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Background/objective: In anterior cruciate ligament ​reconstruction, a tendon graft, anchored by interference screws (IFSs), is frequently used as a replacement for the damaged ligament. Generally, IFSs are classified as being either metallic or polymeric. Metallic screws have sharp threads that lacerate the graft, preventing solid fixation. These constructs are difficult to image ​and can limit bone--screw integration because of the higher stiffness of the screw. Polymeric materials are often a better match to bone's material properties, but lack the strength needed to hold grafts in place. Magnesium (Mg) is a material of great promise for orthopaedic applications. Mg has mechanical properties similar to bone, ability to be seen on magnetic resonance imagings, and promotes bone healing. However, questions still remain regarding the strength of Mg-based screws. Previous ex vivo ​animal experiments found stripping of the screw drive when the full torque was applied to Mg screws during surgery, preventing full insertion and poor graft fixation. The similar design of the Mg screw led to questions regarding the relationship between material properties and design, and the ultimate impact on mechanical behaviour. Thus, the objective of this study was to analyze the stresses in the screw head, a key factor in the stripping mechanism of IFS, then use that information to improve screw design, for this material. Methods: Using finite element analysis, a comparison study of six drive designs (hexagonal, quadrangle, torx, trigonal, trilobe, and turbine) was performed. This was followed by a parametric analysis to determine appropriate drive depth and drive width. Results: It was observed that with a typical torque (2 ​Nm) used for screw insertion during anterior cruciate ligament reconstruction, the maximum von Mises and shear stress values were concentrated in the corners or turns of the drive, which could lead to stripping if the values were greater than the yield stress of Mg (193 ​MPa). With a four-time ​increase in drive depth to be fully driven and a 30% greater drive width, these maximum stress values were significantly decreased by more than 75%. Conclusion: It was concluded that improving the design of a Mg-based screw may increase surgical success rates, by decreasing device failure at insertion. The translational potential of this article: The results of this work have the potential to improve designs of degradable IFSs, allowing for greater torque to be applied and thus greater screw fixation between host bone and the graft. Such a fixation will allow greater integration, better patient healing, and ultimately improved patient outcomes. Keywords: Finite element analysis, Interference screw, Screw design, Screw insertion

The anterior cruciate ligament (ACL) is the most frequently injured ligament of the knee. Its surgical management has been using a soft tissue graft as a replacement. The results of ACL reconstruction have been successful for many patients as it can restore joint stability and relieve pain. However, longer term follow up studies have shown unsatisfactory outcomes with as many as 25% of patients having complications that include prevalence of osteoarthritis. With the advances in tissue engineering and regeneration, there is a renewed interest in healing an injured ACL in order to preserve its complex anatomy and proprioceptive responses of the native ACL. Biotherapies including microfracture have been used clinically for healing of an injured ACL. In animal studies, the use of hyaluronic acid, mesenchymal stem cells, platelet-rich plasma, and extracellular matrix bioscaffolds have also resulted in neo-tissue formation to a varying degree. In general, the ACL heal rate is slow and the lack of loading to the healing ACL has deleterious effects on its attachments to bone. Thus, in our research center, mechanical augmentation that could maintain joint stability while simultaneously loading the healing ACL has been used in combination with biological augmentation to incite more robust tissue healing. Using a goat model, the ACL was found to be well healed at 12 weeks. Robotic measurements revealed good stifle joint stability while the tensile strength of the healing ACL was 3 times greater than suture repair and 1.8 times greater than ACL reconstruction. For the future, novel biotherapies, together with precision medicine that is “big data” based, in combination with mechanical augmentation will be explored for human application. In the end, surgeons will be able to tailor biotherapies to heal the ACL on selected patients for more positive outcomes in the long term.