Summary

Functionally aligned TKA generates an intra-operative medial pivot kinematic pattern through soft tissue balance significantly more often than adjusted mechanical alignment, where lateral pivot kinematic patterns are more commonly found, medial pivot kinematics are associated with improved clinical outcomes.

Abstract

Introduction

Functional alignment (FA) and adjusted mechanical alignment (aMA) are both recognised techniques for performing total knee arthroplasty (TKA). The native femur is known to disproportionately roll back further on the lateral tibial plateau than the medial side during flexion, resulting in a medial pivot pattern of movement. This study primarily compares FA to aMA as to whether there is a difference in the kinematic pattern observed, following a robotic-assisted (RA)TKA. Secondary outcomes assessed are whether the kinematic pattern produced affected Patient Reported Outcome Measures (PROMs) at 12 months post surgery.

Methods

60 consecutive patients were randomised to an RA-TKA with either FA or aMA. All received a cruciate retaining TKA implant. After definitive implantation of the prostheses, a trial insert with a pressure monitor (Verasense) was inserted and the capsule closed. The contact points between the femoral component and the pressure monitor were recorded in the medial and lateral compartments as the knee was taken through a range of motion and the kinematic pattern observed. Additionally, contact pressures in extension, mid-flexion and 90 degrees were measured. One year PROMs were assessed and the two groups compared.

Results

The FA-TKA group produced a medial pivot in 57.7% of cases, symmetrical roll back in 38.5% and a lateral pivot in 3.8%. The aMA-TKA group produced a medial pivot in 17.2% of cases, symmetrical roll back in 48.3% and a lateral pivot in 34.5% (p <0.001). No differences in knee balance were recorded between the two alignment groups at any flexion point. Patients with a medial pivot kinematic pattern had superior one-year PROMs in some measures. Patients producing a lateral pivot had lower kujala scores.

Conclusion

Functionally aligned cruciate retaining TKA generates an intra-operative medial pivot kinematic pattern through soft tissue balance significantly more often than those that utilised adjusted mechanical alignment. Lateral pivot kinematic patterns are more commonly found with aMA. These intra-operative kinematic patterns are related to clinical outcomes with knees with medial pivot performing better than those with lateral pivot.