The anatomy of the anterior cruciate ligament (ACL) has been thoroughly studied to further the understanding of functionality and pursue improvements in anatomical reconstruction [5]. Among the relevant studies are morphometrical investigations that analyze the cross-sectional area and shape of the ligament mid-substance and insertion sites [5][11][15]. The purpose of this study was to analyze additional geometric properties of the ACL along the mid-substance, including the first and second principal second moments of inertia, the major and minor axis lengths, and the major axis rotation relative to the anterior direction. Nine (n=9) fresh-frozen cadaveric knee specimens were tested. A six-degrees of freedom robotic testing system was used with a universal force-moment sensor to collect a passive path of flexion-extension, from full extension to 90°, by minimizing forces and moments about the intact knee joint for every 0.5° [5]. The femur and tibia were fixed in custom cylinders, with the femoral cylinder attached to a rigid base and the tibial cylinder fixed to the robot end-effector. The knee was carefully dissected until the tibial and femoral ACL insertion sites were exposed. Bone was then removed around the insertion sites to expose the ACL for laser scanning, and the cylinders were remounted on the robotic arm, maintaining the tibial-femoral position of the intact knee. A three-dimensional laser was used to scan the ACL at 15° of flexion and the tibial and femoral insertion sites were circumscribed using a 3 mm–diameter ball probe. A three-dimensional model of the ligament was used to calculate the cross-sectional properties. The angle of each cross-section was also varied in two directions in order to observe how the data changed. This study confirmed previous descriptions of the ACL, such as having an hourglass structure [5], and having a “flat-ribbon” mid-substance [11]. It was found that the ratio of major-to-minor axis length decreased from the femoral insertion site to the tibial insertion site. The change in cross-sectional plane angles only showed a significant effect on the geometric data at 20° relative to the original angle
