How High-Index GP Materials Will Impact Your Practice
BY EDWARD S. BENNETT, OD, MSED
One focus of the Annual Meeting of the Contact Lens Manufacturers Association (CLMA) held in November in Alabama pertained to the potential impact of high-refractive-index GP materials. Many of the major material manufacturers have introduced (or will soon introduce) such materials, and it's evident that their potential impact will be significant. What are the benefits of these materials?
GP lens materials in common use today have a refractive index varying from 1.42 to 1.47 and specific gravity values that vary from 1.10 to 1.27. New designs from Contamac (Optimum HR material) and Paragon Vision Sciences (Paragon HDS HI material) have refractive index values ranging from 1.51 to 1.54 and specific gravity values as low as 1.04 (Contamac).
The lower specific gravity values indicate that these materials are lighter; the higher refractive index value results in a lens that can be manufactured in a thinner design. In fact, according to Tim Koch, vice president of Global Professional Services for Contamac, a junction thickness reduction of 5-to-10 percent in minus lenses and a potential weight reduction of 20-to-30 percent for all plus and high minus power lenses can result with high-index materials. These new materials are thinner and lighter, which should help minimize inferior decentration and increase comfort.
The most important immediate benefit of these high-index materials pertains to providing higher add powers. It's clear that, as the material's index of refraction increases, so does the potential add power of the lens. Compared to other materials of similar Dk and standard refractive index values, Contamac claims that its new Optimum HR materials can provide a potential add power increase of 12-to-15 percent. The additional 0.25D to 0.50D often gained from designs manufactured in high-index materials can be the difference between success and failure for moderate-to-advanced presbyopes.
The increased effective add power achieved with high-index lens materials will have important applications in certain patients and with specific lens designs. In their January Contact Lens Case Reports column, Caroline and Andre reported on the higher add power achieved with the Paragon HDS HI material in both posterior and anterior aspheric lens designs. This is especially important for presbyopic patients who have a small pupil diameter, a common challenge as pupil diameter tends to decrease with "chronological maturity." In addition, certain patients require this increase in add power as a result of their refraction and corneal curvature values.
Myopic patients require a higher add power than do hyperopic patients. Likewise, patients who have flatter corneas require higher add powers than do those with steeper corneas.
These lens materials can be especially advantageous in anterior aspheric lens designs. Posterior aspheric designs have been especially popular for several decades. However, their high eccentricity can cause topographical changes, especially if the lens is decentered, and their add power is limited. Higher add powers can be achieved in anterior aspheric lens designs and, combined with the add power benefits of high-index materials, this design/material combination has the potential to meet the vision demands at all distances of most presbyopic patients. CLS
I would like to acknowledge Tim Koch, FCLSA, and Patrick Caroline, FCLSA, for their contributions to this article.
Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and is executive director of the GP Lens Institute.