Contact Lens Design & Materials
Comfort Part 2: Soft Lenses
By Ronald K. Watanabe, OD, FAAO
When we think about comfort with contact lenses, we usually think of soft lenses first. Soft lenses do have the benefit of good initial comfort because of their soft, pliable nature. But other factors are just as important in creating good initial comfort, such as large diameters, thin profiles, minimal movement, and smooth, hydrophilic surfaces. Some lens characteristics, such as edge design and modulus, as well as fitting characteristics can contribute to initial comfort, too.
Yet, even with their good initial comfort, many patients drop out of soft lens wear due to discomfort. Thoughtful lens selection is important in creating happy, comfortable lens wearers.
Silicone Hydrogel Versus Traditional Hydrogel
Several reports (Dumbleton et al, 2008; Dillehay and Miller, 2007; Riley et al, 2006) have shown that silicone hydrogel (SiHy) lenses deliver better end-of-day comfort compared to traditional hydrogel (TH) lenses. Most SiHy materials have low water content and may dehydrate more slowly compared to TH lenses, especially those that have high water content. Less surface dehydration during the day may help lenses retain a smooth, slippery surface that creates less resistance to the upper lid during the blink. Secondly, wetter, smoother surfaces may accumulate less surface deposition.
In addition, the higher oxygen transmission of SiHy lenses presumably decreases hypoxia and its effects on the ocular surface. This may result in less corneal edema, less limbal hyperemia and vascularization, and fewer epithelial microcysts. Though it is unclear how this is related to comfort, it seems reasonable to assume that a healthier, less stressed cornea would feel more “normal.”
But for some patients, TH lenses may be more comfortable. Because silicone is inherently hydrophobic, manufacturers have had to develop novel ways to make SiHys wettable. They have succeeded in doing so, but the inherent hydrophilicity of TH lenses can give them an edge in surface wettability in some cases. Also, SiHys have had some problems with solution compatibility, which may cause discomfort as well. Finally, the higher modulus of some SiHys can sometimes decrease initial comfort as well as increase problems such as papillary conjunctivitis and superior epithelial arcuate lesions.
In my experience, daily disposability has resulted in more consistent vision and comfort, fewer complications, better compliance, and unparalleled ease of use. It is difficult to directly compare daily disposable lenses to other types of lenses because of differences (sometimes slight) in designs and materials. Also, the highest-Dk materials are not available in a daily disposable modality. Nevertheless, it can't be denied that a fresh lens every day leads to more consistently clean lenses, and there are minimal lens care solution-related complications.
Some lenses have wetting agents embedded in the polymer matrix, while others include them in the packaging solution to help enhance on-eye hydration and comfort. Rewetting solutions also come in various formulations to help maintain wettability during lens wear. Finally, most current lens care products include wetting agents to help lenses retain a hydrophilic surface for longer than the lens can by itself.
The Final Answer?
Ultimately, we must consider all of these factors and best match them to individual patients. Soft contact lens material properties, fitting characteristics, and ocular physiology must work together to provide the best possible comfort for our patients. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #189.
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. You can reach him at firstname.lastname@example.org.