158
J. Dent. 1990;
18: 158-l
62
Effect of fatigue upon the interfacial bond strength of repaired composite resins W. P. Saunders Department of Conservative Dentistry, Glasgow Dental Hospital and School, UK
ABSTRACT A comparative study of the fatigue limits of repaired samples of three composite resins was undertaken using the staircase technique. Following storage in water for 21 days at 23°C the surfaces of the specimens to be bonded were ground with a fine Soflex disc, washed and dried. The surface was treated with either Scotchbond or a silane coupling agent, or left untreated. and a composite repair of the same material added. The specimens were thermally cycled and tested sequentially for 5000 cycles at different stress instruments using a transverse impact load. A statistically valid value for mean fatigue limit was determined for each design. This showed that resistance to fatigue forces was lower in repaired specimens than in complete specimens. The use of Scotchbond Dual Cure bonding agent gave the strongest repair. KEY WORDS: Composite resins, Technique, Fatigue testing J. Dent. 1990; 18: 158-l 1990)
62 (Received 27 September 1989;
reviewed 3 November 1989; accepted 14 February
Correspondence shouldbe addressed to: Dr W. P. Saunders, Department of Conservative Dentistry, Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow G2 3JZ. UK.
INTRODUCTION In recent years there has been an increase in the use of composite resins for restoring posterior teeth. It may be
necessary to remove and replact old composite restorations as areas become worn or discoloured and the most conservative treatment for this problem is the replacement of the unsatisfactory part of the restoration with a repair. It has been shown in vitro, however. that the strength of repaired composite is reduced compared with complete specimens (Forsten and Valiaho, 1971; Reisbick and Brodsky, 1971; Causton. 1975; Boyeretal., 1978;Lloyd et al., 1980; Chan and Boyer, 1983; Miranda et al., 1984; Azarbal et al., 1986; Soderholm. 1986). Microleakage has also been detected at the interface between composite and repairs (Chalkley and Chan, 1986). This leakage leads to deterioration of the bond and accelerates failure. It is expected that such repairs should be able to withstand the forces of mastication. These forces may be considered to be impact in nature with a cyclic component. The effect of loading on the teeth is thus a function of volume under stress (Johnson, 1972). When a structure is subjected to repeated stress cycles it may fail at stresses well below the tensile strength by a process known as fatigue (Ashby and Jones, 1980). 0 1990 Butterworth-Heinemann 0300-5712/90/030158-05
Ltd.
Fatigue life may be defined as the number of stress cycles a material will withstand before it fails. At high stresses failure occurs after a low number of cycles but at low stresses failure will occur after a large number of cycles. Below a certain value, the fatigue limit, the material can be subjected to a very large number of cycles without failing. The effect of the fatiguing influence of masticatory load upon repaired composite resin has not been reported. The purpose of this study was to examine the effects of impact fatigue forces upon the transverse strength of repaired posterior composite.
MATERIALS
AND METHOD
Three composite resins were selected for investigation, P50 (3M Co., St Paul, MN, USA), Occlusin (ICI Dental, Macclesfield, UK) and Herculite XR (Kerr, Romulus, MI, USA). These materials are intended for use in posterior restorations and consist of fillers within resin matrices. The principal filler in Herculite XR and Occlusin is a barium glass, and P50 contains zirconia and silica fillers. The filler loading is similar for each material, 87.5 per cent by weight for P50,86 per cent by weight for Occlusin and
Saunders:
Fatigue of repaired composite
resins
159
Fig. 7. The fatigue testing machine with control box (A), steel beam (B), striking pin (P) and loading weight (W). _
75 per cent by weight for Herculite
XR. The average particle size of the filler in these composite resin materials varies between 0.6 urn for Herculite XR, 1.5 urn for P50 and 6 ym for Occlusin, although all the materials could be described as being of hybrid construction. The resin component consists of Bis-GMA in P50 and Herculite XR, and mainly urethane dimethacrylate in Occlusin. All materials are single-paste systems and are polymerized by the application of light. Specimen
preparation
Three groups of 15 rectangular specimens of each composite were made in a split brass mould that was 25 mm long. 3 mm wide and 3 mm deep. Half the mould was filled with a rubber blank of polyvinyl siloxane impression material. The base of the mould was covered with a mylar strip and composite was packed into the mould. The material was cured in two increments with a light source (Aristolite, Wright Health Co., Dundee, UK). The second increment was covered with a mylar strip and compressed with a glass slab prior to polymerization. Each increment was cured in two areas for 60 s each. The specimens were taken from the mould, any excess composite trimmed with a scalpel, and stored in deionized water at 23 “C for 3 weeks. The rubber blank was removed from the mould and full mould sized specimens were made with each of the two incremental layers of resin being polymerized in four areas for 60 s each. Seventeen full mould sized specimens were made for each composite and stored in deionized water at 23 “C for 4 weeks prior to testing. Immediately before adding new composite to create repaired specimens, the half specimens were dried and the surface to be bonded was smoothed with a fine Soflex disc (3M Co.. St Paul, MN, USA). The prepared surface was washed with water from a 3 in 1 syringe and dried with oilfree air. In the first group no other treatment was carried out to the prepared surface, in the second the surface was coated with a silane coupling agent, Scotchprime (3M
