Rotational axes and lever arms for the middle-ear ossicles of several land mammals [electronic resource]
Keywords: Bioengineering.

In this thesis, the possibility of various rotational modes of vibration for the middle ear bones was explored for five land mammals, which are relevant to the field of hearing research. Mammalian hearing is unique because all mammals have three middle ear bones, or ossicles. Furthermore, mammals have a remarkably high upper frequency limit of hearing compared to other types of animals. The overarching goal was to investigate the theory that three ossicles enable more-efficient transfer of sound energy at high frequencies through a reduction of inertial impedance. This study consists of three main parts, which consider the anatomy, functionality, and high frequency sound transmission capabilities of the mammalian middle ear. In the first part, high-resolution micro-CT imaging was used to determine the anatomical morphometry of the ossicles for each species: guinea pig (Cavia porcellus, n = 6), chinchilla (Chinchilla lanigera, n = 2), gerbil (Meriones unguiculatus, n = 6), cat (Felis catus, n = 2), and human (Homo sapiens, n = 1). From this morphological data, moments of inertia and lever arms for five rotational axes of interest were determined for ossicular systems with both mobile and fused joints. In species with mobile ossicles, the lever ratio for the middle ear was found using a novel double fulcrum lever system. These mechanical parameters were used to estimate the angular acceleration of the malleus and incus per unit input force, which was later related to mechanical impedance and predictions of high-frequency hearing limits. In the second part, three-dimensional motion measurements of the malleus and incus were acquired for cadaveric human (n = 3) and cat (n = 2) specimens using three-dimensional laser Doppler vibrometry. The frequency range measured was 0.5 to 23.5 kHz, which includes the upper frequency limit for human. In both species, the measurements indicate that the malleus and incus rotate together in a "hinging" motion about the classical anatomical axis at low frequencies, which is consistent with existing measurements. At higher frequencies, novel rotational modes, with correspondingly reduced inertial impedance, and independent motion of the malleus and incus were observed. For example, at high frequencies the axis of the primary rotational mode of the human malleus (above 8 kHz) and incus (above 7.5 kHz) was found to be most closely aligned with the first principal axis of the ossicle. For cat, the axis of the primary rotational mode of the malleus above 6 kHz was found to be oriented between its first and third principal axes while the axis of the primary rotational mode of the incus above 15 kHz was found to be most closely aligned with its first principal axis. Of note, the first principle axis corresponds to the minimal moment of inertia and the third principal axis corresponds to the maximal moment of inertia. In the third part, the morphometry results and conclusions drawn from the three-dimensional motion measurements were used to estimate the frequency at the upper limit of hearing fH. The new data was applied to an existing characteristic length analysis, which used the cubic root of incudomalleolar complex mass as the characteristic length, and a simplified circuit model of acoustic input impedance to the ear. Both behaviorally and physiologically derived values for fH were used. The original characteristic length analysis predictions of fH were poor for the five species (behavioral, R-squared = 0.06; physiological, R-squared = 0.46, where R-squared is the coefficient of determination for a linear regression fit to the data). The novel characteristic-length model used the quartic root of the incudomalleolar complex lever arm divided by the relevant moment of inertia as the characteristic length, and this resulted in much better predictions of fH (behavioral, R-squared = 0.55; physiological, R-squared = 0.84). Similarly, predictions of fH for the five species using an existing simplified circuit model of cochlear input impedance were poor (behavioral, R-squared = 0.03; physiological, R-squared = -0.40), but were greatly improved with the introduction of rotational mechanical parameters into the model (behavioral, R-squared = 0.92; physiological, R-squared = 0.94). These analyses support the notion that the inertia associated with ossicular movement through various rotational modes generally correlates better with experimentally measured fH than does the commonly accepted treatment of the ossicles as a lumped mass that moves linearly, as assumed by the original characteristic length analysis, or as a body that rotates in a "hinging" motion about the classical anatomical axis, as assumed by the original simplified circuit model.