Introduction: Mechanical factors are thought to be one of the main reasons in determining tunnel enlargement after ACL reconstruction with hamstrings. The purpose of this prospective study was to evaluate how the different techniques may affect the bone tunnel enlargement.

Material and Method: Forty-five consecutive patients undergoing ACL reconstruction with the use of autologous doubled semitendinosus and gracilis tendons entered this study. They were randomly assigned to enter group A (In-Out technique, with cortical fixation and Interference screw) and group B (Out-In technique, metal cortical fixation on the femour and tibia). At a mean follow-up of 10 months, all the patients underwent clinical evaluation and a CT scan exam to evaluate the post-operative diameters of both femoral and tibial tunnels.

Results: The mean femoral tunnel diameter increased significantly from 9.05±0.3 mm (post op) to 10.01±2.3 mm (follow-up) in group A and from 9.04±0.8 mm to 9.3±1,12 mm in group B. The mean tibial tunnel diameter increased significantly from 9.03±0.04 mm to 10,68±2.5 mm in group A and from 9.04±0.03 mm to 10.±0,78 mm in group B. The mean increase in both femoral and tunnel diameters observed in group A was significantly higher than that observed in group B (p< 0.05). Stability evaluated with kt 1000 don’t significantly differ in the two groups

Conclusion: The results of this study suggest that different angular orientation techniques and different hardware devices may affect tunnel enlargement after hamstrings reconstruction. The reason can be reached from the different stiffness of the devices and their backlashes on the tunnels walls.