Contact lenses are medical devices placed directly on the eye’s surface to correct common refractive errors, such as nearsightedness and farsightedness. They provide a wider field of view and an alternative to wearing eyeglasses. Soft contact lenses have become the most commonly prescribed type, preferred by wearers for their comfort and ease of use. This popularity is driven by advancements in material science that have made these lenses highly compatible with the delicate surface of the eye.
Defining Soft Lenses and Their Composition
Soft contact lenses achieve pliability and flexibility using hydrophilic plastic polymers. When hydrated, these materials become soft and gel-like, conforming immediately to the shape of the cornea. This high water content provides the characteristic softness and initial comfort upon insertion.
The first generation of these lenses utilized hydrogel materials, primarily a polymer called HEMA (2-hydroxyethyl methacrylate). With hydrogel lenses, oxygen permeability to the cornea is primarily facilitated by the water within the lens structure. To allow more oxygen to pass through, manufacturers had to increase the water content, which often made the lenses more fragile or susceptible to drying out.
A significant advancement came with the introduction of silicone hydrogels, which now dominate the market, holding over 70% of the global share. Silicone is a highly oxygen-permeable material that allows significantly more oxygen to reach the cornea than water alone. This development was crucial because the cornea, which has no blood vessels, receives its oxygen directly from the air.
Oxygen performance is measured by oxygen transmissibility, known as the Dk/t value. Silicone hydrogel lenses can achieve a Dk/t value up to five times greater than traditional hydrogels, improving eye health, especially for extended wear. This allows the lens to be designed with lower water content for reduced dehydration and higher oxygen flow, offering a better balance of comfort and corneal health.
Key Categories of Soft Lenses
Soft lenses are primarily categorized by their replacement schedule and their specific visual function. The replacement schedule dictates how long a lens can be worn before disposal, balancing hygiene with cost-effectiveness. Daily disposable lenses are worn once and discarded, offering the highest convenience by eliminating the need for cleaning and storage.
Planned replacement lenses (bi-weekly or monthly) are worn for a prescribed period and require nightly cleaning and disinfection. While generally more affordable than daily disposables, they demand a strict care routine to prevent deposit buildup and contamination. Following the exact replacement schedule is paramount for maintaining eye health.
Soft lenses are also classified by their optical design to correct different types of vision problems. Spherical lenses are the most common, used to correct simple nearsightedness (myopia) or farsightedness (hyperopia). These lenses have a uniform curvature across their surface.
Toric lenses are necessary for individuals with astigmatism, which is caused by an irregularly shaped cornea. These lenses are engineered with different power meridians and a stabilization feature to ensure the lens remains in the correct orientation on the eye. Multifocal or bifocal lenses feature different power zones for near, intermediate, and distance vision, providing a solution for presbyopia.
The Difference Between Soft and Rigid Lenses
The distinction between soft lenses and Rigid Gas Permeable (RGP) lenses lies in their material and flexibility. Soft lenses are large, draping over the entire cornea and conforming to its shape, which provides excellent initial comfort. RGP lenses are made from firm, durable plastic and are smaller, sitting on a layer of tears over the cornea.
The difference in material directly impacts the fitting process. Soft lenses are simpler to fit because they cover the entire eye and require less precise measurements of the corneal curvature. RGP lenses require highly customized fitting to ensure the lens rests correctly and allows for sufficient tear exchange underneath its surface.
While soft lenses offer better comfort immediately, RGP lenses often provide sharper, more stable vision, especially for complex prescriptions or high astigmatism, because they maintain their shape perfectly. RGP lenses are also more durable, resistant to tear film deposits, and have a much longer lifespan.
The mechanism for delivering oxygen varies significantly. Soft lenses rely on oxygen passing through the lens material (water or silicone). RGP lenses are highly oxygen-permeable, but a large portion of the oxygen supply is delivered by the continuous flow of fresh, oxygenated tears pumped beneath the lens with every blink.
Basic Principles of Soft Lens Care
Proper hygiene is essential for all soft lens wearers to minimize the risk of eye infections and maintain long-term eye health. The necessary care routine involves cleaning, rinsing, and disinfection for any lens worn for more than one day. This process must begin with thoroughly washing and drying hands before handling the lenses.
The cleaning step requires placing the lens in the palm of the hand, adding a few drops of solution, and gently rubbing the lens for several seconds. This mechanical action is necessary to remove protein, lipid, and microbial deposits, even when using modern “no-rub” solutions. The lens must then be rinsed thoroughly with fresh solution before being placed in the storage case.
Disinfection occurs when the lens is submerged in fresh multipurpose or hydrogen peroxide solution for the time specified by the manufacturer. Always use fresh solution and never “top off” old solution, as this dramatically reduces disinfectant effectiveness. The lens case itself should be cleaned, rinsed, air dried daily, and replaced every few months.
Wearers must strictly follow the prescribed replacement schedule and never expose soft lenses to tap water. Tap water contains microorganisms, such as Acanthamoeba, that can cause serious eye infections if they contact the lens material. Adhering to these steps ensures the lenses remain safe and comfortable to wear.