Okajimas Folia Anat. Jpn., 75(6): 319-322, March, 1999

Surgical Anatomy and Morphometric Analysis of the

Optico-Chiasmatic Apparatus, Optic Canal and Sphenoid Ridge

By

Adnan OZTURK, Mustafa BOZBUGA, Bulent BAYRAKTAR, Zafer ARI,Kayihan SAHINOGLU, Gursel POLAT and Isik GUREL

Department of Anatomy, Istanbul Medical Faculty, Istanbul University, Istanbul, Turkey

Neurosurgical Clinic of Kartal Research and Teaching Hospital, Istanbul, Turkey

-Received for Publication, April 20, 1998-

Key Words: Cranial base approaches, Measurements, Optic nerve, Skull, Surgical anatomy

Summary: An anatomical study was performed in order to obtain help for orientation regarding the cranial base ap-

proaches to the anterior cranial base. Cranial base approaches were studied in 8 adult cadaver heads, and morphometric measurements critical in these approaches were achieved in 76 dry skulls. Importance of the surgical anatomy of the opticochiasmatic apparatus, optic canal, sphenoid ridge, and anterior clinoid was emphasised in this study. Observations

from the dissections and operative approaches, and measurements between various points were recorded in a specifi-

cally designed software, and these data helped both to understand the local anatomy and the relationship to the in-

timate structures better and to decide the head position, the degree and direction of safe bony removal, and the direc-

tion of the operative approach during the surgery. Considerations important in the selection of these structures,

anatomical landmarks and distances were discussed.

The opticochiasmatic apparatus, optic canal, an-

terior clinoid and sphenoid ridge, which are fre-

quently included in surgical field of the neuro-surgical operations, are most important anatomical

structures due to their critical functions and central

locations. The optic canal transmits the optic nerve

with its dural and arachnoid coverings and also the

ophthalmic artery from the orbit to the cranial cav-ity. Surgery of the optic canal, opticochiasmatic ap-

paratus, and related structures requires detailed knowledge of the anatomy of the area and their

variations. This study aims to provide an ori-

entation to the surgical anatomy and the morpho-

metric analysis of the region.

Materials and Methods

Eight adult human cadavers, embalmed with

formalin, and seventy six adult human dry skulls

were used in this study. The dissection and identi-

fication of the neurovascular structures, and cranial

base approaches were performed, and also bony

and soft tissue measurements were done in the ca-

davers. All observations and measurements were recorded in a specially designed software. Seventy six adult human dry skulls (constituting 152 speci-mens) were studied; the landmarks were identified

(Fig. 1), and the measurements were taken of the various relationships, and these data were also re-corded in detail, and statistical analysis of the data from the series was applied. All measurements were taken using standard calipers in millimeters.

This morphometric analysis included the follow-ing measurements (Fig. 1): 1. distance between the optic nerves at the en-trance to the optic canal; 2. length of the roof of the optic canal; 3. length of the lower wall of the optic canal; 4. length of the medial wall of the optic canal; 5. length of the lateral wall of the optic canal; 6. axis of the optic canal relative to the Frankfurt Horizontal Plane (FHP); 7. angle between the optic canal and the sagittal

plane; 8. distance of the medial wall of the intracranial opening of the optic canal from the midline; 9. distance of the medial wall of the anterior open-

Correspondence to: Adnan OZTURK, MD., Assistant Professor, Department of Anatomy, Istanbul Medical Faculty, Istanbul Uni-

versity, Capa 34390, Istanbul, Turkey

320 A. Oztiirk et al .

Fig. 1. The distances are shown in this figure.

ing of the optic canal from the midline; 10. distance from the junction of the floor of the anterior cranial fossa and the anterior wall of the fossa to the anterior border of the prechiasmatic sulcus; 11. intracranial length of the optic nerve on the medial side of the nerve; 12. intracranial length of the optic nerve on the lateral side of the nerve; 13. angle between the optic nerves; 14. distance between the anterior margin of the optic chiasm and the prechiasmatic sulcus in the midsagittal plane; 15. length of the optic chiasm; 16. width of the optic chiasm; 17. distance between the prechiasmatic sulcus and the tuberculum sella in the midsagittal plane; 18. distance between the lateral end of the superior orbital fissure and the apex of the anterior clinoid; 19. distance between the lateral end of the superior orbital fissure and the crista alaris (lateral end of the sphenoid ridge); 20. angle between the sphenoid ridge and the sag-ittal plane; 21. distance between the lateral end of the superior orbital fissure and the internal surface of the supe-

rior orbital margin in the parasagittal plane.

Results

The data from the measurements of our series

are given in Table 1.

