The biomechanical aspects of reconstruction for segmental defects of the mandible: a finite element study to assess the optimisation of plate and screw factors.

A bone plate is required to restore the load-bearing capacity of the mandible following a segmental resection. A good understanding of the underlying principles is crucial for developing a reliable reconstruction. A finite element analysis (FEA) technique has been developed to study the biomechanics of the clinical scenarios managed after surgical resection of a tumour or severe trauma to assist in choosing the optimal hardware elements. A computer aided design (CAD) model of an edentulous human mandible was created. Then 4 common segmental defects were simulated. A single reconstruction plate was designed to span the defects. The hardware variations studied were: monocortical or bicortical screw fixation and non-locking or locking plate design. A standardized load was applied to mimic the human bite. The von Mises stress and strain, spatial changes at the screw-bone interfaces were analysed. In general, the locking plate and monocortical screw fixation systems were most effective. Non-locking plating systems produced larger screw "pull-out" displacements, especially at the hemimandible (up to 5% strain). Three screws on either side of the defect were adequate for all scenarios except extensive unilateral defects when additional screws and an increased screw diameter are recommended. The simplification of screw geometry may underestimate stress levels and factors such as poor adaptation of the plate or reduced bone quality are likely to be indications for bicortical locking screw fixation. The current model provides a good basis for understanding the complex biomechanics and developing future refinements in plate or scaffold design.