Contact Lens Practice Pearls
When Bigger Is Not Better: Sizing Down Scleral GP Lenses
BY JESSICA H. MATHEW, OD, PHD, FAAO
With the increasing popularity of large-diameter (scleral) GP contact lenses, practitioners are having to learn how to make these lenses practical for patients. The theory is that bigger is better, at least in Texas, but just how big are we talking about?
In most instances, scleral lenses are reserved for patients who have some type of compromised corneal condition. In such cases, a larger lens is preferred to ensure that it vaults over the diseased or damaged corneal region so as to not induce further damage. The larger the lens, the more control you have over this because you can spread the weight of the lens over more of the sclera rather than the cornea.
While corneal diameters are mostly consistent across patients, palpebral fissure size and conjunctival fornix depth may not be. For these reasons, we often need to tailor the size of the lens to individual patient eyes.
When Your Diagnostic Lens Is Bigger Than What You Need
Although large-diameter GP lenses are available in a variety of sizes, it is unlikely that one practitioner would have multiple fitting sets covering all of the different diameters offered. Because it is difficult to empirically fit large scleral GP lenses, it would make sense to have a larger-diameter set (for example,18.2mm), if you had to choose.
You would know right away whether this size fits or not. But don’t be afraid to size down the lens just because your set is 18mm. Shaving off a millimeter here or there can provide better comfort, easier application, and overall more convenience to your patients. Additionally, I would venture to say that, more often than not, a slightly smaller lens tends to fit better. Although you get the desired “larger footprint” with scleral GP lenses, you also run into the issue that the sclera is often toric, which can make the peripheral fit of the lens challenging. Making just a 1mm change in diameter can often get you further away from this toric zone and alleviate areas of extreme peripheral blanching.
Larger changes (i.e., >2.5mm) to the diameter of the lens can also be made, although the outcome may be less predictable. Three things to take into account when doing this are:
1) As you decrease the diameter, you need to steepen the base curve. This is because less of the lens will be sitting on the sclera, making the landing zone smaller and causing the lens to vault less over the cornea. By increasing the base curve, you give more vault back to the lens.
2) You will also need to decrease the corneal chamber diameter size to allow room for the landing zone.
3) Standard peripheral curves should not need to be adjusted, but can always be tweaked later.
Because each company has its own proprietary designs, it is not possible to fully understand all of the dynamics and consequences of making these types of changes. Don’t hesitate to call the company consultants and discuss these on a case-by-case basis. Consultants are more than willing to help and want to have this interaction with practitioners. CLS
Dr. Mathew is a research assistant professor at the University of Houston College of Optometry. She manages patients who have severe corneal distortions and require specialty contact lenses, and she is also involved in basic science and clinical trial research with The Ocular Surface Institute. She has received research funding from Allergan, CooperVision, Clearlab, Essilor, Shire, TearLab, Menicon, and Vistakon. You can reach her at firstname.lastname@example.org.