prescribing for presbyopia

Fitting Flat and Steep Corneas With RGP Multifocals

BY DAVID W. HANSEN, OD
JUNE 1998

Until the recent introduction of corneal topography, contact lens base curve selection was based on keratometry measurements of the anterior curvatures of the cornea. Using this less-than-accurate, three millimeter zone could be the reason why clinicians have traditionally reported low success in fitting patients with RGP bifocal contact lenses.

When selecting the base curve, it's important to consider the entire shape of the cornea, not just the central curvature. Before the advent of the topographer, we classified corneas as "flat," "steep" or "average." These terms misrepresent the shape of the cornea, but since many practitioners still rely on central corneal curvature to determine initial base curve, the following clinical pearls may improve fitting success.

Flat Corneas

Corneas that have an overall flat contour (41.00D and less) present a challenge because contact lenses tend to ride superiorly or drop to the inferior limbus (Fig. 1). Large diameter lenses will usually compensate for the flat curvature. If you use a simultaneous aspheric RGP design, select a base curve that is flatter than usual to achieve centration. The lens must also provide enough aspheric change to produce appropriate add power. Remember, if you flatten the base curve to compensate for the flat cornea, you must add the near power to the front surface for good near acuity.

Translating bifocals usually work best on flat corneas (Fig.2). With this design, you can achieve better visual acuity by independently adjusting the power in each segment. It may be necessary to reduce the prism, thereby reducing the mass, to help the lens move upward on the cornea.

Steep Corneas

Corneas that centrally measure 46.00D or greater are usually considered steep. Steep corneas have a profile that adversely affects the fit of most spherically based, translating design contact lenses. The lens-to-cornea relationship and the fluorescein pattern usually will not reflect the true fit of the contact lens. Smaller diameters may improve the fit of spherically based single-vision designs but may not be appropriate for bifocal designs because the distance and near segment sizes are too small for patient viewing.

Aspheric multifocals are usually the best choice for steep corneas. The laboratory can modify the eccentricity value of the posterior contact lens surface to accommodate the steepness. Aspheric designs require centration over the pupillary axis, and you can usually change their parameters to create this optimum effect. Fluorescein often illustrates central pooling, and it may be necessary to steepen or flatten the base curve of the lens to optimize the fit.

Diagnostic lenses remain the best method for fitting both flat and steep corneas.