Enhancement of tendon-bone healing of anterior cruciate ligament grafts by blockage of matrix metalloproteinases

Background: The use of soft-tissue grafts for anterior cruciate ligament reconstruction delays the healing process. This delay may be due to biochemical and/or biomechanical insults. We hypothesized that the blocking effect of alpha(2)-macroglobulin on synovial matrix metalloproteinase activity may enhance the healing of tendon graft in a bone tunnel. Methods: The study was performed on twenty-eight healthy, skeletally mature New Zealand White rabbits. Each rabbit underwent bilateral anterior cruciate ligament reconstruction with use of the ipsilateral semitendinosus tendon. Alpha-2-macroglobulin (alpha(2)-macroglobulin) was injected into the knee joint in one limb, and the contralateral limb served as a control. The rabbits were killed two weeks (fourteen rabbits) or five weeks (fourteen rabbits) after the operative procedures. The presence of matrix metalloproteinases in synovial fluid, and the blocking effect of alpha 2-macroglobulin on them, were determined with enzymatic assays. Healing between the tendon and the bone tunnel was assessed morphologically by determining the presence of fibrovascular tissue and collagen fibers. Healing also was assessed quantitatively by measuring the ultimate load to failure of the reconstructed complex. Results: There was an increase in matrix metalloproteinases in the control group; in contrast, there was a decrease in the study group (p < 0.05). In the control specimens, the fibrovascular tissue at the bone-tendon interface had developed into dense connective tissue with poor vascularization. In the treated specimens, the bone tunnel had more areas of denser connective-tissue ingrowth. The interface tissue was more mature and contained numerous perpendicular collagen bundles (Sharpey fibers). The ultimate load to failure was significantly greater in the alpha(2)-macroglobulin-treated specimens than in the untreated controls at both two and five weeks. Conclusions: The present study demonstrated that alpha(2)-macroglobulin blockade of matrix metalloproteinases can enhance bone-tendon healing. This effect of alpha(2)-macroglobulin could occur through its effect solely on collagenase or on a subset of matrix metalloproteinases that are present at the healing interface.