A three-dimensional boundary element-finite element analysis of the porous coating-bone interface
Description
Stable fixation of total joint replacements relative to the host bone is essential if they are to function as designed. This is especially true for porous coated implants used without bone cement. The failure of these implants to achieve stable fixation is an important issue in orthopaedics and unfortunately the mechanisms that cause the failure are poorly understood. However, it is thought that the loads at the bone-implant interface are an important factor in achieving implant fixation. The bone-implant interface of porous coated implants is very complex, consisting of many smaller structures. This study uses computational and large scale experimental models to characterize the loads on these structures at the interface A three-dimensional coupled boundary element-finite element analysis program, BEFE3DELS, was written to study the computational models. It uses both boundary elements and finite elements in the models. Boundary elements were used to model the porous coating of the implant. This meant that only the surfaces, not the whole volume, of the complex structures in the porous coating had to be meshed. This made it easier to construct the models and made them smaller The experimental models consisted of 15 mm steel balls soldered to a steel plate to form a large scale porous coating similar to those on actual implants. They were used to provide validation of the computational models. There was a strong correlation (r$\sp2$ = 0.851) between experimental and computational strains. The results from the computational and experimental models indicate that the mechanical environment at the interface of porous coated implants is very non-homogeneous. There are areas of both strain intensification and strain relief on the substrate of the implant. The largest strains were found at the ball-plate and ball-ball sinternecks. The strains there were found, with the computational models, to be two to three times the far field strains. This study has demonstrated the steps needed to develop valid three-dimensional computational models of the porous coating-bone interface. Loads on the porous coating that will likely lead to its mechanical failure have been observed in the models