o
16.95*3-78 15*35*3.93
8 8
2 cohesive, 3 adhesive, 3 mixed cohesive/ adhesive All adhesive All adhesive
o-05
18.85*4-5
10
Failure characteristics
1 cohesive, 1 adhesive, 8 mixed adhesive/ cohesive
Results taken from Rider et al. (1977). Table Il. Effect of adhesive thickness on tensile bond strength of Concise to etched enamel Adhesive thickness fmml
Mean tensile failure stress *s.d. IMN/m’l
No. of specimens tested
1*0* 0.5 o-1
19.02i4.89 16.13k2.96 20.74* 3.68
9 7 8
Failure characteristics All adhesive All adhesive All mixed adhesive failures with material on both enamel faces
* Results taken from Rider et al. (1977).
RESULTS The tensile bond strength results for Concise with Enamel Bond, Adaptic with Enamel Bond and the two bonding agents alone are compared with the previously determined bond strengths of Concise and Adaptic (Rider et al., 1977) in Table Z. Analysis of variance established that there was no significant difference between the three bond strength values for the Concise system or for the Adaptic system. As many adhesive systems exhibit an increase in bond strength with decreasing adhesive layer thickness (Meissner and Baldauf, 1951), it is necessary to eliminate any discrepancies arising from comparing bond strength results with different adhesive thicknesses. The bonding agent specimens had a thickness of O-05 mm whereas all the other results in Table Z were for specimens with l-mm thick layers. Therefore, the bond strength for one composite material was determined for various adhesive thicknesses. These results are given in Table ZZ and analysis of variance revealed that there was no significant difference between the three results. The modes of failure throughout were predominantly adhesive, with some mixed fractures and occasional cohesive failures for the bonding agent specimens and the Concise specimens
Rider et al: Enamel bonding agents
299
with a 0.1~mm layer. A residual layer was observed on all enamel fracture surfaces. Typical scanning electron microscope photographs of these surfaces after bond failure can be seen in Fig. 3 compared with an etched enamel surface. Tensile and shear bond strengths after thermal cycling are given in Table III. Although there was a large decrease in the mean tensile bond strength for the Concise specimens after cycling compared with the non-cycled specimens, this difference was not statistically significant. The mean tensile bond strength for the cycled Concise with Enamel Bond specimens did not decrease so drastically, and again there was no significant difference compared with the non-cycled specimens. In addition, there was no significant difference between the tensile bond strength values for the Concise and Concise with Enamel Bond specimens after thermal cycling. However, for the shear bond strength results after cycling, a significant decrease, at the 1 per cent level, for the Concise specimens compared with the non-cycled specimens was observed. Again there was no significant difference in shear bond strength between the cycled and non-cycled Concise with Enamel Bond specimens, or between the two batches of specimens after cycling.
DISCUSSION It would appear from the results in Table I that as there was no significant difference in the bond strengths of the two composite systems with or without the bonding agents, these bonding agents did not improve the tensile bond strength. In addition, there was no difference in bond strength between the values for the bonding agents alone and the values for the composites alone. Mitchem and Turner (1974) also established that there was no significant difference in retentive strength between Concise and Concise with Enamel Bond, or between Adaptic and Adaptic with Enamel Bond. However, they did imply that superior wetting and penetration were achieved when the bonding agents were used, as the mode of failure changed from adhesive to cohesive and enamel failure. By contrast, Meurman and Nevaste (1975) established that Concise with Enamel Bond produced a significantly stronger bond than Concise alone. The reason for this discrepancy in results is uncertain, but Meurman and Nevaste (1975) prepared specimens from bovine teeth whereas the present study and the work by Mitchem and Turner (1974) used human teeth. In the scanning electron microscope photographs of the enamel surfaces after bond failure (Fig. 3), the surface appearance of the composite and composite with bonding agent is similar, but that of the bonding agents alone is somewhat different. If each surface is then compared with the etched enamel surface before the application of adhesive, it is apparent that although adhesive modes of failure were recorded, a residual layer of material still covers the enamel. This layer appears to contain no filler particles. It is therefore suggested that the same mechanism of adhesion and mode of failure occurred in all cases. An unfilled resin layer, either matrixresin or enamel sealant, penetrated the etched enamel surface, providing the necessary retention, but failure was also initiated within this layer. However, this does not imply that the degree of penetration of the surface layer of unfilled material was the same for all combinations of materials. It can only be stated here that whether or not superior wetting and penetration were achieved by the low viscosity bonding agents, this has no effect on the tensile bond strength. The enamel-composite system did not show a decrease in bond strength with increasing adhesive layer thickness (Table II); thus, it was valid to compare the results of Tuble I.
