Anterior cruciate ligament (ACL) injuries are occurring at increasing rates in pediatric and adolescent populations, and current treatment approaches have sub-par results in this young demographic including high secondary tear and injury rates. These outcomes may be improved by the development of age-specific treatments designed to work with the natural growth of the musculoskeletal tissues in the knee. However, there is currently a lack of studies in the orthopaedic field on age-specific changes in the structure and biomechanical function of the ACL during skeletal growth.

The objective of this dissertation was to study changes in the angular orientation, morphology, and biomechanical function of the ACL in both healthy and injured states during skeletal growth. Specifically, ACL growth was studied in a pre-clinical large animal model, the Yorkshire pig, from birth through late adolescence. First, magnetic resonance imaging was performed on porcine stifle (knee) joints from different age groups. From these images, we found that the angular orientation of the ACL increases in both sagittal and coronal planes during growth in a similar manner to previously published findings in humans. These images were further analyzed by measuring length and cross-sectional area to determine that the ACL grows in an allometric manner, with relative cross-sectional area and length proportions changing between age groups. Additionally, we found that the ACL grows in a disparate manner compared to other soft tissues in the joint. Within the ACL, there were varied responses to increasing age between the two primary bundles, the anteromedial and posterolateral bundles. Specifically, the anteromedial bundle continued increasing in cross-sectional area throughout adolescence whereas the size of the PL bundle plateaued at the onset of adolescence. These bundle-specific findings were echoed in initial biomechanics studies where the two bundles had roughly equal functional roles under applied tibial loads and moments during youth, but the functional demands on the anteromedial bundle increased in adolescence. Finally, we studied the immediate impact of partial and complete ACL injuries on joints from juvenile through adolescent ages. Here we found an age-dependent response to partial ACL injury, as only late adolescent joints had a detectable change in kinematics in response to applied anterior tibial loads. Complete ACL injury resulted in changes to kinematic response to both anterior loads and varus-valgus moments regardless of skeletal age. These functional changes were further assessed by calculating in situ joint and ACL stiffness and in situ joint slack, where we found an increase in overall in situ stiffness with increasing age. The results of this work can be used to motivate in vivo surgical studies on age-specific clinical treatments for pediatric ACL injuries.