Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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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 31 a GP lens design to optimize the patient's vision. Often, when we come to this crossroad, we consider the patient's history of contact lens wear, vocation, and hobbies. The largest population of patients who will benefit from custom contact lenses are those with a history of irregular astigmatism. Regardless of the cause of the astigmatism — injury, surgery or disease — the irregular corneal topography can be a challenge to fit with tradi- tional contact lens designs. The visual effects left behind by these conditions may be devastating and often do not allow the patient to function in their daily lives with tra- ditional spectacles or contact lenses. Therefore, it is im- perative that we as the prescribing eyecare professionals are knowledgeable of the various custom contact lens op- tions and modalities to provide the patient with creative contact lens designs that best match their unique corneas or refractive errors. One of the more common conditions treated with specialty contact lenses is keratoconus. Owing to the asymmetrical nature of the disease, considerations must be made for the different visual and anatomic challenges of each eye. In an initial effort to determine if a patient is ideally served with traditional corneal GP lenses or if scleral lenses would be necessary, it is best to use the el- evation display topographical map. Traditionally, we are accustomed to analyzing the cor- neal surface with the axial display map. While the axial larger than 11.8 mm, add 1.00D to the mean keratom- etry value. The base curve of the contact lens can then be determined by factoring in the lens diameter needed to achieve a 1.0 mm drape (Table 1). Online calculators are available to help determine the starting power, base curve, and diameter. Similarly, custom soft lenses are available for patients with irregular astigmatism. Generally, these lenses are designed with an increased center thickness to help manage the irregular corneal astigmatism (Figure 2). Previously, these lenses were manufactured in hydrogel materials only, but with the increased center thickness the oxygen permeability of the lens decreased. Silicone hydrogel is a sticky material and difficult to lathe. Being able to reliably lathe a material is essential, because spin- casting or molding custom parameters for individual patients is not cost effective. Currently, there are a few FDA-approved lathable silicone hydrogel materials avail- able as custom soft contact lenses. DIVERSE CORNEAL SHAPE PROFILES Different corneal shape profiles require some design modifications to achieve an appropriately fitted soft con- tact lens. For example, custom soft lenses specifically designed for keratoconus have a central base curve to al- low for a steeper-than-average central corneal curvature with a flatter "skirt" curve to accommodate the relatively unaffected sclera (Figure 3). Custom soft lenses that are reverse geometry in shape are best for aligning with an oblate, post refractive surgery, or post penetrating kera- toplasty cornea. Both of these lenses will have a center thickness of approximately 0.3 mm, much thicker than the average 0.1 mm (Figure 4). To determine the power of these lens designs, a diag- nostic lens must be placed on the eye and an over-refrac- tion performed. If vision is still not ideal, consider using PRESCRIBING CUSTOM LENSES LENS DIAMETER (mm) FIT FLATTER BY (mm) 12.0 0.00 12.5 0.10 13.0 0.30 13.5 0.50 14.0 0.70 14.5 0.90 15.0 1.10 15.5 1.30 16.0 1.50 16.5 1.70 CALCULATING THE BASE CURVE FOR CUSTOM SOFT CONTACT LENSES As the diameter of a contact lens increases, the sagittal depth increases. The fit factor helps to maintain an appropriate lens-to-cornea fitting relationship. T A B L E 1 Figure 1. Soft contact lens with appropriate scleral drape and centration.

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