Scleral Lens Primer: Part 1
By Edward S. Bennett, OD, MSEd, FAAO, & Christopher Gilmartin, OD
The new generation of scleral lenses is increasingly popular. This first part of a two-part series from Christopher Gilmartin, OD, will focus on scleral lens fitting principles.
1. Lens Vault It is important for the lens to completely vault the cornea while aligning with the bulbar conjunctiva.
2. Overall Diameter Selection In general, larger lenses can hold more fluid in the corneal chamber and tend to be more forgiving, allowing for more clearance over the cornea. For highly irregular corneas, choose a larger mini-scleral design if possible.
3. Initial Trial Lens Selection Follow the manufacturer's fitting guide or try a more simple approach: standing beside the patient, look at the corneal profile. If it is very steep, choose a steeper base curve. If flat, choose a flatter base curve. If average, choose an average base curve. These lenses are fit based on sagittal height, so this method can be very effective when used properly.
4. Examine the Central Clearance With an optic section at high illumination and medium magnification, set the slit lamp housing off axis and sweep from limbus to limbus. You'll see several layers in cross section. The outermost (dark black) band is the lens. The dark area is straddled by two hairline reflections that arise from the front and back surface of the lens. Compare this layer to the (green) tear lens.
Apply diagnostic lenses until you achieve an acceptable central clearance value. Your fitting set will provide acceptable ranges, which vary among designs.
Note that the lens will settle onto the conjunctiva over a 30- to 40-minute period, with full settling after several hours. This will decrease the corneal vault and possibly result in touch in an area that was vaulted upon initial lens application.
A diagnostic lens that shows grossly excessive vaulting of the central cornea initially should be removed and replaced with a flatter base curve. However, if the corneal vault is only mildly excessive upon initial trial lens application, allow the lens to settle because it may yield an acceptable corneal vault after settling.
5. Corneal and Peripheral Fitting Relationships Examine the entire corneal chamber (central fit) under diffuse cobalt blue and high illumination. Note any areas of bearing. If acceptable central clearance is present with a standard geometry trial lens but bearing is observed in the midperipheral or peripheral cornea, you can employ a reverse geometry design to vault over the areas of touch/bearing. Compensatory flattening of the base curve may be required to avoid excessive central clearance. Alternatively, you can use a reverse geometry fitting set to avoid this step. The peripheral or scleral portion of the lens should align with the bulbar conjunctiva. Blanching of the conjunctival blood vessels indicates the scleral curve(s) may be too tight; therefore, flatten the peripheral curve associated with the area of blanching. Excessive movement and/or bubble formation after lens application may indicate that the peripheral curve(s) needs to be steeper.
6. Check for Tear Exchange Proper tear exchange must be demonstrated before dispensing. Apply the lens without fluorescein in the filling media. After the lens has been properly applied, instill a generous amount of fluorescein dye over the top of the lens with a dye strip. Periodically examine the tear lens and check for dye that has made its way behind the lens into the tear chamber. After several minutes, there should be at least a small amount of dye in the tear lens.
Tear exchange does not need to be rapid, but it is critical for a proper fit. If you test for tear exchange and see no fluorescein in the corneal chamber after waiting for several minutes, you may need to flatten the peripheral fit or increase the overall diameter. CLS
Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and is executive director of the GP Lens Institute. You can reach him at firstname.lastname@example.org. Dr. Gilmartin is a graduate of Southern California College of Optometry and is a clinical resident in the Cornea and Contact Lens service at the University of Missouri-St. Louis College of Optometry. Contact him at email@example.com.