Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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Page 33 of 75

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 32 surement taken from 10 mm to 15 mm falls outside of the measuring ability of the corneal topographer, as the human cornea is approximately 12 mm in diameter and the sclera itself is not reflective and cannot be measured by current topographers. In a study by Achong-Cohen and colleagues, an an- terior segment optical coherence tomographer (Visante OCT, Carl Zeiss Meditec) was used to measure the el- evation from 10 mm to 15 mm. 3 This evaluation showed that normal eyes had an elevation of 1,992 microns, and eyes with moderate-to-advanced keratoconus had an el- evation of 1,970 microns. When evaluating the eye to de- termine the overall sagittal height, the initial diagnostic lens may be estimated by using the corneal topographer and measuring 10 mm to the apex, then adding the aver- age sagittal height from 10 mm to 15 mm of 2,000 μm, and then the desired initial clearance (range 300 μm to 400 μm). Our ability to vault the cornea with a con- tact lens has been game-changing for many patients and practitioners, and this has in- tensified research to better understand the shape of the sclera. Clinically, we see that the sclera has asymmetry to it, but is there a way to predict the scleral shape by measur- ing the corneal shape? Unfortunately, this may not be possible. In a small study evalu- ating varying amounts of corneal toricity, there was no correlation to the presence or absence of scleral symmetry as measured by the sMap3D (Precision Ocular Metrology). 4 This evaluation of the sclera has challenged our understanding and description of the sclera, because traditionally, we have described corneal astigmatism in terms of display map is useful for determining the corneal curva- ture, it reveals little about the elevation or height differ- ences within the cornea. The relative height differences in the cornea are important to assess overall shape and are good indicators of whether a cornea can support a corneal GP lens or would be better served with the vault of a scleral lens. In studies by Zheng and colleagues, patients who were successfully fitted with corneal GP contact lenses were evaluated. Elevation differences were measured along the greatest meridian of change. 2 It was determined that if the greatest meridian of change (highest to lowest) was less than 350 microns, patients had an 88.2% chance of being well fitted with — and successfully wearing — a corneal GP lens. If the greatest meridian of change was greater than 350 microns, however, we predicted the cor- neal profile would not ideally support the physical fit of a corneal GP lens and would likely need the significantly larger overall diameter of a scleral contact lens. The sclera is relatively unaffected by most conditions that affect the corneal shape. A scleral lens fits and lands on the sclera and allows the central portion of the lens to vault the cen- tral and peripheral cornea and avoid inter- action with the highly asymmetric corneal elevation differences (>350 microns), there- by allowing the optical correction of the ir- regular astigmatism and providing justifica- tion for a scleral lens. KEY MEASUREMENTS In our academic setting, we have the ben- efit of using instrumentation to measure the sagittal depth of the cornea and the sclera at various chord lengths. In most practices, a topographer will be the primary resource for determining sagittal depth. Many topog- raphers can extrapolate the depth of the cor- nea at a chord length of 10 mm. The mea- Figure 3. The central base curve of the cus- tom soft contact lens is steeper than the "fitting curve," which lands on the more regular sclera. Figure 2. Irregular astigmatism secondary to keratoconus. Note the irregularity of the mires in (A) with no contact lens and a smoothing of the mires in (B) with a custom soft contact lens. PRESCRIBING CUSTOM LENSES A NEW PARAMETER TO CUSTOMIZE F or a contact lens to be "custom," not all param- eters need to be customized, and it could be argued that a lens that has been modified in any way is custom. For this reason, we wanted to mention a surface treatment that recently received FDA approval. It is a 90% water polyethylene glycol (PEG)-based polymer coating that is bound to the front and back surface of the lens, creating a mucin-like layer. This aids in surface wetting of the lens with the potential to aid in comfort and visual quality. This surface treatment is applied on a newly manu- factured lens prior to shipping the lens to your office. Continued on p.39

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