Analysis of astigmatism in anterior segment surgery

Purpose: To demonstrate techniques for analyzing astigmatism in the context of anterior segment surgery. The techniques are applied to refractive and corneal astigmatism and to astigmatism associated with the surgery. Method: The analysis is based on the 3-dimensional first-order ray optics k nown as linear optics, in which astigmatism is axial. Anterior segment surg ery, whose only direct effect is to change corneal curvature, is optically equivalent to placing a thin lens immediately in front of the (preoperative ) eye; the surgery can be described as anterior thin-lens equivalent. In pr inciple, however, surgery may also change the relative axial positions of r efracting elements, in such cases, anterior segment surgery is anterior thi ck-system equivalent and the analysis of astigmatism is considerably more c omplicated, with astigmatism manifesting in power and in other ways. Fundam entally, the method is based on the ray transference, a 4 x 4 matrix that c ontains the 4 fundamental optical properties of a system. Each property may contain an antistigmatic component, and it is this component that is analy zed. Results: The data are not consistent with the anterior thin-lens model of a nterior segment surgery but rather with the anterior thick-lens model. Many components of refraction and corneal power, their means, and their surgica lly induced changes were calculated and are presented in tabular and graphi cal form. Graphical representations include polar profiles and stereo-pair scatterplots in dioptric power space. Results are also presented for all th e fundamental properties of the surgery. Conclusion: Ocular astigmatism can manifest in more ways than just refracti on and corneal power. Traditional approaches, which treat astigmatism as cy linder in some sense and only as manifested in power, are unsatisfactory. I n particular, current approaches weight astigmatism too heavily relative to stigmatism and to other optical phenomena and are inadequate for thick sys tems such as the eye. The ray transference and the 4 fundamental properties it contains provide the basis for the only methods hat can adequately cope with analysis of astigmatism in general and in surgically induced astigmat ism in particular.