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 34 GP SPHERICALS AND TORICS S oft lenses clearly dominate the contact lens world, but there are still patients who would benefit from GP lenses. While much of our clinical attention in the GP realm has moved to scleral and hybrid lenses, corneal GPs are still a great option for many patients. Once you have decided to explore corneal GPs for your patients, how do you get started? In this article, I break down the fitting process into manageable steps to guide you through the process. CHOOSING A GP LENS DESIGN For simplicity, I categorize patients with normal cor- neas as either those who can wear a spherical base curve (BC) or those who need a toric BC. The easiest way to determine this is by looking at the amount of corneal toricity, using a manual or automated keratometer or a corneal topographer. The greater the mismatch between the major corneal meridians, the worse the fit of a spherical BC will be. When an eye has higher amounts of toricity, a spherical lens will not align well with the cornea to provide an ac- ceptable fit. Corneas with toricity of 2.00DC or less can usually be fitted with a spherical BC; those with 3.00DC or more will need a toric BC; and those between 2.00DC and 3.00DC are in a gray area. For those corneas, try a spherical BC first, and if the fit is unacceptable, attempt a toric BC instead. Corneal GPs are well suited for with-the-rule (WTR) corneas (steeper along the vertical meridian). This shape allows unimpeded vertical movement of the lens when the patient blinks. Fitting against-the-rule (ATR) corneas often results in horizontal decentration, as the lens tends to follow the steeper corneal meridian. Keep in mind that keratometry (K) readings represent only the central 3 mm or so. A topographer can refine the choice of lens type. Somewhat counter to what I stated above, a spherical BC may work well for some higher toricity corneas with apical patterns (Figure 1), and lower toricity corneas that are limbus-to-limbus may need a toric BC (Figure 2). The elevation difference at the 8-mm chord also can be used. If the elevation difference along the major meridians is more than about 60 microns, a toric BC should provide a better fit. SPHERICAL GP LENSES If you determine a spherical BC will fit well, you must consider how well the patient will see with a spherical lens. With a spherical lens, all astigmatic correction occurs in the lacrimal/tear lens, and it will equal the patient's corneal astigmatism. If that matches the refractive astigmatism, a spherical lens will provide ideal vision correction. More mismatch means more residual astigmatism and poorer acuity. Step 1: Determine Initial Lens Parameters • Overall diameter (OAD). Typically, the lens diam- eter is determined by a combination of corneal diameter and lid positioning. Start by measuring the horizontal vis- ible iris diameter (HVID), and consider an OAD about 2 mm smaller than the HVID. For example, if the HVID is 11.8 mm, choose an initial lens with a 9.8 mm OAD. Next, modify the OAD based on whether the lids promote lid-attachment or intrapalpebral positioning (Figure 3). An upper lid that covers the limbus promotes lid attachment, where the upper lid provides some sup- port for the lens positioning. This generally enhances comfort. If the upper lid is near or above the limbus, the lens will likely drop more toward the center and be an in- trapalpebral fit instead. Modify the diameter accordingly. A lid-attached lens should be consistent with the initial A s t e p - b y - s t e p g u i d e t o G P s u c c e s s J O H N M A R K JAC KS O N, O D, M S GP SPHERICALS AND TORICS MADE EASY

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