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COVER FOCUS EXTENDED RANGE OF SILICONE HYDROGEL LENSES TO ADDRESS HIGH AMETROPIAS The prevalence of high refractive errors in the population has meant significant unmet needs among contact lens options. BY DESMOND FONN, MOptom, FAAO, AND PAUL CHAMBERLAIN Visual distortions induced by aberrations present in high-prescription spectacle lenses are well known and can often result in unsatisfactory vision. These distortions can be reduced substantially by shifting the plane of the lens to the ocular surface and reducing the thickness of the lens, as with a contact lens. High-prescription spectacles also create physical discomfort for the wearer. However, perhaps the most appealing aspect of contact lenses as an alternative to spectacles for high prescriptions is the elimination of poor spectacle lens cosmetics. PREVALENCE OF HIGH REFRACTIVE ERRORS The literature does not contain a definitive classification of “high refractive error,” and there is a considerable lack of agreement regarding what constitutes “high” for what type of refractive error. Myopia Classification of where “high myopia” begins ranges from -5.00 to -8.00 D. This may be one reason that the reported prevalence of high myopia varies considerably across countries and populations.1-3 For example, in studies conducted in the past 15 years, the prevalence in the United States was 1.6% for -8.00 D and greater,1 and in Israel it was 2.4% for -6.00 D and greater.4 By contrast, the rates appear to be much lower for older adults (age = > 40 years) in Australia, the United States, and Western Europe (2%-4% for > -5.00 D). In Taiwan, two studies have recorded rates of 21% and 38.4% for -6.00 D The younger age of onset and greater,5,6 which will likely result in greater clearly shows the extent progression, leading to higher of the condition in that levels of myopia and increases geographic region. in associated eye diseases Aside from the such as retinal detachment, alarming increase in cataract, and glaucoma. the prevalence of myopia in East Asian and Western countries, where estimates are 70% to 80% and 25% to 40%, respectively, onset of myopia is occurring at an earlier age. The younger age of onset will likely result in greater progression, leading to higher levels of myopia and increases in associated eye diseases such as retinal detachment, cataract, and glaucoma.7 High myopia (> -8.00 D) in the United States appears to have increased by a factor of eight since the early 1970s.7 “ Hyperopia In most reports on hyperopia that may be classified as high (ie, > +3.00 D), the prevalence ranges from 1% to 12%. These publications do not, however, provide further subdivision beyond the classification of greater than +3.00 D.1,8-10 In the group of patients aged 20 to 39 years, the prevalence of high myopia (> -5.00 D) was reported to be 7% to 8%, and approximately the same in the group of 40- to 59-year-olds. The prevalence of high myopia then decreased to 3% in the olderthan-60 group. Comparatively, the prevalence of high hyperopia (≥ +3.00 D) was 1% in the patients aged 20 to 39 years, 2.4% in the 40- to 59-year-olds, and 10% in the over-60 group.1 JANUARY/FEBRUARY 2015 | ADVANCED OCULAR CARE 63 COVER FOCUS Figure 1. Toric optic (5.75 D cylinder) with narrow steep meridian (< 4.5 mm). Astigmatism Young and Hashemi have written the two papers that provide prevalence data on high astigmatism.11,12 The values range from 2% to 3.4% for astigmatism greater than 3.00 D, but, as with the hyperopia reports, they do not provide information beyond the 3.00 D. Astigmatism is rarely an isolated refractive error. Young et al found that there were twice as many patients with myopia and astigmatism as those with hyperopia and astigmatism in the high astigmatic range.11 PRESCRIBING FOR HIGH REFRACTIVE ERRORS Prescribing contact lenses for high refractive prescriptions has traditionally been rewarding for practitoners and patients. Few practitioners would disagree that the professional satisfaction from a successful contact lens fitting in this group is considerable, but the