ROTATING BEARINGS DO NOT IMPROVE EXTENSOR MALALIGNMENT ARISING FROM FEMORAL ROTATION MALALIGNMENT

Abstract

Patellofemoral complications are among the important reasons for revision knee arthroplasty. Femoral component malposition has been implicated in patellofemoral maltracking, which is associated with anterior knee pain, subluxation, fracture, wear, and aseptic loosening. Rotating-platform mobile bearings compensate for malrotation between the tibial and femoral components. It has been suggested that rotating bearings may also reduce the patellofemoral maltracking resulting from femoral component malposition.

We constructed a dynamic musculoskeletal model of weight-bearing knee flexion in a knee implanted with posterior cruciate-retaining arthroplasty components (LifeMOD/KneeSIM, LifeModeler Inc). The model was validated using tibiofemoral and patellofemoral kinematics and forces measured in cadaver knees on an Oxford knee rig. Knee kinematics and patellofemoral forces were measured after simulating axial malrotation of the femoral component (±3° of the transepicondylar reference line). Differences in patellofemoral kinematics and forces between the fixed- and rotating-bearing conditions were analysed.

Rotational malalignment of the femoral component affected tibial rotation near full extension and tibial adduction at higher flexion angles. In the fixed-bearing conditions, external rotation of the femoral component increased patellofemoral lateral tilt, patellofemoral lateral shift, and patellofemoral lateral shear forces. Up to 6° of bearing rotation relative to the tibia was noted in the rotating-bearing condition. However, the rotating bearing had minimal effect in reducing the patellofemoral maltracking or shear induced by femoral component rotation.

The rotating bearing does not appear to be forgiving of malalignment of the extensor mechanism resulting from femoral component malrotation. The rotating bearing may correct tibiofemoral axial malrotation near full extension but not at higher knee flexion angles. These results support the value of improving existing methodologies for accurate femoral component alignment in knee arthroplasty.