KNEE Anterior cruciate ligament: an anatomical exploration in humans and in a selection of animal species Gof Tantisricharoenkul • Monica Linde-Rosen • Paulo Araujo • Jingbin Zhou • Patrick Smolinski • Freddie H. Fu Received: 16 July 2012 / Accepted: 20 February 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Purpose Many anatomical anterior cruciate ligament (ACL) studies have indicated that the human ACL is composed of two functional bundles: the antero-medial (AM) and postero-lateral (PL). The purpose of this study is to compare the ACL anatomy among human and assorted animal species. Methods Twenty fresh-frozen knees specimen were used: five humans, ten porcine, one goat, one Kodiak bear, one African lion, one Diana monkey and one Gazelle antelope. All the specimens were dissected to expose the ACL and to visualize the number of bundles and attachment patterns on the tibia and femur. Following the fibre orientation of the individual bundles, a wire loop was used to bluntly separate the bundles starting from the tibial insertion site to the femoral insertion site. In the human and porcine ACL, each bundle was separated into approximately 2 mm diameter segments and then tracked in order to establish the indi- vidual bundle’s specific pattern of insertion on the femur and tibia. Results It appeared that all human and animal knee specimens had three bundles that made up their ACL. In addition, it was noted that among the various specimens species, all viewed with an anterior view, and at 90° knee flexion, the ACL bony insertion sites had similar attach- ment patterns. Conclusion In all the specimens, including human, the ACL had three distinct bundles: AM, intermediate (IM) and PL. The bundles were composed of multiple fascicles arranged in a definite order and similar among the different species. Keywords Three bundles Á Intermediate bundle Á Insertion site pattern Á Human ACL Á Porcine ACL Introduction The anterior cruciate ligament (ACL) is one of most commonly injured ligaments of the knee [18], and under- standing its complex anatomy is a requisite for any orthopaedic surgeon. Presently, a more anatomical approach to reconstructing the ACL has evoked the interest of and discussion by many surgeons, and many anatomical ACL studies have been done to better understand the ACL anatomy. Yet, with all the anatomical studies, there is still controversy in regards to the number of bundles and the insertion site patterns of the ACL. In the literature, the ACL is reported to have one bundle [22], two bundles the AM (antero-medial) and PL (postero- lateral) [1, 2, 4, 5, 7, 11, 16, 24, 27, 33–36], three bundles the AM, IM (intermediate) and PL [3, 10, 14, 21, 23], and even up to six to ten bundles [20]. It is widely reported that the human ACL is composed of two bundles based on behaviour (the PL bundle being lax and the AM bundle being tight when the knee is flexed), as a basis for the understanding the function of the ACL [1, 2, 4, 5, 7, 11, 16, 24, 25, 27, 33–36], but these studies do not consider anatomy. The studies are not conclusive as to the number G. Tantisricharoenkul Á M. Linde-Rosen Á P. Araujo Á J. Zhou Á P. Smolinski Á F. H. Fu (&) Department of Orthopaedic Surgery, University of Pittsburgh School of Medicine, 3471 Fifth Avenue, Suite #1011, Pittsburgh, PA 15213, USA e-mail: ffu@upmc.edu P. Smolinski Á F. H. Fu Departments of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, USA 123 Knee Surg Sports Traumatol Arthrosc DOI 10.1007/s00167-013-2463-6