Title: Mechanism of improved knee flexion after rectus femoris transfer surgery
One of the most common gait problems in children with cerebral palsy is stiff-knee gait, the inability to sufficiently flex the knee when swinging the leg forward. The cause of stiff-knee gait is commonly thought to be over-activity of the rectus femoris, a muscle that crosses in front of the knee and creates a knee extension moment. A common treatment is rectus femoris transfer surgery, in which the muscle is released from its attachment in front of the knee and transferred behind the knee. Unfortunately, individual patient outcomes vary substantially. It is difficult to improve surgical outcomes because the mechanism by which some patients improve is unknown. The purpose of this study was to investigate the mechanism by which the transferred muscle improves knee flexion. We created muscle-actuated dynamic walking simulations of ten children with stiff-knee gait. The musculoskeletal models were then altered to represent three different types of rectus femoris transfer surgeries. These altered models were then used to create "post-surgical" walking simulations. We compared simulated improvements in knee flexion during leg swing from simulations of different transfer techniques to determine how knee flexion is improved after transfer surgery. Simulations revealed that the primary mechanism for improvement in knee flexion after surgery is reduction of the muscle's knee extension moment, rather than production of a knee flexion moment.