Normal range of exophthalmos values on orbit computerized tomography in Koreans

The value of parameters in Hertel's exophthalmometry was measured by using orbit computerized tomography (CT) in this study. We selected images that revealed the center of the lens, the largest eyeball contour and the optic canal on the axial view of orbit CT. Parameters of exophthalmometry on orbit CT were the distance between the lateral orbital rims of both eyes (A), the shortest distance from the corneal center to line A (B), the B/A ratio, the distance between the lateral orbital rim and the medial orbital rim (C), the length of the line passing through the lens center from the apex to line C (D) and the D/C ratio. To compare the center position of the eye, we measure the axial length (E) and the longest distance between the corneal apex and the posterior pole (F) which is parallel to line B. We also calculated B-F/2 and Hertel-F/2 in order to know if there is a difference between measurements of orbit CT and of Hertel's exophthalmometer. Subjects were classified into group 1 (aged 8-13 years old) and group 2 (aged 20 years old more). Sixteen subjects were enrolled in group 1 (32 eyes). In group 2, 100 subjects were male (200 eyes) and 35 were female (70 eyes). In group 1, the mean value was 89.94 mm for A, 13.49 mm for B, 35.13 mm for C, 10.64 mm for D, 24.40 mm for E, 24.20 mm for F, 0.15 for the B/A ratio and 0.30 for the D/C ratio. In group 2, the mean value was 100.93 mm for A, 15.03 mm for B, 38.78 mm for C, 12.03 mm for D, 24.62 mm for E, 24.33 mm for F, 0.15 for the B/A ratio and 0.31 for the D/C ratio. There was a statistically significant difference between the two groups for parameters A, B, C and D (p = 0.001), but no significant difference for the B/A (p = 0.239) and D/C ratios (p = 0.803). In the males and females of group 2, there was a statistically significant difference for the B/A ratio (p = 0.028). We suggest that the value of the D/C ratio (0.30-0.31) could be used as the index of protrusion. The normal values of exophthalmometry parameters measured by orbit CT could also be helpful to evaluate the exact protrusion in orbital diseases.