Since the introduction of silicone hydrogel contact lenses in 1999, they have become the fastest growing new fit contact lens segment worldwide. Researchers predict that, by 2009, silicone hydrogels will make up more than two-thirds of contact lens sales in the United States. The lens is a favorite of eye care professionals because of its capacity to deliver high levels of oxygen to the cornea and to effectively resist dehydration.
To understand the difference between silicone hydrogel lenses and the conventional alternatives, it is important to note the difference in the lens construction.
In general, contact lenses are manufactured using polymers of varying properties that attract and bind water. Tensile strength or the material’s resistance and surface properties of the polymer help determine if it would make a good contact lens material. Hydrophilic polymers are water loving and bind water up to a predetermined capacity, a property designated by the lens material’s chemical composition. These polymers tend to resist surface deposits, making hydrophilic lenses feel more comfortable on the cornea. In contrast, hydrophobic surfaces reject water and require the application of surface treatments to maintain wettability and comfort for the wearer. Hydrophobic materials tend to attract protein and lipid deposits, leading to a dry cornea, which can result in discomfort for the wearer.
Conventional hydrogel contact lenses are made from a soft, water-loving plastic that binds water. In hydrogel lenses, oxygen travels to the cornea in a couple of ways. The first is the polymer’s transmissibility factor, which is defined by the nature of its physical structure. The other way is via the permeability properties of water, relative to the water content of the material. The more water a lens contains, the higher its permeability, but it has a greater tendency to dry out after long wearing periods.
Silicone hydrogel lenses came about after it was discovered that combining hydrogel with silicone, which is naturally oxygen permeable, produced a hydrophilic material with a unique ability to transmit high levels of oxygen to the cornea.
Silicone hydrogels can permeate up to six or seven times more oxygen than conventional lenses. Because oxygen is essential to the health of the cornea, more oxygen being absorbed through the lens translates into healthier eyes and safer extended lens wear.
One of the biggest challenges for contact lens wearers is finding a lens that delivers high quality vision and comfort at the same time.
It is estimated that about 30 percent of contact lens wearers remove their lenses at the end of the day or stop wearing contacts entirely due to discomfort. Researchers and eye care professionals are striving to improve upon this.
Because silicone hydrogel lenses provide substantial amounts of oxygen to the eye, the lens typically feels more comfortable and the wearer is less likely to suffer eye redness or corneal swelling.
Lens wettability is a crucial component of overall comfort. Since contact lens wearers blink about 11,000 times per day, it is critically important to maintain high wettability to allow for the comfortable interaction between the contact lens surface and the underlid, which rubs against the outside lens surface. Generally speaking, a well-hydrated lens surface helps control the deposition of lipids and proteins found in human tears. Otherwise, dry areas on the lens surface can attract deposits. Lens deposits can come from tears, the environment, or from lens handling. Protein, lipids and naturally occurring salt deposits such as calcium can lead to discomfort.
To help combat deposits, wearers should thoroughly rub and rinse their lenses with solution, use an alcohol-based daily cleaner to dissolve lipids, and, if possible, use lenses with a two-week or less replacement schedule.
Anyone who experiences discomfort or side effects from wearing contact lenses should immediately get in touch with their eye care practitioner.