Arthroscopy: The Journal of Arthroscopic and Related Surgery 10(2):140-147 Publishedby Raven Press, Ltd. © 1994Arthroseopy Association of North America Biomechanical Function of the Human Anterior Cruciate Ligament Yoshitsugu Takeda, M.D., John W. Xerogeanes, M.D., Glen A. Livesay M.S., Freddie H. Fu, M.D., and Savio L-Y Woo, Ph.D. Summary: Knowledge about the biomechanical function of the anterior cruci- ate ligament (ACL) is very important in the treatment of the ACL deficient knee. This article presents an overview of the biomechanical function of the ACL, including its structural and mechanical properties as well as its role in knee stabilization and normal kinematics. Tensile properties of the prospective biological grafts and future directions in ACL research are also discussed. Key Words: Anterior cruciate ligament--Stability--Tensile property--Biological graft--Force---Muscle contraction. During the past decade, the treatment of the an- terior cruciate ligament (ACL) deficient knee has advanced tremendously (1,2). This progress has re- sulted from improvements in surgical technique (3,4), advances in diagnosis (5), surgical apparatus, and postoperative rehabilitation (6,7). These clini- cal advances coincide with the greater understand- ing of the biomechanical properties of the ACL. Decisions regarding graft tissue for ACL recon- struction as well as placement, tensioning, and fix- ation of the graft have benefited from laboratory- derived information about the structural and me- chanical properties of the ACL (8-13). Kinematic studies evaluating length changes of the ACL, strains and forces within the ligament, and the ef- fect of muscle stabilization and external forces on these properties have been helpful in improving ACL graft placement and function. This article pre- sefits an overview of the biomechanical function of the ACL as well as suggesting future directions of ACL-related biomechanical research. From the Musculoskeletal Research Center, Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania. Address correspondence and reprint requests to Savio L-Y Woo, Department of Orthopaedic Surgery, 1010 Kaufmann Building, University of Pittsburgh, 3471 Fifth Avenue, Pitts- burgh PA 15261, U.S.A. STABILIZING ROLES OF THE NORMAL ACL IN KNEE MOTION The knee joint can move in 6 dr: 3 translations (anterior-posterior, medial-lateral, and proximal- distal), and 3 rotations (internal-external, varus- valgus, and flexion-extension). It is often stated that the primary function of the ACL is to prevent anterior translation of the tibia in relation to the femur. Although this is true, the ACL also plays a significant role in other types of knee motion. The concept of primary and secondary ligament restraints was introduced to help describe ligamen- tous stabilization of the knee joint (14). A structure that provides the majority of resistance to a trans- lation or rotation resulting from an externally ap- plied force is considered the primary restraint to that motion. Structures that contribute less to re- sisting the same externally applied load are referred to as secondary restraints. Markolf et al. (15) ap- plied force to an intact knee, then sequentially cut ligaments and repeated the test to determine the contribution of each ligament to a specific knee mo- tion. They reported that sectioning the ACL pro- duced a significant increase in anterior-posterior knee instability, which was the largest at full exten- sion. Butler et al. (16) pointed out that the degree of displacement measured with this method is depen- dent on the order of ligament cutting. Alternatively, 140