Estimation of In-vivo Forces within Anterior Cruciate

The knowledge of Anterior Cruciate Ligament (ACL) forces in-vivo is instrumental for understanding ACL injury mechanisms and for improvement of surgical ACL reconstruction. The goal of this thesis was to develop and implement a non-invasive method to determine the ACL forces under physiological loading using advanced imaging techniques combined with a robotic testing system. First, the in-vivo elongation of the ACL in response to increasing weightbearing was captured by a Dual Fluoroscopic Imaging System (DFIS). Next, the force-elongation curves of the ACL were determined in-situ by the robotic testing system. The in-vivo ACL elongation data were statistically mapped to force-elongation curves and the in-vivo ACL forces were estimated. A gold standard robotic testing protocol was implemented to validate the proposed force estimation method in cadaveric specimens. Moreover, this methodology was extended to the bundles of the ACL i.e., anteromedial (AM) and posterolateral (PL) bundles to determine the force contribution of each bundle. The data showed that the ACL force is greater at lower flexion angles. Generally, the AM bundle carried greater portion of the tension within the ACL at all flexion angles. The data revealed that the load sharing patterns of the two bundles were complementary. The proposed force estimation method was then generalized to measure the contact pressure distribution of the tibiofemoral cartilage. By knowing the tibiofemoral cartilage deformation data in-vivo, and mapping them to in-vitro material property data, one can determine the in-vivo contact pressure inside the tibiofemoral joint. As the first step of application, the DFIS was employed to investigate the time-dependent responses of the tibiofemoral cartilage under a constant bodyweight load (in-vivo creep). The cartilage contact deformation in the lateral compartment was shown to be greater than that in the medial compartment of the knee. The findings of this work provide insight into the biomechanical role of the ACL during in-vivo activities and can be used as quantitative guidelines for the development of optimized surgical reconstruction techniques. The methodology could have a wide application in determination of in-vivo loading of human musculoskeletal joints. Thesis Supervisor: Guoan Li, Ph.D. Professor of Orthopaedic Surgery/Bioengineering, Harvard Medical School Committee Chair: Derek Rowell, Ph.D. Professor of Mechanical Engineering, Massachusetts Institute of Technology Committee Member: Alan J. Grodzinsky, Sc.D. Professor of Electrical, Mechanical, and Biological Engineering Massachusetts Institute of Technology