Mechanical and acoustic analysis of middle ear reconstruction.

Existing mechanical and acoustic models of the human middle ear are used to investigate the expected performance of various middle ear reconstructive procedures. Although the results of these analyses generally are consistent with observed surgical results, such an analytic approach enables the formation of some simple rules for maximizing middle ear function after stapedectomy and tympanomastoid surgery. Model analyses predict the following: (1) stapes prostheses of 0.6 to 0.8 mm in diameter produce air-bone gaps of less than 12 dB; smaller diameter prostheses produce larger air-bone gaps; (2) the mass of a stapedectomy or ossicular replacement prosthesis can be 16 times greater than that of the stapes with little effect on the hearing result; (3) the volume of the middle ear air spaces after tympanomastoid surgery should be at least 0.5 cc for optimal acoustic results; and (4) in a type IV tympanoplasty, the effectiveness of the round window graft (acoustic shield) is an important parameter determining the hearing result; the shield should be as impedant (stiff) as possible for optimal postoperative hearing. The model framework described in this report hopefully can be used to investigate the effect of variations in other surgical and help define new procedures.