Effects of Oxygen and Salt Transport Across Contact Lenses on Lens-Wear Safety and Comfort

There are numerous types of contact-lens materials and designs available for lens wearers today. Regardless of whether the lens is mass produced or customized to an individual patient, oxygen and ion transport across the contact lens is critical to ensure that contact-lens wear is safe and comfortable for the lens wearers. High oxygen-permeable contact lenses allow enough oxygen supply from the environment to the cornea to avoid corneal hypoxia-induced edema. Meanwhile, high ion-permeable contact lenses allow the tear-film between the contact lens and the ocular surface (i.e., post-lens tear film) to be thick enough to avoid lens adherence to the ocular surface. However, high ion-permeable contact lenses could potentially result in post-lens tear-film hyperosmolarity to activate corneal nociceptors and cause discomfort. Effect of contact-lens ion transmissibility (strictly salt transmissibility) on post-lens tear-film hyperosmolarity has not been investigated. Aims of this dissertation is to quantify the effects of contact-lens salt transport properties on post-lens tear-film hyperosmolarity and to quantify central-to-peripheral corneal edema during wear of various types of contact lenses that have yet to be investigated for hypoxic safety. To quantify central-to-peripheral corneal edema for wear of various contact lenses, one-dimensional metabolic-edema model is initially developed to determine central corneal edema for scleral-lens wear. The model utilizes metabolic kinetics and diffusion to determine accurate concentrations of aerobic and anerobic metabolic species. Change in concentrations of metabolic species due to corneal hypoxia affects the hydraulic pressure and, therefore, the corneal thickness. The severity of corneal hypoxia is dependent on the adjustable lens oxygen transmissibility and the post-lens tear-film thickness. Model results show excellent agreement with the clinical corneal edema measurements made from subjects wearing scleral lenses with various lens oxygen transmissibilities and post-lens tear-film thicknesses. Central corneal edema due to wear of scleral lenses made of silicone-acrylate based materials is clinically insignificant while being awake. However, scleral-lens wear during sleep will cause clinically significant swelling and should be avoided. The one-dimensional metabolic-edema model is expanded to two dimensions by incorporating central-to-peripheral cornea and contact lens. Alongside, metabolic support from the limbus is incorporated at the peripheral cornea to accurately model non-central corneal edema. Central-to-peripheral corneal edema is determined for wear of soft contact, scleral, and component embedded contact lenses and is rigorously compared with available clinical data to ensure the accuracy of the metabolic-edema model. Results show that supply of bicarbonate ions and oxygen and removal of lactate ions by the limbal blood supply reduces corneal edema at the midperiphery and the periphery. Due to the limbal metabolic support, central corneal edema measured clinically is a good standard for assessing hypoxic safety for wear of scleral and soft contact lenses despite the variance in lens oxygen transmissibility throughout the lens and higher oxygen demand at the non-central cornea than the central cornea. For component embedded contact lenses, central-to-peripheral corneal edema must be accounted to ensure the hypoxic safety of lens wear. To minimize corneal edema, low oxygen permeable embedments are advised to be embedded in the lens periphery. Post-lens tear-film osmolarity during contact-lens wear has not been measured clinically because the tear film is microns thick and because the tear film is covered by the contact lens that prevents direct probe interaction. To understand the osmolarity of the post-lens tear film, theoretical osmolarity models are developed to predict spatially-averaged and localized post-lens tear-film osmolarities during wear of soft contact lenses with varying salt diffusivity, lens thickness, and salt partition coefficients. Lenses made of acrylate materials are not studied because those hard lenses are practically ion impermeable compared to those of soft contact lenses. Spatially-averaged osmolarity of post-lens tear film is determined by quantifying the transport of salt and water across the contact lens between the pre-lens and the post-lens tear films. The model also incorporates tear production and evaporation rates, tear mixing, and tear produced from the cornea and the bulbar conjunctiva to accurately model the lens-wear osmotic behavior. Meanwhile, localized post-lens tear-film osmolarity due to tear break-up areas on the pre-lens tear film is determined by developing a 2-dimensional model that allows localized hyperosmotic regions to be formed on the pre-lens tear film. Results show that soft-contact-lens wear can protect the cornea from both localized and spatially-averaged hyperosmolarity when the lens-salt diffusivity is low. Realistic ranges of salt partition coefficient and lens thickness for existing soft contact lenses do not have meaningful impact on the post-lens tear-film osmolarity. Conversion of localized post-lens tear-film hyperosmolarity spike values to pain scores based on a previously conducted clinical study suggests that soft-contact-lens wear can mitigate osmolarity-induced discomfort. Understanding salt and oxygen transports across contact lenses allow development of safe and comfortable novel contact lenses. Theoretical models in this dissertation allow optimization of future lens designs to minimize hyperosmolarity and hypoxia for safe and comfortable lens wear.

Advisor: Radke, Clayton J
Lin, Meng C