AXIAL ALIGNMENT OF MOBILE-BEARING TKR: DOES IT AFFECT KNEE ROTATION AND BEARING MOTION?

Abstract

Mobile-bearing total knee replacement (TKR) designs are advocated for their theoretical ability to self-align and accommodate small errors in rotational (axial) alignment. However, for many mobile-bearing TKR, the relationships between axial alignment, knee axial rotation and bearing motion during knee flexion are undefined. This study evaluates whether mobile-bearing TKR with axial alignment outside surgical norms have different rotations and motions compared to well-aligned TKR.

This prospective study included 67 patients implanted with cruciate-retaining mobile-bearing TKR with a rotating platform polyethylene bearing (Scorpio PCS, Stryker). Axial alignment of femoral components relative to the transepicondylar axis and tibial components relative to the medial tibial tuberosity was measured from postoperative CT scans. TKR were categorized as “normal” or “outliers” according to defined tolerances for surgical axial alignment relative to anatomic landmarks (+3° for femur, +10° for tibia) and combined axial mismatch (+5° between femoral and tibial components). Knee kinematics and axial rotation were measured from fluoroscopic images acquired immediately after TKR during 0° to 120° of passive knee flexion. Total knee axial rotation (relative motion between the femoral component and tibial baseplate), femoral component axial rotation on the bearing articular surface, and bearing axial rotation on the tibial baseplate were determined using published shape-matching techniques.

External rotation during knee flexion averaged 8.4°+6.1°, with two phases of axial rotation motion distinguished in all groups. External rotation from 0°–80° occurred primarily due to bearing axial rotation on the tibial baseplate. Beyond 80°, there was combined bearing rotation and external rotation of the femoral component on the polyethylene articular surface, with the latter dominating the motion pattern. Axial rotation varied with the component axial alignment. Among TKR with normal axial alignment, external rotation steadily increased with knee flexion. Among anatomic landmark outliers, there was a transition to internal rotation from 20°–50° and limited (< 1°) axial rotation beyond 80°. Among combined axial mismatch outliers, the magnitude of axial rotation was significantly less than normal TKR throughout the flexion range (p< 0.001) due to opposite rotations between the femoral component and polyethylene bearing.

Achieving appropriate axial alignment using defined bony landmarks remains a challenge. In this study, approximately 30% of TKR did not have suitable axial alignment, with notable combined axial mismatch in tibial-femoral alignment. Axial rotation misalignment affected the kinematics and knee rotation motions over the passive flexion range and appears to result in opposite rotations of the femur-bearing and bearing-base-plate articulations.