Co.), and in the final group the surface was coated with a layer of bonding agent, Scotchbond Dual Cure (3M Co.), thinned with air and light cured for 20 s. The specimens were then returned to the mould and the second half was added using the same brand of composite. Polymerization was again carried out in two increments, each increment being cured for 60 s in two areas. The repaired specimens of composite were removed from the mould and stored in deionized water for 24 h and then subjected to thermal cycling for 8 h through 37°C 55 “C, 37°C and 55°C with dwell times of 10 min at each temperature, prior to testing. Testing Fatigue testing was carried out using apparatus previously described (Saunders, 1987) the design of which was based upon the ‘walking beam’ machine (Gurney. 1965). A simple beam actuated by a motor-driven cam gives a cyclic stress to the specimen by way of a striking pin. The apparatus is shown in Fig. 1. The box (A) controls the operation of the machine and houses the switch gear for the motor, a digital revolution counter and a cut-out device for the motor when the specimen is broken. A steel beam (B) is attached to a metal base by means of a steel block and bolt so that rotation can occur in the vertical axis about this bolt. The free end of the beam rests upon a cam which, in turn, is attached to an AC-powered drive motor. The free end of the beam is fitted with a platform on to which weights can be added during the testing sequence. A striking pin (P), 2 mm in diameter, is situated over the specimen holder such that the force applied to the specimen was directly over the repair, and at 90” to the repaired surface. The force acting on the beam could be adjusted in 1.5 kg increments by the addition or removal of a 0.5 kg lead block from the end of the beam (w). The fatigue test was carried out using the staircase method. This has been shown to give a good measure of mean fatigue limit (Draughn, 1979). By this method a specimen is tested for a prescribed number of cycles at a
J. Dent. 1990;
160
18: No. 3
‘s 3
4.5-
.
.
6.0 -
.
.
.
0
RESULTS
0
An example of the sequence of failures and non-failures at different stress levels for the Herculite repaired with Scotchbond group is shown in Fig. 2. The analysis of data is based upon the least frequent event. In the example there were seven failures and eight non-failures. The mean fatigue limit was calculated by arranging the data as in Table 1. The lowest load level at which failure occurred is denoted by i = 0 the next level as i = 1 and so on. The mean fatigue limit was calculated using the formula:
0
y” ^: 3.0-
.
0
.
0
0
ar -$j 1.5-
0
0
s “0
1
2
3
4
5
6
7
Specimen
8
9
10
11
12
13
14
15
number
Fig. 2. Sequence of failures and non-failures for Herculite with Scotchbond group. W, Failures (n = 7); 0, non-failures (n = 8).
X=X+d
stress close to the estimated mean fatigue limit. If failure occurs within this number of cycles, then the next specimen is tested at a stress level lowered by a fixed increment. If failure does not occur, then the next specimen is tested at a stress level increased by the same increment. Each specimen is tested sequentially and the stress applied depends upon whether failure or nonfailure occurred with the previous specimen. The number of specimens required in this method is less than with other fatigue tests, a minimum of 15 specimens is required for accurate data analysis (Dieter, 1961). The stress acting upon each specimen when subjected to a cyclic fatigue force is complex, however, as the top surface of the repaired specimen will be in compression and the bottom surface in tension. In addition the elastic moduli of the materials was not known. It was decided, therefore, to compare the resistance of each material to such forces in terms of load levels rather than in terms of fatigue stress. The speed of cycling was set to 200 rev./min. This is faster than normal cyclic chewing speed and therefore represented a severe test for the materials. The specimen was placed such that the striking pin was directly over the centre of each specimen and, in those that had been repaired, the pin was positioned directly over the repair in the long axis of the interface between the old and new materials. The prescribed number of cycles was set at 5000. All specimens were tested when immersed in water at room temperature (23°C + 3°C). Table II. Transverse fatigue pretreatments
Treatment
limit
)
whereX is the lowest load level considered in the analysis and d is the load increment employed. The positive sign is used for non-failures and the negative sign for failures. The standard deviation was derived from the formula NB-A2
S = 1.62 d
+ 0.029 )
iv2
(
This formula is an approximation, but when AJB - AZ/ accurate. The standard deviations of fatigue limits in this study were calculated with this formula. The results for the fatigue tests are shown in Table ZZ. The mean fatigue limit of each group was compared using analysis of variance and Duncan’s Multiple Range Test with a significance level of 0.05. The full mould sized specimens of the P50 and Occlusin groups were more resistant to fatigue than the repaired specimens. The use of silane did not improve the fatigue resistance of the P50, but the Scotchbond-treated groups performed significantly better. There were no statistically IV2 is larger than 0.3, it is sufficiently
Table 1. Analysis of staircase test procedural data Load level Ml f.5 3
P50
i
ni
ini
i lli
2
2 3 2 N=ZE ni = 7
4
8
; A = Z ini = 7
; B = Z ini = 11
:,
(kg) of repaired
Specimens fn0.l
(“N + ;
composites
Occlusin
after various
Herculite
Whole
17
6.0
(0.22)
8.63
(1.74)
3.94*
No treatment Silane Scotchbond
15 15 15
2.89 C2.89 3.75
(0.67) (0.67) (0.37)
2.5 2.5 2.68
(0.41) (0.41) (0.57)
No result No result 3.75* (1.31)
Vertical lines connect groups that are not significantly different. Figures in parentheses are standard deviations. *Means showing no statistically significant difference.
