|Year : 2015 | Volume
| Issue : 3 | Page : 204-208
A morphometric evaluation of the optic canal: Comparative study between computerized tomographic study and direct anatomic study
Sneha Kalthur1, Radhakrishnan Periyasamy1, Sandeep Kumar2, Chandni Gupta1, Antony S D'souza1
1 Department of Anatomy, Kasturba Medical College, Manipal University, Manipal, Karnataka, India
2 Department of Radiology, Kasturba Medical College, Manipal University, Manipal, Karnataka, India
|Date of Web Publication||3-Aug-2015|
epartment of Anatomy, Kasturba Medical College, Manipal University, Manipal - 576 104, Karnataka
Objective: The objective of this study is to probe into the normal anatomy of the optic canal by comparison of dry skull and computed tomography (CT) images of live subjects.
Materials and Methods: The optic canals of 107 normal subjects were examined by thin layer CT scan images in axial and coronal sections. 25 dry skulls were examined by using sliding Vernier caliper. The width, height of optic canal in both opening (cranial and orbital side) were measured. The medial, lateral walls (LWs) of optic canal were also measured. By using mathematical formula, the area of optic canal was calculated. Student's t-test was applied to compare the results statistically.
Results: The height of optic canal at orbital and cranial opening in both CT and direct study was 4.12 ± 0.63 mm, 3.6 ± 0.76 mm and 5.7 ± 0.76 mm, 5.17 ± 0.81 mm, respectively. The width at orbital and cranial opening in both CT and direct was 2.98 ± 0.56 mm, 4.59 ± 0.83 mm and 4.74 ± 0.47 mm, 5.48 ± 0.76 mm, respectively. The length of medial and LW in both CT and direct study was 10.63 ± 1.72 mm, 9.2 ± 1.33 mm and 9.1 ± 1.46 mm, 8.66 ± 1.31 mm, respectively. The area was 1.43 cm in direct study and in CT study it was 1.19 cm.
Conclusion: Given parameters may be helpful for surgeons to plan the proper way to approach the optic canal.
ملخص البحث :
هدف هذا البحث إلى دراسة الوضع التشريحي الطبيعي للقناة البصرية بمقارنة جماجم بشرية مع صور الأشعة المقطعية لأحياء. تم قياس طول وعرض القناة والجدران الداخلية والجانبية للقناة. ومن ثم حساب مساحة القناة البصرية باستخدام معادلة حسابية. ربما تساعد القياسات التي توصل إليها الباحثون الجراحين في الطريقة الصحيحة للوصول للقناة البصرية.
Keywords: Computed tomography scan, direct, measurements, optic canal
|How to cite this article:|
Kalthur S, Periyasamy R, Kumar S, Gupta C, D'souza AS. A morphometric evaluation of the optic canal: Comparative study between computerized tomographic study and direct anatomic study. Saudi J Med Med Sci 2015;3:204-8
|How to cite this URL:|
Kalthur S, Periyasamy R, Kumar S, Gupta C, D'souza AS. A morphometric evaluation of the optic canal: Comparative study between computerized tomographic study and direct anatomic study. Saudi J Med Med Sci [serial online] 2015 [cited 2020 Nov 27];3:204-8. Available from: https://www.sjmms.net/text.asp?2015/3/3/204/161997
| Introduction|| |
Knowledge of the normal and variant positions of the canals and foramina of the skull base is important for radiologists, neurosurgeons and anatomists, while doing surgery in these regions. The lesser wing of the sphenoid is connected to the body of the sphenoid by a thin, flat anterior root and thick triangular posterior root. The optic canal lies between these roots and connects the orbit to the middle cranial fossa. The optic nerve and the ophthalmic artery are transmitted through the canal. Neuro-arterial relations in the region of the optic canal need more attention from the practical medical point of view. Since injury to any part of the optic pathway results in visual defects, knowledge of the gross features of the optic canal and related structures is very important for surgeons who are approaching for vascular lesions of this area. Tumors of the posterior orbit such as angioma, neuroma, malignant tumor, meningioma, pneumosinus and tumors of the greater and lesser wings of the sphenoid bone are operated at the posterior part of the orbit, optic canal, or superior orbital fissure. 
