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Title: Effect of ocular higher order aberrations induced from orthokeratology lenses with different compression factors on axial elongation
Authors: Lau, Ki-kit Jason
Degree: Ph.D.
Issue Date: 2020
Abstract: Higher order aberrations (HOAs), optical imperfections that persist despite traditional spherocylindrical refractive correction, contribute to approximately 10% of the total ocular aberrations and vary with numerous factors, including, but not limited to, age, refractive error, pupil size, and accommodation. Changes in the HOA profile can improve or deteriorate retinal image quality, and potentially affect the vision dependent emmetropisation process. While orthokeratology (ortho-k) is one of the most effective myopia control interventions, the mechanism by which it slows axial elongation remains unclear. Since HOAs are substantially elevated following ortho-k treatment, investigating the association between ocular HOAs and axial elongation in paediatric ortho-k may provide new insights into its mechanism of action. This study aimed to investigate this association in normal (untreated, spectacle-wearing) children and ortho-k-treated children using retrospective and prospective data. Modifications to ortho-k lenses by adjusting the compression factor were performed in order to manipulate the levels of induced HOAs. The short-term (one month) and long-term (two years) effects were examined using the changes in choroidal thickness and axial length, respectively. The predictive value of induced ocular HOAs after one month of treatment for axial elongation across all study visits in the two-year period was also investigated. In the retrospective analysis, the association between ocular HOAs and axial elongation measured annually over a two-year period in 137 children was analysed. Using a 6-mm pupil size and a sixth order Zernike polynomials expansion, it was shown that higher levels of ocular HOAs, particularly spherical aberration, were associated with longer axial length and slower axial elongation, after adjusting for other known confounding factors such as age, sex, and refractive error.
In the analysis of another 103 ortho-k-treated subjects, as expected, there were approximately three- to nine-time increases in HOAs after treatment. Adjusting for the influence of baseline ocular HOAs on axial elongation, similar associations were observed between HOAs and axial elongation as reported for spectacle-wearing children. Higher levels of HOAs and spherical aberration were associated with slower elongation. Based on statistical modelling, every micron increase in spherical aberration was associated with 0.46 mm slower axial elongation per year. A contralateral eye, self-controlled study of 28 children was performed to investigate the effect of modifying the ortho-k lens compression factor on ocular HOAs. Ortho-k lenses of different compression factors (0.75 and 1.75 D) were randomly fitted on the fellow eyes of each subject and monitored weekly for one month. It was shown that increasing the compression factor by 1.00 D induced approximately 40% more ocular HOAs, without significantly altering the refractive correction of the subjects. There was no significant difference between the changes in choroidal thickness between the two eyes. Considering paired-eye data, the subfoveal choroidal thickness transiently increased in early lens wear (mean change at week 1: +9.1 ± 12.6 μm), but gradually regressed to the baseline level at the end of the study. However, on average this change approached the coefficient of repeatability of the instrument and analysis procedure (8.0 - 9.0 μm). Measuring the choroidal thickness in early ortho-k lens wear may not provide sufficient information to inform future axial elongation in eyes fitted with different compression factors. A two-year longitudinal, randomised (using different compression factors of ortho-k lenses) study was conducted to examine the influence of different compression factors (0.75 and 1.75 D) on axial elongation. A total of 75 subjects (control: 11; ortho-k [0.75 D]: 29; ortho-k [1.75 D]: 35) completed the study. However, the high dropout rate (63%) and potentially biased control group was not suitable to include in between-group comparisons. Increasing the compression factor improved the myopia control effectiveness by about 30%, compared with the conventional compression factor. In addition, increasing the compression factor of ortho-k lenses significantly increased the induced HOAs, particularly spherical aberration, which was associated with slower axial elongation. It was therefore speculated that the myopia control effect of ortho-k treatment in Chinese children may be improved by increasing the compression factor of ortho-k lenses by 1.00 D. In conclusion, higher levels of ocular HOAs were associated with slower axial elongation in both spectacle-wearing and ortho-k treated children. Increasing the compression factor of ortho-k lenses by 1.00 D induced more ocular HOAs without adversely affecting visual performance and improved the myopia control effect of the ortho-k treatment.
Subjects: Orthokeratology
Hong Kong Polytechnic University -- Dissertations
Pages: xxii, 175 pages : color illustrations
Appears in Collections:Thesis

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