Dynamic Salt Accumulation in the Post-Lens Tear Film with Soft-Contact-Lens Wear: Implications for Protection Against Corneal Hyperosmolarity

Abstract

Purpose : Evaporative-salinity increases in the pre-corneal tear film can lead to hyperosmotic stress-induced discomfort [Liu et al. 2013]. By shielding the cornea, soft contact lenses (SCLs) can, in principle, protect the cornea against such hyperosmotic stresses. Because SCLs are hydrogels, salt can permeate through the lens [Guan et al. 2011] and post-lens tear film (PoLTF) to reach the cornea [Kim et al. 2021]. Permeation through the lens requires time for the salt to diffuse across the lens. If the time required for salt to transport across the lens is of order hours, lens wearers could remove and reinsert the lens during the midday to forestall PoLTF hyperosmolarity. Here, to ascertain whether midday reinsertion is suitable method to mitigate PoLTF hyperosmolarity, we determine the time it takes for the PoLTF osmolarity to reach a periodic steady state.

Methods : The SCL osmolarity model of Kim et al. [2021] was modified to determine the time it takes to reach steady state for various lens salt partition coefficients (k_s), lens salt diffusivities (D_s), and lens thicknesses (h_lens). The mathematical model is scripted and computed with Matlab R2019b. Modified script counts the number of iterations required to reach periodic steady state and converts iteration number into the time to reach steady state. Analyses are conducted for both normal and dry-eye scenarios.

Results : Figure 1 provides calculated PoLTF osmolarity for normal eye for various D_s and fixed interblink period (t_ib), k_s, and h_lens (specified in Figure 1). The D_s range in Figure 1 accounts for most commercially available silicone-hydrogel and hydrogel contact lenses today. PoLTF osmolarity reaches steady state within the first 30 min of lens wear. This is also the case for the dry-eye scenario (i.e., increased tear-evaporation and decreased tear-production rates). Time to attain steady state was still less than 30 min even after varying h_lens and t_ib over physically realistic values.

Conclusions : Time to steady state for the PoLTF osmolarity occurs within the first hour of lens insertion for all adjustable parameters. Therefore, midday lens removal is not a viable way to regulate PoLTF osmolarity. Lowering D_s is the most effective way to mitigate PoLTF hyperosmolarity.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Figure 1: Post-lens tear-film osmolarity histories for different lens-salt diffusivities. Time = 0 is when the lens is inserted.