(0.94)
Saunders:
significant differences between the repaired groups of the Occlusin. The untreated and silane-treated specimens of Herculite XR did not survive the staircase procedure. There was no statistically significant difference between the whole specimens of this material and the Scotchbond group. The unrepaired Occlusin group had a statistically significantly higher fatigue resistance than the P50 which, in turn, had a significantly higher limit than Herculite. The mode of failure of all the repaired specimens was interfacial through the repaired surface.
DISCUSSION The results show that repaired composite samples are less resistant to impact fatigue stress compared with the whole specimens. These values varied from between 29 per cent of the fatigue load applied to the whole samples, in the Occlusin repaired with silane, and with no treatment, to 95 per cent for the Herculite after repair with Scotchbond. This does not include, however, the Herculite repairs made without treatment and with silane where no result was obtained. It is uncertain whether the strength of repaired specimens would be sufficient to survive the fatigue effects of mastication. The data recorded for the mean fatigue limit in this study may be applied only to specimens of the dimensions tested. The reason for this is that calculations were based upon the load applied to the specimen and no account was made of the area of repair. The study was thus comparative in nature. Such tests are frequently applied in fatigue studies and are considered to be of benefit (Osgood, 1970). Five thousand cycles is a small number compared with the number of stress cycles a repaired composite would be required to withstand in tivo. To withstand cyclic stresses in excess of 5000 cycles, it could be argued that the applied load must be less than the mean fatigue limit. On examination of the failed specimens there was no evidence of damage to the surface of the composite as a result of the impact of the pin. Clinically, surface damage may be caused as a result of the increased number of masticatory cycles and this could affect adversely the fatigue resistance of the material. Factors which are most likely to influence the bond between composite and its repair include the ability to wet the surface of the material, the amount and chemical activity of available resin for bonding and the effect of surface treatment prior to repair. The amount of resin present on the repaired surface for adequate bonding has been shown to be important (Lloyd and Dhuru, 1985) the more resin present the better the bond. Vankerckhoven et al. (1982) studied the effect of various parameters on the number of unreacted methacrylate groups on the surfaces of composite resins. They found that all types of surface treatment and manipulative variables reduced the number of these unreacted groups. In addition the degree of saturation of microtilled
Fatigue of repaired composite
resins
161
composites makes the adhesion between matrix and repair questionable. In the present study all the composites used had a high tiller to resin ratio and contained a matrix of very small particle size. This could have affected adversely the ability of each composite to repair to itself. It is also considered that the age of the composite has a detrimental effect on its reactivity, the first 24 h providing the greatest reactivity at the surface. The specimens in the present study were stored for 3 weeks to simulate an aged restoration. In addition the specimens were stored in water. The bonding potential of ‘wet matured’ composite has been shown to be lower than ‘dry matured’ composite (Causton, 1975). Polishing the surface of the composite to be repaired has also been shown to cause a reduction in bonding ability (Davidson et al., 1981; Chiba, 1983). The effect of heat during the polishing procedure influences polymerization and not only is this surface denuded of chemically reactive groups (Davidson et al., 1981) but the cut surface of the tiller is probably inadequately wetted by the added resin (Vankerckhoven et al., 1982). Surface debris created by polishing may also affect bonding (Boyer et al., 1978; Miranda er al., 1984). It is common clinical practice, however, to freshen the surface of an existing composite restoration to remove stained and roughened material and this is the reason the surface to be repaired was disked before the repair was placed. Causton (1975) considered that differences in the bond strength of composite repairs were related to the rheology of the curing mix, the least viscous material being better able to wet the surface of the adherend. Herculite was found to be more viscous than either P50 or Occlusin, and P50 was only slightly less viscous than Occlusin. This could account, in part for the differences in performance with the untreated groups. Scotchbond was most effective in promoting bonding of the composite additions for the P50 and Herculite materials. Azarbal etal. (1986) found that Scotchbond was more efficient than a silane coupling agent in increasing the transverse strength of repaired composites and this was considered to be due to the polar nature of the phosphate groups which may contribute to bonding with the inorganic tiller of composites. Lloyd and Dhuru (1985) found that bonding the repaired composite was also enhanced by Scotchbond, but only if the surface had been previously contaminated with saliva. The finding that the Scotchbond-repaired groups performed least well with Occlusin may be explained by the fact that the resin of this composite consists mainly of urethane dimethacrylate. Scotchbond is a halogenated phosphate ester of BisGMA, which reacts best chemically with Bis-GMA resincontaining composites. Silane treatment did not improve the fatigue resistance of any of the materials. It is possible that the polishing procedure covered exposed filler particles with smear debris which prevented complete silanation of these particles taking place. The poor performance of the
162
J. Dent. 1990; 18: No. 3
silane-treated and untreated Herculite is difficult to explain, but it may be due to a combination of factors of poor reactivity of the surface, difficulties in wetting of the repair and the presence of smear layer preventing adequate bonding. The results of the effects of impact fatigue on the full mould sized specimens showed that Occlusin performed significantly better than the other materials. This is contradictory to the results published by Drummond (1987) who found Herculite to have an increased resistance to stress corrosion. The durability of posterior composites under repeated stress has also been studied by Horie and Hosoda (1987). These workers concluded that the durability of submicron and microfilled resins under stress will be lower than the estimated value for hybrid composites, providing the compressive strength was the same. In conclusion, the results of this study indicate that, for the composite resins investigated, resistance to fatigue forces was lower in repaired specimens than in complete specimens. The use of Scotchbond Dual Cure bonding agent gave the strongest repair.
