Photoelastic evaluation of two different sagittal split ramus osteotomies in advancement surgery.

The aim of this study was to establish the influence of the design of the sagittal split ramus osteotomy (SSRO) on stress distribution on the osteosynthesis in a photoelastic resin model. Two polyurethane hemimandibles were used to perform the osteotomies, tilted in the lateral sector of the first/second molar (group I) and the other descending downwards and laterally from the first molar (group II), with no higher angle. Six replicas of each were made in photoelastic resin and stabilized with a plate and 5 mm monocortical screws in a standardized way. Stabilization was done in the SSRO without advancement, with 3 mm advancement and with 7 mm advancement. Compressive loads were applied at the level of the lower first molar in an Instron machine (model 4411) with a speed of 1 mm/min until reaching 3 mm of displacement, at which point the data was recorded with a camera to identify the stress distribution bands. The results showed stress distribution in different places: for group I it was observed mainly in the screws of the proximal segment, being more intense closer to the osteotomy; in group II I it was observed mainly in the screws of the proximal segment furthest from the osteotomy, also being distributed towards the upper area of the plate. It may be concluded that under standard osteosynthesis conditions, modifications to the SSRO produce changes in the location and distribution of stress.