No description available.

Premium intraocular lenses (IOLs) aim to surgically correct astigmatism and presbyopia following cataract extraction, optimising vision and eliminating the need for cataract surgery in later years. It is usual to fully correct astigmatism and to provide visual correction for distance and near when prescribing spectacles and contact lenses, however for correction with the lens implanted during cataract surgery, patients are required to purchase the premium IOLs and pay surgery fees outside the National Health Service in the UK. The benefit of using toric IOLs was thus demonstrated, both in standard visual tests and real-world situations. Orientation of toric IOLs during implantation is critical and the benefit of using conjunctival blood vessels for alignment was shown. The issue of centration of IOLs relative to the pupil was also investigated, showing changes with the amount of dilation and repeat dilation evaluation, which must be considered during surgery to optimize the visual performance of premium IOLs. Presbyopia is a global issue, of growing importance as life expectancy increases, with no real long-term cure. Despite enhanced lifestyles, changes in diet and improved medical care, presbyopia still presents in modern life as a significant visual impairment. The onset of presbyopia was found to vary with risk factors including alcohol consumption, smoking, UV exposure and even weight as well as age. A new technique to make measurement of accommodation more objective and robust was explored, although needs for further design modifications were identified. Due to dysphotopsia and lack of intermediate vision through most multifocal IOL designs, the development of a trifocal IOL was shown to minimize these aspects. The current thesis, therefore, emphasises the challenges of premium IOL surgery and need for refinement for optimum visual outcome in addition to outlining how premium IOLs may provide long-term and successful correction of astigmatism and presbyopia.

Computational models of human accommodation hold the promise of an improved under- standing of the mechanism and of the development of presbyopia. A detailed and reliable model could greatly assist the design of treatments to restore accommodation to presbyopic eyes. However, a large quantity of data is required for such an endeavour. Currently, the details of the age-related increase in the stiffness of the lens is a major source of uncertainty as the published data differ markedly depending on the form of testing employed. A new version of the spinning lens test is presented, based on the method originated by Fisher, R. F. (1971) 'The elastic constants of the human lens', Journal of Physiology, 212(1):147-180. This test assesses the stiffness of the lens substance by photographically measuring the deformations induced by rotation of the lens about its axis of symmetry. The principal changes introduced in the present version are the removal of the capsule from the lens prior to testing, the synchronization of the photography with the orientation of the lens, and the use of a hyperelastic finite-element model of the test coupled with a numerical optimization procedure to quantify the heterogeneous stiffness of the lens. These alterations, together with further improvements, provide a substantially more accurate means of measur- ing the stiffness of the lens 'substance'. Measurements made with the new test on a series of human lenses are reported. Good- quality tests were obtained for 29 lenses aged from 12 to 58 years. The older lenses were found to be much stiffer than younger lenses. In younger lenses the cortex of the lens is found to be stiffer than the nucleus, but the nucleus stiffens more rapidly, surpassing the cortex by about 44 years. These results differ substantially from those of the original spinning test. The stiffness values calculated for the lens substance are used in a series of hyperelastic finite-element models of the accommodation mechanism. Models corresponding to subjects aged 29 and 45 years follow clinical measurements of the decline in accommodation am- plitude between these ages. Adjusting the material parameters values indicates that it is the increase in stiffness which is largely responsible for the modelled fall in accommodation am- plitude. The 45-year model is adapted to represent the effect of laser lentotomy, a proposed presbyopia treatment. Among the lentotomy options trialled, the best result is a modest O.4D increase in the modelled accommodation amplitude.

No description available.

Peripheral retinal characteristics have been shown recently to have an influence upon refractive error development, specifically myopia. Studies of peripheral refraction have shown that emmetropic subjects typically display a myopic peripheral refraction, whereas myopic subjects have been found to have relative hyperopic refraction in the periphery. Myopic and hyperopic subjects were recruited for the experiments detailed in this thesis, from the student population at Glasgow Caledonian University. Previous studies on human subjects have generally made comparisons between myopic and emmetropic subject groups. Our study will contribute significantly to the knowledge in this area by extending the investigations to include hyperopic subject groups. Our results show significant differences in peripheral retinal shape between hyperopic and myopic subjects. Peripheral retinal shape in hyperopia shows symmetry between the temporal and nasal retina. In contrast, the myopic subjects show significant asymmetry between temporal and nasal retinal shape (Chapters 4 and 5). We also show that eye rotation significantly alters peripheral retinal shape in myopic subjects but not in hyperopic subjects. These findings indicate structural differences between myopic and hyperopic eyes worthy of further investigation. Monochromatic higher order aberrations (MHOA) were investigated in myopic and hyperopic subjects. Analyzed Zernike coefficients of the third, fourth and fifth orders show the myopic and hyperopic subjects display a similar profile. Interestingly when the lens is forced to either relax as a result of the instillation of cyclopentolate hydrochloride (Chapter 7) or accommodate (Chapter 6) the MHOA are significantly altered. The most significant change is seen in the fourth order spherical aberration. This work has identified significant structural and internal differences between subjects with myopia and those with hyperopia thus giving further insight into the optical and physical differences that exist between the myopic and hyperopic eye. With hyperopic eyes often displaying a stable refractive error and rarely going on to develop myopia the key to investigating myopic development may be further investigation of hyperopia and any differences that may exist.

No description available.

The aim of this thesis was to develop standards of best practice for the subjective assessment of near visual function in presbyopia. Near visual acuity (VA) is a quick and simple measure but an assessment of the maximum reading speed and the smallest print size that can maintain this are equally important, to gain a better reflection of real world visual function. These metrics are dependent on the amplitude of accommodation (AoA) and often this must be evaluated using subjective techniques. Defocus curves are less susceptible than the push-up/push-down test to the influence of blur tolerance but their implementation must be standardised such that letter sequences and the order of lens presentation are randomised, to avoid memory effects, whilst the AoA should be quantified as the range of defocus for which only the best VA is maintained. In addition to such clinical assessments, subjective questionnaire evaluations are also important, to determine whether at least an individual's needs are met. The Near Activity Visual Questionnaire (NAVQ) developed in this thesis can be used for this. Using these standardised near vision metrics it is shown that visual performance with monovision and multifocal contact lenses is comparable whilst initial outcomes of single optic 'accommodating' intraocular lens implantation are unlikely to be sustained in the long-term.

The objective of the present studies was to assess whether there were predictable differences in the thickness of the human cornea in relation to stable manifest refractive error using a scanning slit optical topography method (Orbscan II).