Journal of Dentistry, Vol. ~/NO. 4
a
c Fig. a, b, c, 0:
b
d
3. Scanning electron microscope photographs of enamel surfaces after bond failure. Etched enamel surface (magnification X 1440). Enamel surface after failure of the Adaptic bond (magnification X 1440). Enamel surface after failure of Concise with Enamel Bond (magnification X 1380). Enamel surface after failure of Concise bonding agent (magnification X 1200).
Again these results suggested that only the unfilled surface layer was involved in bonding, as altering the composite layer thickness had no effect on bond strength. Thermal cycling followed by either tensile or shear loading revealed that Concise with Enamel Bond may have produced a more durable bond than Concise alone. However, there were decreases in the shear and tensile bond strengths for both the Concise and Concise with Enamel Bond specimens after cycling compared with the non-cycled results, but the
301
Rider et al: Enamel bonding agents
Tab/e /I/. Tensile and shear bond strengths for thermally cycled and noncycled specimens
Material Concise Concise with Enamel Bond Concise Concise with Enamel Bond
Mode of testing 1 t J
Tensile
) Shear
Cycled Mean failure No. of specimens stress+s.d. tested itWN/ma)
Non-cycled Mean failure No. of stressis:d. specmens tested (MN/m’)
13.37*8-38
16
19.02*4.89*
9
16.85i6.91
13
19.67+4*76*
8
1O-40+-4-18
9
17.0 *4-01t
7
14.66+7-l 1
10
16.29*2-59t
5
?? Results taken from Rider et al. (19771 t Results taken from Rider (1977).
Concise specimens showed a more marked decrease, with the shear bond strength after cycling significantly lower than the non-cycled value. It may be, therefore, that the bonding agents, while not affecting the in vitro bond strength, would give a more durable bond in the clinical situation. The reason for the lower mean tensile and shear bond strengths after the cycling procedure adopted here is uncertain. Lee and Swartz (1970) and Asmussen (1974) concluded that the difference in coefficient of thermal expansion between composite materials and tooth structure was unlikely to have any detrimental effects in the temperature range 2-60 “C. Thus a weakening in bond strength after cycling in the present study is not due to fatigue in the resin induced by thermal cycling stresses. It has been suggested that either polymerization shrinkage (Lee and Swartz, 1970) or the ability of a material to adsorb water and close any marginal gaps (Asmussen, 1974) is critical in determining behaviour during and after thermal cycling. Equally, absorption of water by the resin could have produced a decrease in the cohesive strength of the material and a subsequent decrease in bond strength. As polymerization shrinkage is now less of a problem with established composite materials, it seems likely that water absorption and its effects on the material properties govern the resulting bond strengths. CONCLUSIONS
There was no significant improvement in the tensile bond strength of the composites Concise and Adaptic when enamel bonding agents were employed. The same mechanism of adhesion and mode of failure operated in all cases. These were dependent on an intermediate unfilled resin layer penetrating the etched enamel surface irregularities to provide retention, but equally this layer was observed to initiate failure. Thermal cycling results indicated that the importance of the bonding agents may be in improving the durability rather than the strength of the bond. Acknowledgement The authors are grateful to the Department of Metallurgy, University of Sheffield, for the
use of the Stereoscan.
302
Journal of Dentistry, Vol. ~/NO. 4
REFERENCES Asmussen A. (1974) Odontol. &and.
The effect of temperature
changes on adaption
of resin fillings. Acta
32, 161-171.
