contact lens design and materials
Effect of Contact Lens Material on Solution Performance
By Neil Pence, OD, FAAO
With a number of available options, it seems fair to ask which contact lens multipurpose disinfecting solution is best. That might be similar to asking, “which car is best.” To answer that, you need to ask a number of other questions. Best for what—gas mileage, acceleration, cornering, overall ride, interior space, luxury items? Likewise, to determine which MPS might be best, we need to answer two more questions—best for which contact lens material and best for which patient?
CL Material Considerations
Different materials will have different polymer structures with different spacing or mesh-like openings. They may have different surface treatments or characteristics as well. This not only affects how substances might bind in or on the lens (Zhao, 2009), but also causes materials to have different uptake and release rates of the MPS (Sentell, 2004), and may cause varying on-eye, solution-related phenomena (Jones, 2002).
Hydrogel contact lenses were traditionally divided into four subgroups, with lenses within the group more likely to react similarly with solutions. Silicone hydrogels (SiHys) are generally considered a category to themselves, and with regard to possible MPS interactions, it's useful to consider several subgroupings within this category. SiHys with surface treatments may interact differently than those which rely on a wetting agent to keep the surface hydrophilic, while a third group seems to employ a slightly different strategy with changes to the polymer chains. It's reasonable to believe each of these SiHy subgroups may interact differently with different solutions.
Not all patients are alike and not all solutions will work for all patients. One key factors could be whether they perform the rub-and-rinse step. It's easy to see how this may supplement the cleaning action of the MPS, but it also can be a factor in the disinfecting efficacy and possibly even some on-eye effects related to lens-solution interactions.
The noncompliant patient who doesn't change his solution daily, but re-uses it and perhaps tops it off, may benefit from one MPS over another. The preservative uptake characteristics of the lens material-MPS interaction could lead to less efficacy on subsequent overnight soaks (Rosenthal, 2006). There can be effects from not recapping the solution after each use, and again, these effects could vary for each MPS.
The solution manufacturer's task is to offer an MPS that's effective against all types of microorganisms, helps keep the lens surface clean and deposit free, enhances the wetting and comfort of the contact lens, and is gentle enough on the eye to cause no irritation or other ill effects. Add to this the challenge of needing to perform in this way on all types of contact lens materials, and for all types of patients who introduce a number of variables.
Three new MPS products have been introduced in the past 10 months. Since the majority of CL patients are wearing SiHys, it's safe to say the new products were designed to perform well with SiHy lenses. This is good news for patients and practitioners.
But which MPS is best? Best for which contact lens material and best for which patient? We can't address these questions until we have more first-hand experience and clinical data from well-designed trials. Until then, keep an open mind and remember to consider how material and patient variability can alter the of effectiveness CL care solutions. CLS
For references, please visit www.clspectrum.com/references.asp and click on document SE2011.
Dr. Pence is director of the Contact Lens Research Clinic, Indiana University School of Optometry in Bloomington, Indiana. He is a consultant or advisor to Bausch + Lomb, Ciba Vision and Vistakon, and has received research funding from AMO.