References Ashby M. F. and Jones D. R. H. (1980) Fatigue failure. In: Engineering Materials: An Introduction to their Properties and Applications. Oxford, Pergamon, pp. 135-142. Azarbal P., Boyer D. B. and Chan K. C. (1986) The effect of bonding agents on the interfacial bond strength of repaired composite. Dent Mater. 2, 153-155. Boyer D. B., Chan K. C. and Torney D. L. (1978) The strength of multilayer and repaired composite resin. J. Prosthet. Dent. 39,63-67. Causton B. E. (1975) Repair of abraded composite fillings. An in vitro study. Br. Dent. J. 139, 286-288. Chalkley Y. and Chan D. C. N. (1986) Microleakage between light-cured composites and repairs. J. Prosthet Dent. 56, 441-444.
Chan K. C. and Boyer D. B. (1983) Repair of conventional and microfilled composite resins. .I. Prosthet. Dent. 50, 345-350. Chiba K (1983) Adhesion of the subsequently added composite resin. J. Dent Res. 62, (Abstr. 38). Davidson C. L., Duysters P. P. E., De Lange et al. (1981) Structural changes in composite surface material after dry polishing. J. Oral Rehabil. 8,431-439. Dieter G. E. (1961) Mechanical Metallurgy. New York, McGraw-Hill, pp. 446-449. Draughn R. A (1979) Compressive fatigue limits of composite restorative materials. J. Dent. Rex 58, 1093-1096. Drummond J. L. (1987) Cyclic fatigue and stress corrosion of composite restorative materials. J. Dent. Res. 66, (Abstr. 611). Forsten L. and Valiaho M-L. (1971) Transverse and bond strength of restorative resins. Acta Odontol. Stand. 29, 527-537. Gurney T. R. (1965) Fatigue testing. In: Fatigue of Welded Structures. Cambridge, Cambridge University Press. p. 5. Horie K. and Hosada H. (1987) Durability of posterior composites under repeated stress. J. Dent. Res. 66, (Abstr. 610). Johnson B. E. (1972) Effect of impact loading upon Class II amalgam restorations. J. Dent. Child. 39, 206-214. Lloyd C. H., Baigrie D. A. and Jeffrey I. W. (1980) The tensile strength of composite repairs. J. Dent. 8, 171-177. Lloyd C. H. and Dhuru V. B. (1985) Effect of a commercial bonding agent upon the fracture toughness (Ks) of repaired heavily tilled composite. Dent. Mater. 1, 83-85. Miranda F. J., Duncanson M. G. and Dilts W. E. (1984) Interfacial bonding strengths of paired composite systems. J. Prosthet. Dent. 51, 29-32. Osgood C. C. (1970) Fatigue Design. New York, Wiley. Reisbick M. H. and Brodsky J. F. (1971) Strength parameters of composite resins. J. Prosthet. Dent. 26, 178-185. Saunders W. P. (1987) The effect of fatigue impact forces upon the retention of various designs of resin-retained bridgework Dent. Mater. 3, 85-89. Soderholm K-J. M. (1986) Flexural strength of repaired dental composites. Stand. J. Dent. Res. 94, 364-369. Vankerckhoven H., Lambrechts P., Van Beylen M. et al. (1982) Unreacted methacrylate groups on the surfaces of composite resins. J. Dent. Res. 61, 791-795.
