Middle-ear mechanics of Type III tympanoplasty (stapes columella): I. Experimental studies

Objective: To investigate the mechanics of Type III tympanoplasty by developing a cadaveric temporal bone model. Background: Type Ill stapes columella tympanoplasty involves the placement of a tympanic membrane graft, usually made of temporalis fascia, directly onto the stapes head. The procedure is usually done in conjunction with a canal wall down mastoidectomy. Postoperative hearing results vary widely, with air-bone gaps of 10 to 60 dB. The structural features responsible for the wide range in hearing results have not been systematically investigated. Methods: Canal wall down Type Ill procedures were performed in eight cadaveric temporal bones. Acoustic stimuli were presented in the ear canal, and round window velocity V-RW (used as an index of hearing) was measured, while systematically varying stapes mobility, mechanical properties of tympanic membrane graft, and tightness of connection between tympanic membrane graft and stapes. The effect of interposing a thin cartilage disc between the tympanic membrane graft and stapes head was also assessed. Results: When the middle ear was aerated and the stapes was mobile, VRW was 15 to 30 dB lower than in an intact, normal ear. Stapes fixation led to a significant reduction in V-RW; reduction was greatest at low frequencies. There was little effect of varying the tightness of connection between the tympanic membrane graft and stapes head. Sound energy was transmitted from the graft to the stapes as long as the graft was in physical contact with the stapes head. Different tympanic membrane graft materials with a range of mechanical properties (stiffness and mass) resulted in little variation in V-RW. Interposing a thin cartilage disc between the tympanic membrane graft and stapes improved V-RW in the lower frequencies by 5 to 10 dB. The authors hypothesize that the disc acted to increase the effective vibrating area of the graft. Conclusions: The feasibility of using a cadaveric temporal bone model to study the mechanics of Type Ill tympanoplasty was demonstrated. A mobile stapes and aerated middle ear were essential for a successful Type Ill tympanoplasty. There was little effect of varying the mechanical properties of the tympanic membrane graft or changing the tightness of connection between the graft and stapes head. Improved results were achieved by interposing a thin cartilage disc between the graft and stapes head to increase the effective vibrating area of the graft.