prescribing for presbyopia

Choosing the Correct RGP Bifocal Segment

BY DAVID W. HANSEN, OD
March 2000

The America's Cup Yacht race blends science, experience and professional judgement. After the science of designing and building the boat, it's up to the professional judgement of the crew to sail to victory amid the constantly changing conditions. The ability to visualize the end result wins the championship.

Translating RGP bifocals have been touted for their crisp visual acuity, providing that the bifocal segment is positioned properly. When selecting the appropriate translating segment, it's important to review and measure the eyelid structures, pupil size and vertical palpebral fissure.

Pupil Interference

Pupils greater than 5mm with mesopic conditions present a challenge with most designs. Select a bifocal segment positioned inferiorly, rather than a central annular design. You can select any translating design, as long as the patient can move his eyes into the segment for near vision.

Width Considerations

The width of bifocal lenses is not as crucial as it is with spectacles. Segment width is affected when the lens moves excessively or when the eyelids capture the lens edge, thereby causing rotation or misalignment. These movements create subjective visual disturbances; consequently requiring the alteration of the base curve or diameter parameters. When lenses rotate nasally, flatten the base curve 0.50D. When segments rotate temporally, you usually need to steepen the base curve 0.50D.

Segment Heights

Aside from each translating lens' own fitting recommendations, including the suggested segment height positions, there are basic rules for fitting.

1 Annular designs require the distance power segment to align centrally over the pupillary aperture, but the lens needs to provide movement for good near visual acuity. The same is needed for Universal's Decentered DeCarle Design (Fig. 1).

2 The Tangent Streak lens (Fused Kontacts) is fit 0.3mm above the inferior pupillary edge into the pupillary zone.

3 The Solitaire bifocal segment (Tru-Form Optics, Inc.) uses a slab-off design and is set at the lower pupil margin.

4 The Crescent designs, including the Solutions Bifocal (X-Cel Contact Lens, Inc.), are positioned with the top of the segment near or 0.1mm below the lower pupil margin (Fig.2).

5 The Fluoroperm ST design segment (Paragon Vision Sciences) is set 0.4mm to 0.7mm below the inferior pupil border.

FIG. 1: Centration over pupil with the Decentered DeCarle Design.

FIG. 2: Solutions Bifocal fit 0.1mm below the lower pupil margin.

Trifocals

Most trifocal designs, including Llevations (Tru-Form), Tangent Streak Trifocal, and Presbylite (Lens Dynamics) require the segment position at or 0.1mm above the lower pupil border.

As the science of correcting presbyopia advances, new bifocal contact lens fitting methods will change. Professional judgement determines success.

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