How accurate is current TKR instrumentation?

Mechanical total knee replacement (TKR) alignment systems have fundamental limitations that limit their ultimate accuracy. This study uses an image-free computer-assisted navigation system to assess the accuracy of a currently available conventional mechanical, intramedullary instrumentation system. Mechanical instrumentation allows reasonably accurate and reproducible (within 3 degrees varus-valgus) frontal and sagittal limb alignment. There is a tendency to leave the knee in slight flexion, to hyperextend the femoral component and to posteriorly tilt the tibial implant with mechanical AP guides. There also is a consistent tendency to internally rotate the femoral implant. Although almost all of the 20 TKRs resulted in final limb alignment within 3 degrees of the frontal and sagittal mechanical axes, only four TKRs were done in which all of the measured steps were within 3 degrees of the optimal positions. If implant longevity, pain-relief, and function are related to the accuracy with which TKRs are done, this study emphasizes that current mechanical instrumentation does not result in a high incidence of accuracy when each step of the procedure is measured. Computer-assisted techniques permit measurement of the steps of the TKRs procedure with a high degree (error < 1 degree) of accuracy.