Correlation between fracture properties and clinical performance of composite resins in Class IV cavities Martin J. Tyas, BDS, PhD, GradDipHlthSc, FADM*
Key words: Clinical performance, composite resins, fracture properties. Abstract One hundred and two Class IV cavities were restored randomly with four composite resins. The restorationswere assessed at six months, one year, two years and three years for surface chipping, bulk fracture and incisal wear. The data were correlated with various mechanicalproperties of the materials. A significant correlation was found between surface chipping/bulk fracture and fracture toughness (P = 0.002),elastic modulus (P = 0.006) and tensile strength (P = 0.045). There was a trend towards an association between incisal wear and both elastic modulus and inherent flaw size. Fracture toughness may be a useful indicator of the clinical performance of composites in Class IV cavities.
IV cavities, the microfine materials may be more prone to fracture. Limited clinical evidence has been published in this respect; in one study of 435 re~torations,~ 22 chip fractures were found, all in Class IV cavities. Of the four fractures described as cohesive, all occurred in microfilled resins. T h e purpose of the present study was to determine if any correlation existed between fracture properties and clinical performance of four composite resins in Class IV cavities.
Introduction The commercial introduction of composite resins in the late 1960s, with improved mechanical properties compared with silicate cement, led to the widespread use of composites in the restoration of Class IV cavities. Currently available composites may be classified' as small particle (otherwise known as fine particle), hybrid, and microfilled. T h e microfilled materials are generally more brittle than small particle or hybrid products, as evidenced by their lower fracture t o ~ g h n e s s . Thus ~ . ~ it may be postulated that in highly stressed restorations such as those in Class
Materials and methods A total of 102 Class IV cavities was restored using four proprietary composite resins. T h e materials used are listed in Table 1, and the distribution of materials to cavities is listed in Table 2. Materials were used according to the manufacturers' instructions. Generally, a lingual lock was placed to aid retention. T h e use of dentine pins was avoided because of their weakening effect on the composite. Bevelling of cavity margins was not usually carried out, in order to conserve maximum enamel. Restorations were completed in one visit, Soflex discst being used for contouring and finishing. After finishing, labial colour transparency photographs were taken at 1:1 magnification of all restorations. Patients were recalled at six months, one, two and three years, and the restorations were examined. Any restorations which had sustained surface chipping or bulk fracture were recorded as failed and deleted from the study. T h e cumulative failure rate for each material at each time interval was calculated using a life table analysis.' Colour photographs at 1:l magnification were taken of intact restor-
'Former Director, Australian Dental Standards Laboratory. Currently Senior Lecturer, School of Dental Science, University of Melbourne.
t3M, St. Paul, Minnesota, USA
(Received for publication February 1988. Revised September 1988. Accepted September 1988.)
Australian Dental Journal 1990;35(1):46-9.
ations at three years, and the incisal edge of each restoration assessed against a standard set of photographs. A semi-quantitative measurement of incisal wear was thus obtained, where a score of zero indicated no wear, and a score of four indicated approximately 1 mm wear. The mean wear for each material was calculated, and a one-way analysis of variance applied to determine differences between materials. Marginal staining and colour match were also assessed from standard photographsY6and one-way analysis of variance applied. A regression analysis of various fracture properties3 (Table 3) against the three-year cumulative failure rate was carried out.
Table 1. Materials Name
Kulzer GmbH (West Germany) Kulzer GmbH (West Germany) Kulzer GmbH (West Germany) Johnson & Johnson (USA)
Microfine, self cure
Durafill Estilux Miradapt
Small particle, light cure Hybrid, self cure
Distribution of restorations
Upper incisors Upper canines Lower incisors Lower canines Totals
Microfine, light cure
21 1 3 0 25
25 3 1
17 4 1
19 3 2 2 26
Results The cumulative failure of restorations at each time interval is given in Table 4 and Fig. 1, and the mean incisal wear, marginal staining and colour match scores are given in Table 5.
Fracture properties of materials3 K,,
(MN.I~-'-~) 0.70 0.71 1.45 1.41
Estic MF Durafill Estilux Miradapt
E x 10-'O
0.42 0.31 1.33 1.33
105 146 149 138
0.17 0.17 0.27 0.38
K,, = Fracture toughness. E = Modules of elasticity. oT = Diametral tensile strength. G,, =Critical strain energy release rate. a,, =Inherent flaw size.
Table 4. Cumulative failure of restorations (per cent) Material
Estic MF Durafill Estilux Miradapt
40 42 5 9
There was no significant difference between the incisal wear, marginal discoloration and colour match of the four materials either for a given material at base line or between materials at the three-year time interval. It was evident, however, that there was a tendency towards the appearance of some marginal staining, and that products other than Miradapt became slightly lighter, over the three-year period.
lncisal wear (net), marginal discoloration (net) and colour match at three years ~
Estic MF Durafill Estilux Miradapt
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0 0.21 0.29
0.75 0.08 0.39
9.4 8.9 8.6 8.4
8.0 8.5 8.2 8.4
DURAFILL ESTlC MF
MIRADAPT ESTILUX I
Fig. 1. -Cumulative failure of restorations (per cent).
0' ' 0'0 6
1'0 112 FRACTURE TOUGHNESS
MN.ml 5 Fig. 2. -Regression of three-year cumulative failure (per cent) with fracture toughness (MN.m-' ".
