Learning Objectives: To learn from a study of minimally and maximally pneumatized temporal bones, the depth of the sinus tympani relative to the adjacent facial nerve and to the round window.

Background: Cholesteatoma involving the sinus tympani is notoriously diffiuclt to assess and control. Otologists would be happy for every sinus tympani to be shallow. Correlates of sinus tympani depth are unknown, although some suggest that increased depth correlates with large mastoid pneumatization.

Objective: To describe the depth of the sinus tympani, relative to both the adjacent facial nerve and distance from the round window, and how depth correlates with mastoid size.

Methods: Ten clinically ear-normal crania underwent computed tomography in a custom non-metallic positioning device that referenced the Frankfort horizontal plane. The crania, from a series of 41, were the five with the largest mastoids, and the five with the smallest mastoids, as assessed by plain lateral radiograph. Each landmark (midst of round window [RW], apex of sinus tympani [ST] and midst of facial nerve [FN] is that slice) was twice independently identified in xyz Cartesian space. The midst of the facial nerve was chosen even though not surgically accurate, so as to better consistently landmark the facial nerve for this study.

Results: The mean direct distance from RW to ST ranged from 3.4 to 7.7 mm, median 6.1 mm for right ears; 4.1–8.0, 5.0 left. For FN to ST, the range was 1.6–4.0 mm, median 3.2 right; 1.8–3.2, median 2.5 left. Neither bilateral symmetry nor relationship with mastoid size was found.

Discussion: Using a technique free of proximity bias, the depth of the sinus tympani is variable and unpredictable.

Conclusion: From one ear to another ear, the depth of the sinus tympani varies and is not predicatable.