Size of the optic nerve scleral canal and comparison with intravital determination of optic disc dimensions.

Diameters, area, and form of the optic nerve scleral canal were measured in 107 freshly enucleated, unfixed, human donor eyes. Macrophotographs of the sectioned posterior fundus pole were provided with a millimeter scale and evaluated planimetrically. They revealed a surprising variation: area: 2.59 +/- 0.72 mm2 (minimum 0.68 mm2, maximum 4.42 mm2); minimal diameter: 1.67 +/- 0.72 mm; maximal diameter: 1.92 +/- 0.32 mm. The mean form factor was 0.92 +/- 0.11 and the quotient of minimal to maximal diameter 0.86 +/- 0.11, indicating a slightly oval nearly round form. The coefficients of variation of the method's reproducibility were 0.005 for intraobserver and 0.02 for interobserver determination. The size values of these optic nerve scleral canals were not significantly (Wilcoxon-Mann-Whitney test) different from than those of 100 unselected optic discs that had been determined intravitally using Littmann's method. This indicates the reliability of Littmann's method for the intravital measurement of the optic nerve head in absolute dimensions.