Peripheral Corneal Staining:

A Survey of RGP Laboratories

BY JOHN R. BUCH, O.D.
MAY 1997

Several RGP labs share their experiences to help you determine which lens parameters are most likely to reduce the incidence of peripheral corneal staining.

TOP 7 PARAMETERS

An informal survey of rigid gas permeable contact lens laboratories was conducted recently to ascertain the lens parameters that most influence the development of PCS, and how those parameters are modified to reduce it. Lab consultants were polled because their viewpoints represent the culmination of many trial-and-error experiences.

Twenty-seven RGP lab consultants responded to the survey. They ranked the parameters in order of importance with "1" being the most influential. Next, they indicated how they usually modify each parameter to reduce PCS. Finally, they reported any other experiences they may have had related to PCS.

According to the the results of this survey (Fig. 1), axial edge lift is the most influential lens parameter in the development of peripheral corneal staining, followed closely by overall diameter. The remaining design factors, ranked in descending order of importance, were: edge thickness; base curve radius; the use of an aspheric back surface; optic zone diameter; and lens material.

AXIAL EDGE LIFT

Seventeen respondents said they would reduce axial edge lift (AEL) by an average of 0.06mm, while eight suggested increasing AEL by an average of 0.05mm. One consultant would not change this parameter, while one would either increase or decrease it by an average of 0.03mm, depending upon the situation.

OVERALL DIAMETER

Nine consultants said they would reduce overall diameter (OAD) an average of 0.25mm, while another nine said they'd increase OAD an average of 0.50mm to reduce PCS. Seven said they would either increase or decrease OAD by an average of 0.30mm depending on the situation, and two lab consultants recommended no change to this parameter.

EDGE THICKNESS

Most of the survey respondents recommended reducing edge thickness by an average of 0.04mm. One lab consultant suggested increasing edge thickness by 0.05mm, while the rest stated that they would not change this parameter.

BASE CURVE RADIUS

Fifteen consultants recommended flattening the base curve radius (BCR) by an average of 0.50 diopters. Three said they would steepen the base curve radius by 0.30 diopters, while eight said they would not change the base curve radius. The remaining respondent would either flatten or steepen the base curve radius by 0.50 diopters.

ASPHERIC BACK SURFACE

When an aspheric back surface lens is used, 12 consultants recommend steepening the BCR by an average of 0.45 diopters. Seven reported that they typically would not change the BCR at all, while eight recommended that the BCR be flattened an average of a half-diopter.

OPTIC ZONE DIAMETER

Among the consultants who would change the optic zone diameter (OZD), most said they would reduce this parameter an average of 0.30mm. Ten consultants would not change the OZD, while four said they typically recommend increasing the OZD 0.30mm. Two respondents said they would either not change the OZD or they would reduce it, depending on the situation.

LENS MATERIAL

According to the survey, the parameter that is least important in the development of peripheral corneal staining is the lens material. Most consultants (23) recommended using a fluorosilicone acrylate material with an average Dk of 38. Others chose a silicone acrylate material with an average Dk of 24, and one recommended alternative materials such as NovaLens, CAB and Fluoropolymer.

OTHER PCS FACTORS

Thirteen of the 27 lab consultants who responded to this survey commented on their experiences related to peripheral corneal staining. They emphasized that the patient's tear production must be adequate in quantity and quality, and blinking must be frequent and complete for PCS resolution.

Regarding RGP lens factors, one consultant emphasized the significance of proper blending. The four people who commented on lens positioning recommended that the lens be either central or superior-central on the cornea, and that the lens be attached to the upper lid to decrease edge sensation.

Finally, opinions regarding choice of lens material ranged from being viewed as entirely unrelated to the development of PCS, to the belief that one lens material (silicone) causes peripheral corneal staining.

TO RESOLVE 3 & 9 O'CLOCK STAINING, MOST RGP LABS RECOMMEND REDUCING AXIAL EDGE LIFT.

TEARS, THE BLINK AND LENS DESIGN

Of course every case is unique, and asking lab consultants to single out one all-encompassing parameter as the most important relating to PCS formation and resolution is impractical. It's also important to note that a change in one lens parameter ultimately affects how other parameters contribute to the fit of the lens. Therefore, the reason a certain modification is successful may not be well understood.

The relationship between PCS and rigid lens wear indicates that this problem is multifactorial and patient-dependent. What seems to work well with one patient may be ineffective with others. Most studies of PCS have demonstrated that normal tear production and proper blinking are essential for resolution. It's very likely that a change in lens design brings about a change in blinking habits, therefore affecting PCS indirectly. Such changes in lens design result in reduced edge sensation and typically involve a centered or superior-centered lens, thin edges and moderate edge clearance. A change in lens design may favorably affect PCS by reducing peripheral corneal bearing, promoting tear pump efficiency, or blanketing the area of staining.

AEL & OAD: THE LOGICAL CHOICES

It seems reasonable that axial edge lift and overall diameter were the lens parameters most often modified to reduce PCS since these factors are closely associated with the peripheral cornea. More than half the consultants recommended that axial edge lift be reduced. This may be to reduce the gap between the peripheral cornea and the lid at the lens edge, allowing the lid to better coat this area with mucin. It may also promote improved blinking habits through reduced lens awareness. A lower edge lift also reduces the tear meniscus at the lens edge, which may minimize tear thinning at the lens edge. Also, a lower edge lift may promote better lens centration.

The issue of overall diameter was the most controversial in this survey. It was ranked as the second most influential parameter, yet recommendations for modification were mixed between whether the diameter should be increased or decreased. Smaller diameters may have the advantage of increasing lid-to-cornea contact and increasing tear pump efficiency. Larger diameters may blanket the area of staining with post-lens tears and improve blinking habits through better lens comfort.

This survey shows that among the RGP labs responding, axial edge lift and overall diameter are thought to be the most influential parameters in the development of peripheral corneal staining. The true mechanism by which these and other parameters influence the staining is not known. Choice of lens material has the least impact, according to the labs, indicating that proper lens design is more important than lens composition. A frequent response was that any modification in lens design is situation-dependent, and that modifications typically involve changing two or more parameters simultaneously. This illustrates the patient-dependent nature of peripheral corneal staining which is why one lens modification cannot be solely responsible for its resolution. CLS

The author would like to thank Lee Wagoner, B.S., M.H.A., for her review of this article.