Three-Dimensional Topographic Analysis of Bill's Bar in Relation to Semicircular Canal Planes: Implications for Facial Nerve Identification

Bill's bar, a bony ridge at the fundus of the internal auditory canal, has been proposed as a stable landmark; however, quantitative data describing its spatial relationship to adjacent labyrinthine structures remain limited. This study aimed to define the relationship between Bill's bar and semicircular canal planes using three-dimensional imaging analysis. This is a retrospective radiological study. A single secondary referral center. A total of 100 ears from 50 adult patients undergoing high-resolution temporal bone computed tomography were analyzed. The position of the apical point of Bill's bar (Pb) was evaluated relative to the intersection point (Pv) of the horizontal and superior semicircular canal planes. Measurements included the linear distance between Pb and Pv (Lvb), the signed vertical distance from Pb to the horizontal semicircular canal plane (LPLhb), and the angles between the Pb–Pv line and the horizontal and superior semicircular canal planes (APLhvb and APLsvb). The mean Lvb was 4.38 ± 0.98 mm, and the mean absolute vertical distance from the horizontal plane was 0.45 ± 0.41 mm. The mean APLhvb was 4.83 ± 7.17 degrees, indicating that Pb was near-parallel or slightly superior to the horizontal plane, whereas the mean APLsvb was 70.80 ± 10.69 degrees. Measurements were symmetric by laterality, whereas Lvb was significantly greater in females (p < 0.001). These findings reinforce Bill's bar as a reliable landmark for early facial nerve identification during the translabyrinthine approach and may aid surgical orientation in cases with anatomically distorted auditory canals.