Soft Tissue Tendon Graft Fixation in Serially Dilated or Extraction-Drilled Tibial Tunnels

Background: Tibial tunnel preparation may contribute to improved soft tissue graft fixation. Hypothesis: Step dilation produces greater tunnel wall bone volume than does extraction drilling and increases fixation strength. Bioabsorbable interference screw divergence decreases fixation strength, regardless of tunnel preparation method. Study Design: Controlled laboratory study. Methods: Twenty porcine tibias were divided into 2 groups of 10 with matching mean apparent bone mineral density. One group received 9-mm-diameter extraction-drilled tunnels, and the other group received 7-mm-diameter extraction-drilled tunnels followed by step dilation to 9 mm. High-resolution quantitative computerized tomography scans and voxel analysis techniques determined tunnel wall bone volume fraction. Screws secured 8.5-mm-diameter porcine grafts in the tunnels. Repeat scans were used to determine screw divergence. Cyclic loading was performed in a servohydraulic device before load to failure testing. Results: The step dilation group had greater tunnel wall bone volume/total volume than did the extraction drilled group; however, a significant increase in fixation strength was not detected. Specimens with screw divergence angles less than 15° had superior fixation and insertion torques compared with specimens with angles 15° or more. Screw divergence correlated more strongly with fixation strength than did mean apparent bone mineral density or screw insertion torque. Conclusion: Step dilation increased tunnel wall bone volume/total volume, but fixation strength did not improve. Screw divergence ≥15° decreases graft–bone tunnel fixation whether or not step dilation is performed. Clinical Relevance: Screw alignment plays a greater role in anterior cruciate ligament graft fixation than does extraction drilling or step dilation tunnel preparation methods in healthy bone. Keywords: anterior cruciate ligament (ACL) reconstruction bone density porcine model biomechanics bioabsorbable screw