Finite element analysis of dental implant loading on atrophic and non-atrophic cancellous and cortical mandibular bone - a feasibility study.

The first aim of this study was to assess displacements and micro-strain induced on different grades of atrophic cortical and trabecular mandibular bone by axially loaded dental implants using finite element analysis (FEA). The second aim was to assess the micro-strain induced by different implant geometries and the levels of bone-to-implant contact (BIC) on the surrounding bone. Six mandibular bone segments demonstrating different grades of mandibular bone atrophy and various bone volume fractions (from 0.149 to 0.471) were imaged using a micro-CT device. The acquired bone STL models and implant (Brånemark, Straumann, Ankylos) were merged into a three-dimensional finite elements structure. The mean displacement value for all implants was 3.1 ±1.2 µm. Displacements were lower in the group with a strong BIC. The results indicated that the maximum strain values of cortical and cancellous bone increased with lower bone density. Strain distribution is the first and foremost dependent on the shape of bone and architecture of cancellous bone. The geometry of the implant, thread patterns, grade of bone atrophy and BIC all affect the displacement and micro-strain on the mandible bone. Preoperative finite element analysis could offer improved predictability in the long-term outlook of dental implant restorations.

Finite element analysis of customized reconstruction plates for mandibular continuity defect therapy.

Large mandibular continuity defects pose a significant challenge in oral maxillofacial surgery. One solution to this problem is to use computer-guided surgical planning and additive manufacturing technology to produce patient-specific reconstruction plates. However, when designing customized plates, it is important to assess potential biomechanical responses that may vary substantially depending on the size and geometry of the defect. The aim of this study was to assess the design of two customized plates using finite element method (FEM). These plates were designed for the reconstruction of the lower left mandibles of two ameloblastoma cases (patient 1/plate 1 and patient 2/plate 2) with large bone resections differing in both geometry and size. Simulations revealed maximum von Mises stresses of 63 MPa and 108 MPa in plates 1 and 2, and 65 MPa and 190 MPa in the fixation screws of patients 1 and 2. The equivalent strain induced in the bone at the screw-bone interface reached maximum values of 2739 micro-strain for patient 1 and 19,575 micro-strain for patient 2. The results demonstrate the influence of design on the stresses induced in the plate and screw bodies. Of particular note, however, are the differences in the induced strains. Unphysiologically high strains in bone adjacent to screws can cause micro-damage leading to bone resorption. This can adversely affect the anchoring capabilities of the screws. Thus, while custom plates offer optimal anatomical fit, attention should be paid to the expected physiological forces on the plates and the induced stresses and strains in the plate-screw-bone assembly.

Inaccuracies in additive manufactured medical skull models caused by the DICOM to STL conversion process.

INTRODUCTION: The process of fabricating physical medical skull models requires many steps, each of which is a potential source of geometric error. The aim of this study was to demonstrate inaccuracies and differences caused by DICOM to STL conversion in additively manufactured medical skull models. MATERIAL AND METHODS: Three different institutes were requested to perform an automatic reconstruction from an identical DICOM data set of a patients undergoing tumour surgery into an STL file format using their software of preference. The acquired digitized STL data sets were assessed and compared and subsequently used to fabricate physical medical skull models. The three fabricated skull models were then scanned, and differences in the model geometries were assessed using established CAD inspection software methods. RESULTS: A large variation was noted in size and anatomical geometries of the three physical skull models fabricated from an identical (or "a single") DICOM data set. CONCLUSIONS: A medical skull model of the same individual can vary markedly depending on the DICOM to STL conversion software and the technical parameters used. Clinicians should be aware of this inaccuracy in certain applications.