ORA (Ocular Residual Astigmatism) measurement. Comparison of total ocular astigmatism and corneal astigmatism
Jacek Pniewski, Małgorzata Patrzykont
Wydział Fizyki, Uniwersytet Warszawski
Astigmatism is a monochromatic aberration of the optical system and it occurs when the optical system is not symmetric about the optical axis. In the human eye, this results in distorted or blurred vision at any distance. Total ocular astigmatism is the combined outcome of corneal astigmatism that refers to an astigmatism of the anterior corneal surface, and ocular residual astigmatism (ORA), which is defined as an astigmatism of the posterior corneal surface or both surfaces of the crystalline lens. The study included ORA measurement. The estimated total astigmatism was determined by Javal's rule and compared to refractive astigmatism. 20 young adult participants were examined. ORA was determined by the difference of refractive astigmatism measured by subjective refraction and corneal astigmatism measured by the Visionix L80 Wave + aberrometer. It was shown that ocular residual astigmatism was classified against-the-rule in each case, and most frequently ranged from -1.00 to -0.75 diopters. There were statistically significant differences and also a significant correlation between estimated and manifest astigmatism. Javal’s rule does not provide accurate prediction of refractive astigmatism, but it can be used as an indication at the initial stage of examination.