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Otologic surgery is performed for a variety of reasons including treatment of recurrent ear infections, alleviation of
dizziness, and restoration of hearing loss. A typical ear surgery consists of a tympanomastoidectomy in which both the
middle ear is explored via a tympanic membrane flap and the bone behind the ear is removed via mastoidectomy to treat
disease and/or provide additional access. The mastoid dissection is performed using a high-speed drill to excavate bone
based on a pre-operative CT scan. Intraoperatively, the surface of the mastoid component of the temporal bone provides
visual feedback allowing the surgeon to guide their dissection. Dissection begins in "safe areas" which, based on surface
topography, are believed to be correlated with greatest distance from surface to vital anatomy thus decreasing the chance
of injury to the brain, large blood vessels (e.g. the internal jugular vein and internal carotid artery), the inner ear, and the
facial nerve. "Safe areas" have been identified based on surgical experience with no identifiable studies showing
correlation of the surface with subsurface anatomy. The purpose of our study was to investigate whether such a
correlation exists. Through a three-step registration process, we defined a correspondence between each of twenty five
clinically-applicable temporal bone CT scans of patients and an atlas and explored displacement and angular differences
of surface topography and depth of critical structures from the surface of the skull. The results of this study reflect
current knowledge of osteogenesis and anatomy. Based on two features (distance and angular difference), two regions
(suprahelical and posterior) of the temporal bone show the least variability between surface and subsurface anatomy.
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