Spatial orientation of the adult cochlea: rotation, tilt, and angle theta 3

Objective: Quantitative description in adult crania of (1) angular orientation of the basal turn of the cochlea relative to the sagittal (termed “rotation”) and Frankfort horizontal (termed “tilt”) planes, and angle theta 3 [angular relationship of the line defined by the cochlea’s spiral center and cochlear (round) window, to the cochlear window]; (2) orientation of the cochlea relative to the plane defined by the horizontal and vertical portions of the facial nerve; (3) orientation of the basal turn of the cochlea relative to the plane of the posterior semicircular canal; and (4) the association of these orientations with the extent of mastoid pneumatization. Methods: Postmortem material analysis. From 41 bequeathed anatomical ear-normal cadaveric cranial, high-resolution CT scans were performed of the five crania with the largest and the five with the smallest mastoids. Eleven points in three-dimensional Cartesian space were appointed and studied with the software program FIJI. Results: The median angle values (and ranges) for right ears were: “rotation” 52° (range 47–61); and, “tilt” 84° (79–89). The planes of the cochlear basal turn and facial nerve approximated superimposition: median 15° (2–19). Angle theta 3 for right ears was median 40° (28–44). Bilateral symmetry was found for the relationships between the planes. However, no association of any planar relationship with mastoid pneumatization was suggested. Conclusion: Considering the range of angles found in clinically normal adult specimens, spatial orientation of the cochlea may explain some of the difficulties in implantation.