Sức mạnh của postfixation trong femurs là 274 N, trong khi cố định can thiệp, nó là 543 N. Trong tibias, sức mạnh postfixation 343 N, và các ốc vít giao thoa sức mạnh 527 N. Có một sự khác biệt có ý nghĩa thống kê giữa hai loại định hình. | 262 Hill An and Young The strength of the postfixation in femurs was 274 N whereas for interference fixation it was 543 N. In tibias the postfixation strength was 343 N and the interference screw strength 527 N. There was a statistically significant difference between the two types of fixation. All grafts failed at the point of fixation. Shapiro et al. 47 investigated the effect of screw size on the pullout strength of in vitro ACL reconstruction using bovine knees. Results showed no notable difference between 7- and 9-mm interference screws. However in another study it was found that a 9-mm tibial interference screw disengaged from the bone tunnel at significantly higher load to failure than a 7-mm screw. This same study also showed that a failed bone plug fixed by a 7-mm screw could be refixed successfully with a 9-mm screw 57 . The screw diameter alone may not be as important as its relationship to the gap that exists between the bone block and bone tunnel. A biomechanical study in a porcine model showed that a 7-mm screw in a 1- or 2-mm gap gave equal failure strength to a 9-mm screw in a 3- or 4-mm gap 58 . Studies using human cadaver bone yielded similar results 59 . Additionally a study by Brown et al. 60 showed that the amount of interference defined as the screw outer diameter minus the tunnel-bone block gap correlated with failure load but gap size alone did not. The metallic interference screw length of 20 mm appears to be sufficient for routine bone plug fixation 61 62 . Screw divergence from the bone plug is commonly seen on postoperative radiographs particularly with endoscopic placement however its clinical relevance remains controversial 54 63 64 . A biomechanical study in fresh paired bovine knees showed no variation in strength or stiffness when the femoral screw had a divergence of 15 from the bone plug 65 but Jomha et al. 66 demonstrated that there was a significant weakening of fixation for a screw-bone plug angle equal to or greater than 20 .
