Hard contact lenses are still available today, though in a significantly improved form. The original “hard” lenses, made from Polymethyl Methacrylate (PMMA), are obsolete due to safety concerns. They have been superseded by modern Rigid Gas Permeable (RGP) lenses. RGP lenses maintain the optical benefits of a rigid structure but allow oxygen to pass through the lens material. This newer generation of rigid lenses is actively manufactured and prescribed for specific visual and medical needs.
The Evolution from Hard to Rigid Gas Permeable Lenses
The first commercially available rigid lenses, introduced in the 1940s, were constructed from PMMA, a durable plastic similar to Plexiglas. While PMMA offered clarity and rigidity, it was completely impermeable to oxygen. Since the cornea requires a constant supply of oxygen to remain healthy, this led to issues like corneal swelling and discomfort.
To compensate for the lack of oxygen flow, older PMMA lenses were designed to be smaller. They relied on lens movement with each blink to pump oxygenated tears underneath. The late 1970s and 1980s saw the development of modern RGP materials, composed of silicone and fluoropolymers. These polymers contain microscopic pores that permit oxygen molecules to pass directly through the lens to the corneal surface, eliminating oxygen deprivation and improving wearability.
Why RGP Lenses Provide Superior Vision
The advantage of RGP lenses stems from their rigid, stable structure. Because the lens maintains a smooth curve, it creates a uniform gap between the back surface of the lens and the cornea. This space fills with the natural tear film, which acts as a “liquid lens.”
This tear-filled layer neutralizes irregularities in the underlying corneal shape, providing a spherical optical surface for light to pass through. The result is sharp, high-contrast vision, often superior to standard soft contact lenses, especially for high or complex vision errors. RGP lenses are highly durable and resist the buildup of protein and lipid deposits, making them easier to clean and allowing them to maintain optical clarity over a longer lifespan than soft lenses.
Essential Uses in Specialized Eye Conditions
RGP lenses are used for specialized eye conditions that cannot be corrected by soft lenses. The most common is Keratoconus, a progressive condition where the cornea thins and bulges into a cone shape, causing irregular astigmatism and distorted vision. A soft lens conforms to this irregular shape, but the rigid RGP lens vaults over the cone, using the tear layer to create a new, smooth refractive surface.
RGP lenses correct irregular astigmatism resulting from corneal trauma or complications following refractive surgeries like LASIK. When the corneal surface is uneven, light scatters and vision blurs. The stable curvature of the RGP lens bridges these surface imperfections, masking the irregularity and restoring clear vision where spectacles or soft lenses fail.
Patient Adaptation and Lens Care
While RGP lenses offer superior vision and excellent eye health benefits, they require a longer period of adaptation compared to soft lenses due to their firmness. New wearers typically experience an initial awareness or discomfort, often described as feeling the edge of the lens when blinking. This sensation generally subsides as the eye adapts, with most patients achieving comfortable full-time wear within one to three weeks.
New wearers are advised to gradually increase their wearing time each day to allow the eyelids and cornea to acclimate. Proper lens care is straightforward, requiring specific solutions to clean and soak the lenses. Tap water should never be used due to the risk of serious eye infections. Because RGP lenses are resistant to deposits and tearing, a single pair can often be maintained and worn for a year or longer before replacement, making them a cost-effective option over time.