Regression analysis showed a significant correlation between cumulative failure at three years and fracture toughness (Fig. 2; P = 0.002), elastic modulus (P = 0.006) and tensile strength (P = 0.045). There was some correlation, but not significant, between the incisal wear score and both elastic modulus (P=O.29) and inherent flaw size ( P = 0.30).
Discussion There is anecdotal evidence that the microfine composites are more susceptible to chip and bulk fracture than small particle or hybrid composites, when placed in anterior high stress areas such as Class IV cavities. The published evidence for this is, however, limited.4 In the report of Lambrechts et a1.,4 22 chip fractures were found in 455 compo40
site resin restorations; all chip fractures were in Class IV restorations, and 17 of the 22 were of microfine materials. No data were given on the number of Class IV restorations examined. Lambrechts et al.4 suggested that low tensile strength of the composites was associated with susceptibility to fracture. In the present study, there was a positive correlation between resin failure and each of tensile strength, elastic modulus and fracture toughness. The strongest association with failure was fracture toughness, which is intuitively reasonable and supports the proposal of Goldmad that at high stress levels materials with high fracture toughness will perform best, since this property is a measure of the brittleness of a material. The data of Fig. 2 should be interpreted with caution, since the points are near the limits of both axes. These data, Australian Dental Journal 1990;35:1.
however, show a clear difference in clinical failure between the microfilled products (Estic MF and Durafill) and the small particle/hybrid products (Estilux and Miradapt),with the latter two having a considerably lower failure incidence. There was essentially no difference in failure between the self-cure and light-cure products of the same type, i.e., Estic MF compared with Durafidl and Estilux compared with Miradapt. Fracture toughness values for these material pairs are similar, and the only obvious difference between the light and self-cure products is in the inherent flaw size, a,. Although there was no significant difference in incisal wear for the four materials, the clinical data suggest a trend towards Miradapt wearing the most and Durafill the least. The rank order of wear is similar, but not significantly associated with, the rank order of inherent flaw size (a,) and elastic modulus (E). Goldman3 suggested that in the low stress situation, materials with low a, should have longer lifetimes. If it is theorized that composites in Class IV cavities will wear if subject to low stress, but will fracture if subject to high stress, then the results of the present study are consistent with such a hypothesis. In a highly stressed restoration, a high K,, is required to prevent fracture; Estilux and Miradapt were found to have low fracture incidence and also had high K,, values. In a restoration subject to low stress, a low a, is required to prevent fatigue wear; Estic MF and Durafidl were found to wear the least and also had the lowest a, values. Durafill (a light cure material) had the least wear, as has also been found in a study of other light and self-cure composites in posterior teeth.' The marginal discoloration scores were not significantly different for the four materials. The highest score was Durafill, which may be because either the bond strength to enamel is less for microfilled materials than for other composites,8 or because of the difficulty of finishing the restoration at the limit of enamel e t ~ h i n g . ~ The colour of the materials, with the exception of Miradapt, became slightly but not significantly
Australian Dental Journal 1990;35:1.
lighter over the three-year period. For Estilux and Durafidl, this may be a result of the bleaching effect of the camphoroquinone photocatalyst system." The fracture and wear results of this study are based on a small number of restorations (maximum 29 for any product), and on extreme values of K,, and a,. The results do, however, support the general conclusions of GoldmanY3as discussed earlier. There exists, therefore, a basis for further study in this area, and it may ultimately be possible to prescribe a minimum value for K,, and a maximum value for a, in order to make specific recommendations for composites in various levels of intra-oral stress.
References 1. Lutz F, Setcos JC, Phillips RW, Roulet JF. Dental restorative resins. Types and characteristics. Dent Clin North Am 1983;27:697-712. 2. Lloyd CH, Mitchell L. The fracture toughness of tooth colored restorative materials. J Oral Rehabil 1984;11:257-72. 3. Goldman M. Fracture properties of composite and glass ionomer dental restorative materials. J Biomed Mater Res 1985;19:771-83. 4. Lambrechts P, Ameye C, Vanherle G. Conventional and microfilled composite resins. Part 11: Chip fractures. J Prosthet Dent 1982;48:527-38. 5. Tyas MJ, Beech DR. Clinical performance of three restorative materials for non-undercut cervical abrasion lesions. Aust Dent J 1985;30:260-4. 6. Tyas MJ, Burns GA, Byrne PF, Cunningham PJ, Dobson BC, Widdop FT. Clinical evaluation of Scotchbond: one year results. Aust Dent J 1986;31:159-64. 7. Wilder AD, May KN, Leinfelder KF. Three year clinical study of UV cured composite resins in posterior teeth. J Prosthet Dent 1983;50:26-30. 8. Boyer DB, Chalkley Y, Chan KC. Correlation between strength of bonding to enamel and mechanical properties of dental composites. J Biomed Mater Res 1982;16:775-83. 9. Tyas MJ. The restoration of fractured incisors in children; a comparative study of a conventional and a microfine material. Aust Dent J 1982;27:77-80. 10. Cook WD, Chong MP. Colour stability and visual perception of dimethacrylate based dental composite resins. Biomaterials 1985;6:257-64.
Address for correspondenceheprinrs: School of Dental Science, University of Melbourne, 71 1 Elizabeth Street, Melbourne, Vic., 3000.