Scleral lenses are large-diameter contact lenses used to address complex vision problems and manage ocular surface diseases. They are effective for individuals with irregular corneal shapes, such as those caused by keratoconus or post-surgical changes. Unlike conventional contacts, the material composition must balance rigidity for vision correction with the necessity of maintaining ocular health. Specialized polymers are engineered to deliver sharp vision while facilitating the biological needs of the eye beneath the lens.
What Makes Scleral Lenses Unique?
Scleral lenses are significantly larger than standard contacts, often measuring between 14.5 and 24 millimeters in diameter. This size allows the lens to completely bridge or “vault” over the entire corneal surface, which is the most sensitive part of the eye. The lens is instead supported by the less sensitive, white outer layer of the eye known as the sclera.
This vaulted design intentionally creates a space between the back surface of the lens and the front surface of the cornea. This space is filled with a sterile saline solution upon insertion, forming a continuous fluid reservoir. The fluid chamber protects the delicate corneal tissue from the lens material and environmental irritants. By resting solely on the sclera, the lens provides superior stability and comfort, making it suitable for eyes that cannot tolerate traditional contacts that sit directly on the cornea.
The Primary Composition Materials
Scleral lenses are manufactured from highly durable, rigid plastics generically classified as Gas Permeable (GP) materials. These polymers are specifically chosen for their ability to hold a precise shape, which is necessary to correct vision irregularities, while also allowing gases to pass through the material. The modern composition of these lenses relies heavily on advanced polymer chemistry, primarily involving fluorosilicone acrylates.
The backbone of the material is an acrylate polymer, which provides the required strength and optical clarity. Silicone is incorporated into the chemical structure to dramatically increase the lens’s oxygen permeability. Further refinement is achieved by adding fluorine atoms, resulting in fluorosilicone acrylate polymers. This fluorination not only boosts oxygen flow but also improves the surface properties of the lens, such as resistance to deposits and overall wettability.
Specific high-performance materials are used, primarily fluorosilicone acrylate polymers. Examples include the Roflufocon family of polymers, which offer oxygen permeability (Dk) values exceeding 100. Another material, Tisilfocon A, features a siloxanylstyrene structure and achieves Dk values up to 163. This careful chemical engineering ensures the lens remains rigid and optically effective while permitting gas exchange.
Ensuring Eye Health: The Role of Oxygen Permeability
The specialized material composition is directly tied to the biological requirement of the cornea, which needs a constant supply of oxygen to remain clear and healthy. The cornea receives its oxygen directly from the air, but a scleral lens effectively places a barrier over the entire surface. Because the lens is large and creates a semi-sealed system, it limits the normal exchange of tears and air beneath it.
For this reason, the material must possess an exceptionally high oxygen transmissibility, often expressed as the Dk/t value, which accounts for both the material’s permeability (Dk) and the lens’s thickness (t). Research indicates that scleral lenses should use materials with a Dk value of 125 or higher for safe daily wear to minimize the risk of corneal swelling. The tear reservoir beneath the lens also plays a role in oxygen distribution, but the primary pathway is diffusion directly through the lens material itself.
Utilizing materials with high Dk values counteracts the effect of the fluid reservoir, which impedes oxygen flow. By selecting highly permeable polymers, manufacturers ensure enough oxygen diffuses through the lens and the tear layer to reach the corneal cells. This design protects the long-term health of the cornea while allowing the patient to benefit from the lens’s vision correction and protective properties.