The Effect of Interference Screw Diameter on Fixation of Soft-Tissue Grafts in Anterior Cruciate Ligament Reconstruction

Abstract

The purpose of this study was to evaluate the effect that interference screw diameter has on fixation strength of a soft-tissue anterior cruciate ligament (ACL) graft. We prepared 32 fresh-frozen bovine tibiae with 9-mm ACL tibial tunnels. Accompanying 9-mm soft-tissue bovine Achilles grafts were also prepared. Bioabsorbable interference screws of increasing diameters were used for tibial fixation. There were 4 groups, consisting of 8-, 9-, 10-, and 11-mm screws for fixation of the 9-mm graft in the 9-mm tunnel. Tensile testing and cyclic loading from 50 to 250 N at 2 Hz for a total of 1,500 cycles were performed with a hydraulic biaxial materials testing machine. Graft slippage was measured with a video analysis technique with photo-reflective markers. At the end of cyclic testing, the grafts were loaded to failure, and the ultimate strength was recorded. All grafts failed at the tendon-bone-screw interface. The ultimate strength (+/- SD) was greatest for the 11-mm screw (624 +/- 133 N), with slightly decreased strength for the 10-mm (601 +/- 54 N), 9-mm (576 +/- 85 N), and 8-mm (532 +/- 185 N) screws. Graft slippage (+/- SD) was least for the 9-mm screw (2.65 +/- 2.38 mm). There were no statistically significant differences in ultimate strength and graft slippage between screws (P = .45 and P = .34, respectively). All interference screws tested provided adequate fixation strength. The results of this study show no statistical significance for ultimate strength or graft slippage with variable screw diameter. Aperture fixation with the interference screw technique provides adequate stability for soft-tissue grafts in ACL reconstruction. Although no statistical significance was found, there was a trend toward less graft-site motion when we used a screw diameter equal to tunnel size.