The internal acoustic meatus. Anatomic variations and relations to other temporal bone structures.

The internal acoustic meatus was evaluated in 97 temporal bone specimens, half of which were radiographed in different projections. Part of the material was also examined by multidirectional and computed tomography. Casts of the specimens were prepared and the meatuses were measured in different planes. The anatomic variations, especially in the transverse plane, and their relations to other structures in the petrous bone, were assessed. The meatus varied in shape, being mainly either long and thin, short and broad, funnel-shaped or pear-shaped. The pear-shaped meatus was practically always broad. The angle formed by the long axis of the meatus and the posterior wall of the pyramid varied inversely with the meatal length, a long meatus forming a small angle and vice versa. Large anteroposterior diameters were correlated to large meatal angles, but there was no direct correlation between meatal length and width. In addition, the meatal length was correlated to the perilabyrinthine and mastoid pneumatization and also to the dimensions of the pyramid.