Anatomical variations of the tympanic and mastoid portions of the facial nerve canal. A radioanatomical investigation.

Purpose: To obtain information on the morphology of the tympanic and mastoid portions of the facial nerve canal, and on the appearance of unusual vascular channels and anomalies such as dehiscences.

Material and methods: One hundred and two temporal bone preparations were examined by conventional radiography to evaluate mastoid pneumatization. Of these, 73 were examined by high resolution CT in order to test the ability of the method to detect dehiscences in the bony wall of the tympanic portion of the canal and the accompanying channels along its mastoid portion. Subsequently produced plastic preparations were used to measure the length and width of these 2 portions of the facial canal.

Results: Our study reports observations on the three-dimensional morphology of the canal in the plastic casts. The study shows variations in the course and dehiscences of the tympanic portion. Additional bony channels along the mastoid portion are described. These results supplement those in previous investigations.

Conclusion: The measurement results agree with those of previous investigations. The course of the tympanic portion is S-shaped and has an impression on its upper surface. High resolution CT reproduces dehiscences of the bony canal in a percentage similar to that of microscopical methods and in relevant sites. Pneumatization does not influence the dimensions of the 2 portions.