Setting Standards
What is a Peripheral Curve?

BY CAROL A. SCHWARTZ, O.D., EDWARD S. BENNETT, O.D., M.S. ED., & CARL MOORE, F.C.L.S.A.
OCT. 1997

This new survey shows why you should understand your RGP laboratory's fitting philosophy and how it relates to peripheral curve design.

Although most texts still teach us how to calculate even the most minute RGP contact lens parameters, today's computerized lathes can make these calculations more efficiently than any human -- a welcome relief to those of us already pressed for time by managed care. However, a recent survey of Contact Lens Manufacturers Association member laboratories revealed that many practitioners assume that certain parameters are now standard thanks to computerization. This is an unfortunate assumption that can compromise a patient's vision and comfort.

SURVEY SAYS . . .

The 50 CLMA member labs that responded to our survey indicated that nearly three-fourths of their clients rely on them to design the periphery of a lens by simply specifying "standard" for the peripheral design. But is there such a thing as a standard peripheral curve? If so, what is it? What variables do contact lens manufacturers take into consideration when designing lens peripheries?

The type of lens design that individual labs provide is quite variable. Most of the labs we polled provide a tricurve lens, but aspheric and bicurve designs are also popular (Table 1). Knowing which of these your lab prefers to use can impact your fitting success. The net optic zone and final edge lift (clearance) may vary from lab to lab, and the differences can have a significant effect on fit and comfort.

Based on the responses to our survey questions, you may want to ask your lab more than simply, "Do you provide a bicurve or a tricurve?" because terminology about peripheral curves is not standard among all manufacturers.

WHEN IS A CURVE A CURVE?

We asked the labs if they consider a cut-in curve plus a blending lap (a heavy blend) a tricurve or a bicurve lens. Of the 49 labs that responded to this question, 43 call this design a bicurve and six say it's a tricurve. Yet, when we asked if a blending curve is a secondary curve, 38 labs said no, while 12 said yes. In other words, not only do labs not agree with each other about what constitutes a secondary curve, but some of them disagree with themselves.

Edge lift is a major factor in lens comfort because it expresses how much clearance the lens periphery generates as it flattens. Measured at the edge of the diameter, it is the distance between the surface of the peripheral curve and the point in space where the base curve would be if it continued uninterrupted. Lid-attached designs typically require a higher edge lift, while interpalpebral fits on normal eyes usually have lower values. Two-thirds of the manufacturers polled use these principles to determine peripheral curves; the remainder use nomograms or other heuristics.

Edge thickness is also critical to patient comfort. Blunt edges (Fig. 1) and sharp, narrow edges (Fig. 2) are less comfortable than a gently tapered surface (Fig. 3). Only about one-fourth of the labs consider edge thickness when designing their standard peripheral curve. Yet, as shown in Table 2, nearly all of them will alter the anterior surface using techniques such as lenticulation to optimize edge profile. They also consider the lens diameter and material that you specify.

FIG. 1: A BLUNT EDGE MAY CAUSE DISCOMFORT. FIG. 2: A SHARP POSTERIOR EDGE WILL ALSO CAUSE DISCOMFORT. FIG. 3: A WELL-SHAPED EDGE.

These differing approaches can create very different lenses. We gave our survey labs three typical contact lens prescriptions and asked them to tell us what peripheral curve system they would apply (Table 3). Many recommended aspheric peripheries; the ranges shown are for those specifying a conventionally manufactured lens. At the extremes of the ranges for curve radius and width, tear layers will differ significantly.

Once peripheral curves are applied to a lens, it's very difficult for the prescriber or the lab to verify them. Blending those curves improves comfort, but doing so makes it difficult to determine how many peripheral curves the lens contains. Just over half the labs we polled use a medium blend when none is specified; most others use a heavy blend. The remainder use a light blend, with the exception of one lab that does not include a blend as part of a standard peripheral curve.

FLATTER THAN WHAT?

The results of our survey indicate that there's no such thing as a universal standard for peripheral curves. Your idea of standard may differ from your lab's, and since you have no way of determining what they ship, it would be wise to ask just what your lab's fitting philosophy is and how it relates to peripheral curve design. Ask the lab to tell you exactly what peripheral curves it has placed on the lens so that you can make intelligent decisions about modifications. For example, if a lens is not providing adequate tear exchange, you'll want the lab to make the peripheral curve flatter. How can you tell the lab how much flatter unless you know the current lens parameters? Knowing this information is also the only way you can guarantee that you'll be able to provide an exact duplicate of the lens when needed. Three-fourths of the labs said they provide information on peripheral curves when the lens is shipped.

If you discover that the laboratory is providing lenses with edges that are significantly different from those on your diagnostic set, you may need to invest in trial lenses that more closely approximate those your patients will ultimately wear.

KEEP YOUR LAB IN THE LOOP

Inform your lab of any unusual factors it should take into consideration when designing a lens. In one forum on keratoconus, a prescriber complained that he had some difficulty getting the lab to provide sufficiently flat peripheries. When questioned, he admitted that he had not told the lab that this lens was for a keratoconus patient. He assumed the lab would identify it as such from the steep base curve. This is not a valid assumption. Many lenses for keratoconus are within normal ranges, albeit at the steep end of the spectrum. In Case #3 (Table 3), only one lab identified the prescription as a possible keratoconus lens. The old saying about never assuming anything clearly rings true when you allow your lab to design lens parameters.

WHEN A LENS IS NOT A LENS IS
NOT A LENS

Just because you're familiar with the lenses your local lab provides, doesn't mean that all branches of the same company work to the same fitting philosophy. We sent our survey to multiple branches of some large, national labs. In one case, branch #1 uses a conventional tricurve design, branch #2 uses all aspheric peripheries, and branch #3 said it provides either one. Three branches of another large manufacturer also disagreed with one another; two provide a tricurve design as their standard and the third provides bicurves.

SAVE TIME, GAIN ACCURACY, BUT
KNOW THE STANDARD

Practitioners often ask their labs to design lens parameters such as edges, front lenticular zones and center thickness to save time and improve accuracy. With today's high technology lathes, the machine that makes the lens can quickly and precisely make calculations that would take a human much longer with less predictable results. The lathe/computer is also more likely to be programmed to account for the manufacturing constraints of certain RGP materials. These factors favor simply requesting a "standard" peripheral curve. Unfortunately, as we learned, a standard does not exist. Until one does, the best course of action is to learn your lab's design philosophy. CLS

Dr. Schwartz is a contact lens consultant in Vista, Calif.

Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis. He is executive director of the Rigid Gas Permeable Lens Institute.

Carl Moore is President of Con-Cise Contact Lens Co. and chair of the communications committee of the Contact Lens Manufacturers Association.