Optical coherence tomographic measurements of the sound-induced motion of the ossicular chain in chinchillas: Additional modes of ossicular motion enhance the mechanical response of the chinchilla middle ear at higher frequencies

Abstract

Wavelength-swept optical coherence tomography (OCT) was used to scan the structure of cadaveric chinchilla ears in three dimensions with high spatial resolution and measure the soundinduced displacements of the entire OCT-visible lateral surfaces of the ossicles in the lateral-tomedial direction. The simultaneous measurement of structure and displacement allowed a precise match between the observed motion and its structural origin. The structure and measured displacements are consistent with previously published data. The coincident detailed structural and motion measurements demonstrate the presence of several frequency-dependent modes of ossicular motion, including: (i) rotation about an anteriorly-to-posteriorly directed axis positioned near the commonly defined anatomical axis of rotation that dominates at frequencies below 8 kHz, (ii) a lateral-to-medial translational component that is visible at frequencies from 2 to greater than 10 kHz, and (iii) a newly described rotational mode around an inferiorly-to-superiorly directed axis that parallels the manubrium of the malleus and dominates ossicular motion between 10 and

Mammalian middle ear mechanics: A review

“…The axis of rotation is no longer as described by Helmholtz or von Békésy but is an "imaginary" axis outside the bodies of the middle ear ossicles. Although not discussed in detail, Figure 8 shows similar results (Rosowski et al, 2020). Their results from the OCT measurements show a lateral-medial whole-body translational component that suggests the incudomalleolar joint is moving in unison without rotation (Figure 8).…”

“…The existence of this rotational axis was confirmed in multiple measurements of middle ear vibration in humans, cats, gerbils, and chinchilla at stimulation frequencies at the lower end of the hearing range ( Helmholtz, 1868 ; Dahmann, 1929 ; Von Békesy, 1941 ; Von Békésy, 1960 ; Guinan and Peake, 1967 ; Hüttenbrink, 1992 ; Rosowski et al, 1999 ; Decraemer and Khanna, 2004 ; Decraemer et al, 2014 ; Rosowski et al, 2020 ). As shown in Figures 8A,B , in chinchilla, the middle ear ossicles move in phase on one side of the rotation axis for stimulus frequencies below 2 kHz, and the displacement of the ossicles increases with the distance from the rotational axis ( Rosowski et al, 2020 ). The movement patterns are complex for higher frequencies, with an additional axis of vibration emerging ( Figures 8E,F ).…”

“…The length of the manubrium and the long process of the incus would introduce lever action in addition to the surface differences of the tympanic membrane and the stapes footplate. The existence of this rotational axis was confirmed in multiple measurements of middle ear vibration in humans, cats, gerbils, and chinchilla at stimulation frequencies at the lower end of the hearing range (Helmholtz, 1868;Dahmann, 1929;Von Békesy, 1941;Von Békésy, 1960;Guinan and Peake, 1967;Hüttenbrink, 1992;Rosowski et al, 1999;Decraemer and Khanna, 2004;Decraemer et al, 2014;Rosowski et al, 2020). As shown in Figures 8A,B, in chinchilla, the middle ear ossicles move in phase on one side of the rotation axis for stimulus frequencies below 2 kHz, and the displacement of the ossicles increases with the distance from the rotational axis (Rosowski et al, 2020).…”

“…A synovial-fluid-filled gap between malleus and incus exists ( Ihrle et al (2017) . Panels (B) and (C) were taken from ( Rosowski et al, 2020 ) and modified. The malleus (M), incus (I), stapes (S), anterior mallear ligament (AML), and posterior incudal ligament (PIL) are shown.…”

“… Three ossicular motion modes are described by Rosowski et al (2020) ; reprinted with permission. (A) lateral and (B) posterior view of the anterior-posterior rotational axis (C) lateral and (D) posterior view of the whole-body translation, and (E) lateral and (F) posterior view of the superior-inferior rotational axis.…”