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

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.