Finite element investigation of the human mandible bone: examination of bone fracture, plate fixation and increasing degrees of atrophy

The mandible is the largest of the human facial bones. It is located inferiorly at the base of the skull. Fracture of the mandible bone is a frequent event due to the positioning of the bone on the face. These fractures are fixed using a number of different methods including miniplating. FEA is useful in the assessment of complex shape changes in structures such as the mandible bone by observing changes in displacements and stresses, as a result of an applied external force. FEA is used in this study. The main aims of this study are to advance the FEA modelling of the mandible to include representative muscle loads and boundary conditions, and to investigate the effect of common bone fractures, plate fixation techniques and bone atrophy on mandibular stresses. A number of finite element models of the mandible were reconstructed from Computed Tomography scan data. A “simple” and a “complex” model were developed. The simple model advanced the muscle loads and boundary conditions used in previous studies. The techniques of fracture reconstruction and plate fixation were then investigated for a number of mandibular fracture locations using the simple model. Complex models were developed which used the Cawood classification (Cawood and Howell 1988) for an atrophic mandible to design six atrophic FEA model geometries, and a severely atrophic mandible model. The effects of increasing degrees of atrophy on mandibular displacements and stresses were then investigated. Simple model results showed that for stability of any mandibular fracture, a double plate will provide the best stability as the superior and inferior borders of the fracture are supported by a plate. However the use of a double plate configuration may not always be clinically feasible due to location and patient recovery. Complex model results showed it was possible to use engineering mechanics concepts to arrive at explanations for the deformations, and peak stresses found. These predictions are important in clinical pre-surgical planning for possible fixation of an atrophic mandible. Fracture fixation results showed good agreement with current studies in the field in relation to deformation, maximum and minimum principal values. The principal stresses in each of the fracture fixation models peaked at ± 45 MPa. The double plate was concluded to be the best plate configuration in terms of stability and reducing the overall stresses in the mandible across each fracture fixation model. For the Cawood Classes of atrophy, deformations, tensile and compressive stresses increased with increasing atrophy, and decreasing sectional dimensions. There was also a common deformation and stress pattern across all of the mandible models. Bending deformations, tensile and compressive stresses were predominantly generated at the rami, angle and body regions. Peak stresses of approximately ± 55 MPa were found in these regions.