CL Design & Materials
Which is Better? Selecting GP Contact Lens Materials
BY NEIL PENCE, OD, FAAO, & FLORENCIA J. YEH, OD
After deciding to fit patients with GP contact lenses, the next step is determining particular lens designs and diameters. When the lens fit is acceptable, practitioners then must decide which GP contact lens materials to use. This article will explore factors to consider in selecting the best GP lens materials for your patients.
Typically, mid-range-Dk GP materials will be the normal or default material for standard corneal GP contact lenses. Midrange- Dk fluorosilicone acrylate lenses are generally in the 50Dk to 65Dk range. The additional oxygen that they provide seems desirable compared to low-Dk lenses, which are usually silicone acrylate materials. The midrange- Dk lenses will transmit two to four times more oxygen compared to their low-Dk predecessors. Mid-range-Dk lenses seem to be able to supply this added oxygen without noticeably sacrificing lens stability, wetting, flexure resistance, or good manufacturing characteristics. Therefore, mid-range-Dk GP materials have become the most common standard material.
Factors to Consider
What patients or situations warrant making different material choices? The most common reason is when even higher oxygen transmission is needed or desirable. Higher-oxygen-transmitting materials might be advisable when the lens design or power will make the lens thicker than an average lens. For example, higher-powered lenses will be thicker in portions of the lens, as will prism-ballasted lenses and some multifocal designs. Using higher-Dk materials will help offset this effect. In other cases, patients may exhibit signs of corneal fatigue from prolonged GP lens wear. These patients may benefit from wearing lenses that transmit higher levels of oxygen.
Higher oxygen transmissibility is needed in all instances of overnight wear. The 2013 Contact Lenses & Solutions Summary supplement available on the Contact Lens Spectrum website lists GP contact lens materials that can be used for overnight wear. All are approved for up to seven days of extended wear with the exception of Menicon Z (Menicon), which is the only GP material approved for continuous wear of up to 30 days. Lenses to be worn on an extended wear basis, as well as overnight corneal reshaping lenses, should be made in high- Dk GP materials.
Scleral GP lenses need to be made in higher-Dk materials as well because they may be relatively thick with very little tear exchange. Not all GP materials come in the large- or extra large-diameter buttons needed for sclerals, but nearly all major manufacturers have at least one high- Dk, large-button GP material.
There are some last factors to bear in mind. For patients who have dry eyes, lower-Dk lens materials might wet more easily. If lens flexure is a problem, lower- Dk materials often address this issue. Occasionally, patients want particular GP lens colors, which may impact the material selection as not all materials are available in all colors. Finally, if current manufacturing and wetting issues can be overcome, high-index GP materials may fulfill the promise of thinner and lighter lenses, and become a more significant factor in GP material decisions. CLS
Acknowledgement: The authors thank Mike Johnson of Art Optical for his technical assistance with this article.
For references, please visit www. clspectrum.com/references.asp and click on document #215.
Dr. Pence serves as associate dean, Clinical and Patient Services, Indiana University School of Optometry in Bloomington, Ind. He is a consultant or advisor to B+L, Alcon, and Vistakon and has received research funding from AMO. You can reach him at email@example.com. Dr. Yeh is a graduate of the New England College of Optometry and currently is the Cornea and Contact Lens Resident at Indiana University School of Optometry.