Surgically-Relevant Anatomy of the External Auditory Canal Bulge and Scutum.

Hypothesis: Anatomic study of the external auditory canal's (EAC) anterior bulge, scutum, and ossicular chain will generate knowledge applicable to safe ear surgery and instrument design.

Background: The EAC contains two structures that obscure view of the middle ear: the anterior bulge and the scutum. The dimensions of these structures and their relationships to the ossicular chain have not been previously described.

Methods: Cadaveric temporal bones underwent computed tomography scanning, and three-dimensional reconstructions were created. Dimensions and angles of the EAC, its anterior bulge and scutum were measured. Distances to ossicular landmarks and the facial nerve were examined.

Results: The anterior EAC had a swan-neck shape. The thinnest portion was located medially and correlated with the canal thickness at the anterior bulge. However the thickness of the anterior bulge was not correlated with its angulation. The scutum averaged 3.8 mm long with a base thickness of 2.3 mm and a mean tip angle of 33 degrees. The short process of the incus was significantly closer to the scutum than other ossicular landmarks.

Conclusion: Prominent anterior canal bulges are formed by posterior temporomandibular joints, not thicker bone. The scutum has asymmetric distances to various portions of the ossicles with the incus short process sometimes as close as 0.2 mm, placing it at risk of injury.