Book Review Oral Medicine: Pocket Picture Guide. Philip John Lamey and Michael A. 0. Lewis. Pp. 78. 1988. London, Gower Medical. Softback, f 5.95. This pocket picture guidebook consists of 146 figures, mostly colour photographs, depicting clinical appearances of oral and dental pathological conditions, with a few radiographs and microphotographs. Each figure is accompanied by a legend describing the salient features of the condition depicted. Differential diagnosis, investigative procedures and treatment are not included. The different pathological conditions are presented under
1 1 headings which relate to site in the mouth or aetiology. It would have been more useful to have all the conditions by site because this is how patients present. The illustrations are clear and of good quality, the legends concise and to the point, and there is a comprehensive subject index at the end of the book which is useful. It is not clear at whom the book is aimed. Undergraduate and postgraduate dental students will find it to be of limited use because of lack of relevant information, but general dental practitioners interested in oral medicine may find it more interesting. M. Basu
J. Dent. 1990;
18: 158-l
62
Effect of fatigue upon the interfacial bond strength of repaired composite resins W. P. Saunders Department of Conservative Dentistry, Glasgow Dental Hospital and School, UK
ABSTRACT A comparative study of the fatigue limits of repaired samples of three composite resins was undertaken using the staircase technique. Following storage in water for 21 days at 23°C the surfaces of the specimens to be bonded were ground with a fine Soflex disc, washed and dried. The surface was treated with either Scotchbond or a silane coupling agent, or left untreated. and a composite repair of the same material added. The specimens were thermally cycled and tested sequentially for 5000 cycles at different stress instruments using a transverse impact load. A statistically valid value for mean fatigue limit was determined for each design. This showed that resistance to fatigue forces was lower in repaired specimens than in complete specimens. The use of Scotchbond Dual Cure bonding agent gave the strongest repair. KEY WORDS: Composite resins, Technique, Fatigue testing J. Dent. 1990; 18: 158-l 1990)
62 (Received 27 September 1989;
reviewed 3 November 1989; accepted 14 February
Correspondence shouldbe addressed to: Dr W. P. Saunders, Department of Conservative Dentistry, Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow G2 3JZ. UK.
INTRODUCTION In recent years there has been an increase in the use of composite resins for restoring posterior teeth. It may be
necessary to remove and replact old composite restorations as areas become worn or discoloured and the most conservative treatment for this problem is the replacement of the unsatisfactory part of the restoration with a repair. It has been shown in vitro, however. that the strength of repaired composite is reduced compared with complete specimens (Forsten and Valiaho, 1971; Reisbick and Brodsky, 1971; Causton. 1975; Boyeretal., 1978;Lloyd et al., 1980; Chan and Boyer, 1983; Miranda et al., 1984; Azarbal et al., 1986; Soderholm. 1986). Microleakage has also been detected at the interface between composite and repairs (Chalkley and Chan, 1986). This leakage leads to deterioration of the bond and accelerates failure. It is expected that such repairs should be able to withstand the forces of mastication. These forces may be considered to be impact in nature with a cyclic component. The effect of loading on the teeth is thus a function of volume under stress (Johnson, 1972). When a structure is subjected to repeated stress cycles it may fail at stresses well below the tensile strength by a process known as fatigue (Ashby and Jones, 1980). 0 1990 Butterworth-Heinemann 0300-5712/90/030158-05
Ltd.
Fatigue life may be defined as the number of stress cycles a material will withstand before it fails. At high stresses failure occurs after a low number of cycles but at low stresses failure will occur after a large number of cycles. Below a certain value, the fatigue limit, the material can be subjected to a very large number of cycles without failing. The effect of the fatiguing influence of masticatory load upon repaired composite resin has not been reported. The purpose of this study was to examine the effects of impact fatigue forces upon the transverse strength of repaired posterior composite.
MATERIALS
AND METHOD
Three composite resins were selected for investigation, P50 (3M Co., St Paul, MN, USA), Occlusin (ICI Dental, Macclesfield, UK) and Herculite XR (Kerr, Romulus, MI, USA). These materials are intended for use in posterior restorations and consist of fillers within resin matrices. The principal filler in Herculite XR and Occlusin is a barium glass, and P50 contains zirconia and silica fillers. The filler loading is similar for each material, 87.5 per cent by weight for P50,86 per cent by weight for Occlusin and
Saunders:
Fatigue of repaired composite
resins
159
Fig. 7. The fatigue testing machine with control box (A), steel beam (B), striking pin (P) and loading weight (W). _
75 per cent by weight for Herculite
XR. The average particle size of the filler in these composite resin materials varies between 0.6 urn for Herculite XR, 1.5 urn for P50 and 6 ym for Occlusin, although all the materials could be described as being of hybrid construction. The resin component consists of Bis-GMA in P50 and Herculite XR, and mainly urethane dimethacrylate in Occlusin. All materials are single-paste systems and are polymerized by the application of light. Specimen
preparation
Three groups of 15 rectangular specimens of each composite were made in a split brass mould that was 25 mm long. 3 mm wide and 3 mm deep. Half the mould was filled with a rubber blank of polyvinyl siloxane impression material. The base of the mould was covered with a mylar strip and composite was packed into the mould. The material was cured in two increments with a light source (Aristolite, Wright Health Co., Dundee, UK). The second increment was covered with a mylar strip and compressed with a glass slab prior to polymerization. Each increment was cured in two areas for 60 s each. The specimens were taken from the mould, any excess composite trimmed with a scalpel, and stored in deionized water at 23 “C for 3 weeks. The rubber blank was removed from the mould and full mould sized specimens were made with each of the two incremental layers of resin being polymerized in four areas for 60 s each. Seventeen full mould sized specimens were made for each composite and stored in deionized water at 23 “C for 4 weeks prior to testing. Immediately before adding new composite to create repaired specimens, the half specimens were dried and the surface to be bonded was smoothed with a fine Soflex disc (3M Co.. St Paul, MN, USA). The prepared surface was washed with water from a 3 in 1 syringe and dried with oilfree air. In the first group no other treatment was carried out to the prepared surface, in the second the surface was coated with a silane coupling agent, Scotchprime (3M
