Magnesium alloy based interference screw developed for ACL reconstruction attenuates peri-tunnel bone loss in rabbits

Peri-tunnel bone loss after anterior cruciate ligament (ACL) reconstruction is often observed clinically, which may detrimentally affect tendon graft integration with surrounding bone tissue. Biodegradable magnesium (Mg) based fixators in terms of interference screws may be suitable for fixation of the tendon graft due to their favorable effects on promotion of new bone formation. However, the poor mechanical strength of Mg is still one of the major challenges for its clinical applications. The addition of alloying elements into Mg is one of the strategies to improve their mechanical properties. Here, we prepared magnesium (Mg)-(4 and 6 wt%) zinc (Zn)-(0.2, 0.5, 1 and 2 wt%) strontium(Sr) alloys and tested their potential for attenuating peri-tunnel bone loss in ACL reconstruction. The optimal (6 wt%) Zn and (0.5 wt%) Sr contents were screened with respect to the microstructures, mechanical properties and corrosion behavior of these alloys. As compared to pure Mg, Mg-6Zn-0.5Sr rods and screws showed significantly higher torque and torsional stiffness in both numerical and experimental analysis. The in vitro cyto-compatibility of Mg-6Zn-0.5Sr alloy was assessed with MTT test and fluorescence assay. The Mg-6Zn-0.5Sr interference screw was designed for fixation of the tendon graft to the femoral tunnel in a rabbit model of ACL reconstruction, with a commercially available poly-lactide (PLA) screw for comparison. In vivo high resolution peripheral quantitative computed tomography (HR-pQCT) scanning was performed to measure the degradation behavior of Mg-6Zn-0.5Sr interference screws and peri-tunnel bone quality at 0, 6, 12 and 16 weeks post-surgically. Mg-6Zn-0.5Sr interference screw was completely degraded within 12 weeks after surgery. The peri-tunnel bone loss was significantly attenuated in the Mg-6Zn-0.5Sr group when compared to the PLA group. Importantly, the bony ingrowth rapidly filled the cavity left by the complete degradation of Mg-6Zn-0.5Sr screws at 16 weeks. In histological analysis, more bone formation was observed in peri-tunnel region in the Mg-6Zn-0.5Sr group in comparison to the PLA group at 6 and 16 weeks after surgery. The femur-tendon graft-tibia complex was harvested at the end of week 6 and 16 post-operation for tensile testing. The maximum load to failure was significantly improved in the Mg-6Zn-0.5Sr group at week 16 post-operation. Therefore, our results indicate the potential clinical application of MgZnSr based interference screws in ACL reconstruction.