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 38 GP SPHERICALS AND TORICS and the vertical BCR is 46.00D. If LTS and a smaller lens, the BC radii are 44.25 (H)/46.25 (V). • Power. Once the BC radii are known, the power in each meridian is chosen to properly correct each one. Figure 6 uses optical crosses to show the powers needed based on the lacrimal/tear lens powers in each meridian. Note that in Figure 6, the lens specifications for BC ra- dii and powers are written as "drum" readings rather than spherocylinder values. Back-surface Toric (BST) These lenses have toricity only on the back. Why? It is the result of an optical quirk. It would seem logical that a lens that has toricity only on the back has cylinder power equal to the BC toricity. For example, if the BCs are labeled as 43.00/47.00, that lens should have 4.00DC. Wrong! These numbers are not the true powers of the back surface. They are the powers of the front of the lacri- mal/tear lens! When we calculate the true surfaces using the index of the lens and not the tears, the powers are closer to 64 and 70, a difference of 6.00DC! The lens has 1.5 times the cylinder we think we should get. This is why bitorics are used. Cylinder is placed on the front to neutralize the "extra" cylinder we do not want. When do you use a BST then? Optically, it works when the refractive cylinder is approximately 1.5 times the corneal cylinder. If you are using a toric BCR and have this relationship, the lens will be a BST. Practically, this makes little difference to clinicians, but it helps to under- needed powers in each meridian. Compared with soft toric lenses, these lenses typically provide better vision for patients with higher astigmatism (typically greater than 2.00D of corneal cylinder). Lens rotation has less impact on visual acuity, making them a better optical choice. Bitoric Lens Designs Most of the time, a bitoric is the design that is need- ed. Bitoric designs have toricity on both sides of the lens for reasons I will discuss later. Fitting a bitoric lens fol- lows all of the steps previously covered in terms of OAD, lid attachment, and so on. The significant difference is that you are selecting two BC radii instead of one. The method described here is one that has been successful for me, although other methods have been published. This method is successful for ATR corneas, too. • BCR. There are two fitting strategies with toric BCs: saddle fit and low toric simulation (LTS). Saddle fit means each meridian is fit with the same alignment; LTS means the vertical meridian is fit somewhat flat- ter than the horizontal to allow for unimpeded vertical movement. This simulates how a spherical lens aligns on a low-toricity WTR cornea (hence the name). Use LTS when the corneal and refractive cylinder are within ap- proximately 1.00DC of each other. Use a saddle fit when the difference is greater. To select the BCR, first determine the BCR in the more horizontal meridian. For larger OADs (in the 9.8-mm range), make the BCR equal to the flatter K read- ing. Next, choose the vertical meridian depending on whether you will use the saddle or the LTS fitting strategy. If you choose saddle, fit the vertical meridian with a BCR equal to the steeper K reading; if LTS, fit it about 1.00D flatter than the steeper K reading. For smaller diameters, adjust each meridian 0.25D/0.05 mm steeper. For example, if using LTS and the Ks are 44.00/47.00 @ 090 and a larger lens, the horizontal BCR is 44.00D Figure 6. Bitoric GP lens design. EMPIRICALLY DESIGN YOUR TORIC LENSES S everal calculators are available if you want to design toric GP lenses empirically. The most well- known calculator is the Mandell-Moore Guide. In addition, the GP Lens Institute Toric and Spherical Lens Calculator provides graphics to view the tear film powers and calculations. This guide also offers design recommendations in addition to BC radii and powers. It also recommends a back-surface toric when indicated and describes how to empirically design a spherical lens. For a comprehensive guide to empirical bitoric design, the Newman GP Toric Guide is a good resource. All three of these empirical guides are available at the GP Lens Institute website: www.gpli.info. Finally, EyeDock has a useful bitoric calculator on its website: www. eyedock.com.

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