Intracochlear pressure as an objective measure for perceived loudness with bone conduction implants

BACKGROUND

The generally accepted method to assess the functionality of novel bone conduction implants in a preclinical stage is to experimentally measure the vibratory response of the cochlear promontory. Yet, bone conduction of sound is a complex propagation phenomenon, depending on both frequency and amplitude, involving different conduction pathways.

OBJECTIVES

The aim of this study is to validate the use of intracochlear sound pressure (ICP) as an objective indicator for perceived loudness for bone conduction stimulation. It is investigated whether a correlation exists between intracochlear sound pressure measurements in cadaveric temporal bones and clinically obtained results using the outcome of a loudness balancing experiment.

METHODS

Ten normal hearing subjects were asked to balance the perceived loudness between air conducted (AC) sound and bone conducted (BC) sound by changing the AC stimulus. Mean balanced thresholds were calculated and used as stimulation levels in a cadaver trial (N = 4) where intracochlear sound pressure was measured during AC and BC stimulation to assess the correlation with the measured clinical data. The intracochlear pressure was measured at the relatively low stimulation amplitude of 80 dBHL using a lock-in amplification technique.

RESULTS

Applying AC and BC stimulation at equal perceived loudness on cadaveric heads yield a similar differential intracochlear pressure, with differences between AC and BC falling within the range of variability of normal hearing test subjects.

CONCLUSION

Comparing the perceived loudness at 80 dB HL for both AC and BC validates intracochlear pressure as an objective indicator of the cochlear drive. The measurement setup is more time-intensive than measuring the vibratory response of the cochlear promontory, yet it provides direct information on the level of the cochlear scalae.