An evaluation of gas permeable contact lens polymers and the effects on the corneal endothelium of long term PMMA contact lens wear

Stevenson, Ronald William Wood (1993) An evaluation of gas permeable contact lens polymers and the effects on the corneal endothelium of long term PMMA contact lens wear. PhD thesis, University of Glasgow.

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Abstract

Purpose: Long term hard contact lens wear may produce corneal changes, the most significant of which are those in the corneal endothelium. A proposed link with hypoxia being the main cause of these changes is fairly strong, and therefore the purpose of this thesis was twofold. To determine; 1. If prolonged hard contact lens wear has deleterious and potentially dangerous effects on the endothelium and 2. If the newer generation of oxygen permeable polymers demonstrated enhanced mechanical and physical properties to those of non-permeable polymethylmethacrylate. Methods: A method of polarography was developed as an electro-chemical means of measuring the diffusion of oxygen through various contact lens materials (n=8). Apparatus was devised to measure stress/strain curves on RGP material samples (n=12) and relate their mechanical properties to oxygen permeability values. High oxygen permeable lenses were fitted to astigmatic corneas to determine the degree of lens flexure and the corresponding induced astigmatism. Contact specular microscopy of the corneal endothelium was carried out on long term PMMA lens wearers (n=57) and a group of normal controls (n=45). Morphometry, using image analysis of the photomicrographs, allowed calculation of the mean cell area, cell density, coefficient of variation in cell area, and the skewness and kurtosis of the cell population of both groups of subjects. Central corneal thickness was measured in both groups of subjects. Results: Polarography was found to be a repeatable method of measuring oxygen permeability in a range of GP materials of varying oxygen permeability. All RGP materials tested, demonstrated significantly greater oxygen permeability than PMMA but less than that suggested by the manufacturers. The Young's Modulus of materials (n=7) varied significantly across the range tested and there was a significant correlation between oxygen permeability and flexibility (r=-.98). The flexing of high Dk lenses on astigmatic eyes was significant, but the degree was unpredictable. Specular microscopy revealed significantly greater polymegethism (t test, p<0.05) in the group of lens wearers (COV=0.4) than the control group (COV=0.28) and an increased skewness index, although no significant difference was found in the parameters of mean cell area, cell density, or central corneal thickness between the two groups of subjects. Conclusions: This project has shown that it is possible to obtain reliable results of oxygen permeability and flexibility of materials, and that increases in lens thickness to offset lens flexure will significantly decrease the oxygen permeability. The results from specular microscopy of the corneal endothelium, obtained from a unique group of lens wearers (n=57) are important to our understanding of the effects of long term (>10 years) contact lens wear. Although no individual cornea showed evidence of decompensation, the widespread endothelial changes observed suggest that this might be a risk following intra-ocular surgery.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Colm M Kirkness
Keywords: Polymer chemistry, Ophthalmology
Date of Award: 1993
Depositing User: Enlighten Team
Unique ID: glathesis:1993-72856
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 Jun 2019 11:06
Last Modified: 11 Jun 2019 11:06
URI: http://theses.gla.ac.uk/id/eprint/72856

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