Discussion

The opticochiasmatic apparatus, and intimate

structures to the opticochiasmatic apparatus, such

as the optic canal, anterior clinoid, sphenoid ridge,

are commonly involved in many neurosurgical

operations, thus the surgical anatomy of this re-

gion should be known in detail. A wide variety of

pathologic processes may invade the region, and require surgical operations. Benign or malign

neoplasms, vascular lesions such as aneurysms or

arteriovenous malformations, and traumatic lesions

are the most common types of the lesions neces-

sitating the surgical operations to this area. The

pterional approach"' 6), subfrontal approach", extended basal frontal approach"), subtemporal

approaches' '7'12'13), cranio-orbital zygomatic ap-

Surgical Anatomy and Morphometric Analysis 321

Table 1. Morphometric data from measurements of the parameters

proach2), anterior and anterolateral craniofacial resection, and facial translocation approach8), all may be used in treatment according to the nature and location of the lesion.

In this study the landmarks and the measure-ments were chosen completely in order to provide a surgical orientation and ease the surgical operation. Parameters 1, 11, 12, 13, 14, and 17 encompass a critical area, and this area is most oftenly involved in some tumors such as tuberculum sellae meningi-omas, craniopharingiomas, optic gliomas, pituitary adenomas or in some large aneurysms of the ante-rior communicating artery, ophthalmic artery, su-

perior hypophyseal artery. The optic canal lies be-tween the two bases of the lesser sphenoid wings. The proximal opening of the optic canal is formed superiorly by a thin fold of dura, and the length of the optic nerve is covered by this fold of dura. The

proximal opening of the optic canal is elliptical in shape and the horizontal width is consistently

greater than its height. The canal is narrow distally as it approaches the orbit, and the medial wall of the distal canal is very dense in comparison to the

proximal segment"). The anatomical features and the detailed measurements of the optic canal are very important surgical considerations in optic nerve decompression, either transethmoidally6) or

transcranially3). The distal portion of the optic canal is both the narrowest and the densest part of the canal, and a special care must be taken to be able to achieve an effective optic nerve decom-

pression. In our material we determined a mean

length of 7.7 mm (on right) and 8.1 mm (on left) for the roof of the optic canal; 4.4 mm (on right) and 4.3 mm (on left) for the lower wall of the optic canal; 10.7 mm (on right) and 10.5 mm (on left) for the medial wall; 9.9 mm (on right) and 10.1 mm (on left) for the lateral wall. To provide further expo-sure, mostly in paraclinoid aneurysm cases or some tumors of the region, opening of the optic canal, incisising the dural sheath of the optic nerve, and dissection and mobilization of the optic nerve are sometimes applied as a quite useful mean. The multiplan direction of the optic canal is also critical in surgical approach. Hence, the axis of the optic canal relative to the FHP, and the angle between the optic canal and the sagittal plane were de-termined for the hope of the appropriate head po-sition and surgical direction. The axis of the optic canal is directed forward and downward averagely 16.9° (on right) and 18.2° (on left) relative to the FHP, and the angle of the optic canal relative to the sagittal plane is 38.8° on the right and 38.1° on the left. Due to this direction, the proximal (intra-cranial) opening of the optic canal (parameter 8) is narrower to the midline than the distal (orbital) opening of the optic canal (parameter 9). The thickest portion of the optic canal is the lateral wall, and in decreasing order the roof, and the distal

portion. The thickness of the medial wall of the optic canal carries a critical surgical consideration when the transethmoidal and transsphenoidal ap-

proaches to the canal. The distance from the junction of the floor of the

322 A. Ozttirk et al

anterior cranial fossa and the anterior wall of the fossa to the anterior border of the prechiasmatic sulcus was found to be 44.5 (34.2-53.7) mm, and this shows the subfrontal surgical distance to the optic apparatus in subfrontal approaches. In the parasagittal plane, the distance between the lateral end of the superior orbital fissure and the internal surface of the superior orbital margin was 35.9 (30.2-40.1) mm on the right, and 36.2 (31.1-39.5) mm on the left. In performing extended frontal

approach"), or another approach requiring orbital osteotomy, anteroposterior diameter of the orbital osteotomy should not exceed 2.5-3 cm from the orbital margin in order not to injure the neuro-vascular content of the superior orbital fissure.

The average length of the optic chiasm was found to be 7.8 mm (range 5.6-9.6), and the width was 12.8 (9.0-16.4) mm. In some cases the intra-cranial parts of the optic nerves are long and the distance between the intracranial apertures of the optic nerve and the tuberculum sellae is short, a condition known as postfixed chiasm. In such cases the pituitary stalk has an unusually flat course. Renn and Rhoton reported this kind of chiasm in

15% of the cases"). The distance between the lateral end of the su-

perior orbital fissure and the apex of the anterior clinoid (parameter 18), the distance between the lateral end of the superior orbital fissure and the crista alaris (lateral end of the sphenoid ridge) (parameter 19), and the angle between the sphe-noid ridge and the sagittal plane (parameter 20) are necessary in lateral approaches through the sphe-noid ridge. In these operations the key point is, using a high-speed drill, resection of the bone as much as possible to provide a smooth and flat sphenoid surface. This extensive bone resection minimizes the brain retraction, and maximize the operative exposure.

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