OBJECTIVETo investigate the safety range of drilling and fenestration on promontory inferior to the oval window in difficult stapedectomy via anatomical study of the relationship between the inferior margin of oval window and the endosteum of basal cochlear turn.

METHODSBy means of histological sections, the distances between the inferior margin of oval window and different locations of endosteum of basal cochlear turn, the corresponding height of the projecting endosteum and the thickness of promontory bone, on three vertical sections through the anterior, posterior and midpoint of the inferior edge of oval window were measured respectively.

RESULTSThe promontory bone inferior to the posterior point of the oval window was thickest, with an average thickness of about 1.1 mm. The endosteum of basal turn at this portion was lowest, with an average height of 0.2 mm. Furthermore, the shortest distance here between the inferior edge of oval window and the endosteum could be no more than 0.3 mm.

CONCLUSIONRelatively safe location of drilling on promontory should be selected on promontory inferior to the posterior point of the oval window, with the direction of drilling outward and downward.