■ OBJECTIVES: We evaluated the anatomical relationships between the mastoid segment of the facial nerve and the mastoid antrum in axial temporal bone CT scans to provide background knowledge to prevent facial nerve injury when performing a posterior tympanotomy.

■ METHODS: One hundred two patients with chronic otitis media without cholesteatoma (COM) and 43 normal subjects were included in this study. The axial CT scan which shows the most visible image of short arm of the posterior semicircular canal (PSC) was identified, and measured the distance between the mastoid segment and the annulus of the EAC and between the PSC and the mastoid segment. Anatomical relationships between the mastoid segment and the antrum were classified into 6 types according to protrusion of the facial nerve into the antrum, status of the facial canal, new bone formation over the facial canal and size of the antrum. Type 1 has protrusion with regular facial canal and type 2 has no protrusion with relatively thin facial canal. Type 3 has protrusion into the antrum with regular facial canal and type 4 has protrusion with relatively thin facial canal. Type 5 has new bone formation around the facial canal and type 6 has new bone formation with small or contracted antrum.

■ RESULTS: Length measurements were done in 62 normal ears and 44 ears with COM. Mean distance between the mastoid segment and the annulus in COM ear was 2.35 ± 0.44 mm and was significantly different from normal ear which measured 2.63 ± 0.51 mm. Mean distance between the PSC and the mastoid segment in COM ear was 3.47 ± 0.59 mm and was significantly different from ormal ear which measured 3.79 ± 0.55 mm. Analysis of anatomical relationships were done in 156 normal ears and 134 COM ears. In normal ears anatomical relationships were 117, 12, 19, 0, 8, 0 cases according to the classification respectively and in COM ears were 76, 6, 9, 1, 16, 26 cases respectively.

■ CONCLUSIONS: The anatomical relationships between the mastoid segment and the antrum can be classified into 6 types and may provide understandings of the facial nerve variations preoperatively.