Kappa Delta Award paper. Anterior cruciate ligament: its normal response and replacement

This article reviews our research studies on the anterior cruciate ligament (ACL). The human ACL was found to be a primary restraint to anterior tibial displacement at both 90 degrees and 30 degrees of flexion. Geometric measurements of the ligament showed it to have a complex macrostructure, consisting of bundles of different lengths, curvatures, and orientations. Similar material properties were measured for subunits from the human anterior and posterior cruciate and lateral collateral ligaments. However, the linear modulus, maximum stress, and strain energy density to maximum stress of the ACL were significantly less than similar properties for patellar tendons. These tissue subunits exhibited nonuniform axial strain during tensile loading, which was partially attributed to differences in bundle crimp period and crimp angle. The structural mechanical properties (stiffness, strength, and energies and elongations to maximum force and failure) of nine commonly used human ACL substitutes were also compared. Only the bone-patellar tendon-bone unit had maximum force and stiffness greater than that of the ACL. The patellar tendon, when used as an ACL replacement in the dog and primate, exhibited significant loss in structural mechanical and material properties early after implantation. These properties showed only a gradual improvement up to 1 year after surgery. Maintaining a vascular supply to these grafts or using intermittent passive motion immediately after surgery produced no significant improvement in graft properties in the primate model.