Contact lenses are medical devices used widely across the globe to correct various refractive errors. These devices rest on the surface of the eye, providing clear vision without the use of spectacles. Modern technology has led to a wide array of lens materials. For new wearers, the classification of gas permeable lenses is often a point of confusion.
Defining Rigid Gas Permeable Lenses
Gas permeable lenses are definitively classified as rigid lenses, not soft, and are formally known as Rigid Gas Permeable (RGP) lenses. This classification is based on the lens’s structural integrity and its ability to maintain a fixed shape on the eye. They should not be confused with older “hard” lenses, which were made from polymethyl methacrylate (PMMA) plastic.
The older PMMA lenses were completely impermeable to oxygen, meaning the eye’s cornea had to rely on tear exchange beneath the lens for oxygen supply. Modern RGP lenses, developed in the late 1970s, incorporate high-tech plastic materials like silicone compounds and fluoropolymers. The “gas permeable” characteristic refers to the fact that these materials allow oxygen to pass directly through the lens matrix to the corneal tissue.
How RGP Lenses Differ from Soft Lenses
The primary distinction between RGP and soft lenses lies in their fundamental material composition. Soft lenses, including hydrogel and silicone hydrogel types, are made from flexible, water-retaining polymers that conform entirely to the shape of the cornea. RGP lenses, conversely, are made from durable, non-water-dependent plastics that hold a precise, custom shape.
The difference in material dictates a completely different fit on the eye. RGP lenses are typically smaller in diameter, covering only a portion of the cornea and floating on a layer of tear film. This small size and rigid structure allow for constant tear circulation and oxygen flow underneath the lens with every blink. Soft lenses, however, cover the entire cornea and move minimally, relying mostly on the lens material’s own oxygen transmissibility.
Their lifespan and replacement schedule also differ. Soft lenses are disposable, with replacement cycles ranging from daily to monthly, because their water-based structure makes them prone to absorbing deposits. RGP lenses are highly durable and resistant to protein and lipid buildup, often lasting for one to three years.
Adaptation and Specific Care Requirements
The rigid nature of RGP lenses means there is a necessary period of adaptation for the wearer, which can last from a few days up to several weeks. This initial period of foreign body sensation is normal, but once adapted, the lenses often provide sharper, more stable vision, particularly for complex prescriptions like high astigmatism.
Fitting Process
Due to their custom nature, RGP lenses require a specific fitting process by an eye care professional. The fitting involves precise measurements of the eye’s curvature, known as keratometry or “K” readings, and the horizontal visible iris diameter. These measurements are used to manufacture a lens with a specific back optic zone radius that ensures a precise alignment with the cornea. This custom fit is essential for promoting tear exchange and long-term comfort.
Maintenance Routine
The maintenance routine for RGP lenses is distinctly multi-step and requires specific products. After daily wear, the lenses must first be cleaned with a dedicated surfactant cleaner to physically rub away surface deposits. Following this crucial cleaning and rinsing step, the lenses must be soaked in a separate conditioning solution overnight. This conditioning step disinfects the lens and also utilizes hydrophilic polymers to improve the lens surface’s wettability, which enhances on-eye comfort and hydration for the next day’s wear.