Mau Jonquil R, Hawkins Kevin M, Woo Savio L-Y, Kim Kwang E, McCullough Matthew B A
Department of Chemical, Biological, and Bioengineering, North Carolina A&T State University, Greensboro, NC, USA.
Musculoskeletal Research Center, Department of Bioengineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA, USA.
J Orthop Translat. 2019 Oct 14;20:25-30. doi: 10.1016/j.jot.2019.09.003. eCollection 2020 Jan.
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 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.
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.
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%.
It was concluded that improving the design of a Mg-based screw may increase surgical success rates, by decreasing device failure at insertion.
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.
背景/目的：在前交叉韧带重建中，常用干涉螺钉（IFS）固定的肌腱移植物来替代受损韧带。一般来说，干涉螺钉分为金属材质和聚合物材质。金属螺钉的螺纹尖锐，会划破移植物，妨碍牢固固定。这些结构难以成像，且由于螺钉刚度较高，会限制骨与螺钉的结合。聚合物材料通常与骨的材料特性更匹配，但缺乏将移植物固定到位所需的强度。镁（Mg）是一种在骨科应用中有很大潜力的材料。镁具有与骨相似的力学性能，在磁共振成像上可见，并且能促进骨愈合。然而，基于镁的螺钉的强度仍存在问题。先前的动物实验发现，在手术过程中对镁螺钉施加全扭矩时，螺钉驱动器会出现剥离现象，导致无法完全插入且移植物固定不佳。镁螺钉的类似设计引发了关于材料特性与设计之间的关系以及对力学行为的最终影响的疑问。因此，本研究的目的是分析螺钉头部的应力，这是干涉螺钉剥离机制的一个关键因素，然后利用这些信息改进这种材料的螺钉设计。
使用有限元分析，对六种驱动器设计（六边形、四边形、梅花形、三角形、三叶草形和涡轮形）进行了对比研究。随后进行参数分析，以确定合适的驱动深度和驱动宽度。
观察到，在前交叉韧带重建手术中使用典型的螺钉插入扭矩（2 Nm）时，最大冯·米塞斯应力和剪应力值集中在驱动器的拐角或转弯处，如果这些值大于镁的屈服应力（193 MPa），可能会导致剥离。将驱动深度增加四倍以实现完全驱动，并将驱动宽度增加30%，这些最大应力值显著降低了75%以上。
得出的结论是，改进基于镁的螺钉设计可能会提高手术成功率，减少插入时的器械故障。
这项工作结果有潜力改进可降解干涉螺钉的设计，从而能够施加更大的扭矩，进而在宿主骨与移植物之间实现更好的螺钉固定。这样的固定将实现更好的融合、促进患者愈合，并最终改善患者预后。