Co.), and in the final group the surface was coated with a layer of bonding agent, Scotchbond Dual Cure (3M Co.), thinned with air and light cured for 20 s. The specimens were then returned to the mould and the second half was added using the same brand of composite. Polymerization was again carried out in two increments, each increment being cured for 60 s in two areas. The repaired specimens of composite were removed from the mould and stored in deionized water for 24 h and then subjected to thermal cycling for 8 h through 37°C 55 “C, 37°C and 55°C with dwell times of 10 min at each temperature, prior to testing. Testing Fatigue testing was carried out using apparatus previously described (Saunders, 1987) the design of which was based upon the ‘walking beam’ machine (Gurney. 1965). A simple beam actuated by a motor-driven cam gives a cyclic stress to the specimen by way of a striking pin. The apparatus is shown in Fig. 1. The box (A) controls the operation of the machine and houses the switch gear for the motor, a digital revolution counter and a cut-out device for the motor when the specimen is broken. A steel beam (B) is attached to a metal base by means of a steel block and bolt so that rotation can occur in the vertical axis about this bolt. The free end of the beam rests upon a cam which, in turn, is attached to an AC-powered drive motor. The free end of the beam is fitted with a platform on to which weights can be added during the testing sequence. A striking pin (P), 2 mm in diameter, is situated over the specimen holder such that the force applied to the specimen was directly over the repair, and at 90” to the repaired surface. The force acting on the beam could be adjusted in 1.5 kg increments by the addition or removal of a 0.5 kg lead block from the end of the beam (w). The fatigue test was carried out using the staircase method. This has been shown to give a good measure of mean fatigue limit (Draughn, 1979). By this method a specimen is tested for a prescribed number of cycles at a
J. Dent. 1990;
160
18: No. 3
‘s 3
4.5-
.
.
6.0 -
.
.
.
0
RESULTS
0
An example of the sequence of failures and non-failures at different stress levels for the Herculite repaired with Scotchbond group is shown in Fig. 2. The analysis of data is based upon the least frequent event. In the example there were seven failures and eight non-failures. The mean fatigue limit was calculated by arranging the data as in Table 1. The lowest load level at which failure occurred is denoted by i = 0 the next level as i = 1 and so on. The mean fatigue limit was calculated using the formula:
0
y” ^: 3.0-
.
0
.
0
0
ar -$j 1.5-
0
0
s “0
1
2
3
4
5
6
7
Specimen
8
9
10
11
12
13
14
15
number
Fig. 2. Sequence of failures and non-failures for Herculite with Scotchbond group. W, Failures (n = 7); 0, non-failures (n = 8).
X=X+d
stress close to the estimated mean fatigue limit. If failure occurs within this number of cycles, then the next specimen is tested at a stress level lowered by a fixed increment. If failure does not occur, then the next specimen is tested at a stress level increased by the same increment. Each specimen is tested sequentially and the stress applied depends upon whether failure or nonfailure occurred with the previous specimen. The number of specimens required in this method is less than with other fatigue tests, a minimum of 15 specimens is required for accurate data analysis (Dieter, 1961). The stress acting upon each specimen when subjected to a cyclic fatigue force is complex, however, as the top surface of the repaired specimen will be in compression and the bottom surface in tension. In addition the elastic moduli of the materials was not known. It was decided, therefore, to compare the resistance of each material to such forces in terms of load levels rather than in terms of fatigue stress. The speed of cycling was set to 200 rev./min. This is faster than normal cyclic chewing speed and therefore represented a severe test for the materials. The specimen was placed such that the striking pin was directly over the centre of each specimen and, in those that had been repaired, the pin was positioned directly over the repair in the long axis of the interface between the old and new materials. The prescribed number of cycles was set at 5000. All specimens were tested when immersed in water at room temperature (23°C + 3°C). Table II. Transverse fatigue pretreatments
Treatment
limit
)
whereX is the lowest load level considered in the analysis and d is the load increment employed. The positive sign is used for non-failures and the negative sign for failures. The standard deviation was derived from the formula NB-A2
S = 1.62 d
+ 0.029 )
iv2
(
This formula is an approximation, but when AJB - AZ/ accurate. The standard deviations of fatigue limits in this study were calculated with this formula. The results for the fatigue tests are shown in Table ZZ. The mean fatigue limit of each group was compared using analysis of variance and Duncan’s Multiple Range Test with a significance level of 0.05. The full mould sized specimens of the P50 and Occlusin groups were more resistant to fatigue than the repaired specimens. The use of silane did not improve the fatigue resistance of the P50, but the Scotchbond-treated groups performed significantly better. There were no statistically IV2 is larger than 0.3, it is sufficiently
Table 1. Analysis of staircase test procedural data Load level Ml f.5 3
P50
i
ni
ini
i lli
2
2 3 2 N=ZE ni = 7
4
8
; A = Z ini = 7
; B = Z ini = 11
:,
(kg) of repaired
Specimens fn0.l
(“N + ;
composites
Occlusin
after various
Herculite
Whole
17
6.0
(0.22)
8.63
(1.74)
3.94*
No treatment Silane Scotchbond
15 15 15
2.89 C2.89 3.75
(0.67) (0.67) (0.37)
2.5 2.5 2.68
(0.41) (0.41) (0.57)
No result No result 3.75* (1.31)
Vertical lines connect groups that are not significantly different. Figures in parentheses are standard deviations. *Means showing no statistically significant difference.
