SCLERAL LENSES: WE DON’T KNOW WHAT WE DON’T KNOW
S. BARRY EIDEN, OD, FAAO
The growth in the interest and incorporation of scleral lens technologies into contact lens practice has been nothing short of amazing over the past decade. More and more practitioners have become recharged on a daily basis since they have begun to manage patients with scleral lenses. The application of scleral lenses initially was focused on managing irregular corneas and ocular surface disease; however, more and more practitioners are considering scleral lenses for what typically would be considered “normal” corneas. Due to their excellent comfort, their ability to center optics over the visual axis, and the diversity in the optical applications that can be designed into scleral lenses (sphere, cylinder, multifocal, and the development of high-order aberration order aberration corrections), we are seeing a trend toward greater use of scleral contact lenses on a more diverse patient population.
However, do we know enough yet about the long-term implications of scleral lens wear on ocular physiology? Have the key questions been answered? Should we perhaps take our foot off of the accelerator a bit and do our due diligence in finding out how best to design and fit these wonderful tools? Only through properly designed research and the evidence-based outcomes of that research can we appropriately move forward and provide our patients with optimized vision and eye health. So, what is research telling us today about scleral lenses, and where are we going with future investigations?
Oxygen Transmission and Scleral Lenses
As we know, oxygen transmission through a contact lens is dependent on lens thickness and the oxygen permeability of the material from which the lens is fabricated. Contact lens oxygen transmission is represented as a Dk/t value (oxygen permeability [Dk] of the material in consideration of the center thickness [t] of the lens). To maintain dimensional stability, current scleral lenses are fabricated with significantly thick profiles. Typical center thickness values will range from 250 to 450 microns, and edge thickness can be upwards of 450 to 600 microns; as such, this can dramatically limit oxygen transmission even with the utilization of hyper-Dk GP lens materials.
With scleral lenses, however, another factor comes into play. The amount of vault over the cornea and the resulting thickness of the post-lens tear film can further limit oxygen availability to the cornea.
So, with scleral lenses, Dk/t simply does not tell the whole story. We need to look at oxygen availability at the corneal surface. Compañ et al (2014) investigated the corneal oxygen availability during wear of scleral lenses of various material Dk values, lens thicknesses, and post-lens tear thicknesses. Additionally, the researchers clinically measured corneal swelling response to scleral lenses fit with variable post-lens tear thicknesses. They simulated the partial pressure of oxygen across the cornea behind scleral lenses made in different lens materials (Dk values ranging from 75 to 200) and different lens thicknesses (100µm to 300µm). Post-lens tear film thicknesses ranged from 150µm to 350µm. Eight healthy subjects were fit randomly with a scleral lens with a thin and a thick post-lens tear layer in two different sessions for a period of three hours under open-eye conditions. The lenses that had a Dk of less than 125 and a center thickness of more than 200µm depleted the oxygen availability at the lens-cornea interface below 55 mmHg when the post-lens tear film thickness was 150µm. For a post-lens tear film thickness of 350µm, no combination of material or lens thickness will meet the criteria of 55 mmHg. Their clinical measures of corneal edema showed that edema was significantly higher (P < 0.001) with the thicker compared to the thinner post-lens tear layer. The authors concluded that scleral lenses must be manufactured in a material of at least 125 Dk and with a limit of up to 200µm thick to avoid hypoxic effects even under open-eye conditions. The post-lens tear film layer should be below 150µm to avoid clinically significant edema.
Jaynes et al (2015) utilized an existing model based on simultaneous two-lens systems (piggyback lenses) as a resistance to oxygen in series and applied this model to scleral GP lens systems. They found that only the best-case scenario for current scleral GP lenses (thickness and Dk)/tear layer values allow sufficient tear layer oxygen tension (approximately 100 mmHg) to preclude corneal hypoxia. They suggested that clinicians would be prudent to prescribe scleral GP lenses manufactured in the highest-Dk lens materials available and to fit without excessive corneal clearance to minimize anterior segment hypoxia.
Michaud et al (2012) found that excessive vault will lead to decreased oxygen available to the cornea. A thick post-lens tear reservoir acts as a resistor to oxygen flow. They recommended that to minimize corneal edema, practitioners should design scleral lenses using the highest-Dk material (Boston XO2 [Bausch + Lomb], Dk of 150), minimal lens thickness (250µm), and to fit with a post-lens tear reservoir that is no more than 200µm.
Giasson et al (2015) found that large-diameter scleral lenses with a 400μm fluid layer thickness delivered 30% less oxygen to the cornea than did a similarly sized lens fit with a 200μm fluid layer thickness.
Tear Exchange with Scleral Lenses
Scleral lenses are fit with little to no observable movement with the blink. The amount of tear exchange that occurs under a scleral lens is quite minimal. Clinically, we can confirm this by observing the highly limited migration of sodium fluorescein under a lens when instilled on the ocular surface.
Research supports this clinical observation. Vance et al presented a poster at the 2015 American Academy of Optometry annual meeting indicating that tear exchange is limited at a rate of 0.2% per minute once the lens settles on the eye. Based on this rate, it would take more than eight hours to replenish the fluid under a scleral lens. Tear exchange is considered important in contact lens wear to reduce post-lens debris. Metabolic byproducts accumulate and stagnate between the lens and cornea if there is no significant tear exchange. This compromised post-lens tear reservoir has been called a “toxic swamp” or a “contact lens-induced cesspool” (as I have personally described it). It is thought that the accumulation of these metabolic byproducts can contribute to the onset of adverse events primarily by altering the epithelial barrier function (Muntz et al, 2015). Without good tear exchange, some have stated that there may be an increased risk of microbial keratitis, contact lens acute red eye (CLARE) response, and contact lens-induced corneal edema (Sonsino, 2015).
In addition to questions regarding oxygen availability and tear exchange in scleral contact lens wear, other concerns that still require further investigation have been described, including long-term influence of compression of the tear film on corneal metabolism and intraocular pressure, influence on corneal limbal stem cells, conjunctival prolapse, epithelial bogging, and potential negative impact on conjunctival goblet cells. Additionally, great concern has been expressed regarding the use of scleral lenses on corneas that have significantly compromised endothelium, such as in many patients who have undergone penetrating keratoplasty.
Eyecare practitioners have quickly become aware of the marvelous advantages that scleral contact lenses can provide to our patients. The natural tendency when introduced to such revolutionary technology is to charge forward with great gusto. However, proper consideration of potential complications and limitations should always be given. Properly designed research will provide the evidence for us to make the most appropriate clinical decisions for our patients. CLS
Acknowledgment: I would like to acknowledge the contribution of Ken Maller, OD, to the content and review of this article.
For references, please visit www.clspectrum.com/references and click on document #251.
Dr. Eiden is president and medical director of North Suburban Vision Consultants, president and founder of the International Keratoconus Academy of Eye Care Professionals, and co-founder of EyeVis Eye and Vision Research Institute. He is an adjunct faculty member at The University of Illinois Medical Center as well as at the Indiana and Illinois Colleges of Optometry and Pennsylvania College of Optometry at Salus University.