Microstructural morphology in the transition region between scar and intact residual segments of a healing rat medial collateral ligament

This study used a rat model to investigate the microstructural organization of collagen through the transition from sear to intact residual segments o f a healing medial collateral ligament (MCL). Twenty-two mate retired breed er Sprague-Dawley rats were randomly separated into two groups. Eleven unde rwent surgical transections of both MCLs and were allowed unrestricted cage activity until euthanized two weeks post surgery. The remaining eleven rat s were used as normal controls. All 44 MCLs were harvested including intact femoral and tibial insertions and prepared for scanning electron microscop y (SEM) imaging. At harvest the scar region in the healing ligaments was mo re translucent than the normal tissue. Ligaments were viewed from femoral t o tibial insertions at magnifications of 100X through 20,000X. Tissue away from the scar region in the transected MCLs was indistinguishable from norm al tissue in uninjured ligaments. Collagen fibers and fibrils in these tiss ues were more aligned along the longitudinal axis of the ligament than in t he scar tissue. Continuity of collagen fibers and fibrils were consistently observed from the residual portions of the transected ligament through the scar region. Bifurcations/fusions, but no anastomoses, in fibers and fibri ls were observed in both normal and scar tissues of ligaments. Qualitativel y, bifurcations were encountered more frequently in scar tissue. In the tra nsition region, larger diameter fibers from the residual tissue bifurcated into smaller diameter fibrils in the scar. This connection between larger d iameter and smaller diameter fibers and fibrils indicates that bifurcations /fusions are likely to be the dominant way in which force is transmitted fr om a region with larger fibrils (residual ligament) into and through a regi on with smaller fibrils (sear).