Abstract
The advent of large-diameter rigid contact lenses (semi-scleral lenses) has necessitated renewed understanding of the concept of ‘the sag’ of a contact lens and/or cornea. This article reviews the concept of ‘the sag’ and how to calculate the sag for spherical and aspherical surfaces. Reviews of empirical studies investigating the sag of the eye are presented. Data for normal and keratoconic corneas for sags as measured with an Oculus Keratograph are also included. How to include empirical measurements obtained in practice, in the determination of a first-trial semi-scleral lens, is suggested.
Highlights
The concept of sagittal height or depth of a surface is first introduced to optometry students when they are in their formative years of study
The advent of semi-scleral, large-diameter, rigid contact lenses has, for those of us who choose to fit these lenses, forced us to dredge our memories, dust off long-forgotten text books and refresh our understanding of what a sag is and the role it might play in our fitting of these new large-diameter lenses
Sorbara et al.,[11] making use of a Visante optical coherence tomographer (OCT), investigated the ocular sag of 40 normal corneas based on a scleral chord of 15 mm
Summary
The concept of sagittal height or depth (the sag) of a surface (in this context, usually of lens surfaces) is first introduced to optometry students when they are in their formative years of study. For a spherical corneal surface having a radius of 8 mm on a chord of 10 mm, the sag of that cornea would be 1.755 mm (or 1755 μm). For the above-mentioned cornea, on a sclera having an assumed (empirically determined6,7) radius of approximately 13 mm, on a corneal chord of 10 mm and a scleral chord of 15 mm, the total sag would be approximately 3137 μm for the spherical cornea and 3068 μm for the aspheric cornea.
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