On Anterior Cruciate Ligament Injury: Biomechanical Studies of In Vitro Knee Kinematics and Bone Morphology

Abstract

The anterior cruciate ligament (ACL) functions as an important checkrein to limit shear and rotation of the tibia with respect to the femur when using one foot to land a jump or make a sudden stop or turn during sports. Given more than 200,000 ACL tears each year in the U.S., insights are needed to help prevent these injuries because half will go on to cause osteoarthritis in the knee within 10 years. Recent findings from our laboratory suggest that some ACL injuries may be due to the accumulation of fatigue damage in the ligament near its femoral enthesis. The goal of this dissertation was to investigate how knee kinematics and certain bone morphologies may contribute to the mechanism of ACL injury. Young adult cadaveric knees were subjected to simulated pivot landings using sub-maximal simultaneous impulsive loads and moments applied axially to the tibia. The resulting 3-D tibiofemoral kinetics, kinematics and simulated trans knee muscle forces were recorded using a laboratory motion capture system. The hypotheses were tested that (1) the resulting tibiofemoral kinematics correlated with those measured using two wearable inertial measurement units (Chapter 2), and (2) lateral tibial slope, measured from 3T MR images, correlated significantly with anterior tibial translation and internal tibial rotation, both known to be proportional to ACL strain (Chapter 3). To check whether localized fatigue can also accumulate in the bone under the ACL femoral enthesis, (3) a finite element model of the subchondral bone and calcified cartilage shell of the femoral enthesis was developed to determine the effect of three common shell profiles (concave, flat, convex) on the distributions of tensile and shear stresses in the shell (Chapter 4). Then, using 3D nano CT sections through the subchondral bone of the ACL femoral enthesis, we tested the hypotheses that (4-a) sub-maximal repetitive knee loading causes fatigue damage of the bone underlying femoral enthesis, (4-b) the thickness of the subchondral bone in the most highly stressed region is consistently thicker than elsewhere, and (4-c) the trabeculae immediately under the ACL femoral subchondral plate are preferentially aligned with the ACL line-of-action near knee extension (Chapter 5). The results show that (a) the wearable IMUs did not reliably measure these 3D dynamic tibiofemoral motions, even in the absence of soft tissue motion artifact; (b) that knees with larger lateral tibial slope exhibited greater anterior tibial translation and internal tibial rotation than those with more moderate slope. Furthermore, (c) a concave ACL femoral enthesis reduced the peak tensile and shear stresses in the subchondral shell: and (d) no overt signs of subchondral bone fatigue microcracks were found at 20 μm resolution. However, long-term bone adaptation was demonstrated in the origin of the more highly loaded posterolateral ACL fibers being significantly thicker than elsewhere, and the trabeculae under the origin of the anteromedial ACL fibers being preferentially aligned with the direction of ACL tension.