Optic Canal: Microanatomic Study
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Konstantin V Slavin, M.D., Manuel Dujovny, M.D., Gelson Soeira, M.D., and James I. Ausman, M.D., Ph.D. Optic Canal: Microanatomic Study The optic canal is a bony channel that connects the the optic canal and distance between the folds; the height anterior cranial fossa and the orbit and contains the optic of the anterior clinoid process (which forms the lateral nerve and the ophthalmic artery. Detailed knowledge of border of the intracranial opening); distances between the the anatomy of the optic canal and adjacent vital struc- anterior clinoid and the tuberculum sellae, between the tures, such as the internal carotid artery and the hypo- anterior clinoids, and between the inferior margin of the physis, is very important in performing different surgical intracranial opening of the optic canal and the superior procedures on the anterior part of the skull base and the orbital fissure; and the diameter ofthe intracranial opening orbit. Recently, anatomic data about the optic canal had of the optic canal. After initial dissections and division of included descriptions of bony structures forming its walls the dural fold over the optic nerve the optic canal was and its size. Improvement in surgical techniques has in- unroofed using the high-speed drill. At this stage the creased interest in the optic canal, but many aspects still second series of measurements was made, which included require investigation. the intracanalicular length of the optic nerve, the length of This study was performed to facilitate understanding the bony roof and floor of the optic canal, the angle of the of the anatomic relationship of the optic canal to the optic canal with the midline in the horizontal plane, and adjacent bone and soft tissue structures, detail the course the course of the ophthalmic artery relative to the optic of the optic nerve and ophthalmic artery within and near nerve. The diameter of the ophthalmic artery and location the optic canal, and assist in developing a convenient and at the distal and proximal parts of the optic canal were also safe approach to different lesions in this location. noted. For the third stage, unroofing the orbit allowed us to measure the distances between the intraorbital opening of the optic canal and the annulus tendinosus and the bulbus MATERIALS AND METHODS oculi, as well as the width and height of the intraorbital opening ofthe optic canal (foramen opticum). After open- The study was performed on 20 cadaveric speci- ing the entire optic canal, the optic nerve with the ophthal- mens, fixed in glycerin, from patients who had never had mic artery was removed, and the shape and diameter ofthe any intracranial or intraorbital pathology. Optic canal optic nerve in the middle of the optic canal and in its measurements were made before, during, and after ana- orbital exit were measured. The last stage of dissection tomic dissections, which were performed under an opera- included noting the degree of pneumatization of adjacent tive microscope (OPMI I-SH, Carl Zeiss, Inc., New York) bone structures, such as the anterior clinoid process, and using microsurgical instruments and a high-speed drill. the sphenoid and ethmoid sinuses, forming the optic canal. The first series of measurements included the diame- All stages of dissections were photographed and ter and shape of the intracranial portion of the optic nerve; drawn, and appropriate samples were used for laser print- the length of the dural fold covering the proximal part of ing from the slides. Skull Base Surgery, Volume 4, Number 3, July 1994 Department of Neurosurgery, University of Illinois at Chicago, Chicago, Illinois Reprint requests: Dr. Dujovny, Department ofNeurosurgery, Neuropsychiatric Institute (MIC 799), University of Illinois at Chicago, 912 South Wood Street, 136 Chicago, IL 60612 Copyright C 1994 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved. OPTIC CANAL-SLAVIN ET AL RESU LTS process had a mean height of 9.23 + 1.56 mm. The distance between the anterior clinoid and the tuberculum The results were divided into three groups according sellae was 13.72 ± 1.29 mm, and the distance between the to anatomic portion. We decided that such a division anterior clinoids was 25.67 + 1.52 mm. The distance would better correlate with clinical needs because differ- between the centers of the optic nerves at the level of their ent pathological processes affect the optic canal differ- entrance into the optic canal was 20.2 + 1.49 mm. The ently. superior orbital fissure was found to