City Research Online

Abstract

Orthokeratology lenses (ortho-k) are worn overnight and act to reshape the cornea to reduce myopic refractive errors and to control myopia in children. The overall aim of the work described in this thesis was to investigate factors that determine the effectiveness of ortho-k lenses for controlling myopia, specifically the four curve EyeDream ortho-k lens from No7 Contact Lenses. Using semi-customised eye models, we investigated the level of the retinal illuminance in the peripheral temporal retina associated with a range of pupil sizes. In conclusion, a low level of retinal illuminance leads to decreased image quality post ortho-k lens wear compared to baseline in all light conditions due to the increase of aberrations.

Corneal topography has become an integral part of ortho-k clinical practice. To be able to fit, design or monitor changes in ortho-k lens wearers, accurate corneal shape data from topographical maps are required. Agreement between the Aladdin biometer and Medmont E300 topographer on a healthy student population was high: mean difference in K flat and K steep were 0.03 mm (95% limits of agreement (LoA) −0.08 to +0.06 mm) and 0.01 mm (LoA -0.09 to +0.11 mm). The mean difference for the astigmatism vectors was 0.01mm for J0 (LoA -0.04 to +0.02 mm) and 0.07mm for J45 (LoA -0.02 to +0.02 mm). However, significant differences were found regarding eccentricity measurements with the Aladdin producing eccentricity values N% lower than the Medmont E300 (range 0 – 36%). Therefore, it may not be appropriate to use these two instruments interchangeably for peripheral corneal topography.

The success of ortho-k lens fitting and its effectiveness is dependent on baseline corneal shape. We explored inter-eye variability of corneal shape in 129 young, healthy, UK-based adults, while also exploring associations with sex and ethnicity. Our study found significant differences in the anterior corneal shape between the eyes, that was associated with sex, but not between white and South Asian ethnicity. Young adult eyes present
some degree of asymmetry, particularly in the mid-and far periphery (> 6mm chord), independent of the refractive error.

Publication Type:	Thesis (Doctoral)
Subjects:	R Medicine
Departments:	School of Health & Psychological Sciences > Optometry & Visual Sciences

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