process can be challenging. Historically, eye care practitioners have tried to fit high ametropes with rigid gas permeable (RGP) lenses. However, RGPs presented centration challenges and resultant visual fluctuations, which can be even worse with highly astigmatic corneas. These difficulties steered practitioners toward hydrogel lenses, which invariably improved centration and stability of the lens, thus eliminating visual complications. Low-Dk hydrogel materials in high powers, and therefore with thicker geometries, are likely to cause hypoxia. However, these clinical effects can easily be observed and managed by practitioners.13-17 Silicone hydrogel lenses have significantly reduced and in some cases eliminated these hypoxic effects without compromising high prescription lens designs. Companies that manufacture molded silicone hydrogel or hydrogel lenses provide a range of spherical lens powers extending from 64 ADVANCED OCULAR CARE | JANUARY/FEBRUARY 2015 Figure 2. Toric optic (5.75 D cylinder) with uncontrolled flat meridian diameter. approximately +8.00 D to -12.00 D. The spherical power range for stock toric lenses is not quite as extensive (+6.00 D to -10.00 D), and the cylinder powers of those lenses range from -0.75 D to -2.25 D typically in 0.50 D steps. For most of these lenses, cylinder axes are available from 0° to 180° in 10° steps. Prescriptions outside this spherical and astigmatic range are available as custom-designed lathed lenses. SUCCESSFUL FITTING IN A CUSTOM POPULATION The ideal contact lens—regardless of refractive power— should fulfill the following criteria: provide excellent vision, as good as or better than spectacle correction; be physiologically undemanding and biocompatible with ocular surfaces; be centered over the pupil with an optical zone larger than the pupil; and possess a posterior surface shape that closely matches the contour of the cornea and limbus without causing localized mechanical pressure. The lens should be free to move during blinking or eye movement (lag) but not excessively so as to cause discomfort or fluctuating vision. Patients should be able to dispose of the lens and replace it with an identical one so as to maintain the described criteria. Contact lens manufacturers of stock molded silicone hydrogel lenses by and large provide products that satisfy those criteria, but only in the limited range of powers described earlier. Reaching each of these performance standards is a greater challenge in the high-prescription group of wearers, from standpoints of both design and reproducibility. DESIGN AND REPRODUCIBILITY CHALLENGES Contact lens companies have traditionally not manufactured stock molded high-prescription lenses because of of geometry involved in both sphere and toric contact lens corrections. To meet the objectives of satisfactory lens performance criteria in high powers, a balance has to be struck between minimizing the optical zone width—by reducing the thickness profile and blending of the junctional curvatures— while maintaining a largely uniform center and edge thickness throughout the power range. The inability to achieve this balance will likely result in a lens with unstable fitting, poor comfort, and/or undesirable visual side effects. These challenges are further exaggerated in toric lenses because of the difference of meridional curvatures on the posterior surface of the lenses. The critical performance characteristics of toric lenses are cylinder axis predictability, on-eye orientation, and, most important, stability during wear.21 On the one hand, having greater differences in radius of curvature between the two principal meridians on the COVER FOCUS the potential for maintaining a large inventory with limited patient demand. Besides the obvious lens design challenges encountered with custom-lathing hydrogel lenses thin enough to maintain oxygen transmissibility and comfort, the lenses are custom-made to order, and therefore reproducibility may be a concern. Manufacturing processes that may result in poor reproducibility will likely give rise to undesirable responses such as inconsistent