Although the effectiveness of computed tomography (CT) in diagnosing bony lesions is uncontested, most of the authors have described optimal positioning without mentioning the correlation between their findings and the actual anatomical position of the osseous structures. In the following study, anatomical and CT measurements, including length, width and height of optic canals and the distances between these structures, are evaluated. The extent to which anatomical variations can be assessed by CT has also been investigated.
| Materials and methods|| |
The study was carried out on 25 dry adult skulls of either sex. Damaged and diseased skulls were not included for the study. The optic canal was measured from cranial as well as orbital sides in direct and CT studies. For direct method, digital Vernier caliper was used.
The following metrical parameters of optic canal were measured:
In CT study, by using Philips 64 CT scanner, 107 (75 males and 32 females) images of normal subjects between the ages of 30 and 80 years were evaluated. Ethical clearance has been taken from the Institutional Ethical Committee. Images were obtained from patients who had no inflammatory changes in the region of optic canal. Any scans in which the bones of the orbit or sinuses were eroded by tumor or fractures were excluded from the study. The CT images were reviewed retrospectively in both the axial and coronal plane.
- Height and width of cranial side
- Height and width of orbital side
- Length of medial wall (MW)
- Length of lateral wall (LW).
The measurements of optic canal length, width were obtained on both right and left sides. Computer-assisted measurements of the medial and lateral canal wall lengths were obtained by measuring the length of the canal formed laterally by the anterior clinoid process and medially by the sphenoethmoidal cell of the posterior ethmoid sinus and the sphenoid sinus in the axial plane [Figure 1]. The width of the canal was measured in the axial plane at the orbital opening anteriorly and the cranial opening posteriorly. Anterior and posterior canal wall heights were measured on coronal images at the orbital opening and the cranial opening of the canal, respectively [Figure 2].
|Figure 1: Measurement of optic canal in axial sections. MW – Medial wall: Length of medial wall, LW – Lateral wall: Length of lateral wall, WO – Width orbit: Width of orbital side, WC – Width cranial: Width of cranial side|
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Mathematical formulas were used to calculate the surface area of the optic canal. As the diameter of the optic canal at the orbital side is less than that at the cranial (sellar) side, the surface area of a cone frustum was utilized: p(R + r)s (2). Here, "R0" is radius of the cranial side opening and "r" is radius of orbital side opening of the optic canal. "S" is the length of optic canal that is average of the medial and LW length of the optic canal [Figure 3]. Student's t-test was used to compare the results of CT and direct study and also to compare the gender, side and area as well.
| Results|| |
A total of 50 (25 skulls) optic canals were analyzed. The mean ± standard deviation (SD) of all the parameters measured are shown in [Table 1]. The average surface areas of the studied subjects were shown in [Table 2].
Hundred and seven CT scans (75 males and 32 females) were reviewed. A total of 214 optic canals were analyzed (150 males and 64 females). The mean ± SD of all the parameters measured are shown in [Table 1]. The average surface areas of the studied subjects were shown in [Table 2].
There was statically significant difference noted in all the parameters measured in males and females with P < 0.001. The mean and range of all the parameters in males and females are shown in [Table 3]. There was statically significant difference noted in all the parameters measured on right and left sides with P < 0.001. The mean and range of all the parameters on right and left sides are shown in [Table 4]. The average area of the optic canal in male subjects were 1.29 and 1.20 cm 2 on right and left sides respectively, whereas 1.08 and 1.01 cm 2 in female subjects. There was a significant difference between direct and CT method (P < 0.01).
| Discussion|| |
The intracranial portions of the optic nerve and ophthalmic artery are susceptible to various diseases and injuries; therefore, knowledge of the gross anatomy of the optic canal and related structures is necessary for proper diagnosis and preservation of the neurovascular structures during subfrontal and intracanalicular procedures.
Hart et al. stated that the average height and width at the orbital side in male subjects were 0.5 cm and 0.46 cm, respectively, and 0.46 cm and 0.44 cm in female subjects (P > 0.05). At the cranial side height and width in male subjects were 0.41 cm and 0.71 cm. respectively, versus 0.40 cm and 0.64 cm in female subjects (P > 0.05). The average medial canal wall length in male subjects was 1.61 cm, whereas it was 1.39 cm (P < 0.001) in female subjects. The average lateral canal wall length was 1.13 cm (range, 0.7-1.6 cm) in male subjects, when compared 1.00 cm (range, 0.6-1.5 cm) in female subjects (P < 0.01). The average area of the optic canal was 2.53 cm on the left and 2.36 cm 2 on right side. 
