Facial canal dehiscence: histologic study and computer reconstruction.

The incidence, location, shape, and dimensions of dehiscences in the facial canal to the middle ear space were studied in 160 temporal bones obtained from 129 individuals 24 gestational weeks to 109 years of age at death by means of light microscopy and our computer reconstruction and measurement method. Dehiscences were observed in 95 of the 129 individuals (74%) and in 119 of the 160 bones (74%). The incidence was found to be quite high among fetuses and newborns, lowest in individuals in their twenties and then again quite high in the geriatric population (chi 2 test, chi 2 = 5.45 and 4.41, p < .05). The most frequent site of dehiscence was the oval window area, particularly in its posterior half (57% of all ears) on the inferior to inferomedial aspects of the canal; these dehiscences were clearly demonstrated in reconstructed images. The incidence of dehiscence in the area of the cochleariform process was 16%, and all these dehiscences were on the lateral to superolateral aspect of the canal. The second genu area and the mastoid portion were sites of dehiscence in 21% and 18%, respectively, of specimens; more than half of the dehiscences in the second genu area and mastoid portion were on the lateral to anterolateral and posterior aspects of the facial canal, respectively. The shape of the dehiscence tended to be oval in the oval window area, but rather irregular in the other areas. The dehiscences ranged from 0.4 to 2.64 mm in length, from 0.12 to 1.59 mm in width, and from 0.03 to 1.87 mm2 in surface area. The proximity of these dehiscences to the field of otologic surgery is stressed.