TO DEMONSTRATE THE EFFECT OF TIBIAL TRAY ROTATION ON A FIXED AND MOBILE BEARING TOTAL KNEE ARTHROPLASTY USING A KNEE JIG

Abstract

Aims We rotated the tibial tray, of a fixed and mobile bearing, total knee arthroplasty and studied the effects on the patellofemoral and tibiofemoral joints, when the arthroplasty was placed through a range of movement

Method and results A specially designed jig allowed us to put the knee arthroplasty under 100N of compression using a single pulley system. We rotated the tibial platform in isolation, in 5° increments of external rotation. At each position, the arthroplasty was put through 90° of flexion. We studied the PFC fixed bearing prosthesis and the LCS, PCL substituting rotating platform pros-thesis manufactured by Johnson & Johnson. We used a translucent custom femoral component for each type. This allowed us to observe dye at the femoral component articulations, recording the results using digital photography.

The mobile bearing prosthesis was surprisingly more sensitive to tibial tray external rotation. It suffered antero-lateral and posteromedial point loading on the polyethylene tray from 5–15° of tray rotation. Further increase caused the medial femoral component to lift off the polyethylene tray at 25° of rotation resulting in lateral femoral component loading. Simultaneously, there was lateral patella facet loading with medial facet lift off at the patellofemoral joint. The fixed bearing prosthesis did not experience impingement until 10° of rotation. At 25° of external tray rotation, there was posteromedial and antero-lateral point loading on the polyethylene but no lift off. The patellofemoral joint showed superior pole loading of the patella button but no medial/lateral loading.

Conclusion External tibial tray rotation caused polyethylene point loading in a knee arthroplasty. This would increase wear debris generation and aseptic loosening rates. Tray rotation does not improve patellofemoral tracking. We advise neutral alignment of the tibial tray to the femoral prosthesis. The mobile bearing prosthesis was more sensitive because the polyethylene tray could not rotate to confer the optimum tibiofemoral alignment.