How Isometric Are the Medial Patellofemoral, Superficial Medial Collateral, and Lateral Collateral Ligaments of the Knee?

Background Ligament isometry is a cornerstone in the description of normal knee function and thorough knowledge is mandatory for successful repair of torn ligaments. Purpose This study was undertaken to validate a novel experimental model for the study of ligament strains and to determine the length changes in the superficial medial collateral, lateral collateral, and medial patellofemoral ligaments. Study Design Descriptive laboratory study. Methods Passive motions and loaded squats of 12 cadaveric specimens were performed while controlling ankle load and optically tracking the motion of the bones. Preexperiment and postexperiment computed axial tomography scans allow the transformation of rigid body motion to relative motion of relevant anatomic landmarks on the femur, tibia, and patella. Results The superficial medial collateral ligament is a near-isometric ligament with a strain of less than 2%. The ligament is a little more slack in midflexion (30° to 50°) and in deep flexion, but length changes are not significant (P > .05). The lateral collateral ligament behaves near isometric (<2% strain) from 0° to 70° of knee flexion. Cartilage compression in a loaded environment relieves tension from the collateral ligaments (P < .05). The medial patellofemoral ligament is nonisometric. The cranial part of the medial patellofemoral ligament is most taut at full extension, while the caudal part is most taut at 30° of knee flexion. Conclusion Ligament insertion sites on the femur, patella, and fibula can be derived from computed axial tomography scans. The described model allows the study of dynamic ligament behavior. The superficial medial collateral ligament is a near-isometric ligament with no significant length changes. The medial patellofemoral ligament behaves differently in its cranial and caudal parts. Clinical Relevance In knees with chronic medial collateral ligament insufficiency, isometric repair of the superficial medial collateral ligament can be attempted. A medial patellofemoral ligament reconstruction with a double fixation on the medial patellar border is supported. The cranial bundle should be tightened at full extension and the caudal bundle at 30° of knee flexion.