Development of a Workflow for Designing Patient-Specific Mandibular Reconstruction Implants and Predicting their Performance through Experimentally Validated Finite Element Models

Abstract

Restoration of the normal mandibular form and function is attempted with reconstructive surgery. The current standard procedure to restore continuity defects of the mandible involves free tissue transfer with an autologous bone flap. Even though the success rates are high, several critical drawbacks have been associated with this procedure, including severe donor site morbidity, long hospital stay and re­covery process, need for high surgical expertise, insufficient bone graft height, and mechanical failure of the plating system. A systematic approach for the designing and testing of patient­-specific implants used as an alternative to the free flap procedure appears to be still lacking. The goal of this thesis project was to develop a semi­automatic workflow for designing patient­ specific mandibular reconstruction implants and assess the effect of topology optimization on the biome­chanical performance of the implant using the finite element analysis (FEA) and experimental validation. Using the proposed workflow, a fully porous implant (lattice implant) and a topology optimized implant (TO implant) were designed according to the anatomy of a synthetic mandible analog subjected to lateral resection, and were additively manufactured using selective laser melting. Both implants were designed in the shape of a cage to allow for the insertion of a bone graft and the integration of dental implants. The mechanical performance of the reconstructed mandibles was predicted with computa­tional FEA and evaluated through quasi­-static and cyclic experimental testing. Digital image correlation was used to validate the finite element model through principal strain field comparison. An excellent fit between the implants and mandibles was established, indicating the capability of the workflow to develop customized implants with accurate dimensions. The results obtained with FEA were in agreement with the DIC measurements and quasi-­static testing results. No significant differences (P < 0.01) in mean stiffness, mean ultimate load, and mean ultimate displacement were found between the non-­implanted control mandibles, lattice­-implanted mandibles, and TO-­implanted mandibles during quasi-­static testing. No implant failure was observed during static nor cyclic testing at loads substantially higher than the average maximum biting force of healthy individuals, indicating the high resistance of the implant designs to mechanical failure. Yet, the lattice implant would likely be preferred over the TO implant for clinical application due to its lower weight (16.5%), higher porosity (17.4%), and shorter workflow time (633.3%). The workflow proposed in this study may offer surgeons and medical engineers the tools to sys­tematically design and evaluate patient­-specific reconstruction implants. This would result in more cost­-effective and time­-effective pre-­surgical planning and result in implant designs that can minimize morbidity and maximize aesthetic and functional outcomes.