Contact Lens Design & Materials
Optimal Features of a Therapeutic Scleral Lens
BY NEIL PENCE, OD, FAAO
Each year, we contemplate what might be left on our “wish list.” This month’s column will discuss the optimal design and material characteristics of a scleral contact lens for some of the most challenging therapeutic application cases.
Scleral lenses benefit many patients who have a wide range of conditions. One particularly difficult group consists of those who need a therapeutic lens to protect the cornea in cases of severe dehydration or extremely challenged corneal healing. In some acute cases in which a persistent epithelial defect will not heal, the protection and comfort provided by a scleral lens are critical.
Often, a soft bandage contact lens has failed. In addition, either due to handling difficulties or to not disrupt the cornea, it is beneficial to leave the scleral lens in place continuously for a number of days. What characteristics would the ideal scleral lens have in these cases?
Overnight use of scleral contact lenses is an off-label application and has been described in several case reports (Jacobs and Rosenthal, 2007; Tappin et al, 2001); it is used out of necessity in severe cases. While removal at night is preferred, leaving the lens in place for non-healing defects, neurotrophic disease, graft-verus-host disease, etc., may be best. Usually, the only other options may be tarsorrhaphy or a corneal transplant.
Focus on Oxygen
After the obvious goals of protecting the cornea from lid interaction, keeping it hydrated, and improving comfort, reducing corneal hypoxia is paramount. In these generally sick or compromised corneas, they may tolerate hypoxia-induced edema even less than normal. Therefore, supplying as much oxygen transmission as possible is key.
A number of factors affect the amount of oxygen that the cornea will receive. The most obvious is the lens material. Menicon Z, at a hyper-Dk of 163 (buttons available for up to 16mm sclerals), and Boston XO2, at a hyper-Dk of 141 (large size scleral buttons available), currently offer the highest oxygen permeability.
The second factor is lens thickness. If thickness is cut in half, twice as much oxygen will be transmitted. Most scleral lenses are made with a center thickness of 300 microns or greater, mainly to avoid flexure. Because visual acuity is not the goal of a therapeutic lens, flexure may not be a concern. Therefore, the thinnest center thickness possible should be employed.
Finally, the oxygen supplied to the cornea is also affected by the thickness of the tear lens (Michaud et al, 2012; Jaynes et al, 2014). A minimum central clearance would best minimize corneal hypoxia. Depending on the lens size and the amount of settling assumed, an initial 150- to 200-micron clearance might be attempted. A reverse curve design would likely assure adequate clearance at the limbus while allowing minimal central clearance.
Using a GP material with the highest oxygen transmission possible, minimizing center thickness, and employing minimal central clearance will result in an optimal therapeutic scleral contact lens. In these cases, patients generally have normal corneal curvatures, so a set with a relatively small number of base curves might suffice. Hopefully, lens designers far more expert than me will develop a small set that specialty fitters could have on hand for these extremely challenging cases that would benefit from at least a short-term overnight use of a scleral lens. CLS
For references, please visit www.clspectrum.com/references and click on document #231.
Dr. Pence serves as associate dean, Clinical and Patient Services, Indiana University School of Optometry in Bloomington, Ind. He is a consultant or advisor to Alcon and Vistakon and has received travel funding from B+L. You can reach him at email@example.com.