Higher-order aberrations in keratoconus

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Abstract

The reduction in visual performance typically found in keratoconic patients is believed to be associated with large magnitudes of uncorrected irregular astigmatism and higher-order aberrations (HOAs). Previous studies indicate that correcting HOAs in keratoconus patients may result in an improvement in visual performance. This thesis explores the correction of HOAs using standard sphero-cylindrical and customised aberration-controlling soft contact lenses in 22 patients with keratoconus. The findings of this work may be useful from a clinical perspective, as some keratoconic patients cannot tolerate rigid gas-permeable (RGP) contact lenses and have few alternatives, excluding surgical intervention, for vision correction.This thesis firstly describes a series of preliminary studies conducted to improve our current understanding of the HOAs manifested in keratoconus. The results of these investigations suggested that alterations in aberrations, due to changes in accommodation or variations in the pre-corneal tear film post-blink, were unlikely to hinder the correction of HOAs for keratoconic patients. Equally, it was ascertained that subjective refraction data provided significantly better visual acuity compared to objective, aberrometry-derived refractions for patients with keratoconus. The findings also show that both lower-order aberrations (LOAs) and HOAs displayed a larger degree of variability in keratoconic eyes compared to those previously reported for visually-normal subjects. Furthermore, significant increases in 3rd-order coma root mean square aberrations were found after temporarily suspending RGP contact lens wear for 16 keratoconic patients.The results of two clinical studies suggested that standard sphero-cylindrical soft lenses can, to some extent, mask HOAs in keratoconic patients; however, the visual performances achieved were found to be poorer compared to RGP lenses. Equally, the results showed that RGP lenses provide superior visual performances compared to customised, aberration-controlling lenses, in spite of the customised lenses providing comparable reductions in uncorrected HOAs. The inducement of superfluous HOAs and LOAs, through customised contact lens translations and rotations, were modelled using MatLab (version 7.6.0.324; The Mathworks, Natick, MA, US). The results confirmed that minimising the decentration of aberration-controlling contact lenses, to less than 5 degrees of rotation and less than 0.50 mm of translation, will help to achieve an optimal correction of HOAs. However, more stringent criteria were required for LOAs, where rotational displacements should be reduced to less than 3 degrees and translational displacements should be limited to less than 0.10 mm.In conclusion, the correction of HOAs for patients with keratoconus is possible using customised, aberration-controlling soft contact lenses; however, several factors will govern their success, including the repeatability and accuracy of HOA measurements for these irregular corneas, and the stability of the customised lenses on-eye.

Layman's Abstract

The reduction in visual performance typically found in keratoconic patients is believed to be associated with large magnitudes of uncorrected irregular astigmatism and optical distortions, also known as higher-order aberrations (HOAs). Previous studies have suggested that correcting HOAs in patients with keratoconus may result in an improvement in visual performance. This thesis explores the correction of HOAs using standard and customised aberration-controlling soft contact lenses in 22 patients with varying degrees of keratoconus. The findings of this work may be useful from a clinical perspective, as some keratoconic patients cannot tolerate rigid gas-permeable (RGP) contact lenses and have few alternatives (excluding surgical intervention) for vision correction.This thesis firstly describes a series of preliminary studies conducted to improve our current understanding of the HOAs manifested in keratoconus. The results of these investigations suggest that changes in aberrations, due to accommodation (a change in the eye’s focus) or variations in the tear film immediately after a blink, were unlikely to hinder the correction of HOAs for keratoconic patients. Equally, it was ascertained that ‘manual’ eye examination prescription data provided significantly better vision compared to ‘automated’, objective prescriptions derived using a clinically-available aberrometer (a specialised instrument used to measure the optical image quality of the eye), for patients with keratoconus.Our findings also showed that automated prescription data and HOA measurements, made using the aberrometer, displayed a larger degree of variability in keratoconic eyes compared to those previously reported in visually-normal subjects. Furthermore, significant increases in 3rd-order coma aberrations were found after temporarily suspending RGP contact lens wear (for 1 week) for 16 keratoconic patients.The results of two clinical studies suggested that standard soft contact lenses can, to some extent, mask HOAs in keratoconic patients; however, the visual performance achieved was found to be poorer compared to RGP lenses; this is likely to be due to the poor correction of astigmatism with soft lenses. Equally, the results showed that RGP lenses provided superior visual performances compared to customised aberration-controlling lenses, in spite of the customised lenses providing comparable reductions in uncorrected HOAs. The inducement of superfluous HOAs through customised soft contact lens movements and rotations on-eye was modelled using MatLab (version 7.6.0.324; The Mathworks, Natick, MA, US). These results confirmed that minimising the movement of customised aberration-controlling contact lenses, to less than 5 degrees of rotation and less than 0.50 mm of movement, will help to achieve an optimal correction of HOAs. However, this level of on-eye lens stability is not possible using current contact lens technologies or materials.In conclusion, the correction of HOAs for patients with keratoconus is possible using customised soft contact lenses; however, several factors will govern their success, including the repeatability and accuracy of the HOA measurements and the stability of the customised lenses on-eye.

Keyword(s)

Coma; Customised soft lenses; Higher-order aberrations; Keratoconus; RGP lenses

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