Orientation of the manubrium mallei: inexplicably widely variable

There is no consensus why the manubrium of the malleus, as viewed clinically through the external ear canal, generally points downward and posteriorly.

To depict the alignment of the handle of the malleus, viewed clinically through the external auditory canal, relative to the zygomatic arch, the Frankfort plane, and a visual plane proxy and relative to the horizontal semicircular canal. Also, to assess bilateral symmetry and manubrium alignment relative to mastoid pneumatization.

Postmortem anatomic dissection of 41 bequeathed adult crania without clinical otitis.

The line of the manubrium as viewed through the external ear canal was measured relative to the Frankfort plane, to a proxy of the visual plane, and to the zygomatic arch. Mastoid sizes were determined radiographically. In a subset of 10 crania additionally studied by computed tomography, the manubrium position was checked relative to the horizontal semicircular canal.

Relative to the zygomatic arch and Frankfort and visual planes, the range of manubrium angles was at least 45 degrees. Bilateral symmetry was found, each r 0.38 or greater (P < .05). Relative to the horizontal canals, the range of manubrial angles was 30 degrees, with symmetry suggested. Alignment did not correlate with mastoid pneumatization.

Manubrium orientation as viewed through the external auditory canal is not obviously explicable. The wide variety of manubrium orientations may explain the manubrium-stapes offset that perplexes manubrium-stapes surgical constructions.