Engineering Location and Shape on Stiffness-Matched Niti Mandibular Graft Fixation Plates

Graft fixation plates that ensure osteotomy site stability during healing are required for skeletal reconstruction surgery to repair bone segmental defects resulting from trauma or illness. However, where graft fixation plate failure is frequent (e.g., large mandibular segmental defects), we hypothesize that fixation that leads to an osteotomy site loading pattern that does not interfere with normal loading of the healed bone will result in less hardware and bone failure and, therefore, less re-operation. Nickel-titanium’s (NiTi) superelastic properties may allow better engagement of the entire fixation device than the current standard-of-care material, Ti6Al4V. Allowing the device to increase its deformation during loading, reduces the stiffness of the entire healing area, thereby avoiding stress concentrations that may lead to failure of the reconstructed device. Herein, we use 4-point bending to test the overall engagement (ductility) of straight NiTi plates as a function of varying pore geometry according to the ASTM F382 standard. Results of 3D printed, NiTi, unbent mandibular fixation plates show no difference between varied pore geometries (i.e., orthogonal struts versus gyroid struts, both with 300 um diameter and 300 um pore size) under single-cycle failure testing. Indeed, all pore geometries studied presented an apparent elastic modulus 〈E〉=23.75±3.33 GPa. While our goal in stiffness matching is regional, it is also useful for the plate to match cortical bone elasticity (E=10-30 GPa), as it helps to engage the whole plate, indeed, the whole healing region. Location and personalization of plate shape are also expected to be helpful in healing and subsequently restoring the normal stress-strain path in the bone when it has healed. Ongoing work is looking at the relative risk of fatigue failure due to cyclic loading conditions for straight, 3D-printed, NiTi mandibular fixation plates with the same two pore geometries.