Local stresses and bone adaption around orthopedic implants

The severest long-term complications of orthopedic joint-prostheses are associated with loosening of the implant-bone interface, due to adverse reactions in bone and subsequent weakening of the connection between the two materials. Mechanical stresses, caused by joint loading, play a key role in the adaption of interface bone and in the loosening process. In the present study, it is shown how local interface stress patterns can be evaluated using advanced methods of computer-based, engineering stress analyses (the Finite Element Method, FEM). The stress patterns are compared with the local bone structure and bone resorption phenomena as found in animal experiments. The results indicate that loosening and bone resorption is associated with high peak stresses at the interface in the immediate post-operative stage. In addition, there appears to be similarity between the local stress patterns and the bone morphology at the interface if resorption does not occur. Finally, it is found that implants of high local stiffness generate lower peak stresses in bone, as compared with low stiffness implants.