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Tympanum-canal angles anteriorly, anteroinferiorly, and inferiorly: A postmortem study of 41 adult crania

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September 1, 2009
by N. Wendell Todd, MD, MPH


The angles formed by the tympanic membrane and the external ear canal anteriorly, anteroinferiorly, and inferiorly can pose surgical challenges, and the spaces they form can harbor iatrogenic cholesteatoma. The range of values previously reported for these angles-27° to 60°-seems not to have been determined in a clinically applicable manner. To clinically describe the tympanum-canal angles, assess bilateral symmetry, assess the angles relative to mastoid pneumatization, and assess the relationship of the angles to manubrium orientation in the skull, the author conducted a postmortem anatomic study of 41 bequeathed adult crania without clinical otitis media. As viewed through the external ear canal, the tympanum-canal angles were measured in 10° increments anteriorly, anteroinferiorly, and inferiorly relative to the line of the manubrium. Mastoid sizes were determined radiographically. In the right ear, the tympanum-canal angles ranged from 40° to 60° anteriorly (median: 55°), from 50° to 70° anteroinferiorly (median: 60°), and from 70° to 80° inferiorly (median: 75°). Bilateral symmetry was found (each rs ≥ 0.69, p < 0.001). Although the angles did not correlate with either mastoid pneumatization or manubrium orientation relative to the Frankfort plane, nonvisualization of the annulus anteriorly was significantly more common in specimens with well-pneumatized mastoids (p < 0.05). The author concludes that the tympanum-canal angles anteriorly, anteroinferiorly, and inferiorly have comparatively narrow ranges, exhibit bilateral symmetry, and are unrelated to both mastoid size and manubrium orientation in the skull.

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