contact lens materials
Contact Lens Deposits — Patient or Material Driven?
BY LYNDON JONES, PHD, FCOPTOM, FAAO, & KATHY DUMBLETON, MSC, MCOPTOM, FAAO
Contact lens deposition with substances from the tear film, particularly proteins and lipids, is a well-known clinical complication, resulting in reduced comfort and vision as well as increased inflammatory responses. Adsorption of tear film contaminants is moderated by a number of factors, including those relating to the contact lens material (water content, surface charge, pore size, surface roughness, wearing period), care regimen (presence or absence of a surfactant cleaner), and to the patients themselves (tear film quality, compliance with their care regimen).
So, what can clinicians do to minimize lens deposition?
Protein deposition is largely mediated by the protein of interest (its size and charge) and the lens material, with relatively small differences among patients. Increasing material water content and/or ionicity greatly increases protein deposition, with lysozyme being detectable on FDA Group 4 lenses after wearing times of as little as one minute. Highly positively charged, small proteins such as lysozyme bind in greater amounts to negatively charged, high-water-content, Group 4 materials while larger, relatively neutral proteins such as albumin (which has a slight negative charge at tear film pH) have a tendency to deposit on more neutral Group 1 and 2 lens materials.
Kinetic studies on protein deposits show that lysozyme deposition on ionic materials typically peaks after one week of wear, with a plateau then occurring. In neutral Group 2 materials the deposition is much less, no peak occurs and the deposition continues to accumulate over time.
More recent work has demonstrated that protein deposition on silicone hydrogel (SiHy) materials is lower than that typically observed with traditional polyHEMA-based materials, and that much of the lysozyme deposited is relatively denatured.
Lipids preferentially deposit onto hydrophobic surfaces, and kinetic studies suggest that lipid deposition tends to become cumulative over time, with no plateau occurring. Data thus far on traditional hydrogel materials indicate that lipid deposition is minimal for ionic materials but is more prevalent with Group 2 materials, particularly those that contain N-vinyl pyrrolidone (NVP).
More recent work has suggested that lipid deposition on SiHy materials can be a problem for certain patients, and it appears that individual patient differences in tear film composition can play a significant role in the degree of lipid deposition. Current market trends show rapid growth in SiHy lens prescribing. If you note deposition on SiHy lenses at the slit lamp, then lipid deposition is more likely to be the issue rather than protein deposition.
To minimize such deposition, first check for and manage any blepharitis or meibomian gland dysfunction to improve the quality of the tear film. In addition, prescribe a surfactant-containing care regimen (whether hydrogen peroxide or multipurpose system) and instruct patients to rub and rinse their lenses prior to disinfection and to replace their lenses as prescribed. In extreme cases, replacing the lens more frequently, adding a supplemental surfactant cleaner containing alcohol (such as MiraFlow from CIBA Vision) or refitting a non-NVP-containing traditional hydrogel material should further minimize such deposition. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #163.
Dr. Jones is the associate director of the Centre for Contact Lens Research and a professor at the School of Optometry at the University of Waterloo, Ontario, Canada. He has received research funding from Alcon, AMO, B&L, CIBA Vision, CooperVision, Johnson & Johnson, and Menicon. Dr. Dumbleton is a senior clinical scientist at the Centre for Contact Lens Research in Ontario, Canada.