contact lens primer
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
Rigid contact lenses come in several aspheric configurationsbase curves, peripheral curves, biaspheric (aspheric base curve and aspheric peripheral curve) and front surfaces. The degree of asphericity is often expressed in terms of an eccentricity or e value. The higher the eccentricity of a surface, the greater the rate of flattening from its center to its periphery.
The Good and the Bad
Aspheric rigid lenses have been recommended to "optimize" the base curve-to-cornea fitting relationship to reduce spherical aberration, to "correct" residual astigmatism and to correct presbyopia. Often aspheric designs may improve lens centration, especially for patients with moderate corneal toricity. Aspheric contact lenses have also been reported to improve lens comfort. Because rigid lenses with aspheric back surfaces are basically junctionless, they may diminish symptoms of visual flare.
If an aspheric rigid contact lens does not center well, unwanted residual astigmatism may result in decreased and variable vision. Difficulty in lens parameter verification is another disadvantage to aspheric lenses.
Contouring the Cornea
Aspheric base curves are often prescribed to mimic the aspheric corneal surface. A low eccentricity or elliptical design may achieve this goal. A base curve that contours the cornea minimizes localized areas of excessive corneal bearing which may result in decreased lens-induced corneal distortion.
Aspheric peripheral curves are prescribed to enhance tear exchange by removing the junctions between the posterior peripheral fitting curves. The result is similar to a well-blended, multi-curve spherical peripheral curve system. If you modify RGPs in your office, it is more desirable to prescribe spherical peripheral curves. The practitioner is seldom privy to the peripheral curvatures of aspheric designs due to the laboratories' proprietary concerns.
Front-surface aspheric lens designs have a reputation of improving vision for patients who have low amounts of residual over-refraction cylinder when wearing spherical base curve lens designs. The over-refraction cylinder is typically not reduced with an aspheric design, but spherical aberration may be decreased leading to an improved quality of vision.
Aspheric front and back surfaces may be prescribed for presbyopic patients. The higher the eccentricity (greater rate of flattening), the greater the add effect. Multifocal aspheric designs generally have hyperbolic base curves. For a back-surface rigid aspheric design, the greater the desired add, the steeper the base curve needs to be prescribed to maintain lens centration. Patients needing lower adds are better candidates for back-surface aspheric multifocal designs. A combination aspheric front and back surface design may be indicated for presbyopic patients needing adds of more than 2.00D.
Some aspheric multifocals are designed to translate on down-gaze in order to achieve the desired add effect. These designs are fit substantially flatter than traditional aspheric multifocal simultaneous vision design lenses.
Basic Fitting Strategies
Back-surface aspherics generally need to be fitted steeper than spherical base curve rigid contact lenses. If you are prescribing an aspheric back-surface design for presbyopic correction, the more add desired, the steeper the lens needs to be fitted. Excessively steep base curves may lead to decreased tear exchange or corneal distortion due to areas of excessive apical clearance and mid-peripheral lens bearing. Aspheric back-surface designs may be good options for patients with irregular corneal surfaces.
Dr. Edrington is a professor and in the contact lens service at the Southern California College of Optometry. E-mail him at email@example.com.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean for clinical affairs at The Ohio State University.