Search

[EN] The study of oxygen consumption rate under¿ in vivo¿ human cornea during contact lens wear has been technically a challenge and several attempts have been made in the last 20 years to model the physiology of the human cornea during contact lens wear. Unfortunately, some of these models, based on a constant corneal oxygen consumption rate, produce areas on the cornea where the oxygen tension is negative, which has no physical sense. In order to avoid such inconsistency, different researchers have developed alternative models of oxygen consumption, which predict the likely oxygen metrics available at the interface cornea/post lens tear film by determination of oxygen flux, oxygen consumption, and oxygen tension through the different layers (endothelium, stroma, and epithelium). Although oxygen deficiency produces corneal edema, corneal swelling, hypoxia, acidosis, and other abnormalities, the estimation of the oxygen distribution below the impact of a contact lens wear is interesting to know which lens transmissibility was adequate to maintain the cornea and avoid epithelial and stromal anoxia. The estimation of minimum transmissibility for a lens for extended wear applications will be very useful for both clinicians and manufacturers. The aim of this work is to present a complete discussion based on Monod kinetics model that permits give an estimation of oxygen partial pressure distribution, the profile distribution of corneal flux and oxygen consumption rate, and finally the estimation of the relaxation mechanism of the cornea depending on the oxygen tension at the interface cornea/post lens tear film. Relaxation time in this context can quantify the capability of the corneal tissue to adapt to increasing concentrations of oxygen. It is proposed this parameter as a biological meaningful indicator of the interaction between contact lens polymers and living tissues such as the corneal cellular layer.

[EN] Purpose Oxygen is essential for aerobic mammalian cell physiology. Oxygen tension (PO2) should reach a minimum at some position within the corneal stroma, and oxygen flux should be zero, by definition, at this point as well. We found the locations and magnitudes of this ¿corneal equilibrium flux¿ (xmin) and explored its physiological implications. Methods We used an application of the Monod kinetic model to calculate xmin for normal human cornea as anterior surface PO2 changes from 155 to 20 mmHg. Results We find that xmin deepens, broadens, and advances from 1.25 ¿m above the endothelial¿aqueous humor surface toward the epithelium (reaching a position 320 ¿m above the endothelial¿aqueous humor surface) as anterior corneal surface PO2 decreases from 155 to 20 mmHg. Conclusions Our model supports an anterior corneal oxygen flux of 9 ¿L O2 · cm¿2 · h¿1 and an epithelial oxygen consumption of approximately 4 ¿L O2 · cm¿2 · h¿1. Only at the highest anterior corneal PO2 does our model predict that oxygen diffuses all the way through the cornea to perhaps reach the anterior chamber. Of most interest, corneal oxygen consumption should be supported down to a corneal surface PO2 of 60 to 80 mmHg but declines below this range. We conclude that the critical oxygen tension for hypoxia induced corneal swelling is more likely this range rather than a fixed value.

[EN] Purpose. The main goal of this current work is to use an updated calculation paradigm, and updated boundary conditions, toprovidetheoreticalguidelinestoassisttheclinicianwhosegoalistoimprovehisorherscleralgaspermeable(GP)contact lens wearing patients¿ anterior corneal oxygen supply. Methods. Our model uses a variable value of corneal oxygen consumption developed through Monod equations that disallows negative oxygen tensions within the stroma to predict oxygen tension at the anterior corneal surface of scleral GP contact lens wearing eyes, and to describe oxygen tension and flux profiles, for various boundary conditions, through the lens, tears, and cornea. We use several updated tissue and boundary parameters in our model. Tear exchange with GP scleral lenses is considered nonexistent in this model. Results. The majority of current scleral GP contact lenses should produce some levels of corneal hypoxia under open eye conditions.Onlylensesproducingthethinnestoftearvaultsshouldresultinanteriorcornealsurfaceoxygentensionsgreaterthana presumed critical oxygen tension of 100 mmHg. We also find that corneal oxygen tension and flux are each more sensitive to modification in tear vault than to changes in lens oxygen permeability, within the ranges of current clinical manipulation. Conclusions. Our study suggests that clinicians would be prudent to prescribe scleral GP lenses manufactured from higher oxygen permeability materials and especially to fit without excessive corneal clearance. (Optom Vis Sci 2016;93:00Y00

PURPOSE. We defined the theoretical oxygen tension behind modern scleral contact lenses (CLs) made of different rigid gas permeable (RGP) materials, assuming different thickness of the tear layer behind the lens. A second goal was to show clinically the effect of the postlens tear film on corneal swelling. METHODS. We simulated the partial pressure of oxygen across the cornea behind scleral CLs made of different lens materials (oxygen permeability Dk, 75-200 barrer) and different thickness (T-av, 100-300 mu m). Postlens tear film thicknesses (Tpost-tear) ranging from 150 to 350 mu m were considered. Eight healthy subjects were fitted randomly with a scleral lens with a thin and a thick postlens tear layer in two different sessions for a period of 3 hours under open-eye conditions. RESULTS. The CLs with less than 125 barrer of Dk and a thickness over 200 mu m depleted the oxygen availability at the lens-cornea interface below 55 mm Hg for a postlens tear film of 150 mu m. For a postlens tear film thickness of 350 mu m, no combination of material or lens thickness will meet the criteria of 55 mm Hg. Our clinical measures of corneal edema showed that this was significantly higher (P < 0.001, Wilcoxon signed ranks test) with the thicker compared to the thinner Tpost-tear (mean +/- SD, 1.66 +/- 1.12 vs. 4.27 +/- 1.19%). CONCLUSIONS. Scleral RGP CLs must be comprised of at least 125 barrer of oxygen permeability and up to 200 mu m thick to avoid hypoxic effects even under open eye conditions. Postlens tear film layer should be below 150 mu m to avoid clinically significant edema.