Creation of an incus recess for a middle-ear microphone using a drill or laser ablation: a comparison of equivalent noise level and middle ear transfer function

Abstract

Purpose: Studies have assessed the trauma and change in hearing function from the use of otological drills on the ossicular chain, but not the effects of partial laser ablation of the incus. A study of the effectiveness of a novel middle-ear microphone for a cochlear implant, which required an incus recess for the microphone balltip, provided an opportunity to compare methods and inform a feasibility study of the microphone with patients.

Methods: We used laser Doppler vibrometry with an insert earphone and probe microphone in 23 ears from 14 fresh-frozen cadavers to measure the equivalent noise level at the tympanic membrane that would have led to the same stapes velocity as the creation of the incus recess.

Results: Drilling on the incus with a diamond burr created peak noise levels equivalent to 125.1-155.0 dB SPL at the tympanic membrane, whilst using the laser generated equivalent noise levels barely above the baseline level. The change in middle ear transfer function following drilling showed greater variability at high frequencies, but the change was not statistically significant in the three frequency bands tested.

Conclusions: Whilst drilling resulted in substantially higher equivalent noise, we considered that the recess created by laser ablation was more likely to lead to movement of the microphone balltip, and therefore decrease performance or result in malfunction over time. For patients with greatly reduced residual hearing, the greater consistency from drilling the incus recess may outweigh the potential benefits of hearing preservation with laser ablation.

Keywords: Equivalent noise level; Laser Doppler vibrometry; Middle ear microphone; Middle ear transfer function; Noise-induced hearing loss.

Fig. 1

Geometric mean (thick solid line) and 95% confidence interval (shaded grey region) of the 18 METFs that satisfied the inclusion criteria. The modified Rosowski criteria [23, 25] are shown by the dashed lines

Fig. 2

Example waveforms (left panels) and equivalent noise spectra (right panels) of Laser Doppler vibrometry recordings made during formation of an incus recess by drilling (top panels, cadaver C7R-D), or laser ablation (bottom panels, cadaver C10L-L). The waveforms show the velocity of the stapes and the equivalent noise spectra show the sound pressure level (SPL) across frequency of an acoustic signal at the position of the ear insert that would lead to equivalent energy at the stapes due to the drilling or ablation. The vertical reference lines in the plots of stapes velocity show the time windows used in the spectral analysis. Note that the waveforms for the drilling (a) and laser ablation (c) are shown on different axes

Fig. 3

Equivalent noise level in 1/3 octave frequency bands during formation of the incus recess using a drill or laser ablation. Box whiskers are shown for frequency bands with centre frequencies at 500 Hz, 1 kHz, 2 kHz, and 4 kHz. Boxes show the 25 and 75th percentiles with the median shown by horizontal lines. The outliers outside 1.5 times the interquartile range are shown by the diamond symbol

Fig. 4

Change in the normalized velocity of the stapes for individual cadavers following formation of a recess in the incus using a drill and b laser

Fig. 5

Mean change in the METFs following a recess in the incus made with a drill (N = 8) or laser (N = 10). Box whiskers are shown for all frequencies and for three sub-bands: up to 1000 Hz, 1000 Hz up to 4000 Hz, 4000 Hz and above. Boxes show the 25 and 75th percentiles with the median shown by horizontal lines. Outliers outside 1.5 times the interquartile range are shown by the diamond symbols