Analysis of mechanical stress in reconstruction plates for bridging mandibular angle defects.

Introduction: The objective of the present study was to evaluate the mechanical stress in reconstruction plates and the screw-plate-bone interface used in bridging a mandibular angle defect by means of the finite element method (FEM). The influence of plate geometry as well as the configuration and the diameter of the screws on the mechanical stress distribution was to be determined at the same time and was used as the basis for developing suggestions to optimize the design of the reconstruction plates.

Material: Based on the geometrical data of a human mandible, an angle defect bridged by a titanium reconstruction plate was generated and exposed to chewing force. First a reconstruction plate was tightly fixed with M2.7 titanium screws. Then, plate design, screw configuration and screw diameter were varied. The mechanical stress was calculated according to von Mises stress hypothesis.

Results: In the standard reconstruction plate, the result of the finite element analysis revealed stress resulting from the simulated functional loadings which far exceeded the strengths of the components. Possible clinical consequences could be a fatigue fracture of the plate itself, gradual loosening of the osteosynthesis screws and loss of bone. The stress can be reduced to less than half by increasing the diameter of the screw threads 1.5 fold.

Conclusion: Maximizing the interface between bone and reconstruction plate had a favourable effect. As a consequence of the large interface and a triangular or square configuration of the screws, the stresses could be substantially reduced, the plate could be made thinner and thus better adapted to the mandible. As a preliminary result, the newly designed reconstruction plate could be thinned in areas subject to less mechanical stress.