lens fitting, poor vision, and variable comfort. These custom lenses are often nondisposable. Even if they are replaced every 3 months, the chances of surface deposition, notably denatured protein and other contaminants, is greater, increasing the likelihood of ocular complications.18-20 Reproducibility challenges notwithstanding, the likelihood of achieving a successful contact lens fit in the high refractive error range is also encumbered by the extremes TABLE 1. AVAILABLE POWERS BIOFINITY XR SPHERE Power -20.00 Biofinity XR sphere -15.00 -12.50 -12.00 -10.00 -8.00 -6.50 -12.50D to -20.00D in 0.50D steps Base Curve = Diameter = 8.60mm 14.0mm -6.00 -5.00 +5.00 +6.00 +6.50 -6.50D to -12.00D in 0.50D steps -6.00D to +6.00D in 0.25D steps n Biofinity XR sphere +8.00 +8.50 +6.50D to +8.00D in 0.50D steps +10.00 +15.00 +8.50D to +15.00D in 0.50D steps n Biofinity sphere TABLE 2. AVAILABLE POWERS BIOFINITY XR TORIC Biofinity XR toric Power -20.00 -15.00 -12.50 -12.00 -10.00 -8.00 Base Curve = 8.70mm -6.50 -6.00 -5.00 plano +5.00 Diameter = 14.50mm +6.00 +6.50 +8.00 +8.50 +10.00 +15.00 Cylinder -0.75 -1.25 -1.75 -6.50 to -10.00 in 0.50D steps in 10˚ Axis -6.00 to +6.00 in 0.25D steps in 10˚ Axis +6.50 to _8.00D in 0.50D steps in 10˚ Axis -2.25 -2.75 -3.25 -3.75 -6.50D to -20.00D in 0.50D steps in 5˚ Axis -6.00 to +6.00D in 0.25D steps in 5˚ Axis +6.50D to +20.00D in 0.50D steps in 5˚ Axis -4.25 -4.75 -5.25 -5.75 n Biofinity XR toric n Biofinity toric JANUARY/FEBRUARY 2015 | ADVANCED OCULAR CARE 65 COVER FOCUS “ Cast-molded silicone hydrogel lenses with an extended power range will provide a useful addition to the arsenal of practitioners. posterior lens surface (because of high cylinder power) may help with stability of axis orientation. On the other hand, thicker lenses (because of higher powers) may be more prone to movement and rotation because of lid torque. To counteract that effect, all toric lenses have some form of thickness ballasting or differential to stabilize the meridional orientation. Additionally, the optical zone from these two principal meridians must be appropriately centered and larger than the pupil. This fundamental requirement for toric lens performance is more challenging in eyes with high astigmatism. Back optical zones are traditionally oval or elliptical in shape. This is acceptable with low to moderate amounts of astigmatism, as the variations in curvature and thus thickness are not great. However, as astigmatism increases, so does the delta between the radii of curvature. This creates a dual challenge to (1) provide adequate optical zone width (Figure 1) and (2) not infringe on the peripheral stabilization zone of the lens as displayed in Figure 2. A NEW APPROACH CooperVision has enhanced the available power range of its cast-molded Biofinity sphere and toric contact lenses, as depicted in Tables 1 and 2. The base curves and diameters of the extended power range for spheres and torics are the same as for the lower power Biofinity stock lenses: 8.60 and 14.0 mm for sphere, and 8.70 and 14.5 mm for torics. All lenses use the same comfilcon A material and have the same lens design principles as the core range versions and are manufactured to the same thickness standards for edge profile, which is critical for lens stability and comfort. The toric optical zone has been designed to offer both a wide optical zone in each meridian, which is consistent regardless of power and axis, and to not infringe on the peripheral stabilization zone. SUMMARY Cast-molded silicone hydrogel lenses with an extended power range will provide a useful addition to the arsenal of practitioners. In addition to the many patients who may benefit from an extended range of spheri- 66 ADVANCED OCULAR CARE | JANUARY/FEBRUARY 2015 cal lenses in high powers, the unmet needs of patients who fall into the category of high myopia or hyperopia with astigmatism (of both high and low powers) and low myopia or hyperopia with high cylinder should be fulfilled. Also, patients with keratoconus who have struggled with other forms of contact lenses may find these lenses a useful alternative. Advances in lens design and metrology innovations now also allow the manufacture of extended-range multifocal and multifocal toric lenses in silicone hydrogel material. n 1. Vitale S, Ellwein L, Cotch MF, et al. Prevalence of refractive error in the United States, 1999–2004. Arch Ophthalmol. 