Browsing by Subject "anterior cruciate ligament"

More than 400,000 anterior cruciate ligament (ACL) injuries occur annually in the United States, 70% of which are non-contact. A severe consequence of ACL injury is the increased risk of early-onset of osteoarthritis (OA). Importantly, the increased risk of OA persists even if the ACL is surgically reconstructed. Thus, due to the long term physical consequences and high financial burden of treatment, injury prevention and improved reconstruction techniques are critical. However, the causes of non-contact ACL injuries remain unclear, which has hindered efforts to develop effective training programs targeted at preventing these injuries. Improved understanding of the knee motions that increase the risk of ACL injury can inform more effective injury prevention strategies. Furthermore, there is presently limited in vivo data to describe the function of ACL under dynamic loading conditions. Understanding how the ACL functions to stabilize the knee joint under physiologic loading conditions can inform design criteria for grafts used in ACL reconstruction. Grafts that more accurately mimic the native function of the ACL may help prevent these severe long term degenerative changes in the knee joint after injury.

To this end, measurements of in vivo ACL function during knee motion are critical to understanding how non-contact ACL injuries occur and the function of the ACL in stabilizing the joint during activities of daily living. Specifically, identifying the knee motions that increase ACL length and strain can elucidate the mechanisms of non-contact ACL injury, as a taut ligament is more likely to fail. Furthermore, measuring ACL elongation patterns during dynamic activity can inform the design criteria for grafts used in reconstructive surgery. To obtain measurements, 3D imaging techniques that can be used to measure dynamic in vivo ACL elongation and strain at high temporal and spatial resolution are needed.

Thus, in this dissertation a method of measuring knee motion and ACL function during dynamic activity in vivo using high-speed biplanar radiography in combination with magnetic resonance (MR) imaging was developed. In this technique, 3D surface models of the knee joint are created from MR images and registered to high-speed biplanar radiographs of knee motion. The use of MR imaging to model the joint allows for visualization of bone and soft tissue anatomy, in particular the attachment site footprints of the ligaments. By registering the bone models to biplanar radiographs using software developed in this dissertation, the relative positions of the bones and associated ligament attachment site footprints at the time of radiographic imaging can be reproduced. Thus, measurements of knee kinematics and ligament function during dynamic activity can be obtained at high spatial and temporal resolution.

We have applied the techniques developed in this dissertation to obtain novel dynamic in vivo measurements of the mechanical function of the knee joint. Specifically, the physiologic elongation and strain behaviors of the ACL during gait and single-legged jumping were measured. Additionally, the dynamic function of the patellar tendon during single legged jumping was measured. The findings of this dissertation have helped to elucidate the knee kinematics that increase ACL injury vulnerability by identifying the dynamic motions that result in elongation and strain in the ACL. Furthermore, the findings of this dissertation have provided critical data to inform design criteria for grafts used in reconstructive surgery such that reconstructive techniques better mimic the physiologic function of the ACL.

The methodologies described in this dissertation can be applied to study the mechanical behavior of other joints such as the spine, and other soft tissues, such as articular cartilage, under various loading conditions. Therefore, these methods may have a significant impact on the field of biomechanics as a whole, and may have applicability to a number of musculoskeletal applications.

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.

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