Is Refraction a Predictor of RGP Multifocal Success?

BY DAVID W. HANSEN, O.D.
FEB. 1997

Today, we're seeing more presbyopes who want to see without eyeglasses but who have low ametropia. A low refractive error should not be an absolute predictor of RGP multifocal failure, but merely a risk factor to consider. With enthusiastic patients and advanced contact lens technology, we can now prescribe RGP multifocals for many patients who have never worn contact lenses or who wear soft lenses.

Which RGP multifocal design do we start with? Refraction and corneal measurements will help us decide.

EMMETROPIA

It's extremely important to review corneal curvature for emmetropic patients. If there's little or no corneal toricity, you can usually choose either a simultaneous or a translating lens. Remember, these patients are not accustomed to any blur, so keep visual acuity interference to a minimum. For computer users who need correction mainly for the intermediate range, use a simultaneous lens. When fieldwork or driving is the primary activity, use a translating design with a large distance section, such as the Crescent lens, the FluoroPerm ST lens, or the Tangent Streak lens. When the emmetropic patient has significant corneal astigmatism, I use an aspheric or simultaneous lens initially to try to mask the astigmatism.

MYOPIA

Low to medium myopia can be corrected with either a translating or a simultaneous RGP design. Most simultaneous aspheric designs need accurate centration and are fit from one and a half diopters to six diopters steeper than K. This increases the minus lens power, which creates a thicker edge and can complicate the fitting process with potential lid capture. Therefore, medium myopic patients (3.00D to 6.00D) and high hyperopic patients (6.00D and beyond) may be better candidates for a translating bifocal.

HYPEROPIA

Low to medium prescriptions of hyperopia can usually be fit with either a simultaneous or a translating design, but due to the center mass of this design, use an aspheric or simultaneous lens to reduce overall thickness. Even if a larger diameter is needed for lens centration, it's still a more comfortable design.

ASTIGMATISM

The general rule of thumb for fitting patients with astigmatism is:

1. If the refractive astigmatism and the corneal toricity are similar, you can use either a simultaneous or a translating lens, although it may be best to start with the aspheric design.

2. When there is a difference between the refractive astigmatism and the corneal toricity, use a translating lens to compensate for the residual astigmatism. Translating lenses can be fit with back and/or front toric designs to aid overall visual acuity. If the corneal astigmatism is over 2.50D, you may need bitoric translating lenses to compensate for this irregularity.

SUMMARY

To maximize overall lens fit and design, it's important to review corneal topography maps to determine the relationship of the central cornea to the peripheral area. Keratometric readings usually don't provide the subtle nuances for predicting a patient's resultant visual acuity.

Be positive but practical in your attempt to design a multifocal lens for the presbyopic emmetrope. These patients can be the key for increasing awareness of bifocal contact lens success. CLS

Dr. Hansen, a cornea and contact lens diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.