Our values of all parameters are slightly lower than their study, but we also got significant differences between males and females in the average length of medial and LW as their study.
Lang and Gehmann measured the length of the MW of the optic canal as 11.4 (8-16) mm. Berlis et al. measured the distance of the lateral margins as 33.13 ± 2.77 (27.96-40.86) mm. For the cranial aperture, they measured the distance of the lateral margins as 25.15 ± 2.51 (19.83-30.87) mm. ,
Orhan Mağden and Kaynak 1996 measured the width of the orbital aperture as 4.31 ± 0.4 mm on the right side and 4.32 ± 0.4 mm on the left. They measured the width of the cranial aperture as 6.48 ± 0.6 mm on the right side and 6.43 ± 0.6 mm on the left. The height of the cranial aperture was 4.35 ± 0.03 on the right side and 4.36 ± 0.03 on the left. The length of the MW was 9.61 ± 1.6 mm on the right side and 9.84 ± 14.8 mm on the left and that of the LW as 9.22 ± 1.3 mm on the right side and 9.1 ± 1.3 mm on the left. 
Govsa et al. 1999 measured that the height and width of orbital apertures as 5.42 ± 0.73 mm on right side, 5.29 ± 0.89 mm on left side and 4.93 ± 0.73 mm on right side, 4.55 ± 0.75 mm on left side, respectively. The height and width of cranial apertures as 4.7 ± 0.79 mm on right side, 4.55 ± 0.65 on left side and 4.91 ± 0.49 mm on right side, 4.71 ± 0.43 mm on left side, respectively. The length of the MW and LW were 7.5 ± 1.5 mm on right side, 7.72 ± 1.41 mm on left side and 6.48 ± 1.61 mm on right side, 6.94 ± 1.84 mm on left side, respectively. 
Our values are slightly different from these studies. This may be because they have done it on dried skull while we have done on CT.
Maniscalco and Habal 1978 reported that the diameter of orbital and cranial opening of optic canal was 4.7 (1.0-6.0) mm and 7.18 (5.0-9.7) mm, respectively. 
Comparing our results with previous author's shows significant differences particularly in the area of optic canal and it is shown in [Table 5]. The previous authors (most of them) analyzed the morphometry of optic canal by using skull but in our study, we studied through CT of live subjects, which would be more reliable as their values.
Removing the bony border of the optic canal without exposing the neural structures is important in dealing with tumors such as meningiomas that have grown through the bone to compress, but not infiltrate, the structures in the canal. In other cases, tumors such as schwannomas and meningiomas may grow along the nerves, requiring various parts of the optic canal to be opened. Removal of the bone of the optic canal is a common step in exposing tumors and aneurysms in the region. The anterior clinoid process and the lesser wing of the sphenoid are usually removed when dealing with ophthalmic and superior hypophyseal aneurysms. The greater wing of the sphenoid may also be removed in tumors involving the middle cranial fossa and cavernous sinus.  
| Conclusion|| |
The anatomic description of the optic canal should be carefully studied by all neurosurgeons working on the cranial base, especially before operating on meningiomas that may infiltrate along the nerves in this area. A wide variation exists in CT images in all parameters of the optic canal. A significant difference was found among the sex groups and sides in both CT and direct study of optic canal. Our study may help in the development of new strategies by precisely defining the morphological anatomy of the optic canal.
| References|| |
Govsa F, Erturk M, Kayalioglu G, Pinar Y, Ozer MA, Ozgur T. Neuro-arterial relations in the region of the optic canal. Surg Radiol Anat 1999;21:329-35.
Hart CK, Theodosopoulos PV, Zimmer LA. Anatomy of the optic canal: A computed tomography study of endoscopic nerve decompression. Ann Otol Rhinol Laryngol 2009;118:839-44.
Berlis A, Putz R, Schumacher M. Direct and CT measurements of canals and foramina of the skull base. Br J Radiol 1992;65:653-61.
Lang J, Gehmann G. Form development of optic canal, measurements and position with respect to cranial bones. Verh Anat Ges 1976;70567-74.
Orhan Mağden A, Kaynak S. Bilateral duplication of the optic canals. Ann Anat 1996;178:61-4.
Maniscalco JE, Habal MB. Microanatomy of the optic canal. J Neurosurg 1978;48:402-6.
Slavin KV, Dujovny M, Soeira G, Ausman JI. Optic canal: Microanatomic study. Skull Base Surg 1994;4:136-44.
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]