DON'T TAKE RGP EDGE DESIGN FOR GRANTED

BY ROBERT CAMPBELL, M.D. & PATRICK CAROINE, C.O.T., F.A.A.O.

JULY 1997

In the old days, when the edge design of rigid contact lenses was erratic at best, we became quite skilled in the fine art of in-office edge inspection and modification. Today, with improved manufacturing and finishing techniques, we often take edge design for granted. Yet, many patients with well-fitted RGP lenses continue to experience the signs and symptoms of improper edge design.

THE CASE

A 34-year-old nurse who had successfully worn RGPs for 12 years came to us for replacement lenses. We performed a routine contact lens examination, verified the lens parameters and ordered a new pair of lenses with no change in diameter, base curve or posterior optical diameter. We incorporated a slight power change of ­0.25D OU.

After our office staff verified the parameters, we dispensed the new lenses. On follow-up, the patient complained of itching and a slight foreign body sensation in her right eye. Slit lamp examination without and with fluorescein showed a well-fitted lens with no objective signs to explain the patient's symptoms. However, further inspection of the lens edge with the Edge Analyzer (Valley Contax, Eugene, Ore.) clearly revealed an excessive posterior zone (Fig. 1).

FIG. 1: LENS EDGE WITH EXCESSIVE POSTERIOR ZONE.

WHERE IS THE EDGE?

There has always been confusion about where the edge of a contact lens begins in relation to the anterior power curve and the posterior peripheral curve. For purposes of this article, we have divided edge design into three zones (Fig. 2).

FIG 2: THE THREE ZONES OF AN RGP LENS EDGE.

The anterior zone is that portion of the edge that touches the upper lid during the blink. The function of the anterior zone is to taper the lens periphery to minimize the interaction between the upper lid and the lens edge.

The posterior zone is a small reverse curve placed onto the posterior lens surface to flare the edge away from the cornea. Its function is to allow free movement of the lens across the cornea with eye excursion and permit lens removal by the blinking technique. The posterior zone also allows the lens to be safely centered onto the cornea when dislocation has occurred.

The lens apex is the junction between the anterior and the posterior zones. This junction must be well-rounded to minimize lens awareness during the blink.

ANALYZING THE EDGE

The Edge Analyzer offers an accurate and easy-to-use technique for direct examination of the lens edge. The device attaches to the radiuscope base plate and utilizes the instrument's high magnification to view any defects in edge contour.

FIG. 3: THE IDEAL EDGE PROFILE.

Figure 3 illustrates the ideal edge profile. Note the smooth, contoured profile of the anterior zone, the slight regression of the posterior zone away from the cornea and the well-rounded apex. Any deviation from this edge profile can create increased lid awareness, peripheral corneal staining or difficulties with lens removal.

Our ability to recognize and correct subtle abnormalities in edge contour can be the deciding factor between a patient's success or failure with rigid contact lenses. CLS

Dr. Campbell is medical director of the Park Nicollet Contact Lens Clinic & Research Center, Minnetonka, Minn. Patrick Caroline is an assistant professor of optometry at Pacific University, Forest Grove, Ore., and director of contact lens research at Oregon Health Sciences University in Portland.