Astigmatic Corneas—Oblate and Prolate Meridians
In their article, Budak et al.1 provide valuable information about the relationship among refractive and topographic parameters in 287 eyes with refractive astigmatism (mean 0.57 diopter [D] ± 0.65 [SD]; range 0 to 4.50 D), total astigmatism (mean 0.87 ± 0.66 D; range 0.11 to 6.68 D), and regular astigmatism (mean 0.84 ± 0.67 D; range 0.07 to 6.68 D). They report that as the degree of myopia and negative asphericity increased, the corneal radius of curvature decreased. They found no correlation between Q-values and the spherical equivalent refraction (P = .218) and that eyes with moderate myopia (−2.0 to −6.0 D) had more positive Q-values than those with hyperopia (F = 4.34, P = .0032; analysis of variance).
The normal value of asphericity (Q-value) is −0.26, indicating that the normal cornea flattens by about 7% in its radius of curvature relative to a sphere 5.0 mm from the center. We know the Q-value for a sphere is zero.2 A cornea with astigmatism has both prolate and oblate meridians. The prolate axis has a negative Q-value and the oblate axis, a positive Q-value. The EyeSys 2000 topography analysis system calculates mean Q-values in all meridians with the central 4.5 mm diameter zone. So it is possible that a cornea with high astigmatism may have a Q-value near or equal to zero. However, this does not mean that the shape of this cornea resembles a sphere. Actually, its shape is far from this. In astigmatic corneas, when oblate meridians dominate, the mean Q-value becomes positive; when prolate meridians dominate, the mean Q-value becomes negative. For this reason, a sample of 287 eyes with total astigmatism (mean 0.87 ± 0.66 D; range 0.11 to 6.68 D) may not provide reliable information about the corneal asphericity of eyes without astigmatism. We think that including Q-values of eyes with significant astigmatism in this study may have been a flaw. Instead, the magnitude of astigmatism could have been divided into groups and the Q-values of eyes without significant stigmatism analyzed separately. Then, the Q-values for myopic and hypermetropic eyes would be more reliable.
Yaþar Sakarya MD
Sadýk Kavaklý MD
Samed Sýtký Ermiþ MD
Ertuorul Cüneyt Iþýk MD
1. Budak T, Khater TT, Friedman NJ, et al. Evaluation of relationships among refractive and topographic parameters. J Cataract Refract Surg 1999; 25:814-820
© 2000 by Lippincott Williams & Wilkins, Inc.
2. Holladay JT. Corneal topography using the Holladay Diagnostic Summary. J Cataract Refract Surg 1997; 23:209-221