Optical effects of ocular surgery including anterior segment surgery

Abstract

Ocular surgery may alter the refraction and the corneal power of an eye. However, refraction and corneal power are not fundamental optical properties of the eye as such. Refraction is a fundamental property of a lens in front of the eye and an indirect or derived property of the eye, while corneal power is a fundamental property of only part of the eye. Nor are they the only clinically relevant paraxial optical effects of surgery. There are also effects on size, shape, and orientation of retinal images, for example, which are not simply related to refraction and corneal power. Quantitative analyses of the effects of surgery on the eye that account only for refraction and corneal power are necessarily incomplete. This paper offers an approach to the analysis of the optical effects of surgery that is complete within the limitations of paraxial optics. The key is the concept of the ray transference, which contains the 4 fundamental optical properties of an optical system. The surgery is represented by a hypothetical optical system, the anterior surgery-equivalent system, in front of the preoperative eye. The transference of this system, the anterior surgical transference, is a complete paraxial quantification of the optics of the surgery and is what needs to be studied in analyses of surgery. In this approach, ocular surgery in general is optically equivalent to a thick system placed in front of the preoperative eye. A simple model is developed for the case of anterior segment surgery in particular. In terms of the model, anterior segment surgery is shown to be equivalent to placing a thick bitoric lens in front of the preoperative eye. In particular cases, the surgery may be equivalent to an anterior thin lens. Astigmatism is an aspect of optical effects in general. It is not directly amenable to quantitative analysis in isolation; quantitative analyses of astigmatism should be done in terms of the concept of antistigmatism. In thin systems, astigmatism manifests only in power. However, in thick systems such as the eye, astigmatism can manifest in 4 fundamentally different ways. Among them are some phenomena currently described as irregular astigmatism. The foundation is laid for the complete analysis of paraxial astigmatism in general and in ocular surgery in particular.