Application of Dynamic Multimodal Imaging for Identification of Risk Factors Predictive of ACL Injury

Abstract

The anterior cruciate ligament (ACL) is the most commonly injured structure in the knee joint. Importantly, following ACL injury there are high rates of joint degradation and osteoarthritis, and furthermore, athletes are at increased risk for subsequent ACL rupture. However, as injury rates continue to rise, there remains no clear answers as to how to prevent these injuries from occurring. ACL injuries predominantly afflict the young, athletic population, so it is vital to better understand how ACL injuries occur in order to prevent them and their downstream effects. The ACL primarily resists tensile load, so higher strain and force on the ligament place it closer to its ultimate tensile strength. Thus, the ACL is often studied by measuring the kinematics, morphological factors, and other aspects which contribute to altered loading patterns. There are a variety of ways in which to perform these measurements including implanted sensors, finite element modeling, and using cadaveric specimens. However, newer techniques have focused on the use of non-invasive imaging as these provide a relatively high level of detail in an in vivo environment while also capturing normal physiologic movement. Thus the goal of this dissertation was to employ a series of multimodal imaging techniques to evaluate how morphological, biomechanical, and biochemical factors impact in vivo ACL loading. In Specific Aim 1 I compared how changing ACL reference length impacts resultant in vivo strain measurements, and I found that the three methods (supine resting, quiet standing, and an anterior/posterior drawer test) produced comparable strain measurements. I then applied this information to measure how tibio-femoral morphology impacts in vivo ACL loading. This study indicated that the slope and depth of the tibial plateau do not play a large role in elevating ACL strain. Building off of that, as females sustain ACL injuries at higher rates than males, I evaluated a wider range of morphological contributors to ACL loading, and found that the orientation and composition of the ACL may be key to how the ligament withstands the forces of movement. In addition to the inherent morphological differences within the knee joint, how people control the motion of their knees plays an important role in how much strain is placed on the ACL. In particular, in Specific Aim 2, I found that sagittal plane motion more so than coronal plane motion contributes to changes in ACL strain. Furthermore, I found that sagittal plane kinematics can pre-load the ACL even before initial ground contact, such that a single-leg landing with its more extended knee position results in greater ACL strain before landing compared to a double-leg landing, where the knee is in a more flexed position. Importantly, while kinematics and morphology largely dictate how the ACL is loaded, there are additional factors which may help to explain why females sustain ACL injuries at a significantly greater rates compared to males. Namely, there are hormonal differences between the sexes which are thought to play into this. Thus in Specific Aim 3, I began to explore how hormones may impact ACL composition in vivo. My findings suggest a link between estradiol levels and collagen alignment in the ACL, so this should be explored further to more fully understand how these changes alter the load bearing capacity of the ligament. These steps forward in our understanding of how the ACL undergoes loading help to inform changes in injury prevention programs, such as by focusing on increasing knee flexion prior to landing. Furthermore, these results present new methods for studying compositional variations in the ACL in vivo, which will enable improved research into identifying why certain individuals are at an increased risk for ACL rupture.