The maximum equivalent sound pressure output with RW stimulation was determined experimentally in fresh human temporal bones. Experiments were performed in analogy to the ASTM standard (F2504.24930-1) method for the output determination of implantable middle ear hearing devices (IMEHDs) in human cadaveric temporal bones (TBs). ASTM compliant temporal bones were stimulated with a prosthesis having a spherical tip (?0.5?mm) attached to the actuator. The stimulation was performed perpendicular to the round window membrane (RWM) at varying position relative to the RW and the resulting static force on the RW membrane was determined.
At each position the displacement output of the DACS PI actuator and the stapes footplate (SFP) vibration in response to actuator stimulation was measured with a Laser Doppler Velocimeter (LDV). By comparison of the achieved output at the stapes footplate in response to sound and transducer stimulation the equivalent sound pressure level at the tympanic membrane at 1Vrms input voltage was calculated assuming that the SFP displacement in both conditions is a measure of perceived loudness, as it is done in the ASTM standard.
Ten TB preparations within the acceptance range of the ASTM standard were used for analysis. The actuator driven stapes footplate displacement amplitude as well as the resulting equivalent sound pressure level was highly dependent on the static force applied to the RW. The sound transfer efficiency from the RW to the stapes footplate increased monotonically with increasing static load. At a moderate static force load (approx. 3.9?mN) the obtained average sound equivalent sound pressure level was 102-120?eq.?dB?SPL @ nominally 1Vrms input for frequencies ¡Ü4?kHz. At higher frequencies (6-10?kHz) the achieved output dropped to ¡«90?dB SPL.
This output was obtained at loading conditions compatible with the actuator safe operating range, although it was possible to increase the output further by increasing the static force load.
Our results demonstrate for a first time that static force applied to the RW is crucial for sound transmission efficiency. Further we could show that RW stimulation with the DACS PI actuator is possible having a maximum output that is sufficient to treat moderate and pronounced sensorineural hearing losses (SNHL).
This article is part of a Special Issue entitled ¡°MEMRO 2012¡±.