Biomechanics of the facial skeleton.

Several concepts have been discussed in this article. (1) The geometry and muscle attachments of the mandible are such that it is inconsistent with physics as we know today to have this structural arrangement always in tension at the upper surface and compression always at the lower. (2) The idea behind rigid internal fixation (RIF) is to stabilize the fracture to allow the mandible to work (function) as a nonfractured structure. We should therefore attempt to simulate conditions that most accurately approximate the unfractured structure. (3) The larger, older plates worked in most situations if the fracture was reduced anatomically. As physicians use smaller devices and combinations with unicortical screws, there will be less room for error, and the stability provided by the devices will be closer to the critical load characteristics of the fractures. (4) The "neutral" axis does not remain constant in a fixed location during mastication, either in unfractured or fractured mandible scenarios. Even if a standard neutral zone existed, it would be the worst place for plate placement because devices there would not function with mechanical advantage either in tension or compression loading. (5) Tangential and comminuted fractures generally should be addressed with caution and require special attention. The probability of the smaller plates providing adequate stability in these conditions is low, and the likelihood of failure is high. Physician discretion and judgment should be used to select a more stable system if early mobilization is the goal. (6) The plating systems available rely on adequate bone apposition at the fracture site. This, coupled with a degree of compression in many scenarios, provides a stable condition under functional loading. If movement occurs at the fracture site, the load characteristics change dramatically, with much higher demands placed on the plate systems, and failures will increase based on mechanics alone. When biomechanics are not considered, the incidence of infection, nonunion, and tissue injury may increase. (7) Treatment of structural defects of the midface should be directed to the reconstruction of "normal" pretraumatic load paths. (8) When dealing with plating systems in midfacial fractures, the placement of multiple screws on each side of the fracture provides for a more even distribution of loading (load sharing between the plate and the bone). (9) Stabilization of a fracture requires prevention of translation in all three directions and rotation about all three axes. Restraining a point solves translation but not rotation. Plates provide some rotational stability. The best mechanical advantage is obtained during fixation when plates are not placed along the same axis.(ABSTRACT TRUNCATED AT 400 WORDS)