Effect of freezing and thawing on stapes-cochlear input impedance in human temporal bones

Abstract

The use of thawed frozen temporal bones offers advantages over fresh bones in the study of middle-ear and inner-ear mechanical function. We show, however, that freezing and thawing can cause a reduction in the magnitude of the input impedance of the stapes and cochlea Z SC in unfixed temporal bones from human cadavers of as much as a factor of 3–10 over the frequency range 25 Hz–7 kHz. Z SC is considered to be the sum of the impedances of the annular ligament Z S and the cochlea Z C and has been shown to be controlled by Z S below 1 kHz and by Z C at higher frequencies [Merchant et al., 1996. Hear. Res. 97, 30–45]. Experiments in which the inner ear was opened, drained, and refilled identified two mechanisms by which freezing and thawing can cause a reduction in the magnitude of Z SC (| Z SC |). Freezing can allow air to enter the inner ear, with the result that | Z C | is reduced above about 1 kHz; and freezing can reduce | Z S |, which causes a reduction in | Z SC | below 1 kHz. Changes in the phase angle of Z SC induced by freezing were small and were consistent with changes in | Z SC |. Removing air from the inner ear returned Z C to near its value in fresh bones, but | Z SC | remained lower in some thawed bones by a factor of 2–3. Investigations of middle-ear function for which Z SC is critical should use fresh temporal bones only or should allow for the possible reduction in | Z SC | in thawed frozen bones.