Re-Evaluation of the Oxygen Diffusion Model for Predicting Minimum Contact Lens Dk/t Values Needed to Avoid Corneal Anoxia

University of California, Berkeley, Berkeley, California, United States
Optometry and Vision Science (Impact Factor: 1.6). 11/1999; 76(10):712-9. DOI: 10.1097/00006324-199910000-00023
Source: PubMed

ABSTRACT (1) To update Fatt's mathematical model of the distribution of oxygen tension (pO2) across the cornea and contact lens (CL) to include the recent finding that corneal oxygen consumption increases with the acidification that occurs with CL wear. (2) To estimate the minimum transmissibility (CL Dk/t) to avoid epithelial anoxia or to avoid stromal anoxia.
A five-layer static and one-dimensional mathematical model of oxygen diffusion through the cornea based on Fatt's models was used. The relationships between acidosis and increased QO2, and acidosis and CL Dk/t were used to estimate corneal QO2 for a given CL Dk/t.
(1) Revised model predictions are in agreement with direct tear pO2 measurements beneath CLs in the rabbit. (2) For the human eye, the minimum CL Dk/t for oxygen delivery to the basal epithelial cells was determined to be 23 for the open eye and 89 for the closed eye. To prevent anoxia throughout the entire corneal thickness the Dk/t requirements are 35 for the open eye and 125 for the closed eye.
(1) Model predictions of the oxygen distribution beneath contact lenses are significantly lower than previous models that did not include the effect of acidosis on corneal QO2. (2) Minimum Dk/t values that allow oxygen delivery to the basal epithelium are in agreement with the Dk/t needed to avoid corneal edema.

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    • "Therefore, an ideal lens material should integrate appropriate elasticity, good optical performance, anti-fouling properties, and high oxygen permeability. A previous study (Harvitt and Bonanno, 1999) concluded that the minimum oxygen transmissibility value required for SCLs to be worn without cornea swelling is 125 Barrer/mm, far exceeding the maximum limit of conventional HEMA-based hydrogel materials. Silicone-based polymers have appealing features for fabrication of SCLs in virtue of their biological inertness and high oxygen permeability (Morgan and Nathan, 2002). "
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    Chemical Engineering Science 08/2012; 78:236–245. DOI:10.1016/j.ces.2011.11.020 · 2.34 Impact Factor
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    • "However, it does involve overnight wear and, though the risks involved in ortho-k are not fully understood, it can be argued that it is expected to increase the risk over daily wear contact lenses [18]. Although the Dk/t of lens material used is high, it is still below the amount estimated by Harvitt and Bonanno [19] to deliver adequate oxygen to the cornea to avoid hypoxia under closed eye conditions. It is therefore important for prudent practitioners to provide adequate information to the patients before (Information sheets and Consent forms) and after commencing the treatment (Education materials and advice). "
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    Contact Lens & Anterior Eye 03/2008; 31(1):17-28. DOI:10.1016/j.clae.2007.07.003 · 1.37 Impact Factor
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