Effects of tensor tympani muscle contraction on the middle ear and markers of a contracted muscle.

OBJECTIVES/HYPOTHESIS: Many otologic disorders have been attributed to dysfunction of the tensor tympani muscle, including tinnitus, otalgia, Meniere's disease and sensorineural hearing loss. The objective of this study was to determine adequate stimuli for tensor tympani contraction in humans and determine markers of the hypercontracted state that could be used to detect this process in otologic disease. STUDY DESIGN: Multiple types of studies. METHODS: Studies included 1) measuring middle ear impedance changes in response to orbital puffs of air, facial stroking, and self-vocalization; 2) measuring changes in stapes and eardrum vibrations and middle ear acoustic impedance in response to force loading of the tensor tympani in fresh human cadaveric temporal bones; 3) measuring changes in acoustic impedance in two subjects who could voluntarily contract their tensor tympani, and performing an audiogram with the muscle contracted in one of these subjects; and 4) developing a lumped parameter computer model of the middle ear while simulating various levels of tensor tympani contraction. RESULTS: Orbital jets of air are the most effective stimuli for eliciting tensor tympani contraction. As markers for tensor tympani contraction, all investigations indicate that tensor tympani hypercontraction should result in a low-frequency hearing loss, predominantly conductive, with a decrease in middle ear compliance. CONCLUSIONS: These markers should be searched for in otologic pathology states where the tensor tympani is suspected of being hypercontracted.

Prosthetic reconstruction from the tympanic membrane to the stapes head or to the stapes footplate? A laser Doppler study.

BACKGROUND: In the absence of the incus, many surgeons believe that reconstruction from the tympanic membrane to the stapes head is more effective than reconstruction to the stapes footplate. This has rarely been tested empirically. Published better clinical results with reconstruction to the stapes head might simply reflect less underlying disease in ears with an intact stapes superstructure. OBJECTIVE: To compare vibration transmission of these two forms of prosthetic reconstruction. METHODS: A fresh human cadaveric temporal bone model was used. Round window vibrations in response to sound in the ear canal were measured with a laser Doppler vibrometer. After incus removal, the discontinuity was repaired using a titanium prosthesis. Reconstruction from the tympanic membrane to the stapes head was compared to reconstruction to the stapes footplate. RESULTS: Reconstruction of both types decreased round window vibrations by 10 to 15 dB between 500 and 3000 Hz compared to the intact middle ear. Reconstruction to the stapes head performed 5 to 10 dB better at lower frequencies (500-2000 Hz), but this was only statistically significant at 1 and 2 kHz. CONCLUSIONS: There is only a 5 to 10 dB mechanical advantage gained by reconstruction from the tympanic membrane to the stapes head compared to reconstruction to the footplate for frequencies between 1 and 2 kHz.

Effect of Silastic sheeting over the round window niche on sound transmission in the intact human middle ear.

BACKGROUND: Silastic sheeting is commonly used in middle ear surgery to prevent the formation of adhesions between the tympanic membrane and the medial bony wall of the middle ear cavity. This sheeting is often placed, advertently or inadvertently, so as to cover the round window niche. The effects of mechanically shielding the round window niche in the presence of an intact tympanic membrane and ossicular chain have not been empirically studied to date. OBJECTIVE: To investigate the effect of acoustically shielding the round window with 1 mm thick Silastic sheeting on middle ear sound transmission in otherwise intact cadaveric human temporal bones. METHODS: Using a fresh human cadaveric temporal model, a computerized laser Doppler vibrometry system was used to measure vibrations at the umbo and on the stapes footplate in response to sound introduced into the ear canal. Stapes displacement was used as a measure of sound transmission. The measurements were repeated after shielding the round window using 1 mm thick Silastic sheeting. RESULTS: We found that shielding the round window with Silastic produced no significant difference in the measurements at the stapes footplate. At the umbo, a slight increase in vibrations at 250 to 500 Hz was measured after shielding. This was on the order of 3 dB and was not statistically significant. CONCLUSION: In the presence of an intact tympanic membrane and ossicular chain, shielding the round window with Silastic sheeting has no clinically significant effect on sound transmission by the human middle ear.

Linearity and lever ratio of the normal and reconstructed cadaveric human middle ear.

HYPOTHESIS: The linearity and the level ratio are different in reconstructed ears. BACKGROUND: The linearity of the reconstructed human middle ear (ME) has not been previously explored. It is important to analyze if high sound pressure levels (SPLs) result in distortion due to nonlinearities particularly because hearing aids have high-output SPLs. The diseased ME is reconstructed with prostheses. These diseased ears frequently need additional amplification with hearing aids, and it is unclear if reconstruction itself leads to nonlinear ME responses. METHODS: Eight fresh human cadaveric temporal bones were used. Pure tones of 70, 90, and 110 dB SPL at 500, 1,000, and 3,000 Hz were presented to the ear canal. Umbo and stapes displacements were measured by means of a laser Doppler vibrometer. After removing the incus, the tympanic membrane assembly to the stapes head prosthesis was placed, and measurements were repeated. RESULTS: Stapes footplate vibrations in the reconstructed ears are 10 to 15 dB lower than those of the normal ears. In both normal and reconstructed ears, the footplate vibrations are linearly related to SPL at the tympanic membrane between 70 and 110 dB SPL at the frequencies tested. For the lever ratio, intact ears are more efficient at transmission of umbo vibrations to the stapes compared with reconstructed ears. CONCLUSION: To within acceptable limits, the ME seems to be linear between 70 and 110 dB SPL input levels, across the speech frequencies, and this does not change with reconstruction. The reconstructed human ME seems to have a less efficient lever ratio than the intact ME.