Contemporary Mechanics of Conductive Hearing Loss

Abstract

The middle ear plays a critical role for the conversion of acoustic energy to mechanical vibrations that subsequently enter the cochlea. It is middle ear impedance matching through ossicular coupling that has enabled land-dwelling vertebrates to hear soft airborne sounds. Conductive hearing loss may result from damage to the delicate middle ear structures following infection, trauma or rapid pressure changes. An understanding of the mechanics of the middle ear significantly improves the oto-surgeon's ability to effectively diagnose conductive hearing loss, localize the responsible lesion and then effectively correct the conduction abnormality. This article reviews some of the basic knowledge of middle ear mechanics for sound transmission, highlights recent advances in developing new techniques to assist in diagnosis of middle ear disease, and finally sheds light on future research aimed at improving the diagnosis and management of middle ear pathology.

Keywords: Auditory mechanics; bone conduction; conductive hearing loss; holography; middle ear; optical coherence tomography.

Figure 1.

A medial view of the middle ear. The tympanic membrane (TM). The ossicles: FP, footplate of the stapes; I, incus; M, malleus. The muscles, tendons and ligaments: AML, anterior mallear ligament; PIL, posterior incudal ligament; SM, stapedius muscle and its tendon; TTT, tensor tympani tendon; TTM, tensor tympani muscle. The middle ear air spaces: AD, aditus ad antrum; MA, mastoid antrum; AC, mastoid air cells. Adapted with permission from Rosowski (2012).

Figure 2

(Fig 3.9 from Rosowski 2012). Schematic of the ossicular and acoustic coupling of sound through the middle ear. P_ME: the middle ear cavity pressure; P_RW: the sound pressure outside of the round window; P_OW: the sum of the acoustic and ossicular pressures at the oval window.

Figure 3

(Fig 2 from Gan 2004). Mean peak-peak displacements of the tympanic membrane (TM) and the stapes foot plate (FP) of human middle ears with the input of 90 dB SPL sound in the ear canal: (A) Magnitude; (B) Phase.

Figure 4.

Schematic of three middle ear impedance matching mechanisms: (a) Area ratio transformer; (b) Curved membrane catenary lever action; and (c) Ossicular chain lever action. (Adapted with permission from Pickles, 1988)

Figure 5.

Schematic of high-speed holography setup for the temporal bone experiment (See Tang et al., 2021 for more details)

Figure 6.

Averaged frequency response function (AFRF) of the middle ears with pathologies. Statistically significant differences (p-value < 5%) are marked with asterisks.

Figure 7.

(a) Schematic of OCT setup to measure sound induced vibrations of the middle ear; (b) Demonstration of using OCT beam (red) to resolve in depth information of the middle ear; (c) OCT reconstructed middle ear structures, including the eardrum and the manubrium (umbo), as well as partial incus and stapes; (d) Another view of the OCT reconstructed middle ear with bony cochlea (round window outlined).

Figure 8.

Sound induced motions of the TM at two continuous tones: (A) 1.3 kHz and (B) 4 kHz. The displacement magnitudes are color coded as shown in color bar. Note that the sound pressure levels were adjusted at different frequencies to produce significant motions of the TM.

Figure 9.

A prototype of hand-held middle ear OCT system (picture courtesy from Audioptics Medical, Inc).

Figure 10.

Representative OCT measured umbo mobility (dB re 1mm/s/Pa) between 500 and 3000 Hz from a live subject. The resutls (blue curve, left panel) are within the normal range (gray area) of human ear umbo motion from the literature. The right panel shows an medial view of the TM and the location of the umbo where OCT measurements were performed through the A-line scanning.