Finite element stress analysis of three filling techniques for class V light-cured composite restorations.

An important disadvantage of current dental resin composites is polymerization shrinkage. This shrinkage has clinical repercussions such as sensitivity, marginal discoloration, and secondary caries. The objective of this study was to compare three filling techniques in terms of the transient stresses induced at the resin composite/tooth interface during polymerization. The techniques were: bulk filling (B), three horizontal increments (HI), and three wedge increments (WI). A simple Class V cavity preparation was modeled in finite element analysis. Polymerization shrinkage was simulated by a thermal stress analogy, thereby causing 1% shrinkage due to an arbitrary coefficient of thermal expansion. Interface normal and shear stresses were calculated at nine steps during polymerization, proceeding from 0% to 100% volume of cured resin. The importance of the interface transient stresses was revealed by the finding that, in most cases, their peak values exceeded the final or residual stress. Also, the WI and B techniques consistently exhibited the highest and lowest maximum transient stresses, respectively. These results from the simple model of a Class V restoration suggest that bulk filling of light-cured resin composites should be used in restorations which are sufficiently shallow to be cured to their full depth.

Cement-mantle thickness affects cement strains in total hip replacement.

Bone cement is commonly used to affix femoral implants to the bone during total hip reconstruction. Previous studies suggest that the expected life of a cemented femoral implant may depend on the thickness of the cement mantle surrounding the implant and the implant geometry. The purpose of this study was to determine whether different cement-mantle thicknesses and femoral stem sizes affected strain patterns in the bone cement around cemented femoral stems. Two different sizes of cobalt-chromium stems were cemented into composite femora with varying cement-mantle thickness. Strain gages were embedded in the cement mantle and the implanted stems were loaded axially and under conditions simulating walking and standing. An increase in stem size with the same cement-mantle thickness (approximately 2.2 mm) caused a 65% decrease in proximal medial cement strains. Increasing cement mantle thickness from 2.4 to 3.7 mm caused substantial strain reductions in the distal cement (40-49%). We conclude that increased cement-mantle thickness around femoral stems may increase the fatigue life of a bone-implant system by reducing peak strains within the cement.

Stress analysis of bone modeling response to rat molar orthodontics.

The purpose of this project was to determine if alveolar bone modeling could be associated with altered mechanical environment. Finite element stress analysis of an orthodontically tipped rat molar periodontium was performed. The distributions of mechanical components within the periodontal ligament and cortical bone were compared to the well-documented bone formation and resorption patterns in the alveolus of the tooth. It was concluded that in orthodontically induced bone modeling activity, locations of osteogenesis uniquely coincided with increased tension within the periodontal ligament, while bone resorption areas could be associated with increases in other components (minimum principal and maximum shear stresses, strain energy density, and von Mises) within the bone itself.

Mechanical simulation of the human mandible with and without an endosseous implant.

Clinical research has demonstrated that a high remodelling rate for cortical bone exists around a rigid endosseous implant. This phenomenon may be regulated by change in the mechanical environment. 3D finite element models of the human mandible with and without an endosseous implant have been created to investigate the mechanical environment adjacent to the left retromolar area where the ipsilateral implant was located. A bite force of 100 N was applied in the left premolar region. The mechanical environment before and after implantation were computed. The environment was characterized by the following parameters: the principal stresses, dilatational stress, and von Mises stress. The changes in these parameters due to the implantation were calculated. The results showed that the mechanical environment adjacent to the implant changed drastically due to the implant. The major changes in the mechanical parameters occurred adjacent to the bone-implant interface at the bony surface. The changes of the distribution of the mechanical parameters due to implantation were different. Implantation effects were local, and did not alter the overall mechanical environment.