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 17 corneal edema (Figures 2 and 3). OCT is quickly becoming an important tool for con- tact lens specialists. 3. DILIGENTLY ACQUIRE OTHER OCULAR DATA Refractive error is important in contact lens design, as is the ocular surface data discussed previously, but numerous other measurements differentiate the contact lens specialist. First, there is cornea size, typically referred to as visible iris diameter (VID) or horizontal visible iris diameter (HVID). This measure- ment should be taken into consideration for ev- ery lens whether you are prescribing a stock soft, custom soft, corneal GP, scleral GP, or hybrid lens. Approximately 20% to 30% of corneas are larger or smaller than "average," indicating that the fit of these lenses could be improved with the proper lens-to-cornea size relationship. Next is pupil size, which is critical in most lenses but of particular importance in presby- opia-correcting designs. Noting the patient's photopic and scotopic pupil size, then employ- ing the proper coinciding optical zones will de- crease halos, glare, and other visual disturbance in different lighting conditions. Pupil position and visual axis along with line of sight are also important to note. Last but not least, you should note the patient's lid configuration, position of upper and lower lids, distance from the center and lower portion of the pupil to the low- er lid, and completeness of the patient's blink. have historically been the basis for contact lens design, but actually they supply very little information about the corneal surface. It is nearly impossible to expect measure- ments taken at points approximately 2 mm to 4 mm in the central cornea to tell us much about the corneal shape. Therefore, you must have access to a corneal topogra- pher, and you must learn all of the functionality of that particular instrument, including the differences between axial, tangential, and elevation maps. Note that difference (or subtractive) maps are critical to compare the before and after effects of a lens on the eye (Figure 1). In addi- tion, corneal topography is often used clinically to detect and evaluate the severity of keratoconus. In essence, you must become skilled at corneal topography to become a contact lens specialist. Anterior segment optical coherence tomography (OCT) is playing a more prominent role in specialty contact lens fitting today. This is mainly because the use of scleral lenses has been reintroduced to North American eyecare practitioners along with the impor- tance of sagittal height fitting relationships, which can only be estimated — not calculated — with topography. Additionally, OCT images combined with built-in cali- pers can measure posterior tear-film thickness behind contact lenses, evaluate lens bearing, examine lenses for thicknesses at different points, provide cross sections of design profiles, and demonstrate possible lens flaws, along with the possibility of evaluating pachymetry and PRACTICE BUILDING Figure 1. Essential topography captures to understand corneal shape prior to contact lens fitting and the effects of the lens on-eye after wear. A. axial map; B. tangential map; C. elevation map; D. difference (subtractive map). A C B D Figure 2. OCT profile of hybrid lens on-eye fitting relationship. Figure 3. OCT of scleral lens edge profile and fitting relationship.

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