Buonocore M. G., Matsui A. and Gwinnett A. J. (1968) Penetration of resin dental materials into enamel surfaces with reference to bonding. Arch. Oral Biol. 13, 61-70. Gwinnett A. J. and Buonocore M. G. (1965) Adhesives and caries prevention. Br. Dent. J. 119,77-80. Gwinnett A. J. and Matsui A. (1967) A study of enamel adhesives: physical relationship between enamel and adhesive. Arch. Oral Biol. 12, 16 15- 1620. Jorgensen K. D. and Shimokobe H. (1975) Adaptation of resinous restorative materials to acid etched surfaces. &and. J. Dent. Res. 83, 31-36. Lee H. L. and Swartz M. L. (1970) Scanning electron microscope study of composite restorative materials. J. Dent. Rex 49, 149-158. McLundie A. C. and Messler J. G. (1975) Acid-etch incisal restorative materials. Br. Dent. J. 138, 137-140.
Meissner H. P. and Baldauf
G. H. (195 1) Strength
behaviour
of adhesive joints.
Trans.
Am, Sot. Mech. Eng. 73,697-704.
Meurman J. H. and Nevaste M. (1975) The intermediate effect of low-viscous fissure sealants on the retention of resin restoratives in vitro. Proc. Finn. Dent. Sot. 71,96-101. Mitchem J. C. and Turner L. R. (1974) The retentive strength of acid etched retained resins. J. Am. Dent. Assoc. 89, 1107-l 110. Rider M., Tanner A. N. and Kenny B. (1977) Investigation of adhesive properties of dental composite materials using an improved tensile test procedure and scanning electron microscopy. J. Dent. Res. 56, 368-378. Rider M. (1977) The shear strength of enamel-composite bonds. J. Dent. 5,237-244. Tanner A. N. (1976) A study of the deformation of the margins of gold castings with special reference to pm-retained restorations. PhD Thesis, University of Sheffield.
Printed
in Great Britain
The effect of enamel bonding agents on in vitro composite bond strengths Marion Rider, BMet, PhD B. Kenny, BEng, PhD Department of Mechanical Engineering, University of Sheffield
A. N. Tanner, BDS, PhD Charles Clifford Dental Howital, Sheffield
ABSTRACT The role of the enamel bonding agents in improving composite retention to etched enamel was investigated by comparing the bond strengths of the composites alone, the composites with the bonding agents and the bonding agents alone. The bonding agents were not found to increase composite retention to etched enamel, but thermal cycling results suggested that a more durable bond may form when these agents are employed. For all the materials tested the same mechanism of adhesion and mode of failure operated. This involved the formation of an unfilled resin layer which penetrated the enamel irregularities and remained on the enamel surface after failure.
INTRODUCTION The mechanism of bonding by direct filling materials to etched enamel has been investigated (Gwinnett and Buonocore, 1965; Gwinnett and Matsui, 1967; Buonocore et al., 1968), with the conclusion that penetrating ‘tag’extensions of the resin into the enamel surface irregularities were formed. These tag extensions have been observed on demineralized sectioned interface regions by optical and electron microscopy (Gwinnett and Matsui, 1967; Buonocore et al., 1968), and on resin and composite surfaces, after demineralization to separate them from the enamel, by scanning electron microscopy (Jorgensen and Shimokobe, 1975; McLundie and Messler, 1975). Discussion has arisen as to whether the tag length and ability of a material to penetrate the etched enamel surface is a function of its viscosity. Longer tag lengths and better surface penetration by materials with a low viscosity have been suggested by Buonocore et al. (1968) and McLundie and Messler (1975). However, Jorgensen and Shimokobe (1975) found no such correlation when comparing the tag lengths of low viscosity non-composite resins with those of the more viscous composite resins. Meurman and Nevaste (1975) reported no change in tag lengths with viscosity but did find a significant improvement in bond strength for a composite system incorporating a low viscosity enamel sealant. By contrast, the use of these enamel sealants has been found to produce no such significant increase in bond strength to etched enamel (Rider et al., 1977). Further investigations into the role of the enamel sealants in bonding to etched enamel are described in this paper. The present work reports on the in vitro tensile bond strength and environmental testing of composite systems which include these enamel bonding agents.
296
Journal of Dentistry, Vol. ~/NO. 4
A
t
32
.,
r
I
x
ll-
r
Fig. 1. The adhesion test piece. r, Acrylic resin cylinders; f, tooth cylinders; a, adhesive layer. Dimensions: x = not less than 5 mm for all specimens. For tensile specimens; y = 1 mm for Concise with Enamel Bond, Adaptic with Enamel Bond and Adaptic alone; 1, O-5 or 0.1 mm for Concise alone; O-05 mm for the two bonding agents alone. For shear specimens y = 1.5 mm for Concise and Concise with Enamel Bond.
