OBJECTIVETo investigate significance of the distribution and thickness of cancellous and cortical bone in the mandibular ramus using computed tomography (CT) in the design of osteotomy at the medial aspect of the ramus.

METHODS45 cases with class III prognathism underwent CT before operation. The distribution and thickness of cancellous and cortical bone in the mandibular ramus was measured every 2.5 mm thick from mandibular lingular plane to 20 mm above it.

RESULTSThe upper ramus was separated by cancellous bone in only 37.5% of the cases. The other cases had no cancellous bone in the upper ramus. 5 mm above the lingual, the thickness of cortical bone at the lingual aspect decreased from the front to the behind with an average thickness of 1.55 mm. The distance from lingual to the fusion at the posterior border of ramus was 9.45 mm.

CONCLUSIONSThe medial cortical osteotomy should be located within 5 mm above the lingual and within 9.45 mm posterior to the lingular. The osteotomy could be slightly oblique and become deeper from behind to the front with an average depth of about 2 mm.