Analysis of vestibulo-ocular reflex experiments requires knowledge of the absolute orientations (with respect to skull landmarks) of semicircular canals (SCC). Data relating SCC orientations to accessible skull landmarks in humans are sparse, apart from a classic study of 10 skulls that concluded that the horizontal and anterior SCC are not mutually orthogonal (11± 7.6 degrees). Multiple studies of isolated labyrinths have shown the inter-SCC angles are close to 90 degrees. We hypothesized that a larger sample would yield mean absolute SCC orientations closer to the mutual orthogonality demonstrated for isolated labyrinths. We measured canal orientations with respect to accessible skull landmarks using 3-D multiplanar reconstructions of computerized tomography scans of the temporal bones of 22 human subjects. Images were acquired with 0.5-mm thickness and reconstructed with in-plane resolution of 234 um. There was no significant difference between the left and a mirror image of the right, so data were pooled for the 44 labyrinths. The angle between the ante
rior and posterior SCC was 94.0 ± 4.0° (mean ± SD). The angle between the anterior and horizontal SCC was 90.6 ± 6.2°. The angle between the horizontal and posterior SCC was 90.4 ± 4.9°. The direction angles between a vector normal to the left horizontal SCC and the positive Reid's stereotaxic X (+nasal), Y (+left), and Z (+superior) axes were 108.7 ± 7.5°, 92.2 ± 5.7°, and 19.9 ±7.0°, respectively. The angles between a vector normal to the left anterior SCC and the positive Reid's stereotaxic X, Y, and Z axes were 125.9 ± 5.2°, 38.4 ± 5.1°, and 100.1 ± 6.2°, respectively. The angles between a vector normal to the left posterior SCC and the positive Reid's stereotaxic X, Y, and Z axes were 133.6 ± 5.3°, 131.5 ± 5.1°, and 105.6 ± 6.6°, respectively. The mean anterior SCC-contralateral posterior SCC angle was 15.3 ± 7.2°. The absolute orientations of human SCC are more nearly orthogonal than previously reported.ReferenceDella Santina CC, Potyagaylo V, Migliaccio AA, Minor LB, Carey JP.
Orientation of human semicircular canals measured by three-dimensional multiplanar CT reconstruction.
J Assoc Res Otolaryngol. 2005 Sep;6(3):191-206.
PubMed PMID: 16088383; PubMed Central PMCID: PMC2504595.