contact lens primer
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO & JOSEPH T. BARR, OD, MS, FAAO
November's column discussed the indications for prescribing and benefits of spherical versus toric RGP contact lenses. The bitoric design is the most common toric design. They may be ordered empirically or by trial lens fitting. An advantage of ordering empirically is that the patient's first experience is with a lens that contours the cornea and provides usable vision. On the other hand, trial fitting allows the practitioner to fine-tune fitting and refractive data prior to ordering the initial pair of lenses.
The Mandell-Moore guide is a practitioner-friendly, efficient method of determining a bitoric RGP lens design. The form is available on the RGP Lens Institute web site at www.rgpli.org. In most cases, you will be able to dispense the empirically-designed lens to the patient. If not, the fluorescein pattern and position of the empirical lens will assist you in fine-tuning the fit and a sphero-cylinder over-refraction will allow you to make the vision more precise.
Trial Lens Fitting
Consider a bitoric design when the corneal toricity is 1.50D or more. A bitoric lens is indicated if a spherical lens does not position as desired or if there is an unacceptable amount of residual cylinder in the over-refraction. Even though a lid-attached spherical lens might achieve good comfort and vision, we recommend prescribing a bitoric design when there is 3.00D or more of corneal toricity to minimize any adverse effect on corneal topography.
To select the initial diagnostic base curves, use the Mandell-Moore guide or incorporate your own fitting philosophy. In order to optimize horizontal centration, prescribe the base curve toricity to equal the corneal toricity for against-the-rule corneas. To optimize tear exchange, prescribe approximately 0.50D less base curve toricity than corneal toricity for with-the-rule corneas. For example, if the patient has 3.00D of corneal toricity, you would prescribe 3.00D of base curve toricity if the corneal toricity is against-the-rule and 2.50D of base curve toricity if it is with-the-rule.
Once the trial lens is on the cornea, evaluate the fluorescein pattern and perform a sphero-cylinder over-refraction.
Figure 1. Optical crosses of
the bitoric example.
For example, apply a 45.00/41.00, 6.00D/ 2.00D bitoric diagnostic lens to your patient's with-the-rule cornea and obtain a +1.00 1.25 x 180 over-refraction. Place these values on optical crosses to obtain resultant powers for each meridian (Figure 1).
Assuming the fluorescein pattern reveals an optimal lens-to-cornea fitting relationship, the resulting prescription for base curves and powers would be: 45.00/41.00, 6.25D/1.00D. An axis is not specified, as the flat meridian of the lens should align with the flat axis of the central cornea.
If the base curve(s) need to be adjusted, including modifying the amount of toricity, change the power in each meridian by the amount you change the base curve in the same meridian (SAM, FAP).
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at firstname.lastname@example.org.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean of Clinical Affairs at The Ohio State University College of Optometry.