MATERIALS AND METHODS Two composite systems, Concise (3M Co.) and Adaptic (Johnson & Johnson Ltd), both employing enamel bonding agents, were used in this study. Equal proportions by weight of the pastes and liquids were used for the two materials. Mixing and application were performed as recommended by the manufacturers. Sound human molars and bicuspids were collected and stored in physiological saline immediately after extraction until required for specimen preparation. Specimen preparation Universal adhesion test pieces (Fig. I) were prepared from standard enamel specimens using the method described by Rider et al. (1977). The specimen dimensions are given in Fig. 1. A standard enamel diameter of 3.5 mm was used throughout, but the adhesive layer thickness varied according to the product and the test objective. Enamel surfaces were polished on wet 600grit silicon carbide paper and etched for 60 seconds with either 37 per cent orthophosphoric acid as supplied with Concise for the Concise system specimens, or 50 per cent orthophosphoric acid containing 7 per cent by weight of zinc oxide for the Adaptic system specimens. Completed specimens were stored in a waterbath at 37 “C for 24 hours before testing or thermal cycling. Tensile testing All the specimens were tested on a Dartec testing machine (Dartec Ltd, Stourbridge) at a stroke speed of 1 mm/mm with the load at failure recorded on an XY plotter. The specimen was held during testing in split collet grips which fitted around the acrylic cylinders. The split collet grips were attached to the machine grips via universal joints to minimize any axial misalignment.
Rider et al: Enamel bonding
agents
297
Fig. 2. Thermal cycling apparatus. The baskets are shown here in the top position. a, Hot waterbath with heating-stirrer units; 6, cold waterbath with cooling coils; c, baskets; d, motor; e, constant level device. f, control panel.
The two halves of the fractured specimens were kept together for subsequent examination to determine the mode of failure. Fracture surfaces of two specimens from each test were also studied in more detail by scanning electron microscopy (Cambridge Stereoscan, Cambridge Scientific Instruments Ltd). Shear testing Shear bond strengths were evaluated after thermal cycling using the apparatus described by Rider (1977). The specimens were aligned in the special holder and tested at a punch speed of 5 mm/min with the failure load recorded on an XY plotter. Thermal cycling Prepared tensile and shear test pieces were thermally cycled using the apparatus (Tanner, 1976) shown in Fig. 2. The specimens were contained in the stainless steel baskets which were driven up to the top position by compressed air. A small motor allowed the baskets to turn so that the specimens could be exchanged between the two waterbaths. The hot waterbath was kept at 55+1 “C and the cold waterbath at 15+2 “C. A cycle time of 30 seconds was used throughout and the specimens were cycled for 24 hours, giving a total of 2880 cycles. The specimens were then returned to the 37 “C waterbath for 2 hours before testing following the same procedures as previously described.
298
Journal of Dentistry, Vol. ~/NO. 4
Table 1. Tensile bond strengths for Concise and Adaptic composite systems to etched enamel
Material Concise* Concise with Enamel Bond Concise bonding agent
Adaptic* Adaptic with Enamel Bond Adaptic bonding agent
??
Adhesive layer thickness tmm)
Mean tensile failure stress *rd. (MN/m’)
No. of specimens tested
1-o 1 *o
19.02*4-89 1967*4-76
9 8
All adhesive All adhesive
0.05
20.26t3.06
8
1.0 1-o
16.95*3-78 15*35*3.93
8 8
2 cohesive, 3 adhesive, 3 mixed cohesive/ adhesive All adhesive All adhesive
o-05
18.85*4-5
10
Failure characteristics
1 cohesive, 1 adhesive, 8 mixed adhesive/ cohesive
Results taken from Rider et al. (1977). Table Il. Effect of adhesive thickness on tensile bond strength of Concise to etched enamel Adhesive thickness fmml
Mean tensile failure stress *s.d. IMN/m’l
No. of specimens tested
1*0* 0.5 o-1
19.02i4.89 16.13k2.96 20.74* 3.68
9 7 8
Failure characteristics All adhesive All adhesive All mixed adhesive failures with material on both enamel faces
* Results taken from Rider et al. (1977).
