Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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48 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 own lenses using additional factors such as optimum tear fluid thickness (Figure 3). Currently, ortho-k is most commonly used for myo- pia correction. It is most effective with myopia less than 5.00D, with-the-rule astigmatism less than 2.00D, and against-the-rule astigmatism less than 1.00D. When starting out, select highly motivated patients who have low amounts of myopia and with-the-rule astigmatism. This will help you achieve better outcomes while you learn the basics of ortho-k. With experience, high myo- pia and astigmatism can also be successfully corrected with newer lens designs. These enhanced designs in- clude lenses with five curves, double-reverse curves, and dual-geometric designs. 11,12 Progressive myopia is on the rise globally. 13 High myopia is a leading cause of blindness and is associated with retinal comorbidities, such as early-onset cataract, glaucoma, and macular degeneration. 14 Preventing high myopia through myopia control now offers a solution. Ortho-k and other contact lens-based interventions aim to create a peripheral defocus retinal cue that slows myopic progression. 15 According to the American Op- tometric Association, myopia control can be offered to patients with progressive myopia, 16 and practicing ortho- keratologists are best positioned to combat this epidemic (although this remains an off-label use). Although correction of myopia is the primary indica- this method "orthokeratology." Jessen's method consist- ed of fitting a conventional design polymethyl methacry- late (PMMA) corneal lens flatter than K by the amount of the necessary myopia correction with an additional overcorrection (about 1.00D). This overcorrection, or Jessen factor, compensates for myopic regression toward the end of the day. Because PMMA does not transmit oxygen, the lenses were fitted flat in an effort to deliver more oxygen to the cornea. Unfortunately, lenses fit- ted in this manner were originally thought to flatten the underlying corneal tissue. We now know that the corneal epithelium is redistributed through this process rather than overall corneal flattening. Myopic patients did, however, experience improved uncorrected vision for some time after contact lens wear was discontinued. 1 Overall, early attempts at ortho-k, although safe, were plagued by unpredictable refractive outcomes and mini- mal myopia correction (1.00D on average), and it would take months to achieve the target treatment goal. 2-7 Modern ortho-k employs reverse-geometry gas per- meable (GP) lenses. A reverse-geometry lens has a flat central curve and an adjacent reverse curve that is steeper than the central radius. This reverse curve is connected to a flatter peripheral curve that aligns with the midperipheral cornea. A basic 4-curve reverse ge- ometry lens is shown in Figure 1. This design centers well and is highly stable, allowing rapid correction of higher amounts of myopia. The use of GP materials en- ables overnight wear of ortho-k lenses in a closed eye state, which adds to the convenience and comfort of this procedure. 8-10 Using corneal topography to guide the treatment further differentiates the modern practice of ortho-k. 11 Corneal topographers can map a large area of the cornea, providing an accurate and complete view of the pre- and post-treatment ortho-k region (Figure 2). These data are crucial for troubleshooting any problems when the fit is not optimal. Owing to the advent of computer-driven lathing systems, a wide variety of ortho-k lens designs are now manufactured with high accuracy and repeatability. Ob- taining a lens is as easy as providing keratometry values, manifest refraction, and horizontal visible iris diameter to suitable contact lens laboratories. Ortho-k lens de- signs are informed by data collected from thousands of fits, promising good results. Most manufacturers offer an empirically designed ortho-k lens, and their consultants are available to help with lens modification and trouble- shooting. Ortho-k lens sets are also available to facili- tate same-day fitting and dispensing. As a wide variety of parameters are available instantly, these sets increase initial fitting success, thus reducing chair time. Orho-k design software linked to corneal topography is also available when a high degree of customizability is desired. 11,12 This enables practitioners to design their OR THOKERA TOLOGY Figure 1. Representation of modern reverse-geometry GP lens used for ortho-k. Reverse-geometry lenses have a flat central curve (BOZR) and an adjacent reverse curve (RC) that is steeper than the central radius. This reverse curve is connected to a flatter curve that aligns with the midperipheral cornea (AC). A final peripheral curve is added for tear exchange (PC).

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