The anatomic structural relationships of the presigmoid area: The digastric ridge is a potentially dangerous landmark for identifying the intratemporal facial nerve

Objective: To determine the intratemporal course and relationships of the sigmoid sinus, facial nerve, digastric ridge, and semicircular canals. Methods: Anatomical measurements were taken on eight formalin-fixed human cadaveric heads that were dissected using the operative microscope. Results: The relationship between the distal sigmoid sinus, facial nerve, and digastric ridge was variable. The most variable was the distal sweep of the sigmoid sinus as it approaches the jugular bulb: in one specimen the distance between the facial nerve and distal sigmoid was 1.6 mm, while in the other specimens the distance was greater than 4 mm. In addition, the digastric ridge was of varying prominence making identification difficult. Constant was the relationship between the incus, horizontal semicircular canal, and facial nerve. Conclusion: Some have advocated identifying the digastric ridge as a distal landmark for identifying the facial nerve. Based on the preliminary findings of this dissection sequence the safest method for identifying the facial nerve is the horizontal semicircular canal. This knowledge will facilitate safe temporal bone dissection and preservation of the facial nerve. We plan to extend this dissection sequence to a minimum of 20 cadaver heads/ 40 temporal bones.