Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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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 36 determine the final lens power. Be sure to vertex-adjust the SOR first, then add it to the trial lens power. For ex- ample, if the trial lens is –3.00D and the SOR is –6.00D, at the corneal plane the SOR value would be –5.50D, and the final power is (–5.50) + (–3.00D) = –8.50D. If the visual acuity is not as expected with the SOR, perform a spherocylinder over-refraction (SCOR) to determine if residual astigmatism is causing the issue. Step 2: Fit Analysis A properly fit corneal GP lens should position and move well and have good alignment with the cornea. • Evaluate lens position and movement. A lens with good lid attachment will remain tucked under the upper lid during and between blinks. It will pick up with the lid after a blink, and move to a centered or slightly supe- rior position relative to the pupil. It should stay well-centered horizontally. An intrapalpebral fit should drop to a central position after a blink. • Evaluate lens-to-cornea relationship. Use so- dium fluorescein dye (NaFl) and a cobalt blue light with the slit lamp for this evaluation. A yel- low Wratten filter makes it easier to see the NaFl pattern. Use a small amount of NaFl at first; too much will flood the eye and alter the movement and positioning of the lens. It is easier to add more dye if needed than to wait for too much to dissipate. A good fit should show a relatively even glow of green through most of the lens with a brighter band under the edge, approximately 0.5-mm OAD calculation; an intrapalpebral lens should be about 0.4 mm smaller. If you wish to achieve a lid attachment fitting relationship and the lower lid covers the limbus, adjust the OAD down by about 0.4 mm. The next two parameters assume you are using diag- nostic lenses in your office prior to ordering lenses. You could also perform an empirical fit and call the laboratory to order lenses based on your OAD, K readings, and re- fraction. You can fine-tune the first lens order by perform- ing a diagnostic fitting. • Base curve radius (BCR). If you are using K values, use Table 1 to select your BCR. Note that as the corneal toricity increases, the BCR values become steeper to decrease the amount of lens rocking along the steep me- ridian. Table 1 assumes an OAD of approximately 9.8 mm. For every 0.4 mm away from 9.8 mm OAD, adjust the BC to compensate. If you are using an OAD of 9.4 mm, for example, make the BC steeper by 0.25D/0.05 mm. If a larger OAD is used, go flatter instead. An intrapal- pebral lens may also need to be somewhat steeper to aid centration. • Power. A GP diagnostic kit likely has one power for each BCR, typically –3.00D. If you have extra lenses in plus powers and your final lens will be a plus power, use those. After finding the correct BCR by checking the lens fit (see below), do a spherical over-refraction (SOR) to GP SPHERICALS AND TORICS Figure 2. 1.00 DC limbus-to-limbus astigmatism with 60-micron elevation difference at 8-mm chord. Figure 3. Ideal upper lid positions for lid attached (left) and intrapalpebral (right). Figure 1. 4.50 DC apical astigmatism with 40-micron elevation difference at 8-mm chord. CORNEAL CYLINDER (DC) BASE CURVE 0 to 0.50 0.50D flatter than K 0.75 to 1.25 0.25D flatter than K 1.50 On K 1.75 to 2.00 0.25D Steeper than K 2.25 to 2.75 0.50D Steeper than K (consider toric BC) BASE CURVE SELECTION FOR SPHERICAL GPS T A B L E 1

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