Strategies for attachment site locations and twist of the intraarticular anterior cruciate ligament graft

We modeled an intraarticular anterior cruciate ligament graft and investigated the effects of attachment orientation and twist of the graft on its isometry during quadriceps muscle loading. Physiologic levels of quadriceps muscle loads were applied to 15 intact cadaveric knees. We measured the changes in distance between points on the tibia and femur for knee flexion angles between 0 degrees and 120 degrees using a three-dimensional digitizer. Selected points on the tibia and femur, representing graft attachment sites, allowed us to model the graft as a broad band. Distance was used to approximate graft fiber length. A 180 degrees twist in the graft significantly reduced the maximal range of changes in distance when the graft was attached in the anteroposterior direction. Range is defined as the difference between the largest and smallest changes in distance among the fibers of the graft for a given angle of flexion. This reduction enhanced isometry among the fibers of the graft. Enhanced isometry would be expected to enhance load sharing among these fibers, thereby increasing the overall strength of the graft. For a graft 10 mm wide and 4 mm thick, the dimensions of a typical patellar tendon graft, the best overall isometry was found when the breadth of the graft was attached to the tibia in the mediolateral direction, to the femur along the most isometric line, and with a 180 degrees twist in the graft.