STABILITY AND RANGE OF MOTION OF INSALL-BURSTEIN CONDYLAR PROSTHESES - A COMPUTER-SIMULATION STUDY

The Insall-Burstein Posterior Stabilized Prosthesis (Zimmer, Warsaw, IN) uses an articulation between a femoral cam and tibial spine to provide anteroposterior stability to the knee. Dislocation can occur if the femoral cam translocates anteriorly and over the tibial spine. A computer model was used to examine the effects of design changes made between the Insall-Burstein I (IB I), Insall-Burstein II (IB II), and revised Insall-Burstein II (IB IIR) knees. The effects of these design changes were determined from their influence on knee stability and maximum obtainable knee flexion. Knee stability was characterized by a dislocation safety factor, defined as the vertical distance from the top of the tibial spine to the bottom of the femoral cam. Our analysis showed that the dislocation safety factor is greatest at approximately 70 degrees of knee flexion for all IB knees. As knee flexion is increased from this angle, the dislocation safety factor decreases, reducing knee stability. The simulations highlighted a trade-off between improving knee flexion and improving knee stability. The geometry of the IB II knee allowed greater knee flexion. The maximum flexion achieved with the IB II knee was 125 degrees compared with 115 degrees and 117 degrees for the IB I and IB IIR knees, respectively. However, the simulations indicate that the IB I and IB IIR knees are less likely to dislocate because they have greater dislocation safety factors than the IB II knees.