Most people are familiar with soft, disposable, and flexible contact lenses. However, a specialized and highly effective alternative is known colloquially as “hard lenses.” These contacts offer a level of visual clarity and therapeutic support not possible with softer counterparts. Understanding this form of vision correction is important for individuals whose eyes have specific, complex needs.
What Are Rigid Gas Permeable Lenses?
The modern version of the hard lens is correctly known as the Rigid Gas Permeable (RGP) lens. These lenses are crafted from durable plastic polymers, including silicone compounds that provide rigidity and allow the lens to maintain a consistent shape on the eye. This physical property differentiates them from soft lenses that conform to the eye’s surface.
The “gas permeable” feature is crucial because it distinguishes them from older hard lenses (made of PMMA) that did not allow oxygen to pass through. Modern RGP materials permit oxygen to flow directly to the cornea, which is necessary for maintaining eye health.
Key Differences from Soft Contact Lenses
RGP lenses differ structurally and functionally from soft contacts, starting with their size and fit. Soft lenses are thin, pliable, and cover the entire cornea. RGP lenses are smaller in diameter, typically covering about 75 percent of the cornea while floating on the tear film.
The rigid surface of an RGP lens is its most significant feature. It provides a smooth, perfectly spherical front surface that replaces the eye’s natural, potentially irregular, surface. This rigidity allows RGP lenses to consistently deliver sharper, more stable vision than soft lenses. The stable surface allows for highly precise light refraction, which corrects complex visual distortions.
New wearers must go through a longer adaptation period, which can take a few weeks, because the rigid material is initially felt more on the eye. However, RGP lenses are significantly more durable, resisting tears and lasting for a year or longer with proper care, making them cost-effective over time.
Conditions Requiring Hard Lenses
The specialized design of RGP lenses makes them the preferred or necessary treatment for several specific eye conditions where soft lenses are inadequate.
Irregular Corneas and Astigmatism
One of the most common applications is for eyes with irregular corneas, such as those with high or complex astigmatism. The rigid lens creates a new, perfectly smooth refractive surface. It uses the tear film trapped beneath it to fill in any irregularities of the eye, which dramatically improves visual acuity.
Keratoconus
The condition known as keratoconus, where the cornea progressively thins and bulges outward into a cone shape, is often managed with RGP lenses. Soft lenses would simply mold to the abnormal cone shape and fail to correct the vision distortion. The RGP lens vaults over the irregular cornea, creating a smooth optical surface that allows for clear, focused vision.
Specialized Applications
RGP technology is also used in a procedure called Orthokeratology. Specially designed lenses are worn overnight to temporarily reshape the cornea to correct nearsightedness, allowing for clear daytime vision without lenses. RGP lenses are frequently used for post-surgical correction following procedures like LASIK, or to manage corneal scarring, where a precise, non-conforming surface is required to restore visual clarity.