At A Glance
- Polymethylmethacrylate was a fantastic material in terms of durability, optical quality, and ease of care, but it was not great for the cornea.
- Adding silicone to lens material allowed oxygen transmission through the lens and the addition of fluorine to the mildly oxygen-transmissible silicone-acrylate materials created present-day lenses.
- Surface treatments can improve lens tolerability and patient comfort. The two main surface treatments used today are plasma and Hydra-PEG.
Scleral lenses have made gas permeable (GP) lenses sexy again, and, as a result, there is a renewed focus on finding ways to perfect the process of making GP lenses. Information about how to avoid problems such as fogging and poor wettability and how to optimize comfort is all over social media. This is an exciting time for a potentially sight-saving modality.
The rebirth of GP lenses resurrects conversations about billing medically necessary lenses, GP lens materials and their properties, lens coatings, and, yes, the always stimulating topic of lens solutions. This article focuses on lens materials and coatings, a discussion that, until recently, was about as interesting as watching paint dry.
The evolution of lens material
To understand the lens technology of today and tomorrow, we must first take a brief detour into the past. So, let us all channel our contact lens professors for a moment (for me it was Joel Silbert, OD, at the Pennsylvania College of Optometry), and remember the material with which our predecessors started: I’m talking about polymethylmethacrylate (PMMA).
If you’re saying to yourself, “Wait a second. I used PMMA,” then you, my friend, are showing your age. For those of you who, like myself, never got to use this heralded material, it’s good to know one thing: It was a fantastic material in terms of durability, optical quality, and ease of care, but it was not great for the cornea. PMMA had an oxygen transmissibility (Dk) of 0, so practitioners had to ensure excellent tear exchange under the lens to achieve a “healthy” fit.1
The next evolution of lens material was the addition of silicone, which allowed oxygen transmission through the lens instead of around it. The earliest materials with silicone were referred to as silicone-acrylates (SAs). Lenses made of this material can still be found today, but they are rarely used because of poor wettability, surface deposits, and patient discomfort.1 See the Table for a list of common SA lenses. Increasing the Dk of these lenses was desirable, but it hazarded increased surface deposits, warpage, crazing, and brittleness.2
A major step forward was adding fluorine to the mildly oxygen-transmissible SA materials, essentially bringing us to present-day lenses. The addition of fluorine lowered surface tension, decreased the attraction of polarized tear components to the lens, and reduced the problem of ocular dryness.
Referring to these products as “Boston” lenses is not correct. Although “Boston” has become a household name for GP material, it is not a material name. Rather, it represents just one of several companies that produce the material broadly classified as fluorosilicone-acrylate lenses. See the Table for some examples of these lenses.2
Advances in lens material have greatly improved the overall health profiles of GP lens wearers, but some problems persist that ultimately lead patients to drop out of scleral lens wear. First, I should note that working with contact lenses entails accurately and precisely determining whether a patient has dry eye disease (DED) requiring treatment. I truly believe that simply changing material, adding a lens coating, or switching a lens solution are not enough to resolve discomfort if a patient has DED.
Lens wear induces inflammation, even in a healthy eye, and should be dealt with proactively. My first step is to prescribe topical cyclosporine. By aggressively addressing DED, I have refit scores of patients who have discontinued wearing all types of lens modalities. I proactively talk to my specialty/GP lens patients about DED and treat it prophylactically in nearly all of them.
These days, I only really hear about two competing surface treatments to improve lens tolerability and patient comfort. One is plasma treatment, and the second—the new kid on the block—is Tangible Hydra-PEG (Tangible) treatment, which is quickly becoming the standard in high-end GP lens care.
Plasma is one of the most misunderstood topics in contact lens care. It is not a liquid, a solid, or even a coating. It is a surface treatment that improves a lens material’s wettability and interaction with the tear film. What I like to say about plasma treatment is that lenses are placed in a magical box where rainbows and unicorns collide and hit the lens’ surface, making the lens happy. When the lens is happy, it can better deal with the stresses of its life, such as protein deposits, bacteria, and contaminants.
Obviously, this is neither the true nor the full story. The more accurate and boring story is that plasma treatment ionizes the lens material, allowing it to become more hydrophilic, or water loving (snore). It is important to note that the ionization does not last forever and that it can be reduced with the use of abrasive cleaners.3
Hydra-PEG wins my contact lens add-on award for “most useful in patient comfort.” If you are still working with a laboratory that does not have Hydra-PEG or do not regularly use Hydra-PEG, you are out of date. Plasma treatment became the standard of care 5 to 10 years ago, and Hydra-PEG is quickly doing the same today by filling in the gaps and improving outcomes.
So, What Is It?
Hydra-PEG is mostly (90%) water. The rest is polyethylene glycol or PEG, a common component of many artificial tears that also has a robust history in wound care management. This coating increases lubricity and patient comfort, which can lower dropout rates.4
How Does It Work?
The first step in preparing the lens surface is either to add a functional activator to the monomer mix or to add a short plasma surface treatment. Once either addition is active, lenses are soaked in the Hydra-PEG polymers during the extraction/hydration process. Hydra-PEG permanently bonds to the lens surface, completely encapsulating the lens. This optimizes the surface’s wettability, lubricity, tear stability, and resistance to deposits.5
SCLERAL LENS THE WORLD
Now that you are armed with all the information you need to pick the right material and coating, go forth and scleral lens the world!
- 1. Bennett ES, Henry, VA. Clinical Manual of Contact Lenses. Philadelphia, PA: Wolters Kluwer Medical; 2015.
- 2. Bennett ES, Johnson JD. Materials selection. In: Bennett ES, Weissman BA, eds. Clinical Contact Lens Practice. Philadelphia, PA: Lippincott Williams & Wilkins; 2005:243-253.
- 3. Gu ZW, Han Y, Pan FS, et al. Surface hydrophilicity improvement of RGP contact lens material by oxygen plasma treatment. Materials Science Forum. 2009;610-613:1268-1272.
- 4. Schachet JL, Rigel LE, Reeder KM, et al. Rethinking the link between Dk and RGP lens performance. Contact Lens Spectrum. 1998;13(9):43-47.
- 5. Sindt CW. Tangible Hydra-PEG: A novel custom contact lens coating technology designed to improve patient comfort and satisfaction. White Paper. Tangible Science. 2016. artoptical.com/storage/docs/THP_White_Paper.pdf. Accessed April 25, 2019.