A method to measure sound transmission via the malleus–incus complex

Abstract

BACKGROUND: The malleus-incus complex (MIC) plays a crucial role in the hearing process as it transforms and transmits acoustically-induced motion of the tympanic membrane, through the stapes, into the inner-ear. However, the transfer function of the MIC under physiologically-relevant acoustic stimulation is still under debate, especially due to insufficient quantitative data of the vibrational behavior of the MIC. This study focuses on the investigation of the sound transformation through the MIC, based on measurements of three-dimensional motions of the malleus and incus with a full six degrees of freedom (6 DOF). METHODS: The motion of the MIC was measured in two cadaveric human temporal bones with intact middle-ear structures excited via a loudspeaker embedded in an artificial ear canal, in the frequency range of 0.5-5 kHz. Three-dimensional (3D) shapes of the middle-ear ossicles were obtained by sequent micro-CT imaging, and an intrinsic frame based on the middle-ear anatomy was defined. All data were registered into the intrinsic frame, and rigid body motions of the malleus and incus were calculated with full six degrees of freedom. Then, the transfer function of the MIC, defined as velocity of the incus lenticular process relative to velocity of the malleus umbo, was obtained and analyzed. RESULTS: Based on the transfer function of the MIC, the motion of the lenticularis relative to the umbo reduces with frequency, particularly in the 2-5 kHz range. Analysis of the individual motion components of the transfer function indicates a predominant medial-lateral component at frequencies below 1 kHz, with low but considerable anterior-posterior and superior-inferior components that become prominent in the 2-5 kHz range. CONCLUSION: The transfer function of the human MIC, based on motion of the umbo and lenticularis, has been visualized and analyzed. While the magnitude of the transfer function decreases with frequency, its spatio-temporal complexity increases significantly. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

Comparison of sheep and human middle-ear ossicles: anatomy and inertial properties

“…From the viewpoint of mechanics, this indicates that rotation of the human malleus about the superior-inferior direction is prone to being generated at high frequencies because rotational inertia about the superior-inferior direction is close to the minimum. Such a rotational motion of the human malleus at high frequencies, which is denoted as "torsional motion" of the malleus in the literature, was predicted by Puria and Steele (2010) and was observed in measurements in the human above 3 kHz by Dobrev et al (2016). Considering the fact that the principal axis with the minimum moment of inertia deviates from the superior-inferior direction in the sheep malleus, it is expected that the sheep malleus would not have considerable "torsional" motion at high frequencies.…”

“…However, motion of the malleus at high frequencies is determined by flexibility of the IMJ as well. According to previous works (Willi et al 2002;Puria and Steele 2010;Gerig et al 2015;Dobrev et al 2016), in mammals with deformable IMJ (e.g., human and cat), connection of the malleus to the incus can be considered to be loose allowing relative motion between the malleus and the incus only at high frequencies above the first natural (resonance) frequencies. As for species with a naturally fixed IMJ, Puria and Steele (2010) hypothesized that motion of the malleus-incus complex would maintain hinge-like rotational motion even at high frequencies.…”

Tympanic membrane surface motions in forward and reverse middle ear transmissions

“…The stimulator vibration changes linearly with the driving voltage. Some of the variations with frequency may be caused by compliant connections that allow relative motion between the incus and malleus (Willi et al, 2002;Dobrev et al, 2016) as well as relative motion in the manubrium-TM connection (Graham et al, 1978;Gulya and Schuknecht, 1995;De Greef et al, 2016). Note that the two specimens with the largest reverse driven motions at low frequencies (TB13 and 14) exhibited small sound-driven umbo displacements [ Fig.…”

Minimally invasive laser vibrometry (MIVIB) with a floating mass transducer – A new method for objective evaluation of the middle ear demonstrated on stapes fixation

“…Future experiments may show whether incus velocity measurements can aid to resolve this problem. Overall, it is clear that it is important to compare both absolute velocities (Nakajima et al, 2005;Rosowski et al, 2008) and the incus-umbo velocity ratio (Dobrev et al, 2016;Wales et al, 2017) over the whole frequency range to distinguish between different fixations. The input voltages required for the FMT to generate an umbo velocity that is equivalent to 90dB SPL acoustic stimulation is generally shown to be a smooth curve which requires a higher input in the lower (<1kHz) and higher (>2kHz) frequencies while requiring less input in the middle frequencies (Supplementary figure 1).…”