RESULTS The tensile bond strength results for Concise with Enamel Bond, Adaptic with Enamel Bond and the two bonding agents alone are compared with the previously determined bond strengths of Concise and Adaptic (Rider et al., 1977) in Table Z. Analysis of variance established that there was no significant difference between the three bond strength values for the Concise system or for the Adaptic system. As many adhesive systems exhibit an increase in bond strength with decreasing adhesive layer thickness (Meissner and Baldauf, 1951), it is necessary to eliminate any discrepancies arising from comparing bond strength results with different adhesive thicknesses. The bonding agent specimens had a thickness of O-05 mm whereas all the other results in Table Z were for specimens with l-mm thick layers. Therefore, the bond strength for one composite material was determined for various adhesive thicknesses. These results are given in Table ZZ and analysis of variance revealed that there was no significant difference between the three results. The modes of failure throughout were predominantly adhesive, with some mixed fractures and occasional cohesive failures for the bonding agent specimens and the Concise specimens
Rider et al: Enamel bonding agents
299
with a 0.1~mm layer. A residual layer was observed on all enamel fracture surfaces. Typical scanning electron microscope photographs of these surfaces after bond failure can be seen in Fig. 3 compared with an etched enamel surface. Tensile and shear bond strengths after thermal cycling are given in Table III. Although there was a large decrease in the mean tensile bond strength for the Concise specimens after cycling compared with the non-cycled specimens, this difference was not statistically significant. The mean tensile bond strength for the cycled Concise with Enamel Bond specimens did not decrease so drastically, and again there was no significant difference compared with the non-cycled specimens. In addition, there was no significant difference between the tensile bond strength values for the Concise and Concise with Enamel Bond specimens after thermal cycling. However, for the shear bond strength results after cycling, a significant decrease, at the 1 per cent level, for the Concise specimens compared with the non-cycled specimens was observed. Again there was no significant difference in shear bond strength between the cycled and non-cycled Concise with Enamel Bond specimens, or between the two batches of specimens after cycling.
DISCUSSION It would appear from the results in Table I that as there was no significant difference in the bond strengths of the two composite systems with or without the bonding agents, these bonding agents did not improve the tensile bond strength. In addition, there was no difference in bond strength between the values for the bonding agents alone and the values for the composites alone. Mitchem and Turner (1974) also established that there was no significant difference in retentive strength between Concise and Concise with Enamel Bond, or between Adaptic and Adaptic with Enamel Bond. However, they did imply that superior wetting and penetration were achieved when the bonding agents were used, as the mode of failure changed from adhesive to cohesive and enamel failure. By contrast, Meurman and Nevaste (1975) established that Concise with Enamel Bond produced a significantly stronger bond than Concise alone. The reason for this discrepancy in results is uncertain, but Meurman and Nevaste (1975) prepared specimens from bovine teeth whereas the present study and the work by Mitchem and Turner (1974) used human teeth. In the scanning electron microscope photographs of the enamel surfaces after bond failure (Fig. 3), the surface appearance of the composite and composite with bonding agent is similar, but that of the bonding agents alone is somewhat different. If each surface is then compared with the etched enamel surface before the application of adhesive, it is apparent that although adhesive modes of failure were recorded, a residual layer of material still covers the enamel. This layer appears to contain no filler particles. It is therefore suggested that the same mechanism of adhesion and mode of failure occurred in all cases. An unfilled resin layer, either matrixresin or enamel sealant, penetrated the etched enamel surface, providing the necessary retention, but failure was also initiated within this layer. However, this does not imply that the degree of penetration of the surface layer of unfilled material was the same for all combinations of materials. It can only be stated here that whether or not superior wetting and penetration were achieved by the low viscosity bonding agents, this has no effect on the tensile bond strength. The enamel-composite system did not show a decrease in bond strength with increasing adhesive layer thickness (Table II); thus, it was valid to compare the results of Tuble I.
