Summary

Functionally aligned TKAs more reliably produced a medial pivot pattern than mechanically aligned TKAs, without medial implant constraint.

Abstract

Introduction

A medial pivot pattern has been found in native knees and is desirable in total knee arthroplasty (TKA). But whether it is achieved in TKA is debated and not well understood. Mechanical alignment (MA) and functional alignment (FA) are both recognised techniques for TKA. As a part of an ongoing prospective randomized trial, we assessed whether there was a difference between FA and MA primary cruciate retaining TKAs in producing a medial pivot pattern as measured by pressure sensors.

Methods

60 patients (29 FA TKAs and 31 MA TKAs) were consented to participate in a prospective, randomized controlled trial. At the end of the procedure, MA and FA TKAs were determined to be balanced with equal medial and lateral gap measures as determined with robotic assistance. Then an intra-operative pressure sensor was used to collect (with surgeons blinded) medial and lateral pressures and contact points between the femoral component and the insert through the range of motion (determining medial pivot or lateral pivot patterns).

Results

Both alignment techniques achieved balanced TKAs, with no significant difference in balance measured by the sensors at 10°(p=0.475), 45° (p=0.466) or 90° (p=0.644) of flexion. Soft tissue releases were required more frequently to achieve balance in MA TKAs (36% MA vs 3% FA; p<0.01). FA TKAs produced a medial pivot pattern more consistently (76% FA vs 41% MA; p<0.01).

Conclusion

Intra-operative results from this trial suggest that different alignment philosophies may lead to different contact patterns and kinematics, despite being well-balanced. It is possible to recreate a medial pivot following TKA without medial implant constraint. Functionally aligned TKAs more reliably produced a medial pivot pattern than mechanically aligned TKAs.