Contact Lens Design & Materials
GP Multifocal Lenses for Irregular Corneas
By Ronald K. Watanabe, OD, FAAO
Presbyopia continues to be a challenge for contact lens fitters. We have many good options, though all multifocal lens options—and monovision—have drawbacks. When it comes to presbyopic patients who have irregular corneas, a multifocal design may be the last thing on your mind. However, with today's technology, many labs can manufacture GPs that have a specialty back-surface geometry to fit irregular corneas paired with an aspheric multifocal front surface to correct vision at both distance and near.
The back-surface design of these GP lenses can be toric, keratoconic, or reverse geometry. Scleral lenses can also be the base for these special multifocals (e.g. Art Optical's So2 Clear Progressive). The front surface is most often the lab's best multifocal design. Theoretically, it is possible to combine any front and back surface. However, corneal lenses on irregular corneas often require unusual curvatures and dimensions and may not position or move as they should. This is important because position during various eye movements is critical to a successful multifocal GP fit.
When pairing a complex back surface with an aspheric multifocal front surface, consider these tips courtesy of Jim Slightom of ABB-Concise:
• The lens must maintain good centration on the eye. A lens that does not position well over the pupil will cause vision to blur, especially at distance. If it is impossible to center the lens, a multi-focal design is unlikely to work.
• The lens must translate on downgaze. A lens that does not translate will limit the add power that the patient can appreciate. Because large lens diameters are often used on irregular corneas to maintain centration and stability, translation may be limited.
• A patient who has an irregular cornea may have reduced best-corrected visual acuity even with a GP lens; adding a multifocal aspheric front surface will further degrade acuity and contrast.
In other words, the same principles of successful GP multifocal fitting apply to these designs as well. Therefore, it is important to first fit the cornea and see how well the lens centers and moves. Determine best-corrected visual acuity by performing an over-refraction over a diagnostic lens with a single vision front surface. Then you can make a recommendation for fitting (or not). Keep in mind, however, that a multifocal GP that centers and translates well will not always work, as with “normal” presbyopes.
Several labs I contacted (ABB-Concise, Art Optical, Blanchard, Lens Dynamics, Precision Technology Services, Tru-Form, and X-Cel) said that they can manufacture this type of lens, though the options vary. For example, Blanchard can add its Reclaim HD front surface to its RSS Refractive Surgery Specific back-surface design. This provides multifocal optics of varying add powers and zone sizes to patients needing a good reverse geometry design. Precision Technology Services has developed a multifocal that can be put on the front surface of any of its irregular cornea designs, including the KBA and scleral lenses. Its PTS Multifocal can be generated with any add power over a specified zone size that best matches a patient's pupil diameter.
A Useful Option
Though an irregular cornea patient wanting a multifocal lens may not come into your office often, it's good to know that this specialty GP option is available through continued advancements in lens technology. CLS
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at email@example.com.