The cochlear morphometry compendium: High-resolution synchrotron measurements and normative reference values

Natural variations in cochlear anatomy have substantial implications for both clinical care and research in the fields of otology, neurotology, and audiology. While precise anatomic characterization is essential for a multitude of applications, comprehensive reference dimensions of both osseous and membranous cochlear structure obtained from a large and morphologically heterogeneous sample set do not currently exist. In this study, one hundred healthy human cadaveric temporal bone samples, without historical or visible pathology, underwent high-resolution three-dimensional synchrotron radiation phase-contrast imaging (SR-PCI) to develop a morphometric compendium of the human cochlea. Measurements of both bone and soft tissue in the cochlea were obtained, including basal turn diameter and width, cochlear height, and cochlear length along multiple anatomic paths (lateral wall, basilar membrane, and modiolar wall). The hook region, scalar geometry (diameter, area, tilt, width, and volume), and round window dimensions were also comprehensively characterized. Normative tonotopic frequency distributions of the basilar membrane and spiral ganglion were derived using cochlear length measurements and Greenwood's frequency-position function. These anatomic benchmarks establish invaluable reference data which may be used for anatomically informed, precision medicine approaches, including patient-specific surgical planning, intracochlear pharmaceutical delivery optimization, the development of automated image analysis algorithms, and the investigation of cochlear structure-function relationships in pathological conditions.