Effect of Opening Middle-Ear Cavity on Vibrations of Gerbil Tympanic Membrane

Abstract

This paper presents in vivo experimental measurements of vibrations on the pars flaccida, along the manubrium and at several points on the pars tensa in the gerbil with open middle-ear cavity. The effects of progressive opening of the middle-ear cavity are presented, with up to five different extents of opening. In all manubrial, pars-tensa and pars-flaccida responses, opening the cavity causes an increase in the low-frequency magnitude and a shift of the main middle-ear resonance to lower frequencies and introduces an antiresonance. However, opening the cavity has little or no effect on either the mode of vibration of the manubrium or the breakup frequency of the pars tensa. When the opening is gradually widened, the antiresonance frequency moves to higher frequencies. When the opening is made as wide as anatomically possible, the antiresonance moves to almost 10 kHz. The main increase in the lowfrequency response magnitude happens upon making the smallest hole in the cavity wall, and further progressive enlarging of the opening has little or no effect on the low-frequency magnitude. The antiresonance interferes with the response shapes. An identification method is suggested for eliminating the effect of the antiresonance in order to estimate the ideal open-cavity response. The method is validated and then applied to manubrial and pars-tensa responses. Estimating the ideal open-cavity responses will simplify comparison of the data with numerical models which do not include the air cavity. The data collected at intermediate stages of opening will be useful in validating models that do include the cavity.

Finite-Element Modelling of the Response of the Gerbil Middle Ear to Sound

“…The pars flaccida in the model shows responses similar to those measured by Rosowski et al (1997) and Maftoon et al (2014). Our sensitivity analysis shows that the resonance of the pars flaccida shifts to lower frequencies as its Young's modulus or thickness decreases.…”

“…Furthermore, this study does not try to explore the model response in terms of ossicular vibrations in detail: we have presented the displacements of the manubrium along only a single direction and only the piston component of the stapes displacement. Validation of the ossicular motion will require further study, taking into account the 3-D ossicular motions that have been reported by Decraemer et al 2007,2014) as well as the experimental manubrial motion results by Maftoon et al (2013Maftoon et al ( , 2014. A future study should investigate losses in the middle ear due to radiation impedance and to damping in different middle-ear structures, as well as middleear transmission and power flow from the TM to the cochlea.…”

“…As in our report of experimental measurements , we define the break-up frequency of the pars tensa as the frequency at which the phase divergence of the points is more than 15°. Since the main purpose of this paper is to validate the model results against the multi-point experimental data of Maftoon et al (2013Maftoon et al ( , 2014, we chose to greatly reduce the computation time by not computing frequency responses for all of the ∼84,000 nodes in the model. Instead, we processed only nodes corresponding to where we made experimental measurements: a row of nodes along the manubrium, a row of nodes on the pars tensa on a line perpendicular to the manubrium midway along its length, and a node at the centre of the pars flaccida.…”

“…In our experimental measurements in gerbil ears, we often observed that the pars flaccida is retracted into the middle-ear cavity. Figure 6 shows the experimental open-cavity pars-flaccida responses for ear E from Maftoon et al (2014), the only ear in which we could measure the response of a naturally flat pars flaccida, as well as the one ear from Rosowski et al (1997). At the lowest frequency, the centre of the pars flaccida in the model has a phase of zero and a magnitude of 2.2 μm/Pa.…”

“…In Figure 5 only ears E and K had a naturally flat pars flaccida; in all the other ears the pars flaccida was experimental measurements in 9 gerbil ears with partial openings in the cavity wall (coloured lines). The gerbils (C to K) were those reported by Maftoon et al (2014) and the method proposed in that paper was used here to estimate the ideal open-cavity responses; that is, although the responses were measured with a partial opening in the middle-ear cavity wall, they have been adjusted so they appear as though they were measured with no cavity wall. The simulated responses are shown for baseline parameters (solid line) and adjusted parameters (dashed line), as discussed in the text.…”