Bone conduction in Thiel-embalmed cadaver heads

Abstract

Introduction: Sound can reach the inner ear via at least two different pathways: air conduction and bone conduction (BC). BC hearing is used clinically for diagnostic purposes and for BC hearing aids. Research on the motion of the human middle ear in response to BC stimulation is typically conducted using cadaver models. We evaluated middle ear motion of Thiel-embalmed whole-head specimens in terms of linearity, reproducibility, and consistency with the reported middle ear motion of living subjects, fresh cadaveric temporal bones, and whole-heads embalmed with a Non-Thiel solution of salts.

Methods: We used laser Doppler vibrometry to measure the displacement of the skull, the umbo, the cochlear promontory, the stapes, and the round window in seven ears from four human whole-head specimens embalmed according to Thiel's method. The ears were stimulated with a Baha(®) implanted behind the auricle.

Results: The Thiel model shows promontory velocity similar to that reported in the literature for whole-heads embalmed with a Non-Thiel solution of salts (0- to 7-dB difference). The Thiel heads' relative velocity of the stapes with respect to the promontory was similar to that of fresh cadaver temporal bones (0- to 4-dB difference). The velocity of the umbo was comparable in Thiel-embalmed heads and living subjects (0- to 10-dB difference). The skull and all middle ear elements measured responded linearly to different stimulation levels, with an average difference less than 1 dB. The variability of repeated measurements for both short- (2 h; 4 dB) and long-term (4-16 weeks; 6 dB) repetitions in the same ear, and the difference between the two ears of the same donor (approximately 10 dB) were lower than the inter-individual difference (up to 25 dB).

Conclusion: Thiel-embalmed human whole-head specimens can be used as an alternative model for the study of human middle ear mechanics secondary to BC stimulation. At some frequencies, differences from living subjects must be considered.

Keywords: AC; BC; CP; LDV; RMSD; RW; SK; ST; STD; TM; air conduction; bone conduction; cochlear promontory; laser Doppler vibrometer; root-mean-square deviation; round window; skull; standard deviation; stapes; tympanic membrane.