Soft tissue tendon graft fixation in serially dilated or extraction-drilled tibial tunnels: a porcine model study using high-resolution quantitative computerized tomography

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 degrees had superior fixation and insertion torques compared with specimens with angles 15 degrees 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 >or=15 degrees 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.