TOTAL KNEE ARTHROPLASTY STABILITY ENHANCES STRENGTH

Achieving normal strength after total knee arthroplasty (TKA) remains a major challenge, with recent reports suggesting strength following TKA averages 65% of normal. Gait lab studies have reported the greatest strength (80%–84% of normal) in knees with intrinsic stability, i.e. knees where the implant surfaces or retained ligaments provide definitive control of tibiofemoral motions such that dynamic muscle stabilization is not required. Superior results have been reported for bicruciate retaining arthroplasty, posterior-stabilized arthroplasty with early-engaging cams, and single radius highly congruent posterior-stabilized arthroplasty. The goal of this study was to determine if knees with an intrinsically stable posterior cruciate ligament (PCL) retaining implant design showed strength comparable to these other intrinsically stable types of arthroplasty. Ten patients with unilateral intrinsically stable PCL-retaining knee arthroplasty were studied using full-body motion capture, force platforms and electro myography while they stepped onto and over a 20cm box. The implant design includes an asymmetric tibial bearing surface with a fully congruent lateral articulation (0°–70° flexion). Subjects were recruited on the basis of combined KSS scores greater than 180 one year after surgery. Peak knee flexion torques, normalized by body weight and height, are used as a measure of functional strength. Knees with posterior cruciate retaining, intrinsically stable TKA exhibit functional strength comparable to other intrinsically stable TKA designs and superior to strength in less stable TKA designs. These knees show some reduction of knee flexor activation, indicating that antagonist coactivation is not required for joint stability. Stable joints permit more optimal muscle activation, making patients effectively stronger while reducing loads at the joint.