Contact Lens Design & Materials
Marking Up Lenses to Assess Scleral Lens Fit
BY RONALD K. WATANABE, OD, FAAO
We are all familiar with the markings that laboratories can put on contact lenses. The most common are scribe marks or dots on soft toric contact lenses that allow us to visualize rotational position and stability. Observing the rotational characteristics of a soft toric lens allows us to make adjustments to the cylinder axis or other parameters to improve visual acuity and stability.
Some scleral lens manufacturers have been marking their diagnostic lenses to show where the various zones of the lens begin and end. This is helpful in determining which zones need adjustment to improve lens performance. Without these markings, it can be difficult to know whether each zone is overlying the appropriate area of the ocular surface or whether any of the zones should be changed.
Assessing Scleral Lens Rotation
Lately, one of the biggest trends in GP contact lens fitting is the use of toric landing zones on scleral contact lenses. It appears that many patients have significant scleral toricity, independent of whether or not they have corneal toricity. It also appears that scleral toricity increases as you move further away from the limbus. Therefore, many scleral lens patients would likely benefit from toric landing zones that can better align with the toric sclera, especially if they are fitted with larger-diameter scleral lenses.
Just as with a back-surface toric corneal GP lens, a scleral lens with a toric periphery rotates so that the steeper meridian lines up with the steeper scleral meridian, and the flatter meridian lines up with the flatter scleral meridian. If the sclera has regular toricity, the lens should rotate predictably. However, at times it is difficult to know where the steep and flat toric meridians will end up, especially because most fitters have no way of actually measuring the scleral topography.
To be able to know whether a toric periphery lens aligns properly, the lens must be marked such that the steep and/or flat meridians are identifiable when the lens is on the eye. Toric scleral lens diagnostic sets come with markings that show where the flat and/or steep meridians are. Sometimes the lenses rotate unexpectedly; it is good to be able to see this rotational alignment with a diagnostic lens so that any rotation can be accounted for when ordering lenses for patients.
The final ordered lenses should also come with the same markings so fitters know whether the lenses are rotating to the expected location.
Aligning Front-Surface Optics
Another excellent use of toric periphery scleral lenses is to help align the lens for a front-surface toric power without the need for prism ballasts or thin zones. Commonly, the axis of the front-surface toricity will not coincide with that of the posterior-surface peripheral toricity, so it will be difficult to determine whether the toric correction is properly situated in front of the pupil.
In these cases, the lens can have additional markings similar to those of soft toric lenses to mark the 12 o’clock/6 o’clock positions or the 3 o’clock/9 o’clock positions to assess rotational position. With this combination of markings, a fitter can easily see whether the toric power is where it is supposed to be as well as the location of the flat and/or steep meridians.
A Great Assessment Tool
Lens markings are very useful to contact lens fitters. With scleral lenses becoming more sophisticated, we will likely see more uses for lens markings to allow us to better fit these lenses. CLS
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry’s Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at email@example.com.