GP lenses are always a great option for providing crisp vision, especially in patients who are unable to reach their full visual potential with spectacles. GPs are extremely customizable, which is useful when fitting the unique needs of patients’ eyes. Front-surface-toric (FST) GPs are often indicated to improve visual acuity when residual astigmatism remains with spherical GP optics.
What Are Front-Surface-Toric GP Lenses?
Unlike a spherical GP, in which both surfaces are spherical, an FST has a toric curve on the front surface and a spherical curve on the back surface. This means that the lens will have astigmatic power provided by the front surface in addition to any cylinder provided by the lacrimal lens.
Corneal FSTs are used for eyes that have a mild amount of corneal toricity, typically < 2.00DC (so the spherical base curve will fit the cornea), and significant residual astigmatism on over-refraction with a spherical GP. This happens when there is about 0.75DC or more difference between the corneal and refractive astigmatism. The toricity on the front neutralizes the residual astigmatism. Front-toric optics are also used with scleral lenses to correct residual astigmatism. Additionally, they will work with any amount of corneal astigmatism because the lens doesn’t rest on the cornea.
Optically, an FST is similar to a hydrogel toric in that rotation of the lens causes axis misalignment, induced crossed-cylinder, and decreased acuity. Because the base curve is spherical, the lens can rotate freely; therefore, a method to prevent rotation is necessary.
In corneal designs, the most common method of stabilization is prism ballasting, with base-down prism oriented at the bottom of the lens. In a scleral lens, prism ballasting is done when the scleral landing zone is spherical, but this is not necessary with a toric landing zone, as the scleral toricity prevents excessive lens rotation.
To order an FST, first fit patients with a spherical lens. Once a best-fitting spherical lens is determined, perform a spherocylindrical over-refraction (SCOR) and add this to the diagnostic lens power. For example, if the lens power is –3.00DS and the over-refraction is –1.00 –1.00 x 180, order –4.00 –1.00 x 180. For a scleral lens that has toric landing zones, note the rotation of the diagnostic lens and apply LARS (left add, right subtract) as with a soft toric.
A disadvantage of corneal FSTs is that their small diameters provide less rotational stability, even with the prism ballasting. The prism ballasting also makes lid attachment less likely (Figure 1). Increasing the diameter or increasing the prism amount can decrease rotation. For both corneal and scleral lenses, once you have your ordered lens, note the rotation and SCOR values and use a cross-cylinder calculator. Or, you can talk with your lab consultant to modify the lens power and axis as needed.
Patients count on their practitioners to provide them with the crispest vision possible. FSTs are versatile tools that practitioners can use to improve vision and fit success when indicated. CLS