contact lens practice pearls
Understanding Aspheric GP Multifocals
BY THOMAS G. QUINN, OD, MS, FAAO
Gas permeable multifocals work. A study by Rajagopalan et al (2006) found that a posterior aspheric GP multifocal design, the Essential GP Multifocal (Blanchard), performed similarly to progressive addition spectacles for a group of subjects with a mean add power of +2.00D.
According to manufacturers, use of aspheric multifocal designs significantly outpaces the use of classic translating bifocal designs. A key benefit of aspheric designs is their ability to correct vision at the valued intermediate distance.
Most aspheric multifocal GPs have their posterior surface fabricated to flatten from the lens center to the edge (prolate curve). This results in a lens with distance optics in the center, transitioning to near optics in the periphery.
First-generation posterior aspheric designs such as the VFL 3 (Conforma) are fabricated utilizing curves with high eccentricity values. Consequently, these lenses must be fit from 2.50D to 4.00D steeper than the flat K reading to perform properly.
In addition to creating a multifocal effect, these high eccentricity curves create a significant plus tear lens. This is a benefit when fitting moderately high hyperopes because it reduces the amount of plus needed in the lens, resulting in a thinner, lighter lens more likely to center appropriately.
A downside to fitting steep, high eccentricity posterior surface lenses: they induce topographical changes that result in significant spectacle blur. This has contributed to the development of low eccentricity back-surface designs that fit more aligned with the natural curve of the anterior ocular surface. This reduces spectacle blur, although it can still occur even with these lenses.
How About the Front Surface?
Spectacle blur may be minimized if the aspheric curve is applied to the front surface of the lens.
A prolate curve applied to the front of a lens creates optics that are opposite to that which occurs when the same curve is applied to the posterior lens surface. Rather than a center-distance design, it creates a center-near design.
New technologies enable labs to create aspheric surfaces that steepen from the center of the lens to the edge (oblate curves). When applied to the anterior lens surface, a center-distance optical design is created, mimicking the optical effect of a prolate curve on a posterior surface. The Naturalens Progressive (Advanced Vision Technologies) is an example of a lens that employs this design.
Front-surface aspheric designs are particularly easy to fit on established single vision GP wearers. Assuming the spherical lens centers well on the cornea, you can order the same base curve in the front-surface aspheric design.
Patients whose single vision lenses tend to position superiorly often perform better with posterior aspheric multifocal designs.
Translating to Success
While good centration is important with center-distance GP multifocals, some vertical lens translation is desirable in down gaze. This allows patients to view near objects through a more peripheral, and more plus powered, portion of the lens. This provides a boost to near vision.
The development of lathes that can apply high quality, consistent aspheric curves to either lens surface has taken this mode of correction to the next level. Take advantage of these wonderful developments. CLS
Thank you to Keith Parker of Advanced Vision Technologies for his assistance with this article.
For references, please visit www.clspectrum.com/references.asp and click on document #152.
Dr. Quinn is in group practice in Athens, Ohio. He is a diplomate of the Cornea and Contact Lens Section of the American Academy of Optometry, an advisor to the GP Lens Institute and an area manager for Vision Source.