THE SCLERAL LENS VAULT
MINIMIZING CORNEAL HYPOXIA AND EDEMA
GREGORY W. DENAEYER, OD, FAAO
As scleral contact lenses continue to become more mainstream, we see them used not only to manage irregular corneas and ocular surface disease, but also for patients who have high refractive errors and have been unsuccessful with more traditional lenses. Although scleral lenses are highly successful without complications for most patients, we need to be mindful that they are medical devices that can affect normal ocular physiology.
One concern of scleral lens wear is oxygen transmissibility. Scleral lenses are generally fit with hyper-Dk materials (100 or greater), but they are relatively large in diameter, allowing little to no tear exchange. In addition, they are manufactured with a relatively thick center thickness to avoid flexure. Finally, the trapped reservoir is thought to act as a barrier to oxygen. These factors can significantly decrease surface oxygen levels to a point that results in corneal edema.
Three recent studies have been published that have used theoretical models to predict thresholds of the scleral lens parameters and fitting characteristics that would result in corneal hypoxia (Michaud et al, 2012; Compañ et al, 2014; Jaynes et al, 2015).
Michaud’s findings conclude that to avoid corneal edema, scleral lenses should be manufactured in the highest-Dk materials available, with a maximum center thickness of 250 microns, and they should be fit with a maximum central clearance of 200 microns. Although most practitioners are using hyper-Dk materials, many fits probably don’t meet these recommended values.
Clinical studies have demonstrated corneal swelling with scleral lens wear. Pullum and Stapleton (1997) showed early on that scleral lenses made with GP materials resulted in mild corneal edema (Pullum and Stapleton, 1997). Compañ et al (2014) measured corneal swelling of 1.59% and 3.86% after wear of 15.5mm scleral lenses (paflufocon, Dk of 100) fit with vaults of 150 microns and 350 microns, respectively. In addition, Schipper and Lotoczky (2016) concluded that corneal edema increases with increasing corneal vault.
Vincent et al (2016) found that corneal edema was less than 2% after eight hours of scleral lens wear (16.5mm lenses, Dk 100, central thickness 300 microns). They concluded that mini-scleral lens use does not induce clinically significant edema during short-term wear. Additionally, contrary to previous studies, they did not find a relationship between reservoir thickness and magnitude of corneal swelling.
To minimize hypoxia and corneal edema, use hyper-Dk materials and minimize vault and center thickness. The fact that corneal vault is reduced as a lens settles may mean that under certain fitting conditions, hypoxia may be transient during lens wear (Michaud, 2016). Center thickness can be specified with your lens order; however, there can be constraints depending on the power of the lens.
Additionally, eyes that have reduced endothelial function, such as patients who have a corneal graft, will be at greater risk for corneal edema when fit with scleral lenses despite designing them to maximize oxygen transmissibility. For these cases, a corneal GP lens might be the best option. CLS
For references, please visit www.clspectrum.com/references and click on document #250.
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio. He is a shareholder of Precision Ocular Metrology LLC, has proprietary interest in Visionary Optics’ Europa and Elara Scleral Lenses, and is a consultant to Visionary Optics and Alcon. You can contact him at firstname.lastname@example.org.