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 30 PRESCRIBING CUSTOM LENSES C ontact lenses at their core have the potential to be life-altering de- vices, whether they are molded and manufactured tens of thou- sands at a time or made one at a time to custom parameters. Many patients seek out contact lenses for cosmetic reasons to align with their hobbies or activities or to achieve their best corrected vision. In each case, practitioners must balance the wants and needs of their patients to their own. Typically, the main reason a patient visits an eyecare professional is vision related. As eyecare professionals, we attempt to manage these visual needs and potential frus- trations of our patients by any means necessary. There are times, however, when the exact parameter specifica- tions needed to optimize a patient's visual and wearing experience may escape us. This article explores custom contact lenses and their justification, as to how these lenses may further enhance the lives of current contact lens wearers or those who are deemed "difficult to fit." We demystify these challenges and explain how to overcome them with modern contact lens options, material science, ocular surface measure- ments, and manufacturing capabilities. MAKING A CASE FOR CUSTOM Soft contact lenses are the most often prescribed contact lenses in the industry. Most "off the rack" lenses are available in a wide range of powers but only one or two base curves, often only one diameter, and a center thickness predicated on the power of the lens. These parameters serve most patients well but fall short for patients who have larger or smaller than average corneas, large or very small refractive errors, or irregular astigmatism. The parameter range and reproducibility of custom soft lenses have dramatically improved in the last decade, as has the emergent option of lathing more oxygen permeable soft lens materials. Many times, we encounter a patient who has a long history of unsuccessful contact lens wear. When evaluat- ing these patients, we need to rule out any contraindi- cation to contact lens wear, but once this is done, we should take a step back and take note of the patient's corneal diameter. Is it average (11.8 mm) or larger or smaller than average? Corneas that measure outside the average often go unnoticed as a reason for contact lens intolerance. This seems particularly true for larger-than- average corneas where a soft contact lens may decenter or move inappropriately on the eye. A properly fitted soft contact lens has good centration, drapes approximately 1 mm onto the sclera, and demonstrates approximately 0.25 mm of movement in primary gaze (Figure 1). Larg- er corneas will have a greater sagittal depth and require a lens that is large and deep (larger overall diameter and/ or steeper base curve). Anatomical features that contribute to sagittal depth are corneal diameter, shape factor, and radius of curva- ture. 1 Most traditional soft contact lenses have an overall diameter of 14.0 mm to 14.2 mm, which may not pro- vide adequate coverage or centration for a cornea that is larger than 12.0 mm. These corneas tend to need a custom soft lens that is larger in diameter but also has a steeper base curve to accommodate the increased sagit- tal height. For regular refractive error, the power of the contact lens can be determined empirically by vertexing the spectacle refraction to the plane of the cornea. The initial base curve can be calculated by determining "ef- fective K," which uses the central corneal radius of curva- ture AND the corneal diameter. The general rule is that for every 0.2 mm smaller than 11.8 mm, subtract 1.00D from the mean keratometry value and for every 0.2 mm A n d h o w t o u s e t h e m s u c c e s s fu l l y i n p r a c t i c e M AT T H E W L A M PA , O D, & B E T H K I N O S H I TA , O D WHAT MAKES CUSTOM LENSES CUSTOM

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