[Standardized measurement of sound transmission of different middle ear prostheses]

The sound transmission properties of three different commercially available middle ear implants and the prototype of a complete middle ear prosthesis (CoMEP) were evaluated with a special measurement system that permits standardized conditions. This system uses a mechanical middle ear model (MMM) which approximates the impedances of the tympanic membrane and inner ear. The implants were fitted under defined conditions into the MMM. Displacement of the artificial stapes footplate of the MMM was measured with an optical probe. The measurements of different middle ear prostheses showed that the mass of the implant was the most important factor for optimum high frequency transmission. The lightest implant (4 mg, titanium) showed the best results. The CoMEP revealed the highest sensitivity because of a slight enlargement of the diameter of the artificial tympanic membrane. These findings show that the CoMEP is able to restore sound transmission to a normal range. In a second experiment one of the implants was fitted into the MMM with different forces. The increase of stiffness produced a slightly better high frequency transmission at the expense of low frequency sensitivity. Hence, the fitting of the implant may not be too stiff because of its loss of mobility within a significant frequency range.