lie inferolaterally to The intracranial portion of the optic nerve was a the intracranial opening of the optic canal, the distance horizontal oval shape in all cases (Table 1), with mean between them being 6.41 + 1.34 mm. The dura lining the orbital was to lie over the of greatest diameter of4.25 + 0.98 mm (Table 2). Before the superior fissure found base the optic canal was entered the optic nerve was covered by the anterior clinoid process and continued into the optic canal. falciform dural fold, with a mean length of 3.7 ± 1.9 mm The intracranial opening of the optic canal had an almost (Fig. la-c). The distance between the medial borders of round shape, with a mean diameter of 5.14 + 0.9 mm. The optic canal itself (Table 3) presented as a hollow the dural folds was 10.17 + 1.72 mm. The anterior clinoid cylindrical structure, lined from inside with two layers of Table 1. The Shape of the Optic Nerve (%) dura (Figs. Id and 2). The length of the optic canal was measured as 10.74 ± 1.16 mm, equal to the length of the Horizontal Vertical Oval Round Oval intracanalicular part of the optic nerve. Because of the canal's inclining course the lengths of the bony roof (9.91 Intracranial part 100 Intracanal part 100 + 1.67 mm) and the floor (6.30 + 1.02 mm) were less than Intraorbital part 30 70 the length of the canal (Fig. 2). Inside the optic canal the Table 2. The Intracranial Opening of the Optic Canal Range Average Standard Deviation Parameter (mm) (mm) (mm) Diameter of the ON intracranially 2-6 4.25 ±0.98 Length of the dural fold 1-6 3.7 +1.9 Distance between the dural folds 7-13 10.17 +1.72 Height of the anterior clinoid 6.5-12 9.23 +1.56 Distance between anterior clinoid and tuberculum sellae 12-17 13.72 +1.29 Distance between anterior clinoids 24-27.5 25.67 ±1.52 Distance between centers of ONs 18-23 20.2 +1.49 Distance between the OC and superior orbital fissure 4-9 6.41 ±1.34 Diameter of the intracranial opening of the OC 4-7 5.14 ±0.9 ON = optic nerve; OC = optic canal ... Ti'_>4s ON Figure 1. Photographs of dif- ferent stages of dissection. a: Gen- eral view of intracranial course of the optic nerve. (Figure continued on the next page.) OCh 137 SKULL BASE SURGERYNOLUME 4, NUMBER 3 JULY 1994 S t ~~~DM r _ ~~~~ACLI_ F f~~~~~ON ACUPr PSph ':Figure 1. (Continued). b: Mag- nified view of the intracranial open- ing of the right optic canal. The falci- form dural fold covers the proximal N part of the optic canal. c: Intracranial opening of the right optic canal after excision of the falciform dural fold. d: View of the middle portion of the left optic canal after drilling off its roof. The optic nerve is covered by dura inside the canal. DM = dura mater, PSph = planum sphenoidale, ON = optic nerve, OCh = optic chi- asm, ACLP = anterior clinoid pro- cess, ICA = internal carotid artery, OS = opticstrut, OC = optic canal. 138 OPTIC CANAL-SLAVIN ET AL Table 3. Middle Part of the Optic Canal periorbit, while the inner layer accompanied the optic Standard nerve to the bulb (Fig. 5). Range Average Deviation At the level of intraorbital opening, or foramen op- Parameter (mm) (mm) (mm) ticum, the optic nerve had the shape of a vertical oval Length of intracanal 9-12 10.74 ±1.16 (70%) or a circle (30%) and its diameter was 2.86 + 0.48 part of ON mm (Table 5). The width ofthe foramen opticum was 5.20 Length of bone roof of 6-12 9.91 +1.67 + 0.68 mm and the height was 5.38 + 0.91 mm. After OC Length of bone floor of 5-8 6.30 ±1.02 leaving the optic canal the optic nerve went through the OC annulus tendinosus (Zinn), which was attached to the Diameter of ON within 2-3 2.62 ±0.38 periorbit and the bone in all cases. The length of the OC intraorbital part of the optic nerve was measured as 22.04 Diameter of ophthalmic 0.8-2 1.24 ±0.12 + 4.7 mm. artery After all of these measurements the degree of pneu- ON = optic nerve; OC = optic canal. matization of bone structures adjacent to the optic canal was evaluated (Table 6). The anterior clinoid process was pneumatized in 65% of cases, including two cases of optic nerve had a round shape in all cases, and its diameter luxury pneumatization. Luxury pneumatization of the in the middle of the canal was 2.62 + 0.38 mm. In the sphenoid sinus was found in 15% of cases. No cases of horizontal plane the angle of the optic canal in relation to luxury pneumatization of the ethmoid sinus were found, the midline was 29.56 + 1.520 (220 to 340) (Fig. 3). The but in two cases notable thinning ofthe bone wall separat- ophthalmic artery accompanied the optic nerve during the ing the ethmoid sinus from the optic canal was noted.