Results may lead to the use of a two-bundle technique.

The anterior cruciate ligament (ACL) can be anatomically divided into anteromedial (AM) and posterolateral (PL) bundles. Current ACL reconstruction techniques focus primarily on reproducing the AM bundle, but are insufficient in response to rotatory loads. The objective of this study was to determine the distribution of in situ force between the two bundles when the knee is subjected to anterior tibial and rotatory loads. Ten cadaveric knees (50 k 10 years) were tested using a roboticluniversal forcemoment sensor (UFS) testing system. Two external loading conditions were applied: a 134 N anterior tibial load at full knee extension and 15", 30°, 60", and 90" of flexion and a combined rotatory load of 10 N m valgus and 5 N m internal tibial torque at 15" and 30" of flexion. The resulting 6 degrees of freedom kinematics of the knee and the in situ forces in the ACL and its two bundles were determined. Under an anterior tibial load, the in situ force in the PL bundle was the highest at full extension (67 k 30 N) and decreased with increasing flexion. The in situ force in the AM bundle was lower than in the PL bundle at full extension, but increased with increasing flexion, reaching a maximum (90 f 17 N) at 60" of flexion and then decreasing at 90". Under a combined rotatory load, the in situ force of the PL bundle was higher at 15" (21 k 11 N) and lower at 30" of flexion (14 f 6 N). The in situ force in the AM bundle was similar at 15" and 30" of knee flexion (30k 15 vs. 3 5 2 16 N, respectively). Comparing these two external loading conditions demonstrated the importance of the PL bundle, especially when the knee is near full extension. These findings provide a better understanding of the function of the two bundles of the ACL and could serve as a basis for future considerations of surgical reconstruction in the replacement of the ACL.

Despite the differences compared to the normal coracoclavicular ligament complex, the anatomical reconstruction complex more closely approximates the stiffness of the coracoclavicular ligament complex than current surgical constructs, and the incorporation of biological tissue could improve the overall structural properties with healing.

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Significant advances have been made during the past 25 years in characterizing the properties of ligaments as a tissue and as an individual component in the bone-ligament-bone complex. The contribution of ligaments to joint function have also been well characterized. We have presented many studies that sought to characterize the tensile and viscoelastic properties of ligaments. As a result of these investigations, some of the most important experimental and biologic factors affecting the measurements of these properties have been identified and elucidated. The identification of the tensile properties of normal ligaments can serve as the basis for evaluating their success in healing and repair after injury. Furthermore, characterization of normal ligament function is crucial for diagnosing joint injuries as well as for evaluating reconstruction strategies and developing rehabilitation protocols. The recent introduction of robotic technology to the study of joint kinematics has resulted in significant advances in the understanding of the relative importance of ligaments to joint function. With the more accurate simulation of joint kinematics that include multiple degrees of freedom motion, data on the in situ forces in ligaments can be used to improve the treatment of ligament repair and reconstruction. More complex external loading conditions that mimic sports activities and rehabilitation protocols can also be introduced in the future. Furthermore, this technology can be extended to study other frequently injured joints, such as the shoulder.

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Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.