Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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55 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 HOW TO IMPROVE SOFT LENS FITTING "Everything should be made as simple as possible, but not simpler" is a famous quote by Albert Einstein. For educators, this is the essence of our profession, and this certainly applies when it comes to soft lens fitting. Using just sagittal heights will not solve our fitting issues, as it is too simple. Lens design and geometry also play a role. In theory, a monocurve, a bicurve, or an aspheric lens design could all have the same sagittal height but poten- tially show different on-eye behavior. Many other factors contribute to how a lens fits on the eye, including edge lift design, lens thickness, lens roughness, and front sur- face lens design. In addition, they all can influence lens movement on the eye, which is a well accepted method of lens evaluation. This brings us to another important point, although beyond the scope of this article: lens movement may not be a good predictor of the amount of steepness (or flatness) of the lens on-eye. In addition, when looking at the OC-SAG we should look at the 360-degree value, not the horizontal OC- SAG only, as has been performed primarily to date. Studies are currently being performed, comparing the 360-degree OC-SAG value to the CL-SAG values (which are spherical in nature, and a 360-degree value by nature). 10 The horizontal OC-SAG may be different from the vertical OC-SAG. This would mean that often we are fitting a spherical lens to an oval surface independent of any corneal astigmatism. At least in theory, bisagittal soft lenses (with two different CL-SAGs in principal merid- ians) would make more sense in those cases potentially. SURROGATE MEASURES Contact lens fitting can be described as using different sagittal heights. Although certainly not perfect, students and eyecare professionals alike should be aware that both base curve and diameter are surrogate measures for lens sag, and that increasing sag reduces movement and consequently improves comfort within the limits of ac- ceptable fitting. 3 In order to answer the question, "How can we improve soft lens fitting," it may be preferable to separate soft lens fitting into different groups. To optimally respect the shape of the cornea in soft lens fitting, let's begin by rec- ognizing three different categories of soft lenses, adapted from a classification by Lampa & André. 17 FINDING THE NORMAL EYE Stock lenses are basically off-the-rack designs; these lenses have a fixed shape and the eyecare professional basically attempts to find a lens that fits a given eye. The more variety there is in stock lenses, the more likely we are to find a lens that best matches a given cornea. Like- wise, materials can have varying properties (e.g., modu- lus) that influence fit. These lenses would probably best serve the normal, standard eye or the entire top part of the bell curve of eyes. In most cases, a frequent replace- ment lens can serve the normal eye perfectly, likely bet- ter than any other lens. How do we know we are dealing with a normal eye? As eyecare professionals we should be able to determine that. And we can. New technology can help us find these standard eyes, and companies are now starting to embrace this. Using a database of 10,000 eyes, a company in the Netherlands has developed software to determine whether we are working with a standard eye, based on corneal topography and extending out the information peripherally. Thus, we are estimating the overall (15 mm) sagittal height of the ocular surface. The logarithm then recommends either a standard lens or a specially designed lens. Artificial intelligence is entering our pro- fession and most probably will make a big difference go- ing forward. OUT-OF-STANDARD LENSES This brings us to the second category: out-of-standard lenses. These lenses are recommended if the eye is not part of the top of the bell curve. They have a fixed geom- etry (or maybe a number of geometries, but they are still considered standard lens designs), but they have a pa- rameter range that falls outside that of the standard stock lens range. Examples of this category include higher SOFT LENSES Figure 2. CL-SAG differences for a variety of lenses with 200-micron increments imaged with the is830 instrument (Optimec Ltd). Courtesy of Ben Coldrick

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