prescribing for presbyopia

Fitting Flat and Steep Corneas With RGP Multifocals

BY DAVID W. HANSEN, OD
JULY 1998

I'm sure Paul Simon's song, "Slip Sliding Away," wasn't intended to describe the fitting of RGP bifocals, but there is a message to heed. By sliding over the tears, the distance and near prescription segments can move into position for optimum acuity. Like the song,

scription segments of the lens can move into position for optimum acuity. Like the song, translating bifocals have been around a long time, with Dr. George Tsuitaki's original translating crescent PMMA lens as one of the firsts.

Parameter Selection

The success of translating RGP bifocals depends on appropriate selection of parameters such as prism, peripheral curves, thickness, edge design and segment heights, but most important are base curve and diameter.

Choose the initial base curve using corneal topography or your single-vision approach to achieving a good lens-to-cornea relationship with a uniform fluorescein pattern. Starting with an on-K or slightly steeper design for higher astigmatic corneas will usually provide adequate vertical movement. When determining the diameter, consider the physical characteristics of the eyelids and their relative position to the cornea. If the palpebral aperture is small (<9.0mm), you may have to use a vertical contact lens diameter that's 9.0mm or smaller to avoid lid interference. Larger palpebral apertures can accommodate larger vertical contact lens dimensions, which enhance bifocal segment alignment. Truncation may be needed to decrease rotation and to ensure stable vertical movement over the central cornea. Typical truncation is 0.3mm.

Segment Positioning

Segment placement is critical for maximum acuity. Each manufacturer suggests an ideal segment height for proper positioning of the distant power in front of the pupil (Table 1). Measure the seg position with a biomicroscope on 10x power with an in-eye piece reticule and a small light beam set adjacent to the side of the pupil (Fig. 1). If there's an even fluorescein pattern but the seg height is too high, lower the seg to reduce flare and glare. Always select the base curve before changing the seg height. If fluorescein demonstrates extra pooling, which indicates a steep fit, flatten the base curve. If the pattern indicates a flat fit with little to no pooling, steepen the base curve before modifying the seg height.

Prism and Material Mass

Most translating designs require ballast systems to stabilize the lens and provide predictable vertical movement. New RGP materials provide excellent oxygen transmission, but it's still advisable to keep the mass of the material and prism to a minimum. CLS

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