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Claude J Giasson, Jeanne Morency, Langis Michaud; Measuring Oxygen Levels Under Scleral Lenses of Different Clearances. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):6074. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To measure the percent partial pressure in oxygen under rigid gas permeable scleral lenses of different clearances.
The right eye of 8 normal subjects was equipped with 2 scleral lenses (SL200 and SL400) of equal thicknesses (Boston XO2, diameter:18 mm), but adjusted to produce a post-lens layer thickness (clearance) of 200 and 400 µm. The thickness of the liquid layer beneath the center of these scleral lenses was measured with an optical coherence tomograph after 5 minutes of wear. As shortly as possible after a 5 minute-exposure to gases with oxygen tensions of 0, 1.01, 2.50, 5.00, 10.00 and 20.9%, pO2 levels were measured at the corneal surface with a Clark electrode and an electrometer (PHM 73, Radiometer, Copenhagen) linked to a computer. The exponential decay of oxygen upon corneal application was calibrated with a non-linear regression using the Levenberg Marquardt iteration algorithm. A linear regression was done between the obtained exponential decay coefficients and the corresponding percentage of oxygen in the calibrationg gas. Using this regression, it was then possible to obtain with the decay observed after 5 minute of wear of SL200 and SL400, an estimate of the oxygen partial pressure percentage on the corneal surface under SL200 and SL400. Differences between LS200 and LS400 in terms of oxygen percentage equivalent or liquid layer thicknesses were tested for significance with paired t tests.
Mean equivalent oxygen percentage beneath SL200 and SL400 were 9.0 ± 2.6 and 6.7 ± 1.9 %, respectively. The thickness of the fluid trapped under the lens were 240 ± 35 and 435 ± 33 µm for the SL200 and SL400, respectively. Both of these differences between the SL200 and SL400 were statistically significant (p < 0.05).
As predicted by previous calculations of oxygen transmissibility of scleral lens systems when applying the concept of resistors in series of the lens and of the tear film thickness (Michaud, Contact Lens & Anterior Eye 2012; 35 (6): 266-271), we demonstrate that increased thickness in the layer of fluid trapped under a scleral lens reduces the oxygen tension available to the cornea.
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