Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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54 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 c l s p e c t r u m . c o m tear film, eyelid pressure, and blink forces — will most probably move excessively on the eye. This will result in an unsuccessful fit that would be quite uncomfortable. Our current understanding is that the sagittal height of a soft lens on-eye needs to be somewhere in the range of 200 microns "deeper" to achieve a clinically success- ful fit. This amount, the difference between CL-SAG versus OC-SAG, is called the delta-SAG. The desired delta-SAG value likely varies per lens type and design. The question then becomes, with the given work- bench in OC-SAGs, and the given CL-SAG of lenses in standard silicone hydrogel 2-week and 4-week replace- ment, how many eyes can we serve with those lenses available in our standard arsenal? If the soft lens were rigid in nature, then with the available current lenses we would only be able to fit one-third of all eyes. Obviously, a soft lens is not rigid; it allows for a certain amount of flexure. The amount is debatable, but studies indicate that somewhere between 100 and 250 microns in delta-SAG (increase in sagittal height from that of the ocular surface height) is likely to gain enough grip of the lens without creating excessive pressure and caus- ing problems. 10,11 If we apply these realistic numbers to our database of normal eyes, then approximately 78% of lenses would fit. If the delta-SAG numbers were be- tween 150 and 300 microns, then 68% of lenses would fit. It all depends on how much strain, or mechanical pressure, we are willing to accept on the ocular surface. RELEVANCE The steepness of the lens on-eye, defined as delta- SAG, may have consequences for the visual perfor- mance of that contact lens; lens flexure on the eye will cause a small change in lens optics from what the lens was designed to provide. Soft lenses drape to fit the cor- nea. Researchers found that the power of a soft contact lens on the eye is a function of its off-eye power, the way the lens flexes on the eye, lens hydration changes, and the corneal topography. 12 For a simple spherical –2.50D correction contact lens, this does not appear to have a great effect (probably less than 0.25D). For more complex designs, such as multifocal lenses and wavefront-corrected lenses, lens flexure could indeed have an impact, particularly as the delta-SAG amount is unknown for a given eye. The same is true for soft mul- tifocal myopia control lenses. Before we are potentially fitting millions of children in soft lenses, we'd better get our act together on what we are actually fitting. Furthermore, we know from clinical experience that unwanted corneal topographical changes occur under soft lenses. Corneal deformations resulting from subop- timal soft lens fittings are not as uncommon as many may think. If practitioners would remove patients' soft lenses at every follow-up visit and perform corneal topography, as we often do with our students, the dif- ference map could potentially indicate the amount of unwanted changes. Until recently, this phenomenon was like undefined crop circles in wheat fields that nobody seems to be able to clarify. Where do they come from? Our current working theory now is that the steep lens appearance and, thus, the positive delta-SAG must have something to do with the topographical changes observed in the difference maps. It appears to be the effect of the mismatch between the soft lens shape profile and the ocular surface profile. EDGE STRAIN The previously described phenomenon appears to be in line with the edge strain mechanism. 13,14 A lens that is placed on the eye with a given steepness, by de- fault, flexes on the eye. A mathematical model explains that because of this flexing, every lens causes a certain degree of edge strain, or mechanical pressure, on the ocular surface. A lens that is placed on the eye increases in size on-eye, because of the described flexure. The de- scribed optimum amount of increase in size and there- fore edge strain appears to be 3%. 13 This would mean that a 14-mm lens on the finger would increase in size to 14.4 mm on the eye. This also has implications for changing the base curve of the lens. If we change the base curve from 8.4 mm to 8.8 mm, the overall diameter decreases on the eye (as the delta-SAG decreases). In theory, this would mean we would have to add 0.2 mm to the lens diameter if we wanted to keep the same on- eye lens diameter. To make it slightly more complex — but, again, this is something we must teach our students — another study showed that the reported diameters (on the lens blister pack) of soft contact lenses are conventionally measured at room temperature (20° C); however, all soft lenses shrink when raised to eye temperature (34° C). 15 It would be better, in terms of lens fit, to mark the on- eye lens diameter of soft lenses, at minimum, in addition to the theoretical lens diameter at room temperature. 16 SOFT LENSES A r t i f i c i a l i n t e l l i g e n c e i s e n t e r i n g o u r p r o fe s s i o n , a n d m o s t p r o b a b l y w i l l m a k e a b i g d i f fe r e n c e g o i n g f o r w a r d .

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