Influence of Total Knee Arthroplasty Alignment on Soft-Tissue Balance and Pivot Patterns: A Randomized Controlled Trial of Kinematic Versus Mechanical Alignment

Abstract

Over the last decade alternative alignment techniques in primary total knee arthroplasty (TKA) have been developed in the hope to allow knee prostheses to better replicate normal knee kinematics and improve clinical outcomes. The purpose of this study was to quantify prosthesis soft-tissue balance and pivot patterns based on a restricted kinematic alignment (KA) or mechanical alignment (MA) surgical technique. A total of 109 primary cruciate retaining TKAs were randomized to either a mechanical or KA technique. Medial and lateral compartmental pressures and contact point patterns were quantified at 10, 45, and 90 degrees of flexion using an insert pressure sensor. A significantly greater proportion of KA knees were balanced through a full range of motion (ROM) after the initial bone resections (61 KA versus 12% MA, P < 0.001) and the differences were significant at all positions of ROM. For the unbalanced prostheses, the MA knees required significantly more soft-tissue releases (P = 0.008) and bone alignment adjustments (P < 0.001). The initial and final rollback pivot patterns were not significantly different between techniques (initial P = 0.29, final P = 0.29). The primary driving factor for the pivot patterns was not alignment, but instead the differential pressure between the medial and lateral compartments at 45 and 90 degrees flexion (45° P < 0.001, 90° P < 0.001), with the knee pivoting on the tighter compartment in flexion. In primary cruciate retaining TKA a restricted KA technique achieves a balanced prosthesis with significantly fewer soft-tissue releases or bone recuts. The knee's natural medial pivot pattern can be replicated with a prosthesis by controlling the soft tissue balance to achieve a non-symmetrical flexion gap: equal balance in extension, with medial ligament tension maintained through ROM while allowing increased lateral soft-tissue laxity in flexion. The trial and protocol were registered with the Australian New Zealand Clinical Trials Registry (ACTRN12616001705471).