Deformation of restoration and fracture of luting cement film

Objectives: Functional occlusal loading can generate complicated stresses in the luting cement film between a restoration and its abutment. Such stresses may lead to cement fractures, which can promote cement dissolution and induce clinical problems. The purpose of this study is to determine the function of the cement film under loading, and to clarify the relationship between the deformation of the restoration and the cement fractures. Methods: Eccentric loads were applied onto model extension bridges which were cemented on brass dies with zinc phosphate cement. Strain measurements were recorded using of a strain gauge method, ultimate strength by tensile tests, leakage by dye penetration tests and cement fractures by ultrasonic microscopic analyses. Additionally, the bridges which were just seated on their dies without cementation were loaded, and strain measurements were recorded. Results: The cases for linear strain increment with loading showed high tensile strength and minimal dye penetration. The cases with non-linear strain behavior showed low tensile strength and considerable dye penetration. However, no fracture was observed in the occlusal cement film in both cases. The cement fracture first occurred at the opposite marginal region to the loading side. Conclusions: One of the significant functions of the cement film is diminishing the deformation of restorations during loading. The deformation of the restoration affects initiation and propagation of the cement fracture considerably.