Three-dimensional finite element analysis of strength and adhesion of composite resin versus ceramic inlays in molars.

STATEMENT OF PROBLEM: Previous studies on strength of teeth reconstructed with ceramic or composite resin inlays have not resolved which restoration material provides the highest strength and marginal integrity.

PURPOSE: The purpose of this study was to compare strength of mandibular molars restored with composite resin inlays to those restored with ceramic inlays, according to the Mohr-Coulomb failure criterion, and to analyze contact stresses in cement-tooth adhesive interfaces of these inlays.

MATERIAL AND METHODS: The investigation used a 3-dimensional (3-D) finite element analysis with the use of contact elements. Seven 3-D models of first molars of the same shape and size were created: IT, intact tooth; UT, unrestored tooth with an MOD cavity preparation; CRIT, tooth restored with composite resin inlays (True Vitality) with an elastic modulus equal to 5.4 GPa; CRIH, tooth restored with composite resin inlays (Herculite XRV) (9.5 GPa); CRIC, tooth restored with composite resin inlays (Charisma) (14.5 GPa); CRIZ, tooth restored with composite resin inlays (Z100) (21 GPa); and CI, tooth restored with a ceramic (IPS Empress) inlay with an elastic modulus equal to 65 GPa. Each model was subjected to a force of 200 N directed to the occlusal surface. The stresses occurring in the tested inlays, composite resin cement layer, and tooth tissues were calculated. To evaluate the strength of materials, the Mohr-Coulomb failure criterion was used. Contact stresses in the cement-tissue adhesive interface were calculated and compared to tensile and shear bond strength of the luting cement to enamel and dentin.

RESULTS: In the teeth restored with composite resin and ceramic inlays, the values of the Mohr-Coulomb failure criterion were lower than in the unrestored tooth with a preparation (UT), but still 2.5 times higher than in the intact tooth (IT). For the ceramic inlay (CI), the values of the Mohr-Coulomb failure criterion were nearly 3 times higher than in the composite resin inlays. For the luting agent for the ceramic inlay model, these values were 2-4 times lower than for the luting agents for the composite resin inlay models. At the adhesive interface between the cement and tooth around the ceramic inlays, contact tensile and shear stresses were lower than around the composite resin inlays. In the cervical enamel surrounding the proximal surface of the inlays, the stresses exceeded the tissue strength.

CONCLUSIONS: Adhesively bonded composite resin and ceramic inlays reinforce the structure of prepared teeth, but do not restore their original strength. The proximal enamel surrounding inlays is prone to failure. The value of the Mohr-Coulomb failure criterion for ceramic inlays was higher than for composite resin inlays. With an increase in the elastic modulus of inlay materials, the values of the Mohr-Coulomb failure criterion decrease in the luting cement. Contact tensile and shear stresses on the cement-tissue adhesive interface decrease as well.