2008;126:1111-1119. 2. Sun J, Zhou J, Zhao P, et al. High prevalence of myopia and high myopia in 5060 Chinese university students in Shanghai. Invest Ophthalmol Vis Sci. 2012;53:7504-7509. 3. Jung SK, Lee JH, Kakizaki H, Jee D. Prevalence of myopia and its association with body stature and educational level in 19-year-old male conscripts in Seoul, South Korea. Invest Ophthalmol Vis Sci. 2012;53:5579-5583. 4. Bar Dayan Y, Levin A, Morad Y, et al. The changing prevalence of myopia in young adults: a 13-year series of population-based prevalence surveys. Invest Ophthalmol Vis Sci. 2005;46:2760-2765. 5. Lin LL, Shih YF, Hsiao CK, Chen CJ. Prevalence of myopia in Taiwanese schoolchildren: 1983 to 2000. Ann Acad Med Singapore. 2004;33:27-33. 6. Wang TJ, Chiang TH, Wang TH, et al. Changes of the ocular refraction among freshmen in National Taiwan University between 1988 and 2005. Eye (Lond). 2009;23:1168-1169. 7. Holden BA, Sankaridurg P, Smith E. Myopia, an underrated global challenge to vision: where the current data takes us on myopia control. Eye (Lond). 2014;28:142-146. 8. Kempen JH, Mitchell P, Lee KE, et al; Eye Diseases Prevalence Research Group. The prevalence of refractive errors among adults in the United States, Western Europe, and Australia. Arch Ophthalmol. 2004;122:495-505. 9. Ezelum C, Razavi H, Sivasubramaniam S, et al; Nigeria National Blindness and Visual Impairment Study Group. Refractive error in Nigerian adults: prevalence, type, and spectacle coverage. Invest Ophthalmol Vis Sci. 2011;52:5449-5456. 10. Kim EC, Morgan IG, Kakizaki H, et al. Prevalence and risk factors for refractive errors: Korean National Health and Nutrition Examination Survey 2008-2011. PLoS One. 2013;8(11):e80361. 11. Young G, Sully A, Hunt C. Prevalence of astigmatism in relation to soft contact lens fitting. Eye Contact Lens. 2011;37:20-25. 12. Hashemi H, Rezvan F, Yekta AA, et al. The prevalence of astigmatism and its determinants in a rural population of Iran: the “Nooravaran Salamat” mobile eye clinic experience. Middle East Afr J Ophthalmol. 2014;21:175-181. 13. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Vascular response to extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci. 2001;78:147-151. 14. Dumbleton KA, Chalmers RL, Richter DB, Fonn D. Changes in myopic refractive error with nine months’ extended wear of hydrogel lenses with high and low oxygen permeability. Optom Vis Sci. 1999;76:845-849. 15. Papas EB, Vajdic CM, Austen R, Holden BA. High-oxygen-transmissibility soft contact lenses do not induce limbal hyperaemia. Curr Eye Res. 1997;16:942-948. 16. Keay L, Sweeney DF, Jalbert I, Set al. Microcyst response to high Dk/t silicone hydrogel contact lenses. Optom Vis Sci. 2000;77:582-585. 17. Covey M, Sweeney DF, Terry R, et al. Hypoxic effects on the anterior eye of high-Dk soft contact lens wearers are negligible. Optom Vis Sci. 2001;78:95-99. 18. Pritchard N, Fonn D, Weed K. Ocular and subjective responses to frequent replacement of daily wear soft contact lenses. CLAO J. 1996;22:53-59. 19. Kotow M, Holden BA, Grant T. The value of regular replacement of low water content contact lenses for extended wear. J Am Optom Assoc. 1987;58:461-464. 20. Guillon M, Allary JC. Guillon JP, Orsborn G. Clinical management of regular replacement: Part 1. Selection of replacement frequency. International Contact Lens Clinic. 1992;19:104-120. 21. Snyder C. Evaluation of “high cylinder” toric soft contact lenses. International Contact Lens Clinic. 1997;24:160-165. Desmond Fonn, MOptom, FAAO Distinguished Professor Emeritus at the University of Waterloo School of Optometry in Waterloo, Ontario, Canada n [email protected] n Consultant to CooperVision n Paul Chamberlain Senior manager of clinical research within CooperVision R&D in Pleasanton, California n [email protected] n