[An in vitro study of wear and marginal fracture of posterior composite resins].

The collision-and-abrasion test was performed to reproduce the change of the surface structures of posterior composite resins by the stress of mastication and occlusion. Also the effect of the stress on wear and marginal fracture of composites was estimated qualitatively and quantitatively between the cavities with round bevel, straight bevel and butt joint. Extracted human posterior teeth were used as materials, and round or straight bevels were prepared along the margin of the standardized cavities using the bevel-preparing burs which had been designed by us. Light cured posterior composite resin (Occlusin/ICI Co.) was used as the restoration material. Also the surface structures were examined before and after the collision-and-abrasion test with the scanning electron microscope and laser measuring device for the surface morphology. The following results were obtained. 1. The collision-and-abrasion test with use of the slurry of polymethyl methacrylate (PMMA) powder as abrasive showed better reproducing ability of the in vivo change of the posterior composite restorations. 2. The collision stress was found to be one of the main causes for the wear and marginal fracture of composites. 3. It was found that the measurement for wear and marginal fracture of restorations could be performed three-dimensionally with high precision and in shorter time by using a laser device. 4. Marginal fracture resistance of composite resins is influenced by the adhesion with tooth structure and the marginal thickness. Therefore, the best marginal shape for posterior composite restoration is the round bevel.