Scleral lenses are specialized contact lenses developed to address complex vision problems, such as those arising from an irregularly shaped cornea or severe dry eye disease. These devices offer a therapeutic solution by providing a smooth optical surface and a constant reservoir of fluid to the eye. Their distinct design, which is significantly larger than conventional lenses, raises questions about the material from which they are constructed. Since any contact lens covers the eye, understanding the material composition is necessary to confirm the lens’s safety and effectiveness for long-term corneal health.
Defining Scleral Lenses and Their Material
Scleral lenses are named for the part of the eye on which they rest, the sclera, which is the white, less sensitive tissue surrounding the cornea. Unlike standard contact lenses that sit directly on the cornea, these large-diameter lenses vault completely over the corneal surface without touching it. This vaulting mechanism creates a fluid-filled space between the back of the lens and the cornea, which helps to hydrate the eye and neutralize surface irregularities.
The material used in modern scleral lenses provides a direct answer to the question of gas permeability. They are manufactured using highly breathable, rigid gas permeable (RGP) polymers. This material choice is deliberate, as the rigid nature of RGP allows for the precise, custom shaping needed to create the corneal vault and the lens’s complex optical zones. The material’s ability to maintain its shape offers a significant advantage over traditional soft lenses, delivering superior visual acuity, stability, and durability.
The high-performance polymers used are specifically designed to be permeable, meaning they allow oxygen molecules to pass directly through the solid plastic structure to reach the cornea underneath. This feature is non-negotiable for a lens of this size and design. Older, non-permeable lenses made of polymethyl methacrylate (PMMA) were known to cause chronic corneal oxygen deprivation, a complication virtually eliminated with the transition to modern RGP materials.
The Role of Oxygen Permeability
The cornea, the clear front surface of the eye, relies on oxygen primarily from the atmosphere to maintain its transparency and function, as it contains no blood vessels. When a contact lens covers the cornea, it introduces a barrier that impedes this natural oxygen supply. Therefore, a lens material’s ability to transmit oxygen is quantified by its Dk value, a measure of permeability.
The Dk value is a composite measurement where ‘D’ represents the diffusion coefficient, and ‘k’ represents the solubility coefficient. For scleral lenses, which are larger and thicker than other RGP lenses, a material with an inherently high Dk value is required to compensate for the physical barrier they create. Modern scleral lens materials typically have Dk values of 100 or greater, with many exceeding 125 or even 150.
Insufficient oxygen delivery, known as hypoxia, can lead to adverse physiological changes in the cornea. The most immediate sign is corneal edema, or swelling, which occurs as the lack of oxygen disrupts the metabolic pump that maintains the cornea’s normal fluid balance. Chronic hypoxia can also trigger the growth of new blood vessels into the cornea, a condition called neovascularization. Selecting a high-Dk RGP material minimizes these risks and ensures the cornea receives a healthy supply of oxygen throughout the wearing period.
How Scleral Lens Design Affects Oxygen Flow
While the high Dk value of the RGP material is foundational, the physical design of the scleral lens introduces complexity for oxygen transmission. The lens’s defining feature is the vault, which creates a post-lens fluid reservoir filled with sterile saline solution that bathes the cornea. This tear film layer is therapeutic, especially for dry eye, but it adds a secondary barrier to the oxygen pathway.
Oxygen must first pass through the RGP lens material, then through the fluid reservoir, and finally be absorbed by the cornea. The tear film itself has a fixed oxygen permeability, equivalent to a Dk of approximately 80. Since the oxygen must diffuse through this fluid layer, a thicker reservoir reduces the amount of oxygen reaching the cornea. Studies show that increasing the reservoir thickness from 200 micrometers to 400 micrometers can reduce oxygen delivery to the cornea by as much as 30%.
The total oxygen available to the cornea is determined by the lens’s transmissibility, expressed as the Dk/t value, where ‘t’ is the lens’s central thickness. Scleral lenses are generally thicker than other contact lens types for structural stability, which inherently lowers the Dk/t value. This challenge necessitates using the highest Dk materials available and minimizing the lens thickness without compromising its integrity.
The design also creates a semi-sealed environment, which limits the natural tear exchange beneath the lens. Unlike smaller corneal RGP lenses, which exchange 10 to 20 percent of the tear volume with each blink, scleral lenses typically exhibit less than one percent tear exchange per minute. This minimal fluid movement prevents fresh oxygenated tears from replenishing the reservoir, making the cornea almost entirely dependent on the material’s high Dk/t to pull oxygen from the atmosphere. To maintain corneal health, practitioners aim for a central tear reservoir thickness of 200 micrometers or less after the lens has settled on the eye.