Biomechanics of the weakened mandible: use of image correlation analysis.

Uninterrupted resection of mandibular bone is often necessary during maxillofacial operations for cancer. This weakens the mandible, and increases the risk of fracture. To our knowledge no biomechanical analysis has been made of deformations and strains that occur during chewing if this happens, so we have made such an analysis of the weakened mandible using a new technique: image correlation. Five fresh explanted human mandibles were prepared with black and white lacquer, and placed in a loading device that allowed replication of a physiological biting exercise. Calibrated pieces of bone were resected from the right body of each mandible. Images of the mandibular surface were recorded by 2 cameras and analysed with an algorithm to correlate them, which allowed us to confirm the distribution of strain on the body of the mandible, and to focus on the weak points. Before the bone was resected, we noted tensile strains on the alveolar border of the body, and compressive strains on the basilar border. The intensity of the strains in the posterior angle of the resected bony area then increased, with reduction in the height of the bone until fracture. The orientation of the fracture line started at the lower posterior angle of the resection area and spread in a lower posterior direction until it reached the basilar border of the mandible. Image correlation is a new technique for the study of mandibular biomechanics that provides accurate measurements on a wide bony surface with high definition images and without modification of the structure. Its application to weakened mandible provided reliable images of modifications to strains during simulated biting exercises.