Objectives . The C-Factor is normally used to predict the shrinkage stress at composite-tooth interfaces. A study was outlined for clarifying the elements which allows providing the stress at bonded restorations interface. In theory, the stress varies as a function of the opposition to the shrinkage (substrate stiffness), the compliance, dimensions and mechanical properties of the composite.

Materials and methods . Eight 3D non-linear cubic models of Class I cavities were obtained by the Finite Element Method (FEM), maintaining the volume (20 mm ³ ) and wall thickness (2 mm) constant and varying the C-Factor (1.9 through 13.5). A ‘ glue ’ contact between the composite and the teeth was simulated. The polymerization shrinkage, by thermal analogy of the composite, was 1%. Principal stresses and strains were analyzed. A method for calculating the local compliance by FEM was also idealized and its validation of comparing the obtained result by using this method with the analytical compliance value of a 1-mm dentin cube, fixed on its side at one of its faces. A method for calculating, for each cavity shape, the ‘useful volume’ of the composite, which is able to influence on increasing the stress peak at the interface, was further idealized. And finally, using elements mentioned before, a compliance dependent value, so-called C _A -Factor, was calculated for each cavity, at the peak and ‘useful volume’ of the composite regions. The capability on predicting the stress peak by the correlation test (Pearson) between the stress peak and the C _A -Factor as well as between the stress peak and the C-Factor was also evaluated.

Results . Stress peaks provoked by the shrinkage on Class I cavities tend to decrease as a function of the C-Factor, though the Pearson only revealed significance for the maximum principal stress at the XZ wall and Z axis. The local compliance calculation obtained by FEM was 0.0652 μm/N, which was very close to the analytical one (0.0666 μm/N). The C _A -Factor correlated with significance and positively with the three stresses obtained at the trihedral angle formed by two occlusal edges. (Von Mises, maximum principal and maximum shear stresses).

Conclusions . The increase of the C-Factor does not lead to the increase of the stress peaks in Class I cavities. The method for calculating the compliance is valid for complex structures. The C _A -Factor provides the stress peaks for bonded composites on Class I cavities and this new ‘index’ introduces two new concepts in the Adhesive Dentistry: the ‘useful volume of the composite’ and the ‘local compliance’.

Grant : FAPESP 2006/00186-3.