Rigid gas permeable lens design

Gerald

E. Lowther,

OD,

PhD,

co-editor

I

~

CLC

ommentary

Rigid Gas Permeable Lens Design

Rigid cornea1 contact lenses have been fitted for approximately 45 years. One would assume that over this time all the suhtleties of designing and fitting these lenses would have been defined. Talking to a large number of contact lens fitters and manufacturers over the years I have been amazed at the wide range of opinions on how rigid lenses should be designed and fit, ted. If you ask a number of “experts” either about the best design for the “average” patient or how to solve a fit, ting problem, you are liable to get as many different answers as experts. The literature on the topic offers fewer articles than would be predicted that give actual data on lens performance with different lens designs. In many cases the publications contradict each other. Why are there such diverse opinions on how rigid lenses should be designed and fitted? One possibility is that within the range of lens parameters we typically use there is not an obvious

significant difference in lens performance. Changing the peripheral radii or optical zone diameter a few tenths of a millimeter may not be noticeable to a large percentage of the patients. Anorher problem is the wide variation in peripheral cornea1 geometry, position of the upper and lower lid margins in relation to the limbus, lid tension, and other factors that we can not measure. Why are there conflicting results in published studies? One reason is the difficulty in comparing studies. There are so many interdependent lens parameters that even though two studies may be evaluating the same lens parameter, such as base curve radius, it is very difficult to compare the results. The lenses used in the two studies may have different overall diameters, optical zones, peripheral curve designs, or thickness and edge geometry, which overwhelm the effect of the base curve change. Another factor is the patient variation mentioned previously. Dif-

ICLC, Vol. 23, May/June, 1996 0 Elsevier Science Inc. 1996 655 Avenue of the Americas, New York, NY 10010

ferent studies come from differenr patient populations with respect to ocular anatomy, either by patient selection or by chance. Because most of the studies have rather small patient numbers, the chances of these differences being important are greater. Investigator bias can be another factor; they may want a certain design to perform better. This is why results conflict. This issue begins a two-article series by Sorbara, et al., that investigates RGP lens design comparing centered versus lid attachment fitting. This controlled study specifies the lens parameters and conditions. The patient preference was for centered lenses rather than higher riding lid attachment designs. This study is a move in the right direction to help further delineate scientifically how we should be designing and fitting RGP lenses. More studies are required in this area, with larger numbers of patients, before more patients can be fitted optimally with rigid lenses.

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