Distribution of in situ forces in the anterior cruciate ligament in response to rotatory loads

Abstract

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 ± 10 years) were tested using a robotic/universal force-moment 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 ± 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 ± 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 ± 11 N) and lower at 30° of flexion (14 ± 6 N). The in situ force in the AM bundle was similar at 15° and 30° of knee flexion (30 ± 15 vs. 35 ± 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. Keywords Anterior cruciate ligament In situ force Knee kinematics Introduction The anterior cruciate ligament (ACL) tears frequently during high impact or sporting activities. In the United States alone, approximately 70,000 ACL injuries occur each year [24] . A debilitating outcome of ACL injury is severe knee instability, which can lead to osteoarthritis [16,35] . The ACL is usually surgically reconstructed to restore knee stability. However, literature on both short- and long-term follow-ups reveal that 15–25% of patients continue to suffer pain and instability [1–4,7,19,31] . The ACL can be anatomically divided into two bundles: the anteromedial (AM) and the posterolateral (PL) bundles, named for the orientation of their tibial insertions [12] . The ACL is known to be a restraint against anterior tibial loads by limiting anterior tibial translations [30,32,34] . Surgical reconstruction to replace the ACL restores the limit of anterior tibial translation closer to those for a knee with an intact ACL than if no reconstruction is performed [5,36] . However, under more complex rotatory motions, such as internal tibial rotation and valgus rotation, these reconstruction procedures are less successful [8,17,23] . In a cadaveric study, ACL replacement grafts could not replicate the function of the intact ACL under such a combined rotatory loading condition [36] . A potential limitation of ACL reconstruction could be, therefore, that the reconstruction is designed predominantly to replicate the anatomy of only the AM bundle. The outcome of ACL reconstruction may be improved by better replicating the anatomy of the ligament. Therefore, discussion of an anatomical reconstruction has surfaced because the AM and PL bundles have unique contributions to load transfer across the knee joint [13,14,27,33] . For example, with an externally applied 110 N anterior tibial load, the PL bundle carries the majority of load when the knee is at full extension or 15° of flexion, while the AM bundle carries the majority of load with the knee flexed past 30° [30] . However, the role of each bundle under applied combined rotatory loads has not been well elucidated. The objective of this study was to determine the distribution of the in situ force in the AM and PL bundles of the ACL under two loading conditions: a 134 N anterior tibial load and a combined rotatory load of 10 N m valgus and 5 N m internal tibial torques. Using a robotic/universal force-moment sensor (UFS) testing system, multiple degrees of freedom (DOF) kinematics of the knee as well as the in situ force in the ACL and its bundles were determined. Methods Ten fresh frozen cadaveric knees (50 ± 10 years, range: 46–66 years) were used in this study. The age of the specimens was dictated by cadaver availability; however, ACL reconstructions are now routinely performed for patients over 40 with good results [26] . Each specimen was screened for bony and ligamentous abnormalities using radiographs and arthroscopy. Specimens were stored at −20 °C until 24 h before testing at which time they were thawed at room temperature [37] . In preparation for testing, all soft tissue was removed approximately 10 cm away from the joint line on the femur and tibia, leaving the knee joint intact. Throughout the experiment, the specimens were kept moist with 0.9% saline. The femur and tibia were secured in custom-made aluminum clamps using an epoxy compound (Bond-Tite Products, Cleveland, OH) with transfixing bolts. The femur was rigidly fixed relative to the base of the robotic manipulator, and the tibia was mounted to the end-effector of the robot through the UFS [10] . The robotic/UFS testing system has been developed to enable accurate measurements of the 5 DOF kinematics of the knee (medial–lateral, proximal–distal, anterior–posterior translations, and internal–external and varus–valgus rotations) at a predetermined flexion angle and to determine in situ ligament...