(0.94)
Saunders:
significant differences between the repaired groups of the Occlusin. The untreated and silane-treated specimens of Herculite XR did not survive the staircase procedure. There was no statistically significant difference between the whole specimens of this material and the Scotchbond group. The unrepaired Occlusin group had a statistically significantly higher fatigue resistance than the P50 which, in turn, had a significantly higher limit than Herculite. The mode of failure of all the repaired specimens was interfacial through the repaired surface.
DISCUSSION The results show that repaired composite samples are less resistant to impact fatigue stress compared with the whole specimens. These values varied from between 29 per cent of the fatigue load applied to the whole samples, in the Occlusin repaired with silane, and with no treatment, to 95 per cent for the Herculite after repair with Scotchbond. This does not include, however, the Herculite repairs made without treatment and with silane where no result was obtained. It is uncertain whether the strength of repaired specimens would be sufficient to survive the fatigue effects of mastication. The data recorded for the mean fatigue limit in this study may be applied only to specimens of the dimensions tested. The reason for this is that calculations were based upon the load applied to the specimen and no account was made of the area of repair. The study was thus comparative in nature. Such tests are frequently applied in fatigue studies and are considered to be of benefit (Osgood, 1970). Five thousand cycles is a small number compared with the number of stress cycles a repaired composite would be required to withstand in tivo. To withstand cyclic stresses in excess of 5000 cycles, it could be argued that the applied load must be less than the mean fatigue limit. On examination of the failed specimens there was no evidence of damage to the surface of the composite as a result of the impact of the pin. Clinically, surface damage may be caused as a result of the increased number of masticatory cycles and this could affect adversely the fatigue resistance of the material. Factors which are most likely to influence the bond between composite and its repair include the ability to wet the surface of the material, the amount and chemical activity of available resin for bonding and the effect of surface treatment prior to repair. The amount of resin present on the repaired surface for adequate bonding has been shown to be important (Lloyd and Dhuru, 1985) the more resin present the better the bond. Vankerckhoven et al. (1982) studied the effect of various parameters on the number of unreacted methacrylate groups on the surfaces of composite resins. They found that all types of surface treatment and manipulative variables reduced the number of these unreacted groups. In addition the degree of saturation of microtilled
Fatigue of repaired composite
resins
161
composites makes the adhesion between matrix and repair questionable. In the present study all the composites used had a high tiller to resin ratio and contained a matrix of very small particle size. This could have affected adversely the ability of each composite to repair to itself. It is also considered that the age of the composite has a detrimental effect on its reactivity, the first 24 h providing the greatest reactivity at the surface. The specimens in the present study were stored for 3 weeks to simulate an aged restoration. In addition the specimens were stored in water. The bonding potential of ‘wet matured’ composite has been shown to be lower than ‘dry matured’ composite (Causton, 1975). Polishing the surface of the composite to be repaired has also been shown to cause a reduction in bonding ability (Davidson et al., 1981; Chiba, 1983). The effect of heat during the polishing procedure influences polymerization and not only is this surface denuded of chemically reactive groups (Davidson et al., 1981) but the cut surface of the tiller is probably inadequately wetted by the added resin (Vankerckhoven et al., 1982). Surface debris created by polishing may also affect bonding (Boyer et al., 1978; Miranda er al., 1984). It is common clinical practice, however, to freshen the surface of an existing composite restoration to remove stained and roughened material and this is the reason the surface to be repaired was disked before the repair was placed. Causton (1975) considered that differences in the bond strength of composite repairs were related to the rheology of the curing mix, the least viscous material being better able to wet the surface of the adherend. Herculite was found to be more viscous than either P50 or Occlusin, and P50 was only slightly less viscous than Occlusin. This could account, in part for the differences in performance with the untreated groups. Scotchbond was most effective in promoting bonding of the composite additions for the P50 and Herculite materials. Azarbal etal. (1986) found that Scotchbond was more efficient than a silane coupling agent in increasing the transverse strength of repaired composites and this was considered to be due to the polar nature of the phosphate groups which may contribute to bonding with the inorganic tiller of composites. Lloyd and Dhuru (1985) found that bonding the repaired composite was also enhanced by Scotchbond, but only if the surface had been previously contaminated with saliva. The finding that the Scotchbond-repaired groups performed least well with Occlusin may be explained by the fact that the resin of this composite consists mainly of urethane dimethacrylate. Scotchbond is a halogenated phosphate ester of BisGMA, which reacts best chemically with Bis-GMA resincontaining composites. Silane treatment did not improve the fatigue resistance of any of the materials. It is possible that the polishing procedure covered exposed filler particles with smear debris which prevented complete silanation of these particles taking place. The poor performance of the
162
J. Dent. 1990; 18: No. 3
silane-treated and untreated Herculite is difficult to explain, but it may be due to a combination of factors of poor reactivity of the surface, difficulties in wetting of the repair and the presence of smear layer preventing adequate bonding. The results of the effects of impact fatigue on the full mould sized specimens showed that Occlusin performed significantly better than the other materials. This is contradictory to the results published by Drummond (1987) who found Herculite to have an increased resistance to stress corrosion. The durability of posterior composites under repeated stress has also been studied by Horie and Hosoda (1987). These workers concluded that the durability of submicron and microfilled resins under stress will be lower than the estimated value for hybrid composites, providing the compressive strength was the same. In conclusion, the results of this study indicate that, for the composite resins investigated, resistance to fatigue forces was lower in repaired specimens than in complete specimens. The use of Scotchbond Dual Cure bonding agent gave the strongest repair.
