SCLERAL LENSES AND RESIDUAL ASTIGMATISM
JOHN MARK JACKSON, OD, MS
Residual astigmatism can be a challenge with scleral lenses. Fortunately, there are several options for dealing with this.
Check for Flexure
First, check for lens flexure. GP lenses can flex on-eye, inducing unexpected cylinder. You can check for flexure by performing keratometry or topography over the lens. The reading should be spherical; if it’s somewhat toric, then the lens is flexing.
You can fix flexure by having the lab increase the lens thickness. Because this will also decrease the Dk/t, exercise some caution when trying this. You could also try a toric scleral zone. For larger lenses, a spherical scleral zone on a typically toric sclera can also cause flexure. I use toric scleral zones on most scleral lenses now to improve the lens alignment and to prevent meridional blanching, and flexure has been less of an issue.
What if there is residual astigmatism but no flexure? The easiest approach is to prescribe the residual cylinder in spectacles, if patients agree to this approach. If they need near correction anyway, you could make the spectacles in a multifocal.
If a patient objects to glasses, then you can try a lens with toric optics instead. This approach is trickier, but it can work well. Scleral lenses with toric optics will need some way to prevent lens rotation. There are two ways stabilize front-surface toric (FST) optics: with prism ballasting or with toric scleral zones.
Prism Ballasting For spherical scleral zones, prism ballasting is used to minimize rotation, similar to a soft toric lens. To determine the lens power, over-refract the diagnostic lens and add the toric over-refraction and axis directly to the diagnostic power. For example, if the lens power is –2.00D and the over-refraction is –1.00 –1.50 x 050, you would order –3.00 –1.50 x 050 with FST. Once you have the ordered lens on the eye, axis adjustments can be made for any rotation using the normal LARS (left add, right subtract) technique. Note that you adjust the axis based on the original diagnostic over-refraction rather than on the manifest refraction.
Toric Scleral Zones I tend to get better results with the toric scleral zone option. Here, the lens is prevented from rotating by the way the scleral zone “hugs” the sclera. Their stability is pretty remarkable. Figure 1 shows a diagnostic lens with toric scleral zones. The engraved ovals on the lens are along the flat scleral zone meridian, which usually aligns along the horizontal meridian of the eye. Here the lens has rotated about 20° to the left and is very stable in that position. Apply LARS to the over-refraction axis (we would add 20°) and order the lens. An advantage of having a diagnostic set with toric scleral zones is that you can adjust for the rotation in the first order, improving your chance of success with the first lens.
Figure 1. Left rotation on a lens with toric scleral zones.
Help When You Need It
As always, your GP lab consultants can help you decide which type of stabilization would work best and advise you on changes to make, if needed. CLS
Dr. Jackson is a professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at firstname.lastname@example.org.