Transethmoidal Approach to the Optic Canal: Surgical and Radiological Microanatomy

Background: The purpose of the study is to describe anatomic topographic landmarks for transethmoidal approach to optic canal for optic nerve decompression. The study focuses on microsurgical/radiologic anatomies and their relationships in the region of the optic canal and orbit.

Methods: Human optic canal and related anatomic structures were studied in orbits of 6 formalin preserved adult human cadavers. In addition, anatomic measurements were made with digital vernier caliper on the orbits of 25 adult human skulls. The relation between ethmoidal and sinus was assessed with computed tomography (CT) scan in 25 living human heads needing cranial CT scan for any reason.

Results: The suture on the conjunction of frontal, maxilla, and lacrimal bones with a location in medial side of the orbit was accepted as a landmark. When the measurements were taken from this landmark, the distances to supraorbital margin were: right(R): 16.76 +/- 2.62 mm, left (L): 17.10 +/- 1.97 mm, and to infraorbital margin were R: 20.18 +/- 3.24 mm, L: 18.94 +/- 2.19 mm. The distances to the anterior ethmoidal foramen were R: 19.66 +/- 3.96 mm, L: 19.11 +/- 2.84 mm, and to the posterior ethmoidal foramen were R: 32.01 +/- 2.90 mm, L: 32.62 +/- 3.33 mm. Mean distance between the anterior and posterior ethmoidal foramen were R: 12.55 +/- 3.4 mm, L: 13.51 +/- 4.2 mm. The posterior ethmoidal foramen and optic ring were separated only by the mean distances of R: 5.34 +/- 2.81 mm, L: 4.9 +/- 3.35 mm. The distance from the suture to the distal (orbital) opening of the optic canal was R: 37.35 +/- 2.73 mm, L: 37.52 +/- 3.47 mm and to proximal (intracranial) opening of the canal were R: 49.52 +/- 2.62 mm, L: 50.94 +/- 3.35 mm. The average widths of proximal (intracranial) canal measured were R: 7.43 +/- 1.95 mm, L: 7.38 +/- 2.01 mm and those of distal canal (orbital) were R: 5.12 +/- 1.1 mm, L: 4.95 +/- 1.32 mm. The mean lengths of the optic canal were R: 11.19 +/- 2.68 mm, L: 12.42 +/- 3.38 mm. In radiologic examinations, the mean numbers of anterior group ethmoidal cells were R: 7, L: 6 and those of posterior group ethmoidal cells were R: 4, L: 3. The results of CT demonstrated 7 (14%) Onodi or sphenoethmoidal cells in 50 orbits of living humans.

Conclusion: The examination of radiologic anatomy in addition to microanatomy can significantly contribute to preoperative and postoperative evaluation of the patients.