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[Computer-aided 3-D reconstruction and measurement of optic canal and intracanalicular structures].

OBJECTIVE
To reconstruct the human optic canal and its inner structures, and to provide knowledge of this region in detail for optic nerve decompression and further understanding on the pathologic mechanisms of indirect optic nerve injury.


METHODS
6 optic canals and its inner structures were reconstructed by using a computer-aided 3-dimensional (3-D) reconstruction system. Quantitative measurement for the canal wall thickness, bony canal transverse area, optic nerve transverse area, dual sheath transverse area, subarachnoid space transverse area, and subarachnoid space volume were done by means of the computer morphometric analysis system. The detailed spatial relations among intracanalicular structures were also carefully identified on the 3-D models.


RESULTS
The thinnest portion of the canal was the middle part of the medial wall (0.45 +/- 0.35)mm, and the narrowest space was in the middle part of the optic canal [the transverse area was (18.21 +/- 2.50) mm(2)]. The volume of subarachnoid space which can be considered the compensatory space for distention incurred by the hemorrhage, optic nerve edema, or hematoma was (21.16 +/- 4.31) mm(3). At the cranial opening, the middle part and orbital opening, its transverse area was (4.45 +/- 1.12) mm(2), (2.68 +/- 1.32) mm(2), (1.23 +/- 0.83) mm(2), respectively.


CONCLUSIONS
In the middle part, the restraining of the bony canal on injured nerve may be the most powerful. Since the compensatory space was limited, even a tiny amount of blood or swelling of the nerve may cause optic compression. Because the compensatory space for distention gradually decreases from cranial end to orbital end, the middle part and the anterior part of the optic canal and dural sheath are critical in optic nerve decompression.