VECTORIAL MODELS OF ASTIGMATISM VARIATION S

Several studies have analysed postoperative or spontaneous variations in astigmatism. The methods of quantification of these variations, fre quently used in these studies, are based on different ways of calculat ing:subtraction, vectorial or polar methodes : Naylor, Jaffe and Claym an, Cravy, Naeser, Russell et al. and recently, Holladay et al. Althou gh these methods have the advantage of taking into account the axial c omponent of astigmatism and are useful for a single patient, they have many problems and they are more and more numerous (five in the last t hree years). We have computerized these formulae on a personal compute r and studied their performances when the cylinder power decreases wit hout modification of the axes, when the axes are modified without modi fication of the cylinder power and when the two axes change with the s ame angular difference. These calculations demonstrate that these form ulae are not linear and introduce statistical errors before the statis tical decision tests. Moreover, their application is only possible for evaluation of astigmatism axes between 0(0) to 180(0) and not for sem i meridians and corneal topography.