Abstract
Purpose :
Current scleral-lens (SL) calculations [Compañ et al. 2014, Langis et al. 2012] well characterize corneal oxygen-tension profiles with SL wear. However, these contributions do not directly determine corneal edema; they resort to empirical definitions of oxygen deficiency. To predict actual swelling with SL wear, we quantify hypoxic edema from coupled transport of salts, respiratory metabolites, and water, including an active endothelial pump for bicarbonate [Leung et al. 2011].
Methods :
We extend the steady-state corneal-metabolic model of Leung et al. to include a post-lens tear film (PoLTF) and SL. Because Leung et al. determined that water flux is insignificant, we adopted a zero water flux. Corneal hydration and thickness follow from Kedem-Katchalsky membrane transport relations [Kedem et al. 1958] and the corneal-hydration relation of Hedbys and Mishima [1966]. PoLTF thickness, lens oxygen permeability (Dk), and lens thickness were varied to mimic different lenses and lens fits.
Results :
Figure 1 displays predicted percent change in the central corneal thickness for open eye as a function of lens Dk for varying PoLTF thickness with a 400 µm thick SL. Model predictions show clinically insignificant amounts of corneal thickness change for common PoLTF thickness range of 150-250 µm. This finding is confirmed by clinical in vivo measurements [Tan et al. AAO 2015]. Lens oxygen transmissibility (Dk/t) plays a minimal role beyond 44 Fatt Dk/t units (i.e. Dk of 175 Barrer on Figure 1). Apparently, the Dk/t requirement for safe SL wear is larger than that for soft contact-lens open-eye wear [Holden et al. 1983] due to increased PoLTF thickness. We successfully capture the measured swelling reported by Campañ et al. (symbols in Figure 1).
Conclusions :
For the first time, we directly assess central corneal swelling for a SL wear from metabolic oxygen deficiency caused by lens thickness, PoLTF thickness, and lens Dk. Safety of different designs of SL can now be assessed by quantifying swelling of the cornea rather than empirically correlating swelling against oxygen concentration or against Dk/t.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.