Predicting clinical success for RGP materials
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Joe8. Goldberg, OD, FAA0 Guest Commentary Predicting Clinical Success for RGP Materials I t is customary to use Dk and Dk/L values to predict the clinical per- formance of a gas permeable contact lens material. Although this may be valid for hydrogel contact lenses, is it just as reliable for rigid gas perme- able contact (RGP) lenses? A hydrogel contact lens fits flush on the cornea and depends on the ability of the material to transmit oxy- gen through it. A RGP contact lens fit can furnish oxygen to the cornea by the direct transmission of oxygen through the material and when tear exchange under it brings a fresh sup- ply of oxygen to the cornea after each blink. Thus, a comparison of these mo- dalities is meaningful. RGP contact lenses cannot satisfy the cornea’s need for oxygen when the lens fit in- terferes with lacrimal interchange re- gardless of how much oxygen the material transmits. Consequently, RGP contact lens materials with low Dk values may satisfy a cornea’s need for oxygen when the lens-to-cornea fitting relationship allows good tear exchange under it. When RGP contact lenses are worn for extended wear, they must not in- terfere with the cornea1 metabolic system to a degree that could lower the frequency of cell division or affect the consumption of energy. But how much oxygen must RGP contact lens material transmit so that it will satisfy the prerequisites for extended wear? The initial stages of contact lens wear induce anaerobic metabolism in the cornea1 tissue as the amounts of lactic acid increase and glucose de- crease. The amount of these changes may depend on the gas permeability of the lens material and the lens-fitting relationship.’ 1 Kamiyal determined the level of cornea1 glycometabolic activity by quantifying the amounts of lactic acid and the effects of RGP contact len- ses worn for extended wear. He fitted 1 rabbits with rigid gas permeable len- ses having three different low Dk val- ues. The size for all was 9.2mm, and the center thickness was 0.13mm. Biomicroscopy was made for each eye immediately on lens removal, and 0.2 ml of aqueous was collected with a disposable syringe. The pH of the aqueous was determined with a mi- croelectrode. The amounts of lactic acid and glucose were quantitatively determined. The results indicate that the abnor- mal increases in lactic acid are unlikely when a material’s Dk value is higher than 32 and suggest that a combina- tion of a good lens-tocornea fitting re- lationship and the ability of a contact lens material to transmit oxygen may satisfy the criteria for extended wear regardless of the Dk value. Koetting et al.2 reported that a pa- tient’s ability to wear a RGP contact lens for extended wear is not com- pletely contingent on the oxygen transmission characteristics of the lens. They believe that tear flow un- der a contact lens may be more im- portant than is the oxygen passing through it. Just as we learned that there is no singular hydrogel contact lens ma- terial that will satisfy all clinical needs, we should discover that this is also true for RGP contact lens materials. Although it may be difficult to use the Dk value of a RGP contact lens material to predict its clinical perform- ance, it is safe to predict that we shall always rely on trial-and-error clinical investigation to fit this material as a way to satisfy clinical success. References 1. Kamiya, C: Studies on gas- permeable hard contact lenses: A quantitative analysis of lactic acid in rabbit aqueous humor. CLAOJ. 1984;10(1):96-99. 2. Koetting, R.A., Castellano, CF., Nelson, DW: Extended wear with low Dk hard gas permeables. Contact Lens forum 1985; 1O(2): 77-79. II Volume 16, Numbers 11 & 12 November/December 1989 355
Transcript of Predicting clinical success for RGP materials
Joe 8. Goldberg, OD, FAA0
Guest Commentary
Predicting Clinical Success for RGP Materials
I t is customary to use Dk and Dk/L values to predict the clinical per- formance of a gas permeable contact lens material. Although this may be valid for hydrogel contact lenses, is it just as reliable for rigid gas perme- able contact (RGP) lenses?
A hydrogel contact lens fits flush on the cornea and depends on the ability of the material to transmit oxy- gen through it. A RGP contact lens fit can furnish oxygen to the cornea by the direct transmission of oxygen through the material and when tear exchange under it brings a fresh sup- ply of oxygen to the cornea after each blink.
Thus, a comparison of these mo- dalities is meaningful. RGP contact lenses cannot satisfy the cornea’s need for oxygen when the lens fit in- terferes with lacrimal interchange re- gardless of how much oxygen the material transmits. Consequently, RGP contact lens materials with low Dk values may satisfy a cornea’s need for oxygen when the lens-to-cornea fitting relationship allows good tear exchange under it.
When RGP contact lenses are worn for extended wear, they must not in- terfere with the cornea1 metabolic system to a degree that could lower the frequency of cell division or affect the consumption of energy. But how much oxygen must RGP contact lens
material transmit so that it will satisfy the prerequisites for extended wear?
The initial stages of contact lens wear induce anaerobic metabolism in the cornea1 tissue as the amounts of lactic acid increase and glucose de- crease. The amount of these changes may depend on the gas permeability of the lens material and the lens-fitting relationship.’
1 Kamiyal determined the level of cornea1 glycometabolic activity by quantifying the amounts of lactic acid and the effects of RGP contact len- ses worn for extended wear. He fitted
1 rabbits with rigid gas permeable len- ses having three different low Dk val- ues. The size for all was 9.2mm, and the center thickness was 0.13mm. Biomicroscopy was made for each eye immediately on lens removal, and 0.2 ml of aqueous was collected with a disposable syringe. The pH of the aqueous was determined with a mi- croelectrode. The amounts of lactic acid and glucose were quantitatively determined.
The results indicate that the abnor- mal increases in lactic acid are unlikely when a material’s Dk value is higher than 32 and suggest that a combina- tion of a good lens-tocornea fitting re- lationship and the ability of a contact lens material to transmit oxygen may satisfy the criteria for extended wear regardless of the Dk value.
Koetting et al.2 reported that a pa- tient’s ability to wear a RGP contact lens for extended wear is not com- pletely contingent on the oxygen transmission characteristics of the lens. They believe that tear flow un- der a contact lens may be more im- portant than is the oxygen passing through it.
Just as we learned that there is no singular hydrogel contact lens ma- terial that will satisfy all clinical needs, we should discover that this is also true for RGP contact lens materials.
Although it may be difficult to use the Dk value of a RGP contact lens material to predict its clinical perform- ance, it is safe to predict that we shall always rely on trial-and-error clinical investigation to fit this material as a way to satisfy clinical success.
References
1. Kamiya, C: Studies on gas- permeable hard contact lenses: A quantitative analysis of lactic acid in rabbit aqueous humor. CLAOJ. 1984;10(1):96-99.
2. Koetting, R.A., Castellano, CF., Nelson, DW: Extended wear with low Dk hard gas permeables. Contact Lens forum 1985; 1 O(2): 77-79.
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Volume 16, Numbers 11 & 12 November/December 1989 355
