[Vectorial models for analyzing variations of anterior corneal astigmatism]

Several studies have analysed postoperative or spontaneous variations in astigmatism. The methods of quantification of these variations, frequently used in these studies, are based on different ways of calculating: subtraction, vectorial or polar methods: Naylor, Jaffe and Clayman, Cravy, Naeser, Russell et al. and recently, Holladay et al. Although these methods have the advantage of taking into account the axial component of astigmatism and are useful for a single patient, they have many problems and they are more and more numerous (five in the last three years). We have computerized these formulae on a personal computer and studied their performances when the cylinder power decreases without modification of the axes, when the axes are modified without modification of the cylinder power and when the two axes change with the same angular difference. These calculations demonstrate that these formulae are not linear and introduce statistical errors before the statistical decision tests. Moreover, their application is only possible for evaluation of astigmatism axes between 0 degree to 180 degrees and not for semi meridians and corneal topography.