Anterior Cruciate Ligament reconstruction (ACLR) can be performed with a variety of options. Bone-patellar tendon-bone (BTB) autograft is considered by many to be the gold standard. Prior studies support the use of 7mm or larger interference screws for femoral tunnel fixation. However, with modern materials and fixation techniques may allow for adequate fixation with smaller interference screw size. The use of a smaller screw for femoral sided fixation leads to preservation of the native bone and a larger surface-area to volume ratio for graft incorporation to improve biologic healing. The purpose of this study was to determine the effect of interference screw diameter on the pull-out strength and failure mode of femoral tunnel fixation in ACLR at time zero fixation.
Twenty-four fresh-frozen cadaveric knees were obtained from 17 different donors (United Tissue Network, Phoenix, AZ). Specimens were allocated to three different treatment groups (n=8 per group) based on interference screw diameter: 6mm, 7mm, or 8mm biointerference screw (Arthrex, Naples, FL). All specimens underwent DEXA scanning prior to allocation to ensure no difference in bone mineral density among groups (p=0.99). All specimens underwent femoral sided ACLR with BTB autograft. Specimens subsequently underwent mechanical testing under monotonic loading conditions to failure. The load to failure and failure mechanism were recorded. One-way analysis of variance was performed to assess for differences in load to failure. Chi-squared test was performed to assess for difference in failure mechanism. A priori power analysis was performed to ensure 90% power. Data are reported as mean (standard deviation). Significance was set at p=0.05.
Load to failure (N) for each group was 308.6 (213.3), 518 (313), and 541 (267) for 6mm, 7mm, and 8mm diameter screws, respectively. There was no difference in pull out strength among the three groups (p=0.228). One specimen in the 6mm group, 2 specimens in the 7mm group, and one specimen in the 8mm group failed by screw pullout, the remainder in each group failed by ligament failure. There was no difference in failure mode observed among the three groups (p=0.74).
No difference in pull out strength or failure mode was observed among interference screw diameters. These findings suggest that there is no difference in ACL femoral tunnel BTB pull-out strength and failure mechanism among 6mm, 7mm, and 8mm biointerference screws at time zero fixation.