Dehiscence or thinning of bone overlying the superior semicircular canal in a temporal bone survey

Objective: To determine the incidence and etiology of dehiscences of bone o verlying the superior semicircular canal in a temporal bone archive. Design: A microscopic study was performed of 1000 temporal bones from 596 a dults in a university hospital registry. Specimens were sectioned verticall y in the plane of the superior semicircular canal. Measurements of minimum bone thickness over the superior canal were made in a subset of 108 randoml y chosen specimens. All bones were examined for thinning or dehiscence rela tive to these norms. Clinical histories, when available, were reviewed. Results: Complete dehiscence of the superior canal was identified in 5 spec imens (0.5%), at the middle fossa floor (n = 1) and where the superior petr osal sinus was in contact with the canal (n = 4). In 14 other specimens (1. 4%), the bone at the middle fossa floor (n = 8) or superior petrosal sinus (n = 6) was no thicker than 0.1 mm, significantly less than values measured in the control specimens (P < .001). Abnormalities were typically bilatera l. Specimens from infants demonstrated uniformly thin bone over the superio r canal in the middle fossa at birth, with gradual thickening until 3 years of age. Conclusions: Dehiscence of bone overlying the superior canal occurred in ap proximately 0.5% of temporal bone specimens (0.7% of individuals). In an ad ditional 1.4% of specimens (1.3% of individuals), the bone was markedly thi n (less than or equal to 0.1 mm), such that it might appear dehiscent even on ultra-high-resolution computed tomography of the temporal bone. Sites af fected were in the middle fossa floor or a deep groove for the superior pet rosal sinus, often bilaterally. These abnormalities may arise from failure of postnatal bone development. Thin areas of bone over the superior canal m ay be predisposed to disruption by trauma.