RGP Bifocal Base Curve Selections

BY DAVID W. HANSEN, O.D.
JUNE 1996

Base curve selection is generally the first consideration when designing an RGP single-vision contact lens. In most cases, the posterior curve is fit with an alignment cornea-to-lens relationship based on corneal curvature, lens diameter and the amount of corneal toricity. All of these design decisions are important for an alignment fit. With many RGP bifocal and multifocal designs, however, the posterior curve is determined by the specific type of contact lens.

TRANSLATING DESIGNS

Most translating designs are fit with a lens-to-cornea relationship paralleling the fitting philosophy of a single-vision contact lens. Translating lenses such as the Crescent lens, a centered or decentered DeCarle design, the Solitaire (slab-off) design, the Tangent Streak and the FluoroPerm ST all require fitting the lenses "on K" to 0.50 diopter flatter than K. This base curve allows for easy translation and movement in a 12 o'clock to 6 o'clock position. It's important to assess corneal toricity when fitting these lenses just as you would when fitting single-vision lenses.

SIMULTANEOUS DESIGNS

When selecting the base curve for simultaneous or aspheric design lenses, the eccentricity value of the posterior contact lens surface is an integral part of fitting the effective add power of the prescription. Therefore, these lenses may be fit from 1.50 diopters steeper than K to 5.00 diopters steeper than K.

LIFESTYLE GP FIT 0.1MM FLATTER THAN K. GOOD ALIGNMENT.

TANGENT STREAK FIT TOO STEEP WITH NASAL ROTATION. FIT 0.50D FLATTER.

The Unilens and Unilens Plus RGP multifocals are usually fit between 1.50 and 2.00 diopters steeper than K. The VFL, VFL-II and VFL 3 lenses are usually fit 2.00 diopters steeper than K. Other aspheric designs such as the Multisite are fit 2.50 diopters to 4.00 diopters steeper than K and the V/X lens by GBF Laboratories is fit with a posterior apical radius at least 4.00 to 5.00 diopters steeper than K.

The LifeStyle Gp by the LifeStyle Co., Inc., employs a double aspheric design and is fit 0.1mm flatter than the flattest keratometric reading. This design offers a unique superior positioning of the lens. The fitting zone is called the equivalent base curve (EQBC) rather than the curvature of the central apical radius of the lens. These designs offer powers to fit the emerging presbyope as well as latent presbyopes.

MULTISITE APSHERIC DESIGN FIT 3.00D STEEPER THAN FLATTEST K. GOOD ALIGNMENT.

FLUOROPERM ST FIT TOO FLAT; PATIENT NOT TRANSLATING INTO SEG. FIT 0.50D STEEPER.

All bifocal lenses need to translate, so I suggest starting with base curve parameters that allow adequate movement of the lens. Aspheric designs need to position centrally over the pupillary axis to allow optimal visual acuity. Fluorescein assessment with a yellow filter and the biomicroscope is the appropriate method for evaluating base curves. If acuity or decentration is inappropriate, try changing the curves in 0.50-diopter increments and reviewing manufacturer's guidelines to achieve better success. CLS

Dr. Hansen, a diplomate of the AAO Cornea & Contact Lens Section, is in private practice in Des Moines, Iowa.