Ligament grafts become more susceptible to creep within days after surgery: evidence for early enzymatic degradation of a ligament graft in a rabbit model.

Clinical evidence suggests that some ligament grafts stretch after surgery. Our purpose in this study was to quantify early postoperative creep behavior of ligament autografts in an animal model, and to explore potential mechanisms of that behavior. 38 New Zealand white rabbits underwent a unilateral, fresh, anatomic medial collateral ligament (MCL) autograft procedure and were killed immediately (time-zero), at 2 days, 3 weeks, or 8 weeks after surgery (n = 7-11 in each group). We compared the creep behavior of the autografts to normal MCLs (n = 8). An additional 7 MCL specimens were incubated for 2 days in a low concentration collagenase solution and then similarly creep-tested. All grafts were slower to recover their original length after creep than either normal ligaments or time-zero controls. These grafts started to become more vulnerable to elongation in cyclic and static creep tests within 2 days of surgery, compared to time-zero controls. This vulnerability to creep increased over the next 3 weeks, and was maintained at 8 weeks of healing. 2-day collagenase-soaked MCL specimens had the same creep strains as the 2-day autografts. These results suggest that even fresh anatomic ligament autografts become vulnerable to creep within a few days after surgery by mechanisms that may involve degradative enzymes such as collagenase.