Corneal biomechanical properties and the development of myopia

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Corneal biomechanical properties and the development of myopia

 

Author: Wong, Yin Zhi
Title: Corneal biomechanical properties and the development of myopia
Degree: Ph.D.
Year: 2015
Subject: Cornea -- Mechanical properties.
Myopia.
Hong Kong Polytechnic University -- Dissertations
Department: School of Optometry
Pages: xvii, 160 leaves : illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2806863
URI: http://theses.lib.polyu.edu.hk/handle/200/7935
Abstract: Cornea is a viscoelastic tissue. Collagen fibrils and ground substance in the corneal stroma are the contributors of corneal biomechanical properties. Clinical measurement of these properties is available using Ocular Response Analyzer (ORA). Corneal hysteresis (CH) and corneal resistance factors (CRF) are corneal biomechanical parameters obtainable using ORA. Myopia is a major refractive error that affects people worldwide. It garners research interest because of its pathological complications and direct socioeconomic costs. Although low CH is associated with a long eyeball, its role in axial elongation is unclear. This study comprises three experiments to investigate the influence of corneal curvature on CH and CRF (Study I), the associations of CH and CRF with other corneal parameters (Study II), and to monitor the changes in CH and CRF with axial elongation in children (Study III). The ORA measurement relies on an infrared reflection along the horizontal corneal meridian, but provides CH and CRF for the whole cornea. Study I (n=95) investigated the influence of corneal curvature and astigmatism and meridional differences on CH and CRF measurements, when inter-subject variation on CH and CRF was eliminated. We measured CH and CRF at the default position, and with 10°, 20°, and 30° head tilts. CH and CRF were similar in different amounts of corneal astigmatism. Only CRF had a marginal association with corneal astigmatism (r2=0.04, p=0.047). CH and CRF were the lowest along the horizontal meridian compared with the other meridians, but the difference was clinically small. CH and CRF were unaffected by corneal curvature and astigmatism. No previous studies have considered the contribution of corneal stroma on CH and CRF. Study II (n=80) investigated the associations among CH, CRF, axial length (AL), corneal volume (CV), corneal curvature, full stromal thickness (FST), and epithelial and Bowman's thicknesses (Epi+BT) between high myopes and emmetropes. We first confirmed the good inter-observer reproducibility of FST and Epi+BT measurements with confocal microscopy. High myopes exhibited a longer AL and lower CH compared with emmetropes. FST (standard coefficient, β=0.591) and CV (β=0.575) had a stronger association than did AL (β=-0.1) for CH. Study III involved observing the changes in CH and CRF with axial elongation in children. We conducted school-based vision screenings (n=1199) to measure the CH, CRF, AL, corneal curvature, and astigmatism. Apart from cross-sectional analysis, we conducted 1-year cohort (n=269) and 2-year cohort (n=144) studies to monitor the changes of these parameters. Chinese children had lower CH compared with other ethnic groups. Chinese children in local and international schools shared similar CH and CRF. Among Chinese children, CH increased by an average of 0.21mmHg and 0.28mmHg, and after axial elongation by an average of 0.23mm and 0.46mm, annually and biennially, respectively. However, whether a change in CH is a cause or a result of axial elongation remains inconclusive. In summary, corneal curvature, corneal astigmatism, and Epi+BT had minimal influence on CH and CRF. FST and CV had a stronger association with low CH than did the long AL. CH was found to have increased after axial elongation.

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