Tips for Translating GP Bifocals
BY CATHERINE TUONG, OD, & JOHN MARK JACKSON, OD, MS, FAAO
Aspheric multifocal GP lens designs have mostly eclipsed their older cousin, the segmented or translating bifocal lens. But, there are some benefits to the translating design that make it ideal for the right patients.
One advantage of this design is a function of its optics. Because both the distance and near portions of the lens are spherical and in separate locations, each zone provides crisp vision for that distance alone. For patients who cannot adapt to the spherical aberration with aspheric optics, this can be a welcome change.
Good candidates for translating GP bifocals have a lower lid located very near the bottom limbus and an upper lid that is fairly high or loose enough to discourage lid attachment. The lens must drop quickly to the lower lid after a blink to prevent the near segment from interfering with distance vision.
Start with the lab’s fitting guide to select the base curve. After achieving the desired alignment relationship, evaluate trial lens centration, rotation, translation, and segment height.
Ideally, the segment line should be horizontal. Slight nasal rotation is acceptable because of eye convergence when doing near tasks. Use RALS (right add, left subtract) to fix excessive rotation of the segment. For example, if the segment is rotating 20º to the right, change the prism position from 90° to 110°. This will make the segment line straight even with lens rotation.
Ensure that patients are able to view through the reading segment when they look down; the lens should remain sitting on the lower lid. If the lens is tucking under the lower lid, adjust the amount of lower lid interaction; either add more prism to thicken the lens edge and increase the lens-lower lid interaction, or truncate the lower edge. Flattening the base curve or peripheral curves can also help with translation if the lens fit permits.
Check the fitting guide for where the segment line should sit relative to the pupil. Typically, in primary gaze, the segment height should be near the lower pupil margin in normal room lighting. If the segment is too high or low (Figure 1), adjust the positioning.
Figure 1. Segmented bifocal resting on the lower lid, but with low segment position (indicated with white line).
When dispensing translating GP bifocals to patients, apply the lenses, allow for settling, and present patients with something familiar to view such as their phone or a magazine. Instruct patients to keep their head stationary and to look down for reading activities.
For distance troubleshooting, start by looking for proper segment height and quick lens drop after a blink. For near complaints, check the translation and positioning of the segment before adjusting the add power. Better intermediate vision can be achieved by slightly overplussing the top zone of the lens in the nondominant eye.
Worth the Effort
Segmented GP bifocal contact lenses can require more chair time and troubleshooting compared to aspheric designs, but this is partly due to the high level of customization that you can provide with these lenses. Be sure to consider them in your options for your presbyopic GP lens wearers. CLS
Dr. Tuong is the current cornea and contact lens resident at Southern College of Optometry. Dr. Jackson is an associate professor at Southern College of Optometry, where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at firstname.lastname@example.org.