Contact Lens Design & Materials
Select Materials to Meet Your Patients’ Needs
BY GREGORY J. NIXON, OD, FAAO
Contact lens practitioners have the luxury of choosing from a multitude of materials. The vast majority of current fits and refits are in silicone hydrogel (SiHy) materials (Nichols, 2016). Despite the benefits of SiHy lenses, there are still occasions when hydrogel lenses may be best.
Advantages of SiHy Lenses
Perhaps the greatest advantage of SiHy lenses is their superior oxygen permeability. Traditional hydrogel lenses derive all of their oxygen permeability solely from their water content. Because the highest practical water content for any material is about 80%, the maximum permeability is approximately 40 Dk (Snyder, 2004). By incorporating oxygen diffusion-rich silicone into the hydrogel matrix, Dk values of current SiHy lenses range from 60 to 140.
Unlike traditional hydrogels, most SiHy lenses increase in oxygen permeability as water content decreases secondary to a subsequent increase in silicone content. SiHy’s lower water content provides another advantage because they tend to dehydrate less compared to hydrogels.
Challenges to Performance
Higher silicone content causes an increase in lens modulus, which may affect initial lens comfort and cause persistent lens awareness (Snyder, 2007). The stiffness of the original SiHy lenses also brought about a return of giant papillary conjunctivitis (GPC)-like irritation and superior epithelial arcuate lesions (Jones et al, 2010). The negative impact of high-modulus SiHy lenses on some patients provoked companies to formulate lenses with lower silicone content.
The greatest modifications made to SiHy lenses involve addressing the poor wettability of the lens surface. To have surface wettability, SiHy lenses need to incorporate some degree of chemical alteration for adequate compatibility with the corneal epithelium on the posterior lens surface and with the tear film and palpebral conjunctiva on the anterior lens surface. Failure to create a smooth, water-loving surface can result in poor wettability, increased lens friction, and lens deposition. These factors can lead to clinical manifestations of lens-induced dry eye, corneal staining, lid wiper epitheliopathy, GPC, increased lens awareness, and overall reduced lens tolerance. Each manufacturer has employed methods to limit the amount of exposed silicone on the lens surface to minimize poor wettability.
Hydrogels Still Matter
Despite vast improvements provided by surface plasma treatments, internal wetting agents, or unique bonding processes, some patients who have high mucolipid content in their tears maintain a degree of SiHy lens incompatibility. So, when a variety of SiHy options provide similar wettability problems, consider a refit into a high-water, non-ionic lens (FDA group 2). My experience has been that the neutrally charged surface with the absence of silicone can often provide these patients with a better overall lens surface performance. I also believe the Dk of these high-water lenses is sufficient to maintain good corneal physiology for daytime wearers.
Take Advantage of Options
Ultimately, the goal is to produce a stable, comfortable lens with crisp optics that reduces or eliminates any physiological compromise to ocular health. While SiHy materials have dominated, be aware of the potential benefits of some traditional hydrogels that may address the wettability issues that affect a subset of patients who have silicone incompatibility. CLS
For references, please visit www.clspectrum.com/references and click on document #247.
Dr. Nixon is the associate dean for Clinical Services and professor of clinical optometry at The Ohio State University College of Optometry. He is also in a group private practice in Westerville, Ohio. You can reach him at email@example.com.