The beneficial and detrimental effects of exercise and unloading on OA progression after anterior cruciate ligament injury.

Injury of the anterior cruciate ligament (ACL) is a common sports injury that can lead to post-traumatic osteoarthritis (PTOA) within 10-20 years. Surgical ACL reconstruction is often performed several weeks or months after injury, and this period between injury and ACL reconstruction may be a critical time for determining the risk of long-term PTOA progression. However, few (if any) studies in human patients have investigated the long-term effects of exercise or unloading between ACL injury and surgery. Early mobilization is often recommended to maintain range of motion and muscle strength, which are beneficial for positive outcomes of ACL reconstruction, but it is unknown what effects early mobilization or unloading have on long-term PTOA progression. In preclinical animal studies, a brief period of joint unloading immediately after ACL injury significantly decreased osteophyte formation and articular cartilage degeneration, while longer-term non-weightbearing caused muscle atrophy and articular cartilage degradation. Similarly, preclinical studies have shown that different intensities of exercise after knee injury can have divergent effects on PTOA development. Low intensity exercise was protective against joint degeneration, while higher intensity exercise accelerated PTOA progression. The beneficial or detrimental effects of exercise and unloading following ACL injury are likely dependent on the timing, duration, and intensity of these biomechanical interventions. This review summarizes the effects of these biomechanical interventions after ACL injury in both humans and animal models, with the goal of informing therapeutic and rehabilitation strategies for slowing or preventing PTOA progression after injury.