Four failed polycentric total knee prostheses were evaluated by clinical
examination of the patients and surface analysis of the removed prosthetic
components. The failures were secondary to a poor relationship of the
prosthetic components in three cases and to abutment of the tibial spine
against the femoral condyle in the fourth. A poor relationship of the
prosthetic components was not necessarily secondary to malpositioning,
because the positioning may have been accurate but the relationships became
abnormal when the knee was unstable. Scanning electron-microscope
examination revealed various wear patterns but no significant correlation
was noted between the morphological type and the degree of wear. Accurate
placement of the components and restoration of knee stability were critical
requirements for the polycentric total knee prosthesis to be
successful.