Experimental evaluation of three osteosynthesis devices used for stabilizing condylar fractures of the mandible.

INTRODUCTION: The purpose of the study is to evaluate experimentally the quality of the primary stability achieved in treating low subcondylar fractures by means of three different osteosynthesis devices. MATERIAL AND METHODS: The devices, a standard four-hole plate, an axial lag screw and a three-dimensional rectangular plate were tested on fresh isolated human mandibles. Testing was done on a test bench by reproducing static biting exercises between the first molars on the side of the fracture. The quality of the osteosynthesis was assessed by measuring the macroscopic amount of fragment displacement and on the device's ability to diffuse the mechanical strain within the fractured area by photoelastic stress analysis. RESULTS: The straight plates provided the worst restoration. This was explained by the unfavourable position of the plate along compression lines. The axial lag screws allowed average stability. This was due to the difficulty of intra-medullary positioning of the screw, and by the compression of the fracture line. Rectangular plates allowed good stability associated with rather good restitution of the strains. These good results were assigned to the shape of the plate, one of its arms approximating the tensile strain lines. CONCLUSION: Positioning and shape of the osteosynthesis device are of prime importance for condylar fracture stabilization. None of the three tested devices was optimal but the three-dimensional plate was the best. There is a need to develop the geometry of new plates.

Photoelastic analysis of bone deformation in the region of the mandibular condyle during mastication.

INTRODUCTION: The purpose of this experimental study was to demonstrate the stress patterns arising in the region of the mandibular condyle during mastication. MATERIAL AND METHOD: Stress analysis was performed using reflection photoelasticity to demonstrate deformation occurring on the surface of the mandible. The rami of three fresh dentate human mandibles were coated with a shell of photoelastic resin. Using a novel loading device, these mandibles were then subjected to external forces (muscular traction, resultant mandibular forces and intra-articular reaction force) reproducing a unilateral biting task between the first right molars. Deformations were measured from the working side. RESULTS: The trace of isostatic lines from the isoclinic fringes revealed major differences in stress distributions between the three mandibles. These differences were attributed to differences in shape between the three mandibles. Conversely, we consistently found compressive stress patterns along the posterior border of the ramus and tensile stress patterns along the anterior border of the ramus and in the zone situated below the siqmoid notch. CONCLUSION: These findings suggest that during this particular task, the mandible is subjected to sagittal forces which tend to straighten the mandibular angle. This implies that new concepts are needed for the positioning of osteosynthesis plates in the condylar region, close to the tensile strain lines, as has been recommended also for other parts of the mandible when applying semi-rigid internal fixation.