RGP Bifocal Lenses are Easy as 1-2-3
BY EDWARD S. BENNETT, OD, MSED
One of today's misnomers is that RGP bifocal lenses are too difficult to fit. The truth is, RGP bifocals are easier than butterfly ballots, and over 75 percent of patients preferred bifocals over monovision in a recent study by Johnson et al.
Explain the Options. Single vision contact lenses with reading glasses and monovision are options. The former provides good vision at distance and near but requires frequent application and removal of glasses; the latter will result in some blur out of one eye at both near and distance. Bifocal lenses, although more expensive, may provide satisfactory vision from both eyes at all distances.
Be Realistic. Emphasize that spectacles typically provide the most stable vision at all distances. Contact lenses are dynamic devices and may cause transient blur with certain directions of gaze. A good fit is critical and it may take a lens change or two to obtain such a fit.
A patient with an add power requirement of less than or equal to +1.50D should see well at near with an aspheric multifocal. This thin lens design is also a viable option for current RGP lens wearers now entering presbyopia. Several aspheric multifocals have front surface optics incorporating higher add powers, although distance vision may be somewhat compromised. Higher adds and uncompromised vision at distance and near are certainly possible in a segmented, translating bifocal.
Patients with high intermediate need (moderate computer use) can benefit from either a trifocal translating design or an aspheric multifocal that is somewhat overplussed in one eye (modified monovision).
Patients with large pupils often experience glare at distance with aspheric multifocals at night. Patients with inferior positioned lower lids (>1mm below the lower limbus) are not good segmented candidates because the lens cannot translate (shift upward).
Have at least one translating diagnostic set and one aspheric multifocal set; the larger the sets, the better.
Aspheric multifocals are typically fit 1 to 2D steeper than K with the fluorescein pattern close to alignment. Good centration and limited lag with the blink (1mm) are desirable, although an optimum fitting relationship is not mandatory. Most patients obtain good distance vision, most likely the result of good optical quality and slight translation.
When fitting segmented, translating bifocals, ensure that the inferior lens edge is in close proximity if not aligning with the lower lid. Evaluate the position of the seg line (or crescent) in relation to the lower pupil margin. With the patient viewing straight ahead, the seg line should be in close proximity to the lower pupil margin. Finally, have the patient view inferiorally and lift up the upper lid to observe if the lens has translated.
With aspheric multifocals, a steeper base curve (0.50D steeper) can improve a loose fitting and/or decentering lens.
Segmented translating designs may have excessive nasal rotation with the blink. Change to a 0.50D flatter base curve to reduce or eliminate this problem. If the lens positions too high, add another 0.50D of prism to improve stability. If the lens does not shift upward or fails to do so on every downward gaze, increase edge clearance. Select a flatter base curve (0.50D minimum) or flatten/widen the peripheral curve. If this fails, the patient may have flaccid lids and is not a good translating bifocal candidate.
Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and executive director of the RGP Lens Institute.