Although modern scleral lenses are mainly manufactured in hyper-Dk materials with values of 100Dk or more, hypoxia can still occur. Scleral lenses differ from corneal GP lens designs primarily in size, thickness, and weight. A scleral lens can be as large as 24.00mm and can weigh approximately 10mg to 15mg, with an approximate center thickness between 0.225mm to 0.50mm, depending on the design, power, and customization.1 However, the center thickness may increase to 3.0mm or more in certain designs, such as those for patients who have aphakia.
It is well established that increasing the thickness of a scleral lens decreases its oxygen permeability. In addition, oxygen permeability may be impacted by the thickness of the liquid reservoir. The liquid may act as a potential barrier to oxygen; thus, oxygen permeability is reduced with increasing lens vault. When fitting scleral lenses, practitioners should be mindful of the scleral lens design and fit to mitigate complications such as hypoxia and secondary vascularization. This is particularly important in patients who have undergone corneal transplantation.
Key performance areas of GP lens materials are wettability, stability, and oxygen permeability. With previous hyper-Dk materials, there was variable initial wetting and expected degradation of wetting over time. There was also the possibility of lens flexure. With the introduction of new hyper-Dk materials of 200Dk or more, patients can now benefit from more oxygen and improved ocular health with scleral lens wear.
One such material is a U.S. Food and Drug Administration (FDA)-cleared GP material with the highest combination of oxygen permeability and flexural modulus on the market. According to the company, independent testing demonstrated that the material has an oxygen permeability value of 200Dk. In-house testing determined a high flexural modulus of 1341MPa, which defied the high-Dk/low-flexural-modulus convention. It has a refractive index of 1.438, hardness (shore D) of 81, Class II UV blocker, and a specific gravity of 1.20. The material also is highly stable and reproducible, and it is cleared for a full range of indications. Surface wettability can be further enhanced with the use of a polyethylene glycol (PEG)-based coating.
Another novel hyper-Dk GP material is composed of a complex molecular network based on a highly dynamic fluorosilicone composition with an advanced manufacturing process in which higher oxygen permeability can be achieved with lower silicone content. There is anticipated FDA clearance in early 2020. The material has an oxygen permeability of 211Dk, a refractive index of 1.430, hardness (shore D) of 78, Class II UV blocker, and a specific gravity of 1.18. This results in a stable, reproducible lens with a highly wettable surface and good flexural strength, according to the company.
If hyper-Dk materials can improve the scleral lens experience for patients who have undergone a penetrating keratoplasty or post-radial keratotomy or who have had corneal edema with other lens materials, should we consider these materials for everyone? Is this the future of GP lens fitting? In addition to scleral lenses, consider hyper-Dk materials for other indications such as orthokeratology, the correction of refractive error, ocular surface disease, and corneal molding rehabilitation. By providing enhanced oxygen, a smooth wetting surface, and lens surface stability, practitioners have the opportunity to provide improved ocular health with scleral lens wear.
1. Barnett M, Toabe M. Scleral Lens Handling. In Barnett M, Johns L, eds. Ophthalmology Current and Future Developments (Volume 4): Contemporary Scleral Lenses: Theory and Application. Bentham Science Publishers, 2017 Nov. 6.
Dr. Barnett is a principal optometrist at the University of California Davis Eye Center in Sacramento, specializing in anterior segment disease and specialty contact lenses. She is the past president of the Scleral Lens Education Society. She is an advisor to and/or has received honoraria or travel expenses from AccuLens, Alcon, Alden Optical, Allergan, Bausch + Lomb, Bruder, Contamac, CooperVision, EveryDay Contacts, Johnson & Johnson Vision, Ocusoft, Paragon Bioteck, RaayonNova, ScienceBased Health, Shire, SynergEyes, and Visioneering Technologies.