References Ashby M. F. and Jones D. R. H. (1980) Fatigue failure. In: Engineering Materials: An Introduction to their Properties and Applications. Oxford, Pergamon, pp. 135-142. Azarbal P., Boyer D. B. and Chan K. C. (1986) The effect of bonding agents on the interfacial bond strength of repaired composite. Dent Mater. 2, 153-155. Boyer D. B., Chan K. C. and Torney D. L. (1978) The strength of multilayer and repaired composite resin. J. Prosthet. Dent. 39,63-67. Causton B. E. (1975) Repair of abraded composite fillings. An in vitro study. Br. Dent. J. 139, 286-288. Chalkley Y. and Chan D. C. N. (1986) Microleakage between light-cured composites and repairs. J. Prosthet Dent. 56, 441-444.
Chan K. C. and Boyer D. B. (1983) Repair of conventional and microfilled composite resins. .I. Prosthet. Dent. 50, 345-350. Chiba K (1983) Adhesion of the subsequently added composite resin. J. Dent Res. 62, (Abstr. 38). Davidson C. L., Duysters P. P. E., De Lange et al. (1981) Structural changes in composite surface material after dry polishing. J. Oral Rehabil. 8,431-439. Dieter G. E. (1961) Mechanical Metallurgy. New York, McGraw-Hill, pp. 446-449. Draughn R. A (1979) Compressive fatigue limits of composite restorative materials. J. Dent. Rex 58, 1093-1096. Drummond J. L. (1987) Cyclic fatigue and stress corrosion of composite restorative materials. J. Dent. Res. 66, (Abstr. 611). Forsten L. and Valiaho M-L. (1971) Transverse and bond strength of restorative resins. Acta Odontol. Stand. 29, 527-537. Gurney T. R. (1965) Fatigue testing. In: Fatigue of Welded Structures. Cambridge, Cambridge University Press. p. 5. Horie K. and Hosada H. (1987) Durability of posterior composites under repeated stress. J. Dent. Res. 66, (Abstr. 610). Johnson B. E. (1972) Effect of impact loading upon Class II amalgam restorations. J. Dent. Child. 39, 206-214. Lloyd C. H., Baigrie D. A. and Jeffrey I. W. (1980) The tensile strength of composite repairs. J. Dent. 8, 171-177. Lloyd C. H. and Dhuru V. B. (1985) Effect of a commercial bonding agent upon the fracture toughness (Ks) of repaired heavily tilled composite. Dent. Mater. 1, 83-85. Miranda F. J., Duncanson M. G. and Dilts W. E. (1984) Interfacial bonding strengths of paired composite systems. J. Prosthet. Dent. 51, 29-32. Osgood C. C. (1970) Fatigue Design. New York, Wiley. Reisbick M. H. and Brodsky J. F. (1971) Strength parameters of composite resins. J. Prosthet. Dent. 26, 178-185. Saunders W. P. (1987) The effect of fatigue impact forces upon the retention of various designs of resin-retained bridgework Dent. Mater. 3, 85-89. Soderholm K-J. M. (1986) Flexural strength of repaired dental composites. Stand. J. Dent. Res. 94, 364-369. Vankerckhoven H., Lambrechts P., Van Beylen M. et al. (1982) Unreacted methacrylate groups on the surfaces of composite resins. J. Dent. Res. 61, 791-795.
Book Review Oral Medicine: Pocket Picture Guide. Philip John Lamey and Michael A. 0. Lewis. Pp. 78. 1988. London, Gower Medical. Softback, f 5.95. This pocket picture guidebook consists of 146 figures, mostly colour photographs, depicting clinical appearances of oral and dental pathological conditions, with a few radiographs and microphotographs. Each figure is accompanied by a legend describing the salient features of the condition depicted. Differential diagnosis, investigative procedures and treatment are not included. The different pathological conditions are presented under
1 1 headings which relate to site in the mouth or aetiology. It would have been more useful to have all the conditions by site because this is how patients present. The illustrations are clear and of good quality, the legends concise and to the point, and there is a comprehensive subject index at the end of the book which is useful. It is not clear at whom the book is aimed. Undergraduate and postgraduate dental students will find it to be of limited use because of lack of relevant information, but general dental practitioners interested in oral medicine may find it more interesting. M. Basu
