Performance of a Crescent Design RGP Bifocal

BY BRUCE W. ANDERSON, O.D., F.A.A.O.
SEPT. 1997

This six-month study evaluated the Metro-Seg Bifocal, a translating RGP lens design, to determine its overall success rate and to help define an optimal candidate for multifocal RGPs.Segmented bifocal RGP contact lenses have been available for a number of years, but the Metro-Seg bifocal design incorporates some unique features. We conducted this study to determine an overall success rate for this lens and to define an optimal candidate for bifocal RGP lenses.

LENS DESIGN AND PARAMETERS

The Metro-Seg Bifocal by Metro Optics, Austin, Texas, is a one-piece, crescent-shaped bifocal design made as thin as possible from any FDA-approved RGP material. The center of curvature of the distance and near corrections lies in the optical axis of the lens to facilitate a no-jump transition when the patient focuses from distance to near. The lens is prism ballasted to stabilize the segment orientation. The basic lens design does not include truncation, which is usually not necessary except in high plus corrections or for patients with loose lower lids.

With a crescent design, the junction between the distance and near corrections forms a ledge that helps align the lens during the blink. As the lid descends, the edges of the segment interact with the lid, pushing the lens down into proper position and counteracting rotational effects. Another advantage of a crescent shape is that if the lens rotates, the curve of the segment follows the lower edge of the pupil for more consistent near vision during lens translation in spite of rotation.

The Metro-Seg lens is available in base curves from 6.5mm to 9.0mm; powers from -20.00D to +10.00D; add powers from +1.00D to +3.50D in 0.25D steps; diameters from 8.0mm to 12.0mm; and segment heights from 2.5mm to 6.0mm. The back surface of the lens has a spherical base curve, but there are aspheric secondary and peripheral curves to eliminate junctions. The lens can be made in a base toric, front toric or bitoric curve design for patients with high corneal toricity or residual astigmatism.

STUDY CRITERIA

We prescribed the Metro-Seg Bifocal to 20 patients over a period of about six months (Table 1). We followed them for at least three months after dispensing to be sure that each "success" was truly successful and to enable us to fine-tune any problems. Ideal candidates were current RGP wearers who had a near vision deficit sufficient to warrant this lens and were motivated to adapt to this design. We also accepted some patients who did not meet the first two criteria but were very motivated.

RESULTS

Of the 19 patients who completed the study (one dropped out due to lack of interest), 14 were successful with the Metro-Seg design. Of the five patients who could not wear this design, two were subsequently fitted with an aspheric multifocal RGP lens.

One of the two patients who had never worn contact lenses was successful with the Metro-Seg bifocal. Three of the five current soft lens wearers were successful. Of the 12 current RGP wearers, 10 were successful with the Metro-Seg. The two who were unable to adapt were successful with an aspheric multifocal RGP design. All RGP wearers were successfully refit into a rigid multifocal.

PROBLEM SOLVING AND FITTING TIPS

If a bifocal contact lens candidate wears high-riding RGPs or has a superior lid adhesion fit, a low-riding bifocal such as Metro-Seg will usually be difficult to fit because there's a tendency for new lenses to ride high also. In these situations, more prism or a flatter design may help the lens drop into position. These patients may be more aware of a segmented bifocal contact lens due to increased lower lid interaction when the lens is sitting low, however, the Metro-Seg's non-truncated design minimizes this interaction.

When the lens fits properly, the lower lid pushes it into the reading position as the patient looks down, but if improper translation causes near vision problems, you may need to truncate the lens. In some cases, the lens may slide under the lower lid as the patient looks down to read. This is more likely with hyperopes due to the thinner edge of plus prescriptions. For these patients, order the initial lenses without truncation and have the patient wear them for several days to determine if truncation is needed.

When prescribing a translating bifocal for a computer user, be aware of the proper working distance required for computer work versus normal reading. It may be helpful to decrease a normal reading add by a quarter- to a half-diopter, and you may need to raise the bifocal segment slightly for easier translation into the reading correction so the patient doesn't have to change position. Lowering the monitor also helps maintain good near vision through the bifocal segment.

I recommend using trial lenses because they enable you to assess the lens-to-cornea relationship, measure the seg height accurately and evaluate lens translation. A base curve that's too steep will inhibit proper lens movement and translation into the segment. A base curve that's too flat will be less comfortable and can decenter inferiorly or slide off the cornea.

Measure the segment height only after determining the proper lens size and fit. Position the segment at the lower pupil edge under moderate light conditions when the eyes are in primary gaze. If the patient holds his head in an unusual position such as a chin-up or chin-down, adjust the segment height accordingly. CLS

This study was sponsored by Metro Optics. Dr. Anderson has no proprietary interest in Metro Optics.

Dr. Anderson is a contact lens specialist in private practice in Tampa, Fla. He is director of the Contact Lens and Low Vision Clinic for the Ophthalmology Department at the University of South Florida.