Biomechanics of the ACL: Measurements of in situ force in the ACL and knee kinematics

Abstract

Injury of the anterior cruciate ligament (ACL) can lead to knee instability associated with damage to other knee structures and the increased risk of degenerative joint disease. This has led to the frequent use of intra-articular tissue grafts for ACL reconstruction in an attempt to restore normal knee function. Despite moderate success, the continued failure of ACL reconstruction to restore normal knee kinematics has led many investigators to study the role played by the ACL in normal knee motion. At our research center, we have focused on the development of a new and innovative approach to measure multiple degree of freedom (DOF) knee kinematics and to determine the in situ forces within the ACL. A unique testing system utilizing a 6-DOF robotic manipulator and universal force–moment sensor (UFS) has been developed such that these measurements can be made in a non-contact fashion while allowing a series of experiments to be performed on the same knee. In this manuscript, we will describe the functional and mathematical development of the robotic/UFS system and its use in a series of studies designed to give insight into the function of the ACL and ACL grafts. Our first study investigated the effect of constrained vs. unconstrained knee motion on anterior tibial translation and on the in situ force in the ACL. We found that unconstrained multiple-DOF knee motion significantly increased anterior tibial translation. While the magnitude of the in situ force remained similar to the more constrained condition, its direction, point of application and distribution between the anteromedial (AM) and posterolateral (PL) bundles were found to significantly change. These findings led us to investigate the effect of knee flexion angle and magnitude of anterior applied tibial load on the in situ force in the ACL and its bundles during unconstrained knee motion. We found the PL bundle of the human ACL to carry a greater proportion of the in situ force than the AM bundle near knee extension. Also, the change in magnitude of the in situ force in the PL bundle with changing knee flexion angle was similar to that of the entire ACL. This led us to conclude that the PL bundle must play a significant role in ACL function and in resisting anterior tibial load and that it should receive more serious consideration during ACL reconstruction. Lastly, we used our new testing system to examine two popular ACL reconstruction techniques: bone–patellar tendon–bone (BPTB) and quadruple semitendinosus/gracilis hamstring (QST/G) grafts. We compared them in terms of restoration of anterior tibial translation and reproduction of the in situ force in the intact ACL. Each reconstruction was performed on the same knee, allowing us to minimize interspecimen variability and take advantage of paired statistical analysis. We found that while both reconstructions effectively reduced anterior tibial translation secondary to anterior tibial loading to a level not significantly different from the ACL intact knee, use of a QST/G graft may be advantageous, as it reproduced the in situ forces of the intact ACL more closely. This series of three studies has garnered quantitative data to further our understanding of how the ACL functions and has yielded new concepts to help improve ACL reconstruction. Through the use of a robotic manipulator, our testing system has the capability to reproduce complex physiologic loading conditions that allow us to evaluate the efficacy of various ACL reconstruction techniques and rehabilitation protocols in the same knee. In the future, we intend to acquire in vivo kinematic data of the knee secondary to various functional and sporting activities and reproduce them using our robotic/UFS system such that in vivo in situ forces can be indirectly determined. We believe that such studies will provide a basis for the advancement of ACL reconstruction and rehabilitation.