Prescribing for Astigmatism
Toric Scleral Lens Options
BY SARA BIERWERTH, OD, & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Scleral lenses have increased our success in managing patients who have irregular corneas. GP lens laboratories can now customize scleral lens fits. In this column, we will discuss options for scleral lens patients who would benefit from a cylinder correction to treat residual astigmatism (RA).
Front-Surface Toric Designs
Historically, there are three different methods to achieve stability for front-surface toric corneal GPs: prism ballasting, lens truncation, and double slab-off (or thin zones).
Scleral lenses are significantly larger in overall diameter compared to corneal GPs, so there are certain factors to consider when contemplating toric designs. Because scleral lenses position completely behind the eyelids, a truncated design is not indicated. To stabilize lens rotation, laboratories utilize either prism ballasting or double slab-off designs. It is important to reflect on each patient’s case to determine which stabilization method to pursue.
RA can result from lens flexure. To determine whether flexure is present, perform over-keratometry or over-topography. If toricity is present, consider increasing lens center thickness to minimize lens flexure and reduce unwanted cylinder.
If flexure is not the reason for RA, and the amount of RA results in reduced vision, consider prescribing either a spectacle over-correction or a front-surface toric scleral lens.
Essilor uses a prism-ballasted design to attain lens stability with front-surface toric scleral lenses. A small amount of prism, such as 0.75D or 1.00D, is typically employed to keep the lens from decentering inferiorly. Dennis Neifert, a laboratory consultant at Essilor, shared with us that patients who have axis 90-degree or 180-degree cylinder are generally more successful compared to patients who have oblique cylinder corrections. Not surprisingly, the higher the amount of RA, the more critical the location and stability of the axis become.
Eric Marshall, vice president of operations at Visionary Optics, also recommends prescribing front-surface toric lenses for more visually critical patients. Visionary Optics utilizes a double slab-off design for rotational stabilization of the company’s front-surface toric scleral lenses.
What About Bitoric Sclerals?
It’s important to mention that a scleral bitoric design is rarely indicated. This is because sclerals are fit to completely vault the cornea; therefore, the toricity of the back surface does not “lock into” or align with a patient’s corneal toricity. However, a toric peripheral curve design could be used to stabilize lens rotation if the sclera is highly toric.
While a patient’s vision will often be improved by incorporating a front-surface toric design, at times it may be better to recommend a pair of glasses for wear over the contact lenses. For presbyopic patients already wearing reading or multifocal glasses, you can add the RA correction to the spectacle prescription. Occasionally, the addition of prism to stabilize the rotation of a toric scleral lens can affect the fit of the lens on the eye. “Lens drop” sometimes occurs in spherical scleral lenses and can be worsened by the addition of prism to the lens. You may observe thinning of the tear lens superiorly and occasional resultant superficial punctate keratitis (SPK). Also, with high prescriptions, it may be advantageous to place any RA in an overlay pair of glasses as the lenses are already “heavy” due to lens mass.
While most of your scleral contact lens patients will be ecstatic with a spherical lens design, a portion will benefit from the enhanced vision that the addition a front-surface toric correction can provide. CLS
Dr. Bierwerth received her optometry degree from the Pennsylvania College of Optometry at Salus University. She is currently a cornea and contact lens resident at the Southern California College of Optometry. Dr. Edrington is a professor at the Southern California College of Optometry. You can reach him at firstname.lastname@example.org.