The effect of modern total knee arthroplasty on muscle balance at the knee

Total knee arthroplasty (TKA) may affect the muscles operating at the flexion/extension (FE) or internal /external rotation (IE) axes. This study tested the hypothesis that a modern posterior stabilizing TKA will change the mechanical balance of the knee joint by altering the moment arms of muscles acting about two separate axes of rotation. Moment arms were determined for the normal knee, the knee after resection of the Anterior Cruciate Ligament (the ACL - knee) and the knee after a PCL-sacrificing TKA. Five fresh cadaver hemi pelvis specimens were used with 5 posterior stabilizing prostheses (a single model available from one manufacturer). Moment arms for the individual muscle tendons were multiplied by the muscle's tension fraction (fractional physiological cross-sectional area [PCSA]) to estimate its potential for moment production relative to the other muscles at the knee, and this value was labeled as the muscle's moment potential. Unlike earlier studies that looked at TKA across many manufacturers' types, this study concluded that there were no significant differences in muscle balance when comparing the intact knee and the posterior stabilized TKA.