Optometry Research Today is a free monthly online journal that collates and summarizes the latest research about Optometry, including details on myopia, optometric practice, therapy. | ||||||||
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Corneal oxygen distribution with contact lens wear.Alvord LA, Hall WJ, Keyes LD, Morgan CF, Winterton LC CIBA Vision Corporation, Duluth, GA 30097, USA. PURPOSE: Since 1991, multilayer mathematical in vivo oxygenation models have been created to predict normal corneal oxygenation with contact lens wear. From these models, there have been assertions that most hydrogel contact lenses allow 97%-98% of normal corneal oxygenation compared to no contact lens wear. In light of hydrogel lens-induced neovascularization and limbal hyperemia, to clinicians, this finding seems counterintuitive. This work seeks to validate or refute those preexisting models and estimate the impact of contact lens wear on the oxygen distribution profile across the cornea. To this end, to estimate the impact of contact lens wear on the 3-dimensional (3-D) oxygen distribution profile within the cornea as a function of the oxygen permeability of the contact lens, a two-dimensional axisymetric finite element analysis (FEA) model was constructed for contact lenses, on the cornea, both having varying thickness profiles. METHODS: A two dimensional (2-D) axi-symetric finite element analysis (FEA) model of a -3.00 D contact lens on eye was constructed. The model included the varying thickness profiles of the contact lens and cornea. By symmetry, this 2-D model is equivalent to a full 3-D model. The oxygen permeability, material thickness profile, and oxygen consumption coefficients from Brennan (Optometry and Vision Science, June 2005) were used for this validation. Several different oxygen consumption profiles were also considered. Oxygen partial pressure, flux, and consumption profiles were generated. RESULTS: Profiles of the oxygen partial pressure, flux, and consumption were generated from the central cornea to the limbal junction. CONCLUSION: This FEA model reproduced Brennan 8-layer model (BEL model) results at the central cornea. However, BEL model parameters yielded regions of oxygen deficiency in the corneal periphery, even in the open eye with no contact lens. If the BEL model cannot account for oxygenation across the whole cornea, it may be incorrect or incomplete. This assertion calls into question any conclusions from the BEL model regarding the minimum contact lens transmissibility needed to fully oxygenate the eye. Published 26 June 2007 in Cornea, 26(6): 654-64.
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