Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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53 c l s p e c t r u m . c o m C O N T A C T L E N S S P E C T R U M S P E C I A L E D I T I O N 2 0 1 7 know about the contact lens sagittal height (CL-SAG) of the lenses we fit daily? Not much, to be honest, as that information is currently unavailable to us. A relatively small study shed some light on this. The study aimed at measuring the CL-SAG of commercially available lens- es, beginning with silicone hydrogel 2-week and 4-week replacement lenses. 1 These data show what difference is induced if an 8.8-mm base curve lens of one design is replaced with an 8.4-mm base curve lens of the same design, for example, this could be as much as 275 microns. It also shows that one 8.6-mm base curve design can be quite different from another (up to 170 microns difference), and that lens substitution is not advised, as described by Wolffsohn and colleagues, as well. 9 It merely shows us something about what we are fitting, which could be insightful for students. Because the bottom line is that in the sample of lenses in the study, there was no more than 330 microns in variation from highest to lowest sagittal height of lenses to fit the overall 'workbench' of eyes with a range of 900 microns to 1000 microns, over the same chord. Moreover, in clinical practice, when an eye with a spherical lens has an over-refraction of 0.75D cylinder, we typically reach for the toric version of that lens. Ac- cording to our data, that means a change of lens fit of al- most 500 microns (close to the thickness of the cornea) for one lens type, while for another lens type, this means no change at all. The investigators are not judging one over the other as potentially better, but it appears that we should be aware of this to, at minimum, better under- stand soft lens fitting. Figure 2 shows different CL-SAG heights in a sample of lenses. 1 STRAIN ON THE OCULAR SURFACE If a soft contact lens is aligned with the ocular sur- face (i.e., it has the same sagittal height as the sagittal height of the eye), then that soft lens — influenced by Dropout rates and the reasons for dropout are com- plex, ranging from tear film components and dry eyes to lens material, lens surface friction, and edge design, as well as environmental factors. 4 Eyecare practitioners have little or no control over many of these factors. What we do have control over is the lens fit. This article fo- cuses on that topic, discussing how we can better control the fitting of a soft lens and how we can optimize lens fit to respect and follow the shape of the ocular surface. WORKBENCH If we want to improve soft lens fitting and educate our students concurrently, we need to examine our "work- bench" (the shape of the ocular surface) and our tools (the lens designs that we have available). Let's begin with the workbench. For successful soft lens fittings, we need better ways to image and quantify the ocular surface shape. In daily practice, we individu- ally and meticulously measure the precise power that the eye needs; so why not measure the ocular surface individually, too, to best match that shape of the eye? Instruments to measure eye shape and predict the fit of a soft lens on the eye are not available in our practices currently. However, a new generation of ocular surface topographers that can help analyze the entire anterior surface shape beyond the corneal borders has entered the market. These instruments, based on profilometry (using fluorescein as a "screen" to project height pat- terns) or Scheimpflug systems, can help determine lim- bal and anterior scleral shape, with 360-degree coverage on the eye. Other alternatives include corneal topographers that take data about the measured corneal shape, spe- cifically the peripheral cornea described as an angle in modern techniques, and extend that out into the lim- bal region. Studies show that peripheral corneal angles can be predictive of anterior scleral shape, which could help us to some degree to predict eye shape beyond the corneal borders. 5 From scleral lens fitting, we know that the ocular sur- face and the lens can be defined in sagittal height. The previously mentioned new instruments can measure the sagittal height of the ocular surface (OC-SAG) within 10 microns of accuracy. The average OC-SAG, using op- tical coherence tomography (OCT) and profilometry, is somewhere in the range of 3,750 microns (a micron is 1/1000 of a millimeter) based on a variety of measure- ments. 6-8 Figure 1 shows an overview of sagittal heights of 214 normal right eyes in the horizontal meridian over a 15-mm chord, illustrating the spread of normal eyes. BETTER TOOLS Let's assume that we have this OC-SAG information available to us in our practices in the future. What do we SOFT LENSES Figure 1. Overview of saggital heights of 214 normal eyes (all right eyes) in the horizontal meridian over a 15-mm chord measured with the Eye Shape Profiler (ESP, Eaglet Eye). Courtesy of Reinier Stortelder

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