2003 Papers - Blum


Computational Assessment of Patellar Tethering for Multidirectional Patellofemoral Instability
Gary T. Blum, MD, John J. Elias, PhD, Allen B. Richardson, MD
University of Hawaii School of Medicine, Department of Surgery, Division of Orthopedics

Introduction: No surgical procedure has gained general acceptance for multidirectional instability. Patellar tethering shifts the insertion point of the tendon proximally, shortening the lever arm. Clinical results have been good to excellent, but evidence of the biomechanical effect of patellar tethering is lacking. The current study was performed to computationally evaluate how tethering influences patellofemoral biomechanics using a model that has been shown to accurately characterize how altering patellofemoral loading influences the patellofemoral force and pressure distributions.

Methods: Three knee models were created from CT data. The muscle and tendon points were identified and patellofemoral kinematics and loading conditions were applied to simulate a squatting activity. At 30, 45, and 60 degrees of flexion, a cartilage surface and joint capsule was created between the femur and patella. The patellofemoral pressure distribution was characterized by minimizing the potential energy within springs representing the cartilage and joint capsule. Model parameters were altered to simulate normal knees and knees with multidirectional instability.

Results: In normal knees, tethering decreased the lateral and medial subluxation forces and total force applied to the cartilage by an average of 17, 1, and 11 percent, respectively. These changes were consistent across all degrees of knee flexion, with the most demonstrable decreases at higher degrees of flexion. There was little variation in peak contact pressure. Tethering was particularly effective in decreasing the lateral subluxation force in the medially unstable and multidirectionally unstable models. Discussion: This computational simulation revealed that the procedure favorably alters subluxation and total joint forces while minimizing changes to pressure distribution, confirming clinical experience as an effective stabilizing procedure. Future directions include development of a fourth knee model, validation against a cadaveric study, and use of the model for preoperative planning.