Effect of proximal contour of composite restorations on fracture resistance

Objectives: To compare in vitro the marginal ridge fracture strength of Class II composite resin restorations inserted with help of a flat or contoured matrix band using composite resins varying in modulus of elasticity.

Methods: In 60 artificial first molars standardized MO-preparations including mechanical undercuts were ground. The dimension of the proximal box was 5.0 mm in width and 6.0 mm in height. One of two matrix systems were used: 1- A flat matrix (Standard Tofflemire Matrix, KerrHawe) in a Tofflemire retainer (Produits Dentaire). 2- A contoured matrix (Standard matrix, Palodent, Dentsply). In both groups, a wooden wedge and separation ring (Composi-Tight Gold, GDS) were placed and the matrix was burnished against the adjacent tooth. These techniques were combined with three composite resins (Filtek Supreme: modulus of elasticity 13.3 GPa (3M ESPE); Clearfil AP-X: 16.6 GPa (Kuraray); Clearfil Majesty Posterior: 22.0 GPa (Kuraray)) making six groups (n=10). Teeth were mounted into a MTS servo hydraulic testing machine (Mini Bionix II, MTS System Corp, USA) with the stylus placed on the marginal ridge. Samples were loaded at a crosshead speed of 1.0 mm/min until fracture occurred. Fracture resistance data were statistically analyzed using ANOVA and Scheffˇ post-hoc test for multiple comparison of groups (p<0.05).

Results: Contoured proximal surfaces (365.5 ± 6.0 N) resulted in significant stronger marginal ridges compared to flat surfaces (290.5 ± 11.7 N) (p<0.001). Clearfil AP-X (378.1 ± 21.1 N) provided a higher resistance to fracture than Filtek Supreme (301.4 ± 15.1 N) (p=0.001) and Clearfil Majesty Posterior (304.5 ± 15.8 N) (p=0.002). No differences were found between Filtek Supreme and Clearfil Majesty Posterior (p=0.890).

Conclusion: Use of a contoured matrix results in a stronger marginal ridge of a Class II composite resin restoration. A higher modulus of elasticity does not necessarily lead to a higher fracture resistance.