Medial knee stability at flexion increases tibial internal rotation and knee flexion angle after posterior-stabilized total knee arthroplasty

Background: Soft-tissue balance is an important element for the success of total knee arthroplasty; however, the influence of intraoperative soft-tissue balance on knee kinematics in posterior-stabilized-total knee arthroplasty remains unknown. We investigated whether intraoperative soft-tissue balance could influence knee kinematics and flexion angle after posterior-stabilized-total knee arthroplasty. Methods: This study included 30 patients with knee osteoarthritis and varus alignment who underwent posterior-stabilized total knee arthroplasty; intraoperative soft-tissue balance parameters, such as varus ligament balance and joint component gap, were assessed by an offset-type tensor at knee flexion angles of 0 degrees, 10 degrees, 30 degrees, 60 degrees, 90 degrees, 120 degrees, and 135 degrees. Medial and lateral compartment gaps were calculated. The amount of tibial internal rotation was determined between 60 degrees and 135 degrees flexion by the navigation system. Simple linear regression analysis was used to analyze the effects of intraoperative soft-tissue balance on tibial internal rotation. Linear regression analysis was used to assess correlation between tibial internal rotation and postoperative knee flexion angle. Findings: Medial compartment gaps at 60 degrees (r = -0.57, P < 0.05) and 90 degrees (r = -0.60, P < 0.05) of flexion were significantly negatively correlated with tibial internal rotation. Moreover, tibial internal rotation showed a significant correlation with the 2-year postoperative knee flexion angle (r = 0.50, P < 0.05) and improvement in knee flexion angle (r = 0.61, P < 0.05). Interpretation Thus, smaller medial compartment gaps at 60 degrees and 90 degrees of flexion play an important role in achieving medial pivot motion with tibial internal rotation; moreover, tibial internal rotation provides a better flexion angle after posterior-stabilized total knee arthroplasty.