Do Peripheral Curves
Matter?

BY THOMAS G. QUINN, O.D., M.S.
MAY 1997

When ordering rigid gas permeable lenses, do you specify peripheral curve radii and widths? Should you? Although often overlooked, peripheral curve design will significantly affect RGP lens performance.

INTERPALPEBRAL APPROACH

When you fit a contact lens interpalpebrally, the lid must repeatedly travel over the lens edge when the patient blinks. Selecting the appropriate peripheral curve will enhance lens comfort by optimizing lens thickness and edge profile.

Axial edge lift (AEL) between 80 and 110 microns is generally enough to provide adequate tear exchange beneath the lens without compromising comfort due to excessive lid interaction during the blink. Edge thickness from 0.09mm to 0.12mm is ideal. Thinner edges tend to be too sharp and chip easily.

LID ATTACHMENT APPROACH

A fairly flat peripheral curve system (AEL from 150 to 300 microns) and a thicker edge profile (0.16mm) can help achieve a lid attachment fitting relationship. However, if other patient or lens factors prohibit lid attachment, such a design will promote peripheral corneal staining. This occurs due to one of two mechanisms: an excessively flat peripheral curve system (Fig. 1a) creates "lid gap" which compromises tear distribution to the corneal surface adjacent to the lens edge; or the flat lens periphery tends to align with the flatter extreme corneal periphery, often resulting in inferior lens decentration and poor movement. A low riding lens is often associated with 3 and 9 o'clock staining.

A peripheral curve system that's too steep (Fig. 2b) can lead to poor tear exchange beneath the lens, lens binding and corneal chafing which can manifest as vascularized limbal keratitis. Figure 3a demonstrates an ideal peripheral curve system.

ENSURING OPTIMUM CURVES

Use slit lamp examination to clinically assess the peripheral curve system of a lens. With excessive edge lift, there will be fluorescein under the lens, but none peripheral to the edge (Fig. 1b). Inadequate edge lift will prohibit fluorescein from flowing beneath the lens, but the dye will accumulate at the lens edge and peripheral to it (Fig. 2b). The ideal pattern will show fluorescein under the edge of the lens and peripheral to it (Fig. 3b).

FIGS. 1a & 1b: TOO FLAT.

FIGS. 2a & 2b: TOO STEEP.

FIGS. 3a & 3b: IDEAL.

To ensure you're ordering the optimal peripheral lens geometry, start by specifying the desired edge thickness. If you don't have the tools to calculate the peripheral curves that will give you the optimal axial edge lift, you can tell the laboratory what AEL you'd like, and have the lab do the calculations. Or, you can follow these rules of thumb for a tricurve design: secondary curve = base curve radius + 1.0 to 1.5mm; tertiary curve = secondary curve radius + 1.5 to 2.0mm. Bias your curve selection flatter for smaller lenses. CLS

Dr. Quinn, a diplomate and fellow of the AAO Cornea and Contact Lens Section, has served as a faculty member at The OSU College of Optometry. He is in group practice in Athens, Ohio.