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How rigid gas-permeable lenses supply more oxygen to the cornea than silicone hydrogels: a new model
Hideji Ichijima1, H Dwight Cavanagh
1Menicon Co., Ltd., Nagoya, Japan. h-ichijima@menicon-net.co.jp
Purpose:
To show the superiority of hyper-Dk rigid gas-permeable (RGP) lenses to silicone hydrogels from the viewpoint of total oxygen supply to the entire cornea.
Methods:
The simple model of fitting the cornea with a contact lens was divided into 1-mm steps from the center to the limbus, and the area of concentric ring zones was calculated. Precorneal oxygen tension (PO2) for each zone was calculated using the Brennan oxygen calculator, and by taking into account the area uncovered by the RGP lens and the lens thickness, the average precorneal PO2 was mathematically compared for wear of silicone hydrogel and RGP lenses.
Results:
The average precorneal PO2 of the RGP lens was superior to that of the silicone hydrogel lens under open- and closed-eye conditions because the precorneal PO2 in the uncovered areas of the cornea can supply large contributions. The PO2 of a silicone hydrogel lens with 125 x 10(-9) of oxygen transmissibility at center thickness (Dk/t(c)) was estimated as 45 mm Hg under closed-eye conditions. At the same value, the Dk/t(c) of an RGP lens with a diameter of 11.0 mm was estimated to be 90 x 10(-9).
Conclusions:
To minimize hypoxia in extended wear, clinicians should consider that an RGP lens with a Dk/t(c) value of only 90 x 10(-9) is equivalent to a silicone hydrogel lens with a Dk/t(c) value of 125 x 10(-9), and even high Dk/t(c) values for RGP lenses will provide more total oxygen to the entire cornea than their hybrid silicone hydrogel counterparts.
