prescribing for astigmatism
Revisiting GP Front-Surface Torics
BY JOHN MARK JACKSON, OD, MS, FAAO
For specialty contact lens fits, we often talk about digging in the tool box for a unique lens design. Recently I had to find a design that was stored in mothballs in the attic instead.
This lens is the GP front-surface toric (FST) — toric on the front and spherical on the back. Unlike a bitoric lens, it is for patients with a low amount of corneal cylinder but a high amount of refractive cylinder; a spherical GP would fit well but result in excess residual cylinder. The extra cylinder power needed is built into the front surface of the lens.
Usually an eye like this would be fit with a soft toric lens. In many ways the FST performs like a soft toric: unwanted rotation shifts the cylinder axis and causes blurred vision; rotation is prevented by prism ballasting; and LARS (Left Add, Right Subtract) is used to correct the axis for any observed rotation. However, this is one instance in which soft torics have an advantage over GPs. Keeping a stable axis is much harder with a small, mobile GP than with a large, minimally moving soft toric. The FST usually won't lid attach, either, because of the prism.
My patient had tried soft torics but couldn't get stable visual acuity with them. She also had worn GP lenses and wanted to give them another try.
The right eye was easy. The refractive and corneal cylinder matched and she had no residual cylinder with a GP sphere. However, the left eye had a good fit with the GP sphere but had –1.25D residual cylinder on over-refraction. Thus, the left eye needed an FST for best acuity.
Figure 1. Patient's right eye.
The left eye spectacle Rx was –0.25 –3.25 ×150 and the Ks were 42.00/44.00 @ 70. The best-fitting GP sphere has parameters of 8.0mm BOZR/9.6mm OAD/–3.00D. With this lens the over-refraction was +2.75 –1.25 ×150.
Unlike a soft toric, some refractive cylinder was corrected by the tear film, but not all. I determined the final power by adding the over-refraction to the lens power to get –0.25 –1.25 ×150. We ordered an FST with this base curve, diameter and calculated power with 1.5 prism diopters of ballasting. We also ordered a GP sphere for the right eye.
The right lens had a wonderful lid-attached fit and good acuity, but the left lens dropped too quickly and had about 20 degrees of rotation to the left (Figures 1 and 2). This positioning difference affected comfort and acuity. We reordered the left lens with less prism and power of –0.25 –1.25 ×170 to account for rotation. With the new left lens, acuity was 20/15 and the lens did not drop as much. Comfort was good and the patient adapted well.
Figure 2. Patient's left eye.
A Unique Design
Although rarely used, the FST is a good option for the right patient. It just takes persistence on your part and a little understanding of the mechanics and optics of this unique design.CLS
Dr. Jackson is an assistant professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses and performs clinical research.