Computer-aided implant positioning in knee endoprosthetics. Kinematic analysis to optimize the operation technique

Modern approaches in knee endoprosthetics using computer-assisted navigatio n systems are based on matching procedures which refer to osseous anatomica l landmarks. However, surface replacement techniques require a correct posi tion of the implants in relation to the bone, as well as an optimal postope rative interaction of the ligaments and the surface design of the endoprost hesis. Functional aspects are of elementary importance. Insufficient design of the prosthetic surface or incorrect implantation ove rstresses ligamentous structures and result in functional disturbance, incr eased wear, and early loosening of the components. The present computer mod el permits a quantitative analysis of different implant positions. Distal p ositioning of the femoral component or insufficient resection of the tibial plateau results in a lag of extension, increased distal resection of the t ibial plateau produces marked translatory instabilities in the sagittal pla ne which exceed the absolute height of the resection in millimeters. Monoco ndylar replacements with preservation of the ACL react kinematically more s ensitively than bicondylar endoprostheses sacrificing the ACL. The results of this study are systematically summarized in an algorithm and indicate correction steps in order to avoid functional impairment. The com puter model delivers in addition some basic data for optimized navigation p rocedures in knee surgery and demonstrates the importance of further develo pments in custom-made endoprosthetics.