Effect of freezing and thawing on stapes-cochlear input impedance in humantemporal 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, howe ver, that freezing and thawing can cause a reduction in the magnitude of th e 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 rang e 25 Hz-7 kHz. Z(SC) is considered to be the sum of the impedances of the a nnular ligament Z(S) and the cochlea Z(C) and has been shown to be controll ed 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 thaw ing 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 ab ove about 1 kHz; and freezing can reduce \Z(S)\, which causes a reduction i n \Z(SC)\ below 1 kHz. Changes in the phase angle of Z(SC) induced by freez ing were small and were consistent with changes in \Z(SC)\. Removing air fr om 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. (C) 2000 Elsevier Science B.V. All rights reserved.