The development of custom wavefront-guided soft contact lens corrections for the keratoconic eye

Can custom wavefront-guided soft contact lenses mitigate the optical aberrations associated with the eye disease keratoconus? This general research question was broken down into four specific research questions: (1) How well do the gold standard corrections for keratoconus (RGP contact lenses) reduce optical aberration and improve visual performance (Chapter 2)? (2) What optical aberrations are visually important in keratoconus (Chapter 3)? (3) Can a process for design, manufacture and evaluation of custom wavefront-guided soft contact lenses be developed (Chapter 4)? (4) How do basic, wavefront-guided soft contact lenses perform in terms of reducing optical aberration and improving visual performance in keratoconus subjects (Chapters 5 and 6)?; Data reported in Chapter 2 demonstrate that 5 of the 7 keratoconic eyes exhibit residual 2nd--6th Zernike order aberrations over a 5mm pupil that are elevated when compared to normal values. LogMAR visual acuity was reduced for 5 of the 7 keratoconic eyes and Pelli Robson contrast sensitivity (PRCS) was reduced for 2 of the 7 keratoconic eyes when compared to normal values. Linear regression shows a relationship between the residual eye/RGP optical aberration structures transformed by 4 optical quality metrics (RMSw, RMSs, Bave, D50) and reduced visual performance quantified as logMAR VA and PRCS. Average blur strength (Bave) was the best predictor, with coefficients of determination of 0.62 for logMAR VA and 0.75 for PRCS. Data from Chapter 3 demonstrate that visually significant aberrations exist through the 5th radial order of the Zernike polynomial for eyes with <60D max corneal power. Simulated correction of the first 5 orders of the Zernike polynomial over a 4mm pupil in eyes with <60D max corneal power returns visual performance to near normal levels (<=1 line logMAR) for both low and high contrast VA. Eyes with >60D max corneal power require more orders, or never return to normal levels. Eyes with <60D max corneal power compose 88% of keratoconus cases, making a 5th order correction an excellent initial target for customization. Chapter 4 demonstrates the ability to design, manufacture and test custom optics in the laboratory setting. This basic process allows for a posterior toric lens design to be mated with a fully customizable, wavefront-guided, prism ballasted anterior lens surface. The anterior wavefront patch can be oriented, through quantification of lens rotation and decentration, in front of an offset pupil. Further, the wavefront-defined patch is designed to be resistant to the reduction in performance associated with the movements necessary in soft contact lens wear through the use of weighting coefficients described by Guirao et al. {lcub}Guirao, Cox, et al. 2002 #460{rcub}. Chapters 2--4 find their confluence in Chapters 5 and 6, where it was demonstrated that keratoconus subjects could achieve visual performance equivalent to habitual performance as well as a reduction in both low and higher order aberration. Chapter 5 investigated one habitual soft contact lens wearing keratoconic eye and found that high contrast logMAR VA improved from habitual soft contact lens value of 0.07 to final custom contact lens value of -0.08 over a 5mm pupil. Low contrast values for the same correction modalities improved from 0.73 to 0.62 over a 5mm pupil. Low order RMS over this 5mm zone decreased from 2.08mum for the habitual correction to 0.34mum for the custom correction. High order RMS levels decreased from 0.77mum for the habitual correction to 0.39mum for the custom correction. In Chapter 6, improvements in custom lens design were implemented and tested on three habitual RGP wearing subjects. All subjects reached the established exit criterion of high contrast logMAR VA equivalent to RGP wear. These subjects also experienced increased comfort relative to the habitual RGP correction.