QUANTITATIVE-ANALYSIS OF BONE REACTIONS TO RELATIVE MOTIONS AT IMPLANT BONE INTERFACES

Connective soft tissues at the interface between implants and bone, su ch as in human joint replacements, can endanger the stability of the i mp]ant fixation. The potential of an implant to generate interface bon e resorption and form soft tissue depends on many variables, including mechanical ones. These mechanical factors can be expressed in terms o f relative motions between bone and imp]ant at the interface or deform ation of the interfacial material. The purpose of this investigation w as to determine if interface debonding and subsequent relative interfa ce motions can be responsible for interface degradation and soft tissu e interposition as seen in experiments and clinical results. A finite element computer program was augmented with a mathematical description of interface debonding, dependent on interface stress criteria, and s oft tissue interface interposition, dependent on relative interface mo tions. Three simplified models of orthopaedic implants were constructe d: a cortical bone screw for fracture fixation plates, a femoral resur facing prosthesis and a straight stem model, cemented in a bone. The p redicted computer configurations were compared with clinical observati ons. The computer results showed how interface disruption and fibrous tissue interposition interrelate and possibly enhance each other, wher eby a progressive development of the soft tissue layer can occur. Arou nd the cortical bone screw, the predicted resorption patterns were rel atively large directly under the screw head and showed a pivot point i n the opposite cortex. The resurfacing cup model predicted some fibrou s tissue formation under the medial and lateral cup rim, whereby the m edial layer developed first because of higher initial interface stress es. The straight stem model predicted initial interface failure at the proximal parts. After proximal resorption and fibrous tissue interpos ition, the medial interface was completely disrupted and developed an interface layer. The distal and mid lateral side maintained within the strength criterion. Although the applied models were relatively simpl e, the results showed reasonable qualitative agreement with resorption patterns found in clinical studies concerning bone screws and the res urfacing cup. The hypothesis that interface debonding and subsequent r elative (micro)motions could be responsible for bone resorption and fi brous tissue propagation is thereby sustained by the results.