Injuries to intra-articular tissues, including the anterior cruciate ligament (ACL), menisci, cartilage and tendon, are of great clinical importance. Over 1 million patients each year sustain these injuries and the high rate of failure of repair of these injuries (as high as 90%) leads to loss of tissue and joint integrity and high rates of premature arthritis. Our preliminary work has demonstrated that a collagen-platelet hydrogel can stimulate healing in the wound site of a complete ACL transection. In the work outlined in this proposal, we wish to further understand the mechanisms behind the ability of platelets to stimulate wound healing in the joint in vivo. In our first aim, we propose to study the influence of platelet concentration on the cellular and vascular repopulation of the ACL wound site using in vitro and in vivo assays. The second aim focuses on defining the effects of differences in wound site biology with the return of strength to the ligament as a functional outcome measure. As a secondary outcome, correlations between the biomechanical properties and findings on magnetic resonance imaging (volume measures and signal to noise ratios) will also be sought to enable us to begin to identify non-invasive measure for functional healing of this important intra-articular ligament.