The effect of rim mandibulectomy configuration and residual segment size on postoperative fracture risk: an in vitro study.

PURPOSE: This study evaluated the effect that size and shape of rim mandibulectomy has on residual mandibular strength and resistance to fracture, with the ultimate goal of improving the use of this technique and establishing a threshold for the application of prophylactic internal fixation. MATERIALS AND METHODS: Ten partially dentate dry human mandibles were mounted in a manner replicating the pterygomasseteric musculature. Unilateral midbody serial rim excisions of increasing size were completed in both a curvilinear and right-angled fashion, and physiologic forces were gradually applied to a level of residual segment fracture. Dimensional measurements were taken of the specimens before and after fracture to understand which factors contributed to failure. RESULTS: On average, a curvilinear excision configuration resisted higher occlusal forces with a smaller residual segment than did the right-angled excision configuration. A residual inferior border thickness of less than 9 mm could not predictably withstand force application within the limits of the experimental model. CONCLUSION: These data support the use of a curvilinear excision configuration for rim mandibulectomies. Reproducibility of the location of the residual segment failure and other consistent residual segment dimensional information have implications regarding the limitations of this technique, beyond which the use of adjunctive support in the form of maxillomandibular fixation or prophylactic internal fixation may be indicated.

Orthodontic management of the patient undergoing mandibular distraction osteogenesis.

Distraction osteogenesis is a gradual incremental bone-lengthening technique that requires very precise treatment planning and surgical execution. Subtle variation in the position of the corticotomy/osteotomy or in the position of the distraction device will affect the ultimate position of the tooth-bearing (distal) segment. Other functional and/or anatomic factors (that at this time are not well understood) also have an influence on the direction in which the tooth-bearing segment moves during distraction. These factors combine to create a discrepancy between the planned direction and the actual or observed direction of distraction. The charge for the orthodontist is to plan, as carefully as possible and in concert with the surgeon, the direction (vector) in which the tooth-bearing segment will travel during distraction. Further, the orthodontist must be able to modify the position that the tooth-bearing segment takes by exerting orthopedic and orthodontic forces and by making adjustments to the distraction device. In addition, the orthodontist must prepare the dentition before distraction, manage the dentition during distraction, and finish the occlusion after distraction in a manner that is different from more traditional jaw-respositioning techniques.

Strain gauge analysis of the frontozygomatic region of the zygomatic complex.

PURPOSE: This study evaluates the biomechanical consequences of physiologic forces acting on the frontozygomatic suture region, with the ultimate goal of defining appropriate fixation techniques and improving the surgical management of this complex anatomic region. MATERIALS AND METHODS: Single foil strain gauges were bonded to the cortical surface of the frontal process of the zygoma in 13 subadult Cercopithecus africanus. Subdermal electrodes were used to induce tetanic contractions of the jaw elevator muscles, and bite force was measured using a twin-beam bite force transducer placed between the teeth. Microstrain was simultaneously measured and recorded. RESULTS: Tensile strains predominated in the region of the frontal process of the zygoma, with balancing side strains twice as large as working side strains; incisal strains were intermediate. CONCLUSIONS: The tensile strains observed in the frontozygomatic region contradict the concept of this area as a "stress riser" for compressive occlusal forces, and supports the use of compression plate osteosynthesis for improved stabilization of fractures in this region.