Scleral Lens Fitting: Selecting and Evaluating Your Initial Lens11:31 AM
Since the mid 2000s, no other type of contact lens has seen more growth and excitement than scleral lenses. In last year's annual contact lens trend report from Contact Lens Spectrum, doctors reported fitting scleral contact lenses more than both rigid gas permeable toric and multifocal lenses, with sclerals making up 3% of all contact lenses being fit (including soft, hybrids, and RGPs) in 2015. Why sclerals? The larger contact lens size makes for more stable vision and improved comfort over traditional small diameter RGPs, ideal for irregular corneas, high astigmatism, and conditions like keratoconus. Breaking into the world of fitting scleral contact lenses can have a sharp learning curve, so I strongly recommend attending courses like those offered from the Scleral Lens Education Society, Blanchard, and Vision By Design. Once you feel ready to get started, the fitting set arrives and it's time to put lenses on your patient. What to do? We're talking initial lens selection and evaluation in today's post!
Choosing the Right Diameter
There are a ton of scleral contact lens options out there, with common diameters ranging from as small as 14.3 mm to over 18 mm. The truth is, lens diameter selection depends on your patient. Smaller lenses are typically easier to fit, especially on corneas with lower amounts of irregularity. Advantages of smaller diameter lenses include:
- better oxygen permeability (smaller lenses can be fit with less central vault, meaning better oxygen exchange through the tear film)
- easier patient handling
- easier removal (smaller lenses will fit flatter in general so will have less suction with removal)
- avoiding scleral toricity --- The white of the eye (sclera) tends to have more of it's own innate astigmatism as you venture more peripherally from the limbus. The larger the scleral lens you are fitting, the more likely you'll have to use toric peripheral curves to achieve a good fit.
- minimize fogging -- larger lenses tend to have more issues with lens fogging, theoretically because they are interacting more with the conjunctival goblet cells that produce mucin
|Proper lens diameter should clear the limbus by 2-3mm on either side. The lens above shows inferior fenestration which isn't used by many doctors these days -- at one point fenestrations were popular to improve oxygen transmission. These days, doctors change their fit to improve oxygen transmission by minimizing their central and limbal vault (see below). via|
|How do you assess central clearance? Fill the lens with nonpreserved saline and a swirl of NaFl dye. You can judge the central clearance of the lens by comparing the tear film thickness (green above due to the NaFl dye) to the corneal thickness on your slit beam. Knowing the average corneal thickness is about 550 microns, you have a predictable judge of tear clearance. In the picture above, the tear clearance is almost as thick as the corneal clearance, so I would grade this around 450- 500 microns of clearance. This is TOO MUCH!! Photo via|
|Of course, if you have an OCT you can image the exact amount of corneal clearance with an anterior segment scan. via|
- For example: your patient has a flat K of 44 diopters. Your first lens to try on should be around a 7.67 mm base curve (select the closest thing in your fitting set). You can find a diopter to base curve conversion chart here.
- Need more or less central clearance? As a rule of thumb, changing base curve by 0.1 mm will give you about 35 microns of vault. Steepening the lens gives more vault; flattening the lens gives less.
|Want the perfect limbal clearance? You should just see a faint hint of NaFL dye right at the limbus with the perfect fit of 30-40 microns. Of course, you could also use an OCT to assess this area if you have access! via|
- Need more limbal clearance? Increase the lens diameter will achieve more limbal clearance without changing your central vault
- Too much limbal clearance? You can decrease lens diameter, but likely if you have too much limbal clearance you have too much central clearance as well and you can solve the issue just by flattening your central base curve
|If the edge of the lens is too tight, you'll see blanching of the conjunctival vessels in as little as half an hour. This will cause severe discomfort and binding of the lens when the patient attempts removal. Before sending the lens home with a patient, make sure you check the edge carefully to ensure that the conjunctival vessels are passing underneath the edge of the lens freely, without any pinching or thinning. via|
- "My eyes are red after a few hours" Likely this is a tight edge lens. Tell the lab to flatten the edge
- "I can feel the edges" Likely this is a flat edge, causing some fluting of the lens as it lands on the conjunctiva. Tell the lab to steepen the edge.