Higher-Order Aberrations in Keratoconus A thesis submitted to The University of Manchester, for the degree of Doctor of Philosophy, in the Faculty of Life Sciences. 2012 Amit Jinabhai Optometry Thesis contents List of Figures ............................................................................................................................................... 4 List of Tables ................................................................................................................................................ 5 List of Equations ........................................................................................................................................... 6 Abbreviations list .......................................................................................................................................... 7 Abstract ......................................................................................................................................................... 8 Declaration .................................................................................................................................................... 9 Thesis Format ............................................................................................................................................... 9 Copyright statement ................................................................................................................................... 10 Acknowledgements .................................................................................................................................... 11 1. Overview .................................................................................................................................................. 12 2. Higher-order aberrations ....................................................................................................................... 13 2.1 The higher-order aberrations of the eye ............................................................................................ 13 2.2 Wavefront aberrations ......................................................................................................................... 13 2.3 Zernike polynomials ............................................................................................................................ 14 2.4 Measurement of ocular aberrations ................................................................................................... 17 2.5 Principles and repeatability of the Hartmann-Shack method .......................................................... 19 2.6 Comparing the LRT and Hartmann-Shack methods ......................................................................... 23 2.7 Corneal topography and deriving corneal aberrations .................................................................... 24 2.8 Instrumentation methods to measure corneal aberrations .............................................................. 28 2.8.1 The Placido disc method ............................................................................................................ 28 2.8.2 Slit scanning method .................................................................................................................. 28 2.8.3 Scheimpflug imagery .................................................................................................................. 29 2.9 The repeatability of corneal aberration measurements .................................................................... 29 2.10 The accuracy and limitations of Zernike polynomials for calculating corneal aberrations ........ 32 2.11 Other methods of describing higher-order aberrations ................................................................. 32 2.12 Higher-order aberrations in a normal population ........................................................................... 33 2.13 Compensation of corneal aberrations by the internal optics of the eye ....................................... 34 2.14 Ocular aberrations and retinal image quality .................................................................................. 35 2.14.1 Point spread function (PSF) ..................................................................................................... 35 2.14.2 Modulation transfer function (MTF) .......................................................................................... 36 2.14.3 Visual benefit (VB) scores ........................................................................................................ 36 2.14.4 Strehl ratio ................................................................................................................................ 37 2.14.5 Maréchal’s criterion .................................................................................................................. 38 2.14.6 Wavefront variance (WV) ......................................................................................................... 38 2.14.7 Contrast sensitivity function (CSF) ........................................................................................... 38 2.15 Effect of aberrations on visual performance ................................................................................... 39 2.16 The tear film and its effect on higher-order aberrations................................................................. 40 2.17 Changes in higher-order aberrations with accommodation .......................................................... 42 2.18 Limitations of aberration measurement and representation .......................................................... 43 2.18.1 Spot imaging issues ................................................................................................................. 43 2.18.2 Computational errors ................................................................................................................ 45 3. Higher-order aberrations in keratoconus ............................................................................................. 47 3.1 Aberrations of the keratoconic eye .................................................................................................... 47 3.2 The Visual benefit of correcting the higher-order aberrations ........................................................ 55 3.3 Correcting monochromatic higher-order aberrations ...................................................................... 57 3.3.1 Surgical intervention ................................................................................................................... 57 3.3.2 Adaptive optics ........................................................................................................................... 58 3.3.3 Contact lenses ........................................................................................................................... 58 3.3.4 Neural adaptations ..................................................................................................................... 60 3.4 Considerations when measuring aberrations in patients with keratoconus .................................. 62 3.4.1 The line of sight (LoS) in keratoconus ........................................................................................ 64 3.5 Keratoconus and aberration-controlling contact lenses .................................................................. 65 3.6 Do all aberration terms need correcting? .......................................................................................... 69 3.7 The drawbacks of customised soft contact lenses .......................................................................... 70 3.8 Addressing the gaps in the literature ................................................................................................. 71 4. Repeatability of Ocular Aberration Measurements in Patients with Keratoconus ............................ 73 4.1 Abstract ................................................................................................................................................ 74 4.2 Introduction .......................................................................................................................................... 75 4.3 Methods and materials ........................................................................................................................ 76 4.3.1 Data analysis .............................................................................................................................. 77 4.4 Results .................................................................................................................................................. 77 4.5 Discussion ............................................................................................................................................ 78 4.5.1 Lower-order aberrations ............................................................................................................. 79 4.5.2 Higher-order aberrations ............................................................................................................ 80 5. Dynamic Changes in Ocular Higher-Order Aberrations in Patients With Keratoconus ................... 83 5.1 Abstract ................................................................................................................................................ 84 5.2 Introduction .........................................................................................................................................