A novel trans-petrous vestibular approach for measuring middle ear transfer function could eliminate inner ear interference and provide more accurate assessment of sound conduction in temporal bone specimens, according to a study published in the Journal of Otology.
Researchers evaluated 40 fresh-frozen human temporal bone specimens, with 36 meeting inclusion criteria. Using laser Doppler vibrometry with vertical laser alignment to the stapes footplate, they established a reference range for middle ear transfer function across frequencies from 0.25 to 10 kHz.
"By eliminating inner ear influences and standardizing access to the stapes footplate, the trans-petrous approach establishes a reliable framework for evaluating [middle ear transfer function] in middle ear implant assessment," wrote lead study author Yueqi Li, of the Department of Otolaryngology at Peking Union Medical College Hospital at the Chinese Academy of Medical Sciences in Beijing, China, and colleagues.
The researchers defined middle ear transfer function as the ratio of sound pressure at the tympanic membrane to that within the cochlea. The measure serves as a key parameter for assessing the mechanical properties of temporal bone specimens used in developing middle ear implants.
Traditional methods preserve the structural integrity of the inner ear while accessing the stapes footplate through the tympanic cavity. However, previous studies showed that inner ear impedance could influence middle ear transfer function, and surgical manipulation may cause changes in inner ear pressure, resulting in instability.
The new method involved a trans-petrous approach along the arcuate eminence, preserving the tegmen tympani to gain access to the vestibule while maintaining the integrity of the lateral vestibular wall where the stapes footplate is located. This allowed the researchers to expose the vestibular surface of the stapes footplate and direct the laser beam perpendicularly onto it.
Experiments were conducted in an anechoic chamber. Pure-tone stimuli at 0.25 to 10 kHz at 100 dB SPL were delivered via a plug-in earphone positioned in the external auditory canal, approximately 1-cm lateral to the tympanic membrane. The laser Doppler vibrometer system was aligned perpendicular to the vestibular surface of the stapes footplate at three distinct measurement points: anterior, middle, and posterior.
Further, the researchers performed vibration velocity measurements using laser Doppler vibrometry with a laser spot diameter of 48 µm at a stand-off distance of 25 mm. The filter bandwidth was set to 12.5 kHz, and the sampling frequency was 32 kHz. Stapes footplate velocity and ear canal sound pressure were simultaneously recorded.
Middle ear transfer functions were computed as the ratio of stapes footplate velocity to ear canal sound pressure. Triplicate measurements were averaged for each frequency, and a 95% tolerance interval was determined.
Vibrational analysis conducted at 3 points on the stapes footplate revealed statistically significant variations within the 500 to 5,000 Hz frequency range, underscoring the complex motion characteristics of the stapes footplate.
The researchers compared middle ear transfer function prior to and following surgical interventions involving mastoidectomy, tympanic cavity access, and exposure of the intact ossicular chain. No statistically significant alterations in middle ear transfer function were observed following these procedures.
Similarly, surgical manipulations such as tympanic membrane elevation and partial removal of the attic demonstrated no statistically significant changes in middle ear transfer function profiles. These findings confirmed that the described surgical approaches preserved middle ear acoustic transmission while maintaining measurement validity.
Among the 40 temporal bone specimens initially included, 4 were excluded: 3 because of tympanic membrane perforation and one because of ossicular chain fixation.
The study group comprised specimens from patients aged 20 to 85 years. All specimens were preserved at −20°C postmortem and thawed at 4°C immediately prior to the experimental procedures. Structurally intact temporal bone specimens with a patent external auditory canal and an intact tympanic membrane, as verified through endoscopic examination, were included.
The researchers noted that conventional middle ear transfer function assessments typically involve angular laser Doppler vibrometry irradiation through the tympanic cavity, which introduces variability in laser incidence angles. In contrast, the proposed trans-petrous approach enabled perpendicular laser alignment to the stapes footplate, thereby ensuring more consistent and reproducible measurements.
The multipoint vibrational analysis revealed significant regional variations in stapes motion, corroborating previous observations of frequency-dependent rocking motions. This enhanced spatial resolution exceeded that of single-point measurements employed in the ASTM F2504 protocol.
The stability of middle ear transfer function following surgical interventions underscored the reliability of this method for implant testing. The extended frequency range utilized could address a critical gap in existing literature, which predominantly focuses on frequencies below 6 kHz.
Study limitations included the lack of comprehensive capture of three-dimensional vibration patterns that may be critical for the design of stapes piston implants. The absence of direct comparisons with conventional transmastoid laser Doppler vibrometry methods limited the ability to quantify the absolute improvements in measurement precision afforded by the proposed approach.
The researchers emphasized that the current sample size may not yet constitute a definitive standard for middle ear transfer function reference data. Future studies are planned to expand the cohort to refine these normative values.
Ethical approval was obtained from the Medical Ethics Committee of Peking Union Medical College Hospital.
The authors declared no conflicts of interest.
Source: Journal of Otology
