Femoral component rotation of a modern tka implant does not affect pfj biomechanics

Patellofemoral joint (PFJ) complications, such as anterior knee pain, are a common complaint among knee arthroplasty patients and femoral rotational mal-alignment is thought to be a contributing factor. However, no studies have assessed the effect of femoral internal and external rotation on PFJ biomechanics using simulated physiological loading cycles. The present study aimed to assess the effect of surgical femoral rotational alignment errors on the forces, moment arms and contact areas within the PFJ.
Testing was carried out under physiological loading, with a quasi-static kinematic knee joint simulator, using Scorpio NRG prostheses implanted on synthetic bones. Three scenarios were simulated, to replicate the worst case in terms of surgical error; neutral placement was compared to 5° internal and 5° external femoral rotation.
External rotation caused a significant reduction in the patella moment arm. However, femoral rotational mal-alignments of ± 5° had no clinically relevant effect on the quadriceps force, patella compressive force, or PFJ contact areas. For all scenarios, the PFJ was subjected to over 65% lateral loading and consistent edge loading of the patella button. This study demonstrates that, in terms of PFJ biomechanics, the Scorpio NRG implant used was tolerant of surgically relevant levels of femoral rotational mal-alignment.