Purpose: To evaluate the safety of scleral-lens designs, we model and clinically assess central corneal edema induced by scleral-lens wear for healthy subjects.
Materials and methods: Central corneal swelling during scleral-lens wear is measured using optical coherence tomography (OCT). Transport resistances are modeled for oxygen diffusion through the scleral lens and post-lens tear-film (PoLTF), and into the cornea. Oxygen deficiency in the cornea activates anaerobic metabolic reactions that induce corneal edema. Oxygen permeability, carbon-dioxide permeability, settled-lens PoLTF thickness, and scleral-lens thickness are varied in the calculations to mimic different lens fits.
Results: Transport modeling predicts that for open eyes, increasing PoLTF thickness from 50 to 400 µm increases central corneal swelling by approximately 1-1.5% when oxygen transmissibility (Dk/L) is greater than 10 hBarrer/cm (i.e., hectoBarrer/cm). Although swelling is larger for oxygen Dk/L < 10 hBarrer/cm, PoLTF thickness has minimal impact in this range. For open eye, oxygen transmissibility of the lens plays a significant role in corneal edema, but is negligible when oxygen Dk/L is > 40 hBarrer/cm. For closed eye, central corneal swelling is greater than 5% for an oxygen Dk/L range of 0-100 hBarrer/cm with typical lens-fitting parameters. For carbon-dioxide transmissibilities increasing from 50 to 250 hBarrer/cm and with a fixed oxygen Dk/L of 25 hBarrer/cm, calculated swelling diminishes by an additional 0.5%. Comparison of model calculations to clinical-swelling data is within the error range of the clinical measurements.
Conclusions: Oxygen/metabolite transport calculations for open-eye scleral-lens wear show that typical PoLTF thicknesses fitted by clinicians (i.e., PoLTF thicknesses < 400 µm) with modern scleral lenses (i.e., oxygen Dk/L > 25 hBarrer/cm) produce corneal swelling of less than 2% in agreement with experiment. Therefore, scleral lenses prescribed today evoke less than physiological hypoxic swelling (i.e., less than 4%) for healthy corneas during open-eye. Closed-eye wear, however, appears clinically unsafe.
Keywords: Scleral lens; computational modeling; contact lens; corneal edema; corneal metabolism; hypoxia; oxygen.