Inter-limb differences in impulsive loading following anterior cruciate ligament reconstruction in females.

Anterior cruciate ligament injury and reconstruction (ACLR) dramatically increase the risk of knee osteoarthritis, but the contributing factors, and therefore the targets for intervention, are poorly understood. Differences in loading characteristics between the ACLR and contralateral limbs during routine activities such as walking may elucidate the mechanical pathogenesis of post-traumatic knee osteoarthritis. Twenty-nine females with ACLR (age=21.7±3.1 years; time since ACL injury=48±41 months) performed walking gait at a self-selected speed from which the overall peak vertical ground reaction force (vGRF) in the first 50% of the stance phase and its linear (slope of the vGRF-time curve) and instantaneous (first time-derivative) loading rates were calculated. The magnitude of the vGRF peak immediately following heelstrike and its linear and instantaneous loading rates were also identified. Subjects were further classified as "Impulsive Loaders" or "Normal Loaders" based on whether the transient vGRF peak immediately following heelstrike was objectively classified as a heelstrike transient in the majority of trials. The vGRF magnitude immediately following heelstrike and instantaneous loading rates (both overall and immediately following heelstrike) were greater in the ACLR limb. Additionally, vGRF linear and instantaneous loading rates were greater in subjects classified as Impulsive Loaders. As higher loading rates are associated with greater cartilage degradation in animal models, these data suggest that the greater loading rates in the ACLR limb may play an important role in development of post-traumatic knee osteoarthritis. Additionally, the heelstrike transient appears to be an objective indicator of impulsive loading.