Abstract
Purpose:
To determine the relation between anterior and posterior corneal curvature in Tohono O’odham schoolchildren. About 1 in 4 of these children have more than 3 Diopters (D) of astigmatism that is predominantly corneal in origin and with-the-rule.
Methods:
Subjects were 975 children, 6 to <19 years (mean 12.2 years), 48% boys. The cornea of the right eye of each subject was measured with the Pentacam (Oculus Inc., Wetzlar Germany) rotating Scheimpflug corneal tomographer, and with the Retinomax K-Plus (Nikon, Tokyo Japan) autoref-keratometer. Pentacam Posterior Corneal (PPC), Pentacam Anterior Corneal (PAC) and Retinomax Keratometer (XK) steeper and flatter dioptric powers and meridia were converted into equivalent orthogonal dioptric power vectors M, J0 and J45. Linear regression was performed to determine the relation between PPC and PAC or XK power vectors.
Results:
For PPC, M mean(sd)=-6.15 (0.23), J0=0.25 (0.11), J45=0.15 (0.06). For PAC, M=42.4 (1.38), J0=-1.08 (0.64), J45=-0.20 (0.31). For XK, M=42.5 (1.40), J0=-0.98 (0.66), J45=0.28 (0.27). The linear regression of PPC and PAC had Rsq=0.69 for J0 and Rsq=0.62 for J45. The linear regression of PPC and XK had Rsq=0.68 for J0 and Rsq=0.37 for J45. All correlations were inverse: as posterior astigmatic power became increasingly negative, anterior corneal astigmatism (either Pentacam Anterior Corneal Power or Retinomax Keratometry) became increasingly positive, indicating that the steeper axis of the concave posterior cornea was aligned with the steeper axis of the convex anterior corneal surface. Up to 1.4 D of posterior corneal astigmatism was observed, not detectable by only measuring the anterior corneal surface.
Conclusions:
In this group of subjects, the curvature of the steeper and flatter meridia of the anterior and posterior cornea are aligned with each other, indicating that the process causing astigmatism to develop influenced the shape of both corneal surfaces, suggesting corneal thickness is conserved in the process. As this relation is observed with two independent anterior corneal measurements, correlated errors are ruled out. The corneal posterior astigmatism decreases the astigmatism induced by the anterior corneal toricity.
Keywords: 428 astigmatism •
677 refractive error development •
681 refractive surgery: corneal topography