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Restoring "Normal" Knee Motion after Joint Replacement

Total knee replacements (TKRs) are designed to restore normal knee function. However, there's one slight problem. TKRs today rarely allow patients to bend the knee beyond 120 degrees. Normal knee flexion is around 120 degrees. Most people have more than 120. In some people, the knee is actually able to bend as much as 160 degrees.

Knee flexion is increased by a movement called femoral rollback. This gliding motion occurs inside the knee joint as the upper leg bone (femur) rolls backward on the lower leg bone (tibia). Femoral rollback improves muscle function around the knee and helps increase knee flexion. It does this by keeping knee structures from getting pinched at greater angles of flexion.

Researchers are asking, "What's the best way to get full range of knee flexion?" This study looked at the effect of the posterior cruciate ligament (PCL) on femoral rollback. The PCL is a key ligament inside the knee joint. A robotic testing system was used to measure femoral roll back after a TKR. Cadavers (human bodies preserved for study) were used for the study. The knee joint was tested with a PCL and again after the PCL was cut.

The authors report that femoral rollback occurs at different times in different degrees for the entire range of knee flexion. This is true for the normal knee as well as the knee with a TKR. Femoral rollback in a TKR is increased when the PCL is intact. However, the authors found that normal knee motion needs more than just a healthy PCL. The ligament must be well balanced and the tibia must rotate inward during knee flexion. The other soft tissues that allow or control tibial rotation are important, too.

Improving the design of TKRs is a goal of this research. The hope is to consistently increase knee flexion beyond 120 degrees. Improving posterior femoral translation may be the key to this result. TKRs of the future will need to restore normal joint motion with and without the PCL for this to happen.

References: Ephrat Most, MS, et al. Femoral Rollback After Cruciate-Retaining and Stabilizing Total Knee Arthroplasty. In Clinical Orthopaedics and Related Research. May 2003. Vol. 410. Pp. 101-112.

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