GP Toric Technology and Applications
BY EDWARD S. BENNETT, OD, MSED, & CLARKE D. NEWMAN, OD, FAAO
One of the most challenging patients to a dedicated contact lens practitioner is the highly astigmatic patient who enters your office having failed — due to poor vision — with numerous soft toric lenses fit at another office. This is an excellent opportunity to successfully fit these patients with toric GP lenses. Their ease of fit and high rate of success are enhanced by improvements in manufacturing technology, resulting in thinner, more reproducible and higher-optical-quality lenses than their predecessors.
When to Use a Toric Design
Although often fit to corneas exhibiting ≥2.50D of corneal cylinder, several factors influence selection of a toric lens. If topography shows that the corneal astigmatism is mostly central in location, a spherical design may suffice. This design should also be acceptable if the fluorescein pattern does not exhibit a dumbbell-shaped astigmatism pattern with a spherical diagnostic lens.
Contemporary GP Toric Designs
Back-surface toric lenses — in which the back surface uses toric curves to align with a highly toric cornea — induce cylinder equal to anywhere from 40-to-50 percent of the back toricity of the lens, although it is slightly less with today' lower-refractive-index, high-Dk materials.
The lens design of choice is often a bitoric design in which the back surface has toric curves and the front surface corrects for the induced cylinder. These lenses should result in good vision and a fitting relationship similar to a spherical lens on a very low astigmatic cornea. A spherical power effect (SPE) lens is a bitoric lens that corrects only for the induced cylinder and can rotate on the eye without affecting vision. This concept was popularized with the Polycon 2D, 3D and 4D diagnostic sets (CIBA Vision), which have powers of Pl/–2.00D, Pl/–3.00D and Pl/–4.00D respectively. These lenses are fit slightly flatter than K; if you select a 42/45 Pl/–3.00D SPE lens for a patient who has keratometry values of 42.25/45.25 and obtain an over-refraction of –3.00DS, simply add this value to the diagnostic lens powers to obtain your final powers of –3.00D/–6.00D.
A cylinder power effect (CPE) lens corrects the residual astigmatism on the front surface in cases where this can impact vision. In the aforementioned case, if an over-refraction of –2.00 –1.00 ×180 results in optimum vision, you can simply add the over-refraction in the horizontal meridian (–2.00D) to the diagnostic lens power in that meridian (plano) and the over-refraction in the vertical meridian (–3.00D) to the diagnostic lens power in that meridian (–3.00D).
Peripheral Curves With CNC technologies, you can specify — and receive — exact toric peripheral curves.
Lenticulars The vertical meridian (for with-the-rule astigmatism) may have a high minus power and often needs a plus lenticular. Dr. Bennett utilizes a plus lenticular on all powers ≥–5.00D; Dr. Newman lenticulates every GP lens not between plano and –2.00D to help control lens mass and edge interactions.
Additional Bitoric Resources
The GP Lens Institute (GPLI) has a number of resources including a GP Lens Management Guide. This includes a series of toric cases in the "GP Lens Case Grand Rounds Troubleshooting Guide" and a Mandell-Moore design calculator, with other resources forthcoming. CLS
Information for this column was obtained from a GPLI online symposium held in July 2008.
Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and is executive director of the GP Lens Institute. Dr. Newman has been in private practice in Dallas, Texas since 1986 specializing in vision rehabilitation through contact lenses as well as corneal disease management, optometric medicine and refractive surgery.