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

OBJECTIVE: To reconstruct the human optic canal and its inner structures, and to provide detailed knowledge of this region for optic nerve decompression for further understanding on the pathologic mechanisms of indirect optic nerve injury. METHODS: Six optic canals and their inner structures were reconstructed using a computer-aided 3-dimensional reconstruction system. Quantitative measurement of the canal wall thickness, bony canal transverse area, optic nerve transverse area, dural sheath transverse area, subarachnoid space transverse area, and subarachnoid space volume were done by means of the computer morphometric analysis system. The detailed spatial relationship 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 mm2). 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 mm3. At the cranial opening, the middle part and orbital opening, its transverse area was 4.45 +/- 1.12 mm2, 2.68 +/- 1.32 mm2 and 1.23 +/- 0.83 mm2, respectively. CONCLUSIONS: Since the compensatory space was limited, even a tiny amount of blood or swelling of the nerve may cause optic nerve compression. Because the narrowest space was in the middle part of the optic canal and 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.