Contact Lens Design & Materials
Options for Dry Eye Patients
By Ronald K. Watanabe, OD, FAAO
Perhaps the most common complaint among contact lens wearers is “dryness.” Dryness can mean several things to a patient, including decreased wearing time, sandy or scratchy irritation, and blurry vision. When discomfort is no longer tolerable, the patient may drop out of lens wear. Though it is a common problem, dryness is complex and sometimes difficult to solve. For some, a well thought out material choice can help reduce dry eye complaints.
Water Content and Thickness
For traditional HEMA-based hydrogel materials, water content is an important factor in lens dehydration. In general, the higher the water content, the faster the lens dehydrates, so it makes sense to prescribe low-water-content materials for patients who complain of dryness. This is one reason why silicone hydrogels (SiHys) are thought to reduce dryness symptoms. Because most SiHys have a low water content, there is less water to evaporate.
Another strategy for reducing dryness was to use thick lens designs. The rationale was to have more water present so it would take longer for lenses to desiccate, leaving more water to keep the ocular surface hydrated. Some hydrogels were available in multiple thicknesses, and it was common to prescribe the thicker versions to dry eye patients. Of course, the downside to thicker lenses was the significantly decreased Dk/t in already oxygen challenged materials. It also was found that most lens dehydration occurs at the surface and that the lens loses very little water by volume. Today, it is uncommon to find high-volume lenses available in multiple thicknesses, though custom labs can still make them.
Omafilcon A has phosphorylcholine (PC) in its hydrogel matrix. PC is a biocompatible material that mimics the polar group of lipids found in cell membranes. In contact lenses, PC helps maintain hydration and resist protein deposition. It is also used to make thrombus-resistant stents and may be found in artificial tears and contact lens solutions in the near future.
Hioxifilcon (A and D) is a copolymer of HEMA and glycerol methacrylate. It contains hydroxyl groups that tightly bind water and so it does not dehydrate as much as traditional HEMA materials do. The 59% hioxifilcon A material reportedly loses only 1 percent of its water content over a 12-hour period of lens wear.
SiHy lens manufacturers have the challenge of trying to make a silicone-containing product hydrophilic. First generation SiHy materials were surface treated to turn the hydrophobic components toward the inside of the lens. This process, along with added wetting agents such as Alcon's Aqua Moisture System (polyvinyl pyrrolidone and polyethylene glycol), makes these lenses very wettable.
Polyvinyl pyrrolidone (PVP), a water-soluble hydrophilic polymer, was also bound to the matrix of senofilcon A, galyfilcon A, and narafilcon B materials to make these SiHy products more wettable without needing a surface treatment. Efrofilcon A, a custom latheable SiHy, contains hydrophilic monomers that allow it to maintain a high 74% water content while demonstrating minimal on-eye dehydration.
Material is Only One Factor
Despite these technological advances, materials are only one factor in determining on-eye dryness. Lens care products, underlying dry eye or ocular surface disease, systemic disease and medications, and poor compliance may be more responsible for patient complaints compared to the material, so address these issues before changing lens brands. If the patient is still having problems, look to these “dry eye” materials to relieve dryness. CLS
|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.|