Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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Page 38 of 75

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 37 GP SPHERICALS AND TORICS today with the many slit lamp iPhone adaptors commer- cially available. • Dispense the lenses. Confirm the lens fit, check vision, and perform a SOR to confirm the lens power is correct. If all is well, dispense the lenses and have the patient return for a progress check. If needed, you can consult with the laboratory on how to improve the lens fit. FRONT-SURFACE TORICS If the cornea can be fit with a spherical BCR, but there is excessive residual astigmatism with spherical optics, you can try a front-surface toric. The extra cylinder correction needed is ground onto the front of the lens. This design is prism-ballasted to put the lens axis at the correct location. The laboratory can help you choose the amount of prism and determine the lens power needed based on your SCOR of a spherical lens. With the exception of scleral lenses, these lenses are less frequently used because of dif- ficulty with axis stabilization, and prism ballasted lenses do not lid attach well, possibly decreasing comfort. A soft toric may be a better choice in a case like this. BITORICS AND BACK-SURFACE TORICS Both of these designs have toric BCs, enabling you to independently fit each meridian on higher-toricity corneas. Al- though these designs may seem intimidat- ing at first, it is actually easier to achieve a good fit with them, and they are only a little more challenging than spherical lenses. In fact, the primary difference per- tains to having two tear lens calculations — typically the horizontal meridian and the vertical meridian — as opposed to one with spherical lenses, but these values can be determined empirically with online calculators (See "Empirically Design Your Toric Lenses" on page 38). Figure 5 shows the fit of spherical and toric BCs on a high-toricity cornea. Residual astigmatism is less of an issue as well, as you choose the wide. Figure 4 shows NaFl patterns that are aligned, flat, and steep. There is some tolerance as to what is consid- ered an acceptable pattern; a lens that moves and posi- tions well but is slightly flat or steep is acceptable. Some practitioners prefer a slightly steep pattern to bridge the center of the cornea. • Evaluate lens movement. Lens movement can also be a clue as to whether the lens is too flat or too steep. A too-flat lens tends to drop from the lid in a circular pat- tern, skating along the limbus; it may also have excessive lid attachment. A steep lens will tend to have poorer lid attachment and drop straight down. • Troubleshoot the fit. Table 2 shows recommenda- tions for adjusting lenses to improve the fit. These are listed in the best order to try them. • Choose the final power. After determining the cor- rect BCR, perform a SOR to establish the final lens pow- er. Be sure to vertex-adjust the SOR first, then add it to the trial lens power, as described above. • Place the order. Advise your GP laboratory of the final OAD, BCR, and power. The lab will also want to know the lens set you used, the material to be used, and any modifications to the design, such as changes to the edge lift. If you are unsure about any of these, your lab consultant can provide valuable expertise to guide you. Photos and videos of lens fits are also helpful tools to provide to your lab consultant. This is easy to perform Figure 5. Spherical (left) and Toric (right) base curves on highly-toric cornea. Figure 4. Aligned (left), flat (middle), and steep (right) GP fluorescein patterns. TO RAISE A LOW-RIDING LENS TO LOWER A HIGH-RIDING LENS Flatten base curve Steepen base curve Increase the diameter Decrease the diameter Add lenticulation (plus lenses) Add lenticulation (minus lenses) Flatten the peripheral curves Steepen the peripheral curves Use a lighter material Use a heavier material Decrease center thickness Increase center thickness GP LENS ADJUSTMENTS T A B L E 2

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