contact lens design and materials
When to Go High-Index
BY RONALD K. WATANABE, OD, FAAO
High-index GP contact lens materials have been on the market for more than a year and a half, but adoption of these materials has been slow compared to that of high-index materials for spectacle lenses. A quick survey of a few large GP labs revealed that less than 1 percent of all GP orders are for high-index materials. Practitioners automatically recommend high-index spectacle lenses for high myopes and hyperopes—or for anyone who wants thinner, lighter lenses. So why have high-index GP materials been so slow to catch on?
High-index GP materials can be made into thinner lenses. This is of little significance for the usual range of lens powers, but the difference is significant for powers approaching 10.00D and higher. For example, a −10.00D lens with a diameter of 9.5mm made with Paragon HDS (n = 1.44) (Paragon Vision Sciences) will have a lenticular junction thickness (thickest portion of the lens) approximately 0.04mm greater compared to the same power made with Paragon HDS HI (n = 1.54). This is enough to significantly change upper eyelid interactions and comfort. A +10.00D lens would be 0.05mm thinner in the center with the high-index material, which would help decrease lens mass and is more conducive to lid attachment. Thinner lens profiles also allow for larger optic zones because the front and back surfaces are closer in curvature compared to those with lower-index materials. (Thanks to Mike Johnson of Art Optical for these calculations.)
High-index materials have low specific gravity (as low as 1.04 for Optimum HR, Contamac). This decreases the weight of the lenses, which helps them stay centered on the cornea and allows the upper lid to more easily hold them in place, especially as lenses become thicker and heavier.
High-index materials can increase the effective add power of aspheric multifocal lenses. This is of particular benefit with aspheric multifocal lenses that generate add power through curvature changes across the optic zone. For patients requiring higher adds (+2.00D or more), it is sometimes challenging to deliver enough add with aspheric multifocals. Making the lens in a highindex material can add up to +0.50D more add power, depending on the design of the lens.
High-index materials have problems with surface wettability. The receding contact angles are relatively high compared to comparable fluorosilicone acrylate materials: Optimum HR (33 degrees and 42 degrees) versus other Optimum materials (3 to 12 degrees); Paragon HDS HI (44 degrees) versus Paragon HDS (15 degrees) and FluoroPerm 30/60/92 (13 to 16 degrees) (Paragon). This is especially challenging for patients who have dry eyes, which many presbyopes have. High-index material lenses must be plasma treated to maintain reasonable wettability on the eye. Manufacturers of high-index materials recommend diligent cleaning regimens to maintain surface wetting over time.
High-index materials have lower oxygen permeability, which is a disadvantage especially if they are being used for high-powered lenses that have thicker profiles. Though a well-fit lens can provide the cornea with plenty of oxygen via tear exchange, in this age of hyper-oxygen-permeable materials, high-index materials are toward the low end of the spectrum.
High-index materials also have a modest price premium that may make them less attractive despite their benefits.
Choose the Right Patients
High-index materials can be used to make thinner, lighter lenses that have larger optic zones and greater effective add power. These benefits can make a difference for challenging presbyopes and high ametropes. CLS
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass.