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Title: Tonometry : a study in biomechanical modelling : appraisal and utility of measurable biomechanical markers
Author: Frampton, Peter
ISNI:       0000 0004 7226 3424
Awarding Body: Aston University
Current Institution: Aston University
Date of Award: 2017
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Goldmann Applanation Tonometry (GAT) is the recognised ‘Gold Standard’ tonometer. However this status is refuted by eminent authors. These contradictory views have driven the initial goal to assess, from first principles, the evolution of GAT and to experimentally evaluate its utility and corrections. Subsequently, an important caveat became the evaluation of Corneal Hysteresis and Corneal Resistance Factor. Chapter 1. Biomechanical building blocks are defined and constitutive principles incorporated into continuum modelling. The Imbert-Fick construct is re-interpreted a simple biomechanical model. GAT corrections are also appraised within a continuum framework; CCT, geometry and stiffness. These principles enable evaluation of alternative tonometer theory and the evolving biomechanical markers, Corneal Hysteresis (ORA-CH) and Corneal Resistance Factor (ORA-CRF). Chapter 2 appraises corneal biomechanical markers, CCT, curvature, ORA-CH and ORA-CRF in 91 normal eyes and the impact these have on three tonometers: GAT, Tonopen and Ocular Response Analyser (ORA). Tonopen was the sole tonometer not affected by biomechanics. CCT was confirmed the sole measurable parameter affecting GAT. ORA did not demonstrate improved utility. ORA-CH and ORA-CRF do not appear robust biomechanical measures. Chapter 3 assessed agreement between GAT, the ORA measures and Tonopen. Tonopen is found to measure highest and raises the question should a development goal emphasise GAT agreement or improvement? Chapter 4 assessed repeatability of the three tonometers and biomechanical measures keratometry, pachymetry, ORA-CH and ORA-CRF on 35 eyes. Coefficients of Repeatability (CoR) of all tonometers are wide. Effects assessed in Chapter 5 may be masked by general noise. ORA does not appear to enhance utility over GAT. Isolation of corneal shape change via Orthokeratology (Chapter 5) demonstrate ORACH and ORA-CRF reflect, predominantly, a response to corneal flattening. It is proposed they do not significantly reflect corneal biomechanics. After reviewing models for tear forces (Chapter 6), a refined mathematical model is presented. Tear bridge attraction is minimal and cannot explain under-estimation of IOP by GAT in thin corneas. CCT corrections and the Imbert-Fick rules are incompatible. Chapter 7 summarises findings. The supremacy of GAT is likely to remain for some time, reflecting the sheer magnitude of overturning 60 years of convention, historical precedent, expert opinion as well as the logistical and educational difficulties of redefining standards and statistical norms.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral