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Objectives:To assess the distance between the bony groove created during subannular tubes placement and the chorda tympani, and examine the depth of the hypotympanum and retrotympanum.Method:Grooves drilled in cadaver temporal bones at two levels were imaged to measure: the distance between the chorda tympani nerve and the tympanic sulcus, and the depth of the hypotympanum and the retrotympanum relative to the annulus.Results:The chorda tympani was between 0 and 5 mm from the groove cut across the annulus. The hypotympanum average depth was 2 mm (0.44–6.40 mm) and the retrotympanum average depth was 1 mm (0–2.53 mm).Conclusion:Grooves drilled across the tympanic sulcus should be placed at a point 20 per cent of the height of the tympanic membrane or lower; this will ensure least risk of injury to the chorda tympani nerve. The depth of the hypotympanum and retrotympanum dictates that the posteroinferior part of a subannular tube flange should be approximately 2 × 1 mm. [ABSTRACT FROM PUBLISHER]