The effect of proximal contour on marginal ridge fracture of Class II composite resin restorations

Objectives To compare the marginal ridge fracture strength of Class II composite resin restorations placed with a straight or contoured matrix band using composite resins with different modulus of elasticity. Methods In 60 artificial first molars standardized MO-preparations were ground. Two matrix systems were used: (1) A straight matrix (Standard Tofflemire Matrix, KerrHawe) in 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. Three composite resins together were used (Filtek Supreme: e-modulus 13.3 GPa (3 M ESPE), Clearfil AP-X: 16.6 GPa (Kuraray) and Clearfil Majesty Posterior: 22.0 GPa (Kuraray)), resulting in six groups (n = 10). Teeth were mounted into a MTS servo hydraulic testing machine (Mini Bionix II, MTS, USA) with 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 Schefféʹs post hoc test for multiple comparison of groups (p < 0.05). Results Contoured proximal surfaces (365.5 ± 87.6 N) resulted in significant stronger marginal ridges compared to straight surfaces (290.5 ± 64.2 N) (p < 0.001). Clearfil AP-X (378.1 ± 94.63 N) provided a higher resistance to fracture than Filtek Supreme (301.4 ± 67.3 N) (p = 0.001) and Clearfil Majesty Posterior (304.5 ± 70.6 N) (p = 0.002). No differences were found between Filtek Supreme and Clearfil Majesty Posterior (p = 0.890). Conclusion Within the limitations of this in vitro study it was shown that use of a contoured matrix results in a stronger marginal ridge of a Class II composite resin restoration.