Healing of Knee Ligaments
Animal models of knee ligament injury have shown that collateral ligaments heal very slowly (during a period of months to years) by scar tissue formation. Gross, histological, biochemical, and biomechanical evaluations all suggest that this scar tissue is, in some ways, similar to normal ligament tissue; however, in many ways it is not. Even after many months there are persistent structural, material, and organizational abnormalities in scars (relative to the normal ligament) that appear to be permanent in these models. Scars reach only about 30% of the ultimate failure stress of normal ligaments after 78 weeks of healing, for reasons that are still being elucidated. Possible reasons for scar weakness include the presence of many types of organizational “defects” in the scar matrix, collagen microfibrils that are smaller than normal, abnormal proportions of collagen types, the failure of collagen cross links to mature normally, and the long-lasting presence of abnormal proteoglycans in scars. In addition to high-load abnormalities, scars also show some persisting low-load changes. Their viscoelastic behaviors remain slightly altered for months to years.