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

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 sound-induced displacements of the entire OCT-visible lateral surfaces of the ossicles in the lateral-to-medial 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 16 kHz. This new axis of rotation is located near the posterior edge of the manubrium. The onset of the second rotational mode leads to a boost in the magnitude of sound-induced stapes displacement near 14 kHz, and adds a half-cycle to the accumulating phase in middle-ear sound transmission. Similar measurements in one ear after interruption of the incudostapedial joint suggest the load of the cochlea and stapes annular ligament is important to the presence of the second rotational mode, and acts to limit simple ossicular translation.