MIDDLE-EAR MECHANICS IN NORMAL, DISEASED AND RECONSTRUCTED EARS

A review of the structure-function relationships in normal, diseased a nd reconstructed middle ears is presented. Variables used to describe the system are sound pressure, volume velocity and acoustic impedance. We discuss the following: (1) Sound can be transmitted from the ear c anal to the cochlea via two mechanisms: the tympano-ossicular system ( ossicular coupling) and direct acoustic stimulation of the oval and ro und windows (acoustic coupling). In the normal ear, middle-ear pressur e gain, which is the result of ossicular coupling, is frequency-depend ent and smaller than generally believed. Acoustic coupling is negligib ly small in normal ears, but can play a significant role in some disea sed and reconstructed ears. (2) The severity of conductive hearing los s due to middle-ear disease or after tympanoplasty surgery can be pred icted by the degree to which ossicular coupling, acoustic coupling, an d stapes-cochlear input impedance are compromised. Such analyses are u sed to explain the air-bone gaps associated with lesions such as ossic ular interruption, ossicular fixation and tympanic membrane perforatio n. (3) With type IV and V tympanoplasty, hearing is determined solely by acoustic coupling. A quantitative analysis of structure-function re lationships can both explain the wide range of observed postoperative hearing results and suggest surgical guidelines in order to optimize t he post-operative results. (4) In tympanoplasty types I, II and III, t he hearing result depends on the efficacy of the reconstructed tympani c membrane, the efficacy of the reconstructed ossicular chain and adeq uacy of middle-ear aeration. Currently, our knowledge of the mechanics of these three factors is incomplete. The mechanics of mastoidectomy and stapedectomy are also discussed.