A rotating inlay decreases contact pressure on inlay post after posterior cruciate substituting total knee arthroplasty.

Background: The post/cam mechanism of posterior cruciate substituting total knee arthroplasty, which is intended to achieve maximum range of flexion, offers the risk of failure due to mechanical overload. The purpose of this in vitro study was to investigate load and contact pressure on the inlay post of posterior substituting knee prosthesis with different designs.

Methods: Isokinetic extension/flexion motions of seven fresh frozen left knee specimens were simulated dynamically in a specially designed knee simulator with an extension moment of 31 Nm. After implantation of the knee prosthesis system, which provides a fixed and a rotating posterior cruciate substituting inlay, a pressure sensitive film was fixed on the inlay post surface to measure maximum load and contact pressure.

Findings: Both types of inlays showed nearly the same contact load of up to 480 N on the posterior surface of the inlay post at 120 degrees knee flexion. Contact pressure was measured to be up to 19.7 MPa at 120 degrees flexion on the posterior surface of the post of the fixed inlay, whereas contact pressure was measured to be significantly lower (6.8 MPa, p=0.04) on the inlay post of the rotating inlay.

Interpretation: The modification of a rotating posterior cruciate substituting inlay could not decrease the horizontal load, but offers the possibility to decrease contact pressure on the inlay post to avoid mechanical overload of the polyethylene inlay.