Drill-induced Cochlear Injury During Otologic Surgery: Intracochlear Pressure Evidence of Acoustic Trauma

Abstract

Hypothesis: Drilling on the incus produces intracochlear pressure changes comparable to pressures created by high-intensity acoustic stimuli.

Background: New-onset sensorineural hearing loss (SNHL) following mastoid surgery can occur secondary to inadvertent drilling on the ossicular chain. To investigate this, we test the hypothesis that high sound pressure levels are generated when a high-speed drill contacts the incus.

Methods: Human cadaveric heads underwent mastoidectomy, and fiber-optic sensors were placed in scala tympani and vestibuli to measure intracochlear pressures (PIC). Stapes velocities (Vstap) were measured using single-axis laser Doppler vibrometry. PIC and Vstap were measured while drilling on the incus. Four-millimeter diamond and cutting burrs were used at drill speeds of 20k, 50k, and 80k Hz.

Results: No differences in peak equivalent ear canal noise exposures (134-165 dB SPL) were seen between drill speeds or burr types. Root-mean-square PIC amplitude calculated in third-octave bandwidths around 0.5, 1, 2, 4, and 8 kHz revealed equivalent ear canal (EAC) pressures up to 110 to 112 dB SPL. A statistically significant trend toward increasing noise exposure with decreasing drill speed was seen. No significant differences were noted between burr types. Calculations of equivalent EAC pressure from Vstap were significantly higher at 101 to 116 dB SPL.

Conclusion: Our results suggest that incidental drilling on the ossicular chain can generate PIC comparable to high-intensity acoustic stimulation. Drill speed, but not burr type, significantly affected the magnitude of PIC. Inadvertent drilling on the ossicular chain produces intense cochlear stimulation that could cause SNHL.

Figure 1

Baseline V_Stap, P_SV, P_ST, and P_Diff transfer function magnitudes to air conducted stimuli. Response magnitudes recorded from six specimens are shown normalized to the SPL recorded in the ear canal (P_EAC). Responses are superimposed onto the 95% CI and range of responses (gray bands) observed previously^,.

Figure 2

Example recording during drilling on the incus in a single specimen. V_Stap, P_SV, P_ST, and P_Diff shown in the tracings from top to bottom, with amplitudes autoscaled to the figure to facilitate visual comparison between channels.

Figure 3

Schematic of methods for deriving estimated equivalent ear canal pressure from intracochlear pressures. Intracochlear pressures resulting from acoustic pure-tone stimulation are compared with measured sound pressures in the ear canal to calculate scala-specific transfer functions (A). A 128-tap finite impulse response filter is derived from this transfer function and used to estimate equivalent ear canal pressure for any arbitrary intracochlear pressures measured while drilling on the incus (B).

Figure 4

Summary of peak sound pressure levels observed in all specimens during all drilling conditions. Unfiltered peak intracochlear pressure measurements (A) and estimated EAC pressures (B) are shown for each pressure recording as a function of drilling condition. Box plots represent the median +/−25% of the range of pressures observed, whiskers show the full range of the estimated distribution, and +’s mark outliers.

Figure 5

Average equivalent noise in raw pressure for all specimens by drilling conditions. Mean P_IC and V_Stap were calculated as root-mean-square (RMS) averages in third-octave bandwidths around 500, 1000, 2000, 4000, and 8000 Hz. Grey bars show the mean noise exposure for each corresponding drill speed and burr type combination in P_SV (5A), P_ST (5B), and P_Diff (5C), with solid bars indicating use of the diamond burr, checkered bars indicating the cutting burr, and increasing dark shade corresponding with increasing drill speed. Error bars indicate the SEM for each data set.

Figure 6

Average equivalent noise in estimated ear canal sound pressures level (dB SPL Eq) for all specimens by drilling conditions. Mean P_IC and V_Stap were calculated as root-mean-square (RMS) averages in third-octave bandwidths around 500, 1000, 2000, 4000, and 8000 Hz. Grey bars show the mean noise exposure for each corresponding drill speed and burr type combination in P_SV (6A), P_ST (6B), P_Diff (6C), and V_Stap (6D), with solid bars indicating use of the diamond burr, checkered bars indicating the cutting burr, and increasing dark shade corresponding with increasing drill speed. Error bars indicate the SEM for each data set.