Optometry and Vision Sciences - Theses

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    Oxygen levels at the human cornea during contact lens wear
    Efron, Nathan ( 1981)
    Contact lenses can restrict the amount of oxygen available to the cornea, often resulting in corneal swelling and visual dysfunction. However, despite the obvious importance of corneal oxygen availability, few studies have attempted to measure the level of oxygen at the anterior corneal surface of the in vivo human eye during contact lens wear. The aim of this thesis is to acquire such information. The rate at which the unanaesthetised cornea consumes oxygen from the membrane of a polarographic oxygen sensor forms an arbitrary index of corneal oxygen demand. An equivalent oxygen technique was modified for use in the in vivo human eye; this technique involved equating the corneal oxygen demand following a test condition (such as contact lens wear), to that following exposure to known oxygen levels. Human eye models relating static equivalent oxygen percentages (EOP) to lens oxygen transmissibility (Dk/L) were derived using seven hard and soft gas-permeable contact lenses to allow predictions to be made of the likely physiological response to contact lens wear. The discrepancies between these and other models were to be expected in view of the differing criteria upon which they were based. The effects of blinking and lens fit on the EOP beneath hydrogel lenses were found to be minimal, confirming earlier predictions. The significant increases in rigid lens EOP results upon blinking could be explained in terms of the tear pumping mechanisms known to exist with such lenses. An EOP of 7.7% O2 is available to the cornea of the closed eye. In some instances, a significant amount of oxygen is derived not only from the paipebral conjunctiva, but also from the atmosphere as a result of an imperfect paipebral aperture seal. Attempts to derive and EOP v Dk/L model for contact lens wearing under closed-lid conditions were unsuccessful; individual variations in the supply of oxygen to the cornea under these conditions were thought to contribute to this failure. The validity of similar models was questioned in view of the simplified assumption which were made.
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    Changes in selected physiological parameters of the human cornea with age
    Horton, Peregrine ( 1989)
    The effects of age on corneal oxygen flux and the corneal oedema response to contact lens induced hypoxia were studied by comparing the measurements of these variables made on two subject groups of differing ages. No significant difference in corneal oxygen flux between the groups was detected, although a trend towards a decrease in oxygen flux with age was found. A difference was detected, however, in the amount of corneal oedema which occurred in response to eye closure, both in the presence of a thick hydrogel contact lens, which caused corneal hypoxia, and without a lens, with older corneas swelling significantly more than those of the younger group. The implications of this finding for the clinical management of contact lens wearers are discussed. These findings are of particular relevance to the fitting of extended wear lenses to elderly patients, as they cast doubt on the criteria commonly used to determine "acceptable" levels of corneal oedema during lens wear. An attempt was also made to correlate several aspects of corneal structure and physiology with age and each other. A significant difference in the thickness of the corneal epithelium was found to exist between central and peripheral locations - a previously unreported phenomenon. A mathematical model was constructed, relating the various corneal parameters measured.