Biomechanical analysis of femoral tunnel pull-out angles for anterior cruciate ligament reconstruction with bioabsorbable and metal interference screws

BACKGROUND: Fixation strength of metal and bioabsorbable interference screws has not been evaluated while varying the anterior cruciate ligament graft tension angle. HYPOTHESIS: There is no difference in fixation strength between 2 types of interference screws for anterior cruciate ligament graft fixation while the graft tension angle is varied relative to the femoral tunnel. STUDY DESIGN: Controlled laboratory study. METHODS: Forty-eight anterior cruciate ligament reconstructions were performed using immature porcine femurs stripped of soft tissue and doubled-over porcine flexor digitorum profundus tendon grafts. Specimens were randomized to bioabsorbable or titanium interference screw fixation. Specimens were randomized to one of three pull angles (0 degrees , 30 degrees , 60 degrees ) representing loading at different knee flexion angles (n = 8/group). Reconstructed ligaments were tensioned to 10 N followed by 200 loading cycles between 10 and 150 N and a final failure test. Construct elongation (mm) at 100 and 200 cycles and failure load (N) were analyzed using a 2-way analysis of variance (P < .05). RESULTS: Screw material interacted significantly with graft tension angle, as the bioabsorbable screw specimens demonstrated significantly greater fixation strength when tensioned at greater angles. Specimens fixed with bioabsorbable screws showed significantly less elongation at both 100 and 200 cycles and significantly greater failure load compared with titanium screws. CONCLUSION: Bioabsorbable interference screws acutely have increased fixation strength compared with titanium interference screws for anterior cruciate ligament grafts loaded at greater tension angles. CLINICAL RELEVANCE: The strength of anterior cruciate ligament reconstruction fixation increases with increasing divergence between the tension angle and femoral tunnel, a condition seen when the knee approaches full extension.