Do high-speed drills generate high-frequency noise in mastoid surgery?

Background: The introduction of increasingly high speed drills for mastoid surgery has heightened the concern that cochlea damage may occur in both the operated and nonoperated ear. It has been observed clinically that this damage could be associated with frequencies above 8,000 Hz and that, to observe these changes, high-frequency audiometry should be performed. Previous studies have investigated noise transmission to the cochlea at frequencies below 4,000 Hz only. There having been, until recently, limitations to the equipment available to measure higher frequencies.

Objective: To define the characteristics of noise transmitted to the cochlea during drilling of temporal bone, specifically in the higher frequency ranges up to 20,000 Hz.

Methods: Cleaned temporal bones were fitted with 3 mutually perpendicular accelerometers, capable of measuring frequencies in the range 500 to 20,000 Hz. The system was calibrated using a Kamplex Audio Traveller AA220 pure tone audiometer, and accelerometer outputs were recorded on a personal computer at a sampling frequency of 102.4 kHz per channel. The magnitude of the noise transmitted to the cochlea was determined for a range of burrs.

Results: Maximum transmission of sound was 108 dBA at 4,000 Hz using a 6.5-mm burr on the cortical mastoid bone. The average results showed that the sound transmission tailed off at the higher frequencies dropping to 84 dBA at 8,000 Hz and 40 dBA at 16,000 Hz.

Conclusion: The high-frequency hearing reduction noted in patients after mastoid surgery was shown not to be due to excessive high-frequency noise generated by drilling.