WOUND-HEALING AND BRAIN-STEM AUDITORY-EVOKED POTENTIALS AFTER EXPERIMENTAL VENTRAL TYMPANIC BULLA OSTEOTOMY IN DOGS

Ventral tympanic bulla osteotomy was performed in 13 normal dogs to ev aluate healing and retention of brain stem auditory evoked potentials (BSAEP). Healing was evaluated by gross and microscopic examination of the middle ears after 1 (n = 2), 4 (n = 8), 5 (n = 1), and 6 (n = 1) weeks. One dog was eliminated from the study. Brain stem auditory evok ed potentials were measured using an air-conducted sound stimulus befo re and after surgery and before killing. Nine of 12 dogs re-formed the tympanic bulla by formation of fibrous connective tissue lined with c uboidal epithelium in the osteotomy site. Four of the nine dogs had a reduced tympanic bulla volume (estimated 20% to 40% volume reduction) caused by soft tissue ingrowth through the osteotomy. Nine of 12 dogs had proliferation of subperiosteal new bone from the inner surface of the tympanic bulla that varied in severity. Three of these nine dogs h ad nearly complete obliteration of the tympanic cavity by proliferatin g subperiosteal new bone. Eight dogs had nodules of granulation tissue containing new bone and mineralization diffusely distributed on the s urfaces of the middle ear. The measured BSAEP sensitivity before killi ng was equivalent to preoperative levels in 11 dogs. The remaining dog had no change in auditory sensitivity after surgery but had a markedl y reduced BSAEP detection threshold before killing. This appeared to b e attributable to mechanical impingement on the ossicles and tympanic membrane by proliferating bone within the tympanic cavity. This study showed that after ventral tympanic bulla osteotomy the tympanic bulla rapidly re-forms with no deleterious effect on hearing in most dogs tr eated. However, the internal surfaces of the middle ear appear to be s ensitive to surgical trauma, and extensive new bone proliferation is e asily induced. This response may be extreme, resulting in middle ear o bliteration and reduced auditory sensitivity. (C) Copyright 1995 by Th e American College of Veterinary Surgeons