Revisiting Javal's rule: a fresh and improved power vector approach according to age.

PURPOSE: The scientific community has established Javal's rule as a model linking refractive (RA) and keratometric (KA) astigmatism since its appearance more than 100 years ago. The aim was to improve the accuracy of this relationship according to subject's age by applying the power vector analysis. Posterior corneal curvature has also been studied. METHODS: The IOLMaster 700 optical biometer was used to measure the corneal thickness and the radius of curvature of the anterior and posterior corneal surfaces. Refractive error was determined by a non-cycloplegic subjective refraction process with trial lenses. Linear regression analyses were applied using J0 and J45 power vector components. An evaluation was carried out according to the subject's age resulting into eight regression relationships for each astigmatic vector component for each relationship. RESULTS: A total of 2254 right eyes from 2254 healthy subjects were evaluated. A trend towards against-the-rule astigmatism (ATR) was found with aging, both for refractive astigmatism (RA) and keratometric astigmatism (KA), with 95.2% of subjects under 20 years old having with-the-rule (WTR) KA, and only 22.8% above 79 years old. The following regression equations were found between RA and KA: [Formula: see text] = 0.73 × [Formula: see text] - 0.18 (R = 0.78) and [Formula: see text] = 0.70 × [Formula: see text] + 0.04 (R = 0.69) and between RA and total corneal astigmatism (TCA): [Formula: see text] = 0.73 × [Formula: see text] + 0.13 (R=0.78) and [Formula: see text] = 0.70 × [Formula: see text] - 0.06 (R = 0.68) for the whole sample, but with sensible differences among age groups, both in the slope and in the intercept. CONCLUSION: Ignoring the age of the subject when using Javal's rule could lead to an error in the final cylinder calculation that would increase in high astigmatisms. Applying this new power vector approach based on subject's age could improve the accuracy of the astigmatism prediction.

Vecto-keratometry: determination of anterior corneal astigmatism in manual keratometers using power vectors.

BACKGROUND: A new keratometric routine that employs power vector management for manual keratometers is described. This study evaluates the agreement of the new proposed keratometric technique with the classical one. RESEARCH DESIGN AND METHODS: The applicability of a new keratometric routine was verified using Helmholtz's and Javal's keratometers. Results were obtained by two different and well-trained examiners over two different samples, one including 65 and the other 74 eyes, respectively. Both conventional keratometry and the newly proposed routine (named vecto-keratometry) were used in each eye to obtain the results. The clinical agreement between the methods was evaluated using Bland-Altman and Passing-Bablok analysis. RESULTS: For Helmholtz's keratometer, Bland-Altman plots showed good agreement between methods for both astigmatic components being J0 = 0.04 ± 0.20 D and J45 = -0.07 ± 0.17 D. For Javal's keratometer, Passing-Bablok regression test determined regression line for J0 difference as y0 = 1.03, confidence interval: [0.98, 1.10] and regression line for J45 difference as y45 = 0.97, confidence interval: [0.83, 1.12]. CONCLUSIONS: Vecto-keratometry provides accurate clinical results. It has been demonstrated that there are no significant differences between methods in any of the power vector astigmatic components; thus, both methods can be applied interchangeably.