Handheld laser-fiber vibrometry probe for assessing auditory ossicles displacement

Abstract

Significance: Measurements of auditory ossicles displacement are commonly carried out by means of laser-Doppler vibrometry (LDV), which is considered to be a gold standard. The limitation of the LDV method, especially for in vivo measurements, is the necessity to expose an object in a straight line to a laser beam operating from a distance. An alternative to this approach is the use of a handheld laser-fiber vibrometry probe (HLFVP) with a curved tip.

Aim: We evaluate the feasibility of an HLFVP with a curved tip for measuring sound-induced displacement of the auditory ossicles.

Approach: A handheld vibrometer probe guiding the laser beam with a fiber-optic cable was used for displacement measurements of the incus body and the posterior crus of the stapes. Tonal stimuli at frequencies of 0.5, 1, 2, and 4 kHz were presented by means of an insert earphone positioned in the outer ear canal. The probe was fixed at the measurement site using a tripod or hand-held by one of the two surgeons.

Results: The measurements were carried out on six fresh temporal bones. Multivariate analysis of variance showed statistically significant differences for stimulus frequency (F3,143 = 29.37, p < 0.001, and η2 = 0.35), bone (F5,143 = 4.61, p = 0.001, and η2 = 0.01), and measurement site (F1,143 = 4.74, p = 0.03, and η2 = 0.02) in the absence of statistically significant differences for the probe fixation method (F2,143 = 0.15, p = 0.862, and η2 = 0.001). Standard deviations of the means were 6.9, 2.6, 1.9, and 0.6 nm / Pa for frequency, bone, site, and fixation, respectively. Ear transfer functions were found to be consistent with literature data.

Conclusions: The feasibility of applying HLFVP to measure the displacement of auditory ossicles has been confirmed. HLFVP offers the possibility of carrying out measurements at various angles; however, this needs to be standardized taking into account anatomical limitations and surgical convenience.

Keywords: handheld probe; laser-Doppler vibrometer; laser-fiber vibrometer; middle ear mechanics; middle ear surgery.

Fig. 1

(a) The four channel laser-fiber Doppler vibrometer and (b) the handheld probe. The probe, consisting of a ceramic ferrule with the three optical fibers mounted inside a metal shielding sleeve, was connected to one of the four channels of the laser-fiber vibrometer. Two optical fibers are for the forward and return paths of the 1550 nm laser, and one is for the red navigation spot.

Fig. 2

Noise floor (gray dots) measured on a stationary object and the displacement of the posterior crus of the stapes with a stimulation of 60 dB HL (black line). The probe was fixed on a tripod.

Fig. 3

Photograph of the temporal bone with reflective material (a) placed at the measuring sites: on the posterior crus of the stapes (1) and the incus body (2). The approach to the tympanic cavity was performed by means of the standard antromastoidectomy followed by posterior tympanotomy with preservation of the facial nerve canal and the chorda tympani. (b) Photograph of the measurement on the posterior crus of the stapes (1). The probe was introduced into the tympanic cavity through a posterior tympanotomy.

Fig. 4

The middle ear transfer functions. Single data point represents the interpolated median of 18 measurements of the posterior crus of the (a) stapes and (b) incus body taken on six temporal bones by two surgeons and using a tripod. 95% confidence intervals determined using bootstrapping are marked with whiskers. The gray areas present 95% confidence intervals reported in the literature. A slight horizontal shift at the frequencies 500 Hz, 1, 2, and 4 kHz was introduced to improve symbol visualization. The article by Rosowski et al. presented an average of 13 other studies. The measurements in the research by Nedham et al. were carried out on the stapes head. The analogous chart for (c) temporal bone and (d) the fixation method is presented by means of the interpolated median of 6 and 12 (6 bones × 2 sites) measurements, respectively.

Fig. 5

The displacement of the posterior crus of the stapes in relation to the intensity of the sound stimulus in (a) dB SPL and (b) dB HL. Single data point presents interpolated median of 6 measurements. Whiskers present 95% confidence intervals determined by means of bootstrapping.

Fig. 6

Effects of the angle between the laser beam and the reflecting surface on displacement measurements. Both figures show the laser beam (full arrow) forming a right angle with the direction of stapes displacement (dashed arrow). The measured displacement (bold arrow) is greater in (a) than (b) as it depends on the angle between the laser and the reflective surface.