Enhancement of Bone Implants by Substituting Nitinol for Titanium (Ti-6Al-4V): A Modeling Comparison

Abstract

Mandibular segmental defect reconstruction is most often necessitated by tumor resection, trauma, infection, or osteoradionecrosis. The standard of care treatment for mandibular segmental defect repair involves using metallic plates to immobilize fibula grafts, which replace the resected portion of mandible. Surgical grade 5 titanium (Ti-6Al-4V) is commonly used to fabricate the fixture plate due to its low density, high strength, and high biocompatibility. One of the potential problems with mandibular reconstruction is stress shielding caused by a stiffness mismatch between the Titanium fixation plate and the remaining mandible bone and the bone grafts. A highly stiff fixture carries a large portion of the load (e.g., muscle loading and bite force), therefore the surrounding mandible would undergo reduced stress. As a result the area receiving less strain would remodel and may undergo significant resorption. This process may continue until the implant fails. To avoid stress shielding it is ideal to use fixtures with stiffness similar to that of the surrounding bone. Although Ti-6Al-4V has a lower stiffness (110 GPa) than other common materials (e.g., stainless steel, tantalum), it is still much stiffer than the cancellous (1.5–4.5 GPa) and cortical portions of the mandible (17.6–31.2 GPa). As a solution, we offer a nitinol in order to reduce stiffness of the fixation hardware to the level of mandible. To this end, we performed a finite element analysis to look at strain distribution in a human mandible in three different cases: I) healthy mandible, II) resected mandible treated with a Ti-6Al-4V bone plate, III) resected mandible treated with a nitinol bone plate. In order to predict the implant’s success, it is useful to simulate the stress-strain trajectories through the treated mandible. This work covers a modeling approach to confirm superiority of nitinol for mandibular reconstruction. Our results show that the stress-strain trajectories of the mandibular reconstruction using nitinol fixation is closer to normal than if grade 5 surgical titanium fixation is used.