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
(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.
Available from: Jian Zhou
- "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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ABSTRACT: Grand canonical Monte Carlo combined with molecular dynamics simulation methods are used to investigate the microstructure and oxygen permeability in hydrogels containing interpenetrating poly(bis(trimethylsilyloxy)methylsilylpropyl glycerol methacrylate) (PSiMA) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) networks at varying water contents. Water molecules are classified into hydrophilic-hydration water, hydrophobic-hydration water and intermediate water according to their distribution in the hydrogel. As the hydrogel becomes increasingly swollen, three kinds of water molecules concurrently move faster. This leads to the increased water self-diffusion coefficient as the hydrogel's water content is increased. On the contrary, increasing water content in the hydrogel is associated with the decrease of oxygen permeability. This decrease is mainly attributed to the unique transport path of oxygen molecules in the specific hydrogel. Oxygen molecules mainly transport through the PSiMA networks. While the increasing amount of hydrophobic-hydration water molecules occupy oxygen's sorption sites within PSiMA. In addition, higher amount of hydrophobic-hydration water lessens the continuity of free volume in the silicone phase. As a consequence, both oxygen's solubility and diffusivity in the hydrogel are reduced. This contribution anticipates providing some guidance and inspiration for developments of next-generation hydrogel contact lenses.
Available from: Peter White
- "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 . Although the Dk/t of lens material used is high, it is still below the amount estimated by Harvitt and Bonanno  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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ABSTRACT: Overnight orthokeratology is becoming more and more popular especially in the Asia-Pacific region where the treatment is primarily used for myopic control in young children. Risk of complications in contact lens wear increases during overnight wear and may further increase when the treatment is used on children. The aim of this paper is to provide a comprehensive guideline for practitioners to improve their orthokeratology practice and minimize unnecessary or preventable complications. The fundamental requirement for starting an orthokeratology practice is to have proper education in the area and to equip the practice appropriately. Overnight trial fitting is recommended to confirm the physiological response prior to commencement of the treatment. Practitioners should provide adequate information, both oral and written, to patients before and after the commencement of treatment to avoid any legal dilemmas and to improve patients' compliance. Costs for the treatment should be transparent and provision of an emergency contact number is a must. Patients should be regularly recalled for aftercare visits and all communication with patients should be properly documented. In this paper, patient selection and the clinical procedures were discussed and a standard of practice in orthokeratology proposed. We believe that the key to providing a safe orthokeratology practice is to continually update knowledge in the field, and to practice to the highest professional standards.
Available from: aoa.org
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