Journal of Dentistry, Vol. ~/NO. 4
a
c Fig. a, b, c, 0:
b
d
3. Scanning electron microscope photographs of enamel surfaces after bond failure. Etched enamel surface (magnification X 1440). Enamel surface after failure of the Adaptic bond (magnification X 1440). Enamel surface after failure of Concise with Enamel Bond (magnification X 1380). Enamel surface after failure of Concise bonding agent (magnification X 1200).
Again these results suggested that only the unfilled surface layer was involved in bonding, as altering the composite layer thickness had no effect on bond strength. Thermal cycling followed by either tensile or shear loading revealed that Concise with Enamel Bond may have produced a more durable bond than Concise alone. However, there were decreases in the shear and tensile bond strengths for both the Concise and Concise with Enamel Bond specimens after cycling compared with the non-cycled results, but the
301
Rider et al: Enamel bonding agents
Tab/e /I/. Tensile and shear bond strengths for thermally cycled and noncycled specimens
Material Concise Concise with Enamel Bond Concise Concise with Enamel Bond
Mode of testing 1 t J
Tensile
) Shear
Cycled Mean failure No. of specimens stress+s.d. tested itWN/ma)
Non-cycled Mean failure No. of stressis:d. specmens tested (MN/m’)
13.37*8-38
16
19.02*4.89*
9
16.85i6.91
13
19.67+4*76*
8
1O-40+-4-18
9
17.0 *4-01t
7
14.66+7-l 1
10
16.29*2-59t
5
?? Results taken from Rider et al. (19771 t Results taken from Rider (1977).
Concise specimens showed a more marked decrease, with the shear bond strength after cycling significantly lower than the non-cycled value. It may be, therefore, that the bonding agents, while not affecting the in vitro bond strength, would give a more durable bond in the clinical situation. The reason for the lower mean tensile and shear bond strengths after the cycling procedure adopted here is uncertain. Lee and Swartz (1970) and Asmussen (1974) concluded that the difference in coefficient of thermal expansion between composite materials and tooth structure was unlikely to have any detrimental effects in the temperature range 2-60 “C. Thus a weakening in bond strength after cycling in the present study is not due to fatigue in the resin induced by thermal cycling stresses. It has been suggested that either polymerization shrinkage (Lee and Swartz, 1970) or the ability of a material to adsorb water and close any marginal gaps (Asmussen, 1974) is critical in determining behaviour during and after thermal cycling. Equally, absorption of water by the resin could have produced a decrease in the cohesive strength of the material and a subsequent decrease in bond strength. As polymerization shrinkage is now less of a problem with established composite materials, it seems likely that water absorption and its effects on the material properties govern the resulting bond strengths. CONCLUSIONS
There was no significant improvement in the tensile bond strength of the composites Concise and Adaptic when enamel bonding agents were employed. The same mechanism of adhesion and mode of failure operated in all cases. These were dependent on an intermediate unfilled resin layer penetrating the etched enamel surface irregularities to provide retention, but equally this layer was observed to initiate failure. Thermal cycling results indicated that the importance of the bonding agents may be in improving the durability rather than the strength of the bond. Acknowledgement The authors are grateful to the Department of Metallurgy, University of Sheffield, for the
use of the Stereoscan.
302
Journal of Dentistry, Vol. ~/NO. 4
REFERENCES Asmussen A. (1974) Odontol. &and.
The effect of temperature
changes on adaption
of resin fillings. Acta
32, 161-171.
Buonocore M. G., Matsui A. and Gwinnett A. J. (1968) Penetration of resin dental materials into enamel surfaces with reference to bonding. Arch. Oral Biol. 13, 61-70. Gwinnett A. J. and Buonocore M. G. (1965) Adhesives and caries prevention. Br. Dent. J. 119,77-80. Gwinnett A. J. and Matsui A. (1967) A study of enamel adhesives: physical relationship between enamel and adhesive. Arch. Oral Biol. 12, 16 15- 1620. Jorgensen K. D. and Shimokobe H. (1975) Adaptation of resinous restorative materials to acid etched surfaces. &and. J. Dent. Res. 83, 31-36. Lee H. L. and Swartz M. L. (1970) Scanning electron microscope study of composite restorative materials. J. Dent. Rex 49, 149-158. McLundie A. C. and Messler J. G. (1975) Acid-etch incisal restorative materials. Br. Dent. J. 138, 137-140.
Meissner H. P. and Baldauf
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