Allowable movement of wavefront-guided contact lens corrections in normal and keratoconic eyes.

PURPOSE: The goal was to use SyntEyes modelling to estimate the allowable alignment error of wavefront-guided rigid contact lens corrections for a range of normal and keratoconic eye aberration structures to keep objectively measured visual image quality at or above average levels of well-corrected normal eyes. Secondary purposes included determining the required radial order of correction, whether increased radial order of the corrections further constrained the allowable alignment error and how alignment constraints vary with keratoconus severity. METHODS: Building on previous work, 20 normal SyntEyes and 20 keratoconic SyntEyes were fitted with optimised wavefront-guided rigid contact lens corrections targeting between three and eight radial orders that drove visual image quality, as measured objectively by the visual Strehl ratio, to near 1 (best possible) over a 5-mm pupil for the aligned position. The resulting wavefront-guided contact lens was then allowed to translate up to ±1 mm in the x- and y-directions and rotate up ±15°. RESULTS: Allowable alignment error changed as a function of the magnitude of aberration structure to be corrected, which depends on keratoconus severity. This alignment error varied only slightly with the radial order of correction above the fourth radial order. To return the keratoconic SyntEyes to average levels of visual image quality depended on maximum anterior corneal curvature (Kmax). Acceptable tolerances for misalignment that returned keratoconic visual image quality to average normal levels varied between 0.29 and 0.63 mm for translation and approximately ±6.5° for rotation, depending on the magnitude of the aberration structure being corrected. CONCLUSIONS: Allowable alignment errors vary as a function of the aberration structure being corrected, the desired goal for visual image quality and as a function of keratoconus severity.

Utilising a visual image quality metric to optimise spectacle prescriptions for eyes with keratoconus.

PURPOSE: To compare optical performance, visual performance, and patient-perceived quality of vision with: (1) spectacles determined using subjective refraction and (2) spectacles determined using an objective optimisation method based on wavefront aberration data for eyes with keratoconus. METHODS: Thirty-seven eyes (20 subjects) with keratoconus underwent both subjective refraction and uncorrected wavefront aberration measurement. Wavefront aberration data were used to objectively identify a sphero-cylindrical refraction that optimised the visual image quality metric visual Strehl ratio (VSX). The two refractions were assembled in trial frames and worn by the subject in a random order. High-contrast visual acuity (VA), letter contrast sensitivity (CS), and the patient's short-term subjective preference were recorded for each prescription. RESULTS: Median magnitude of the dioptric difference (a measure of similarity between the subjective and objective refractions) was 2.77 D (range = 0.21-20.44 D, first quartile = 1.02 D, third quartile = 4.36 D). Sixty-eight per cent of eyes had better VA with the objective refraction and 32% of eyes gained more than one line of VA. Monocularly, objective refraction was preferred 68% of the time when looking at a distant acuity chart and 76% of the time when viewing a real-world dynamic scene. CONCLUSIONS: Objective refraction based on visual image quality derived from wavefront aberration data can be valuable in the determination of monocular spectacle refractions for individuals with keratoconus.

Case Report: When Two Is Worse Than One-Stereo Imbalance in a Case of Wavefront-guided Scleral Lenses.

SIGNIFICANCE: Wavefront-guided scleral lenses (WGSLs) reduce visually debilitating residual higher-order aberrations. Although reduced higher-order aberrations lead to improvement in monocular high-contrast visual acuity (VA), the success of the lenses in everyday life depends on additional factors such as retinal contrast, binocular balance, and stereoacuity. PURPOSE: This report describes a case where WGSLs provided improved monocular vision compared with scleral lenses (SLs) but reduced binocularity and stereoacuity. CASE REPORT: A 48-year-old woman with moderate keratoconus right eye (OD) and severe left eye (OS) was fitted with SLs and WGSLs. Visual acuity with best SLs was 20/20 -2 OD and 20/25 -2 OS. Residual higher-order root-mean-square (HORMS) wavefront error (6 mm pupil) was 0.56 µm OD and 1.38 µm OS. Visual acuity with WGSLs was 20/16 -2 OD and 20/25 +2 OS, and residual HORMS was 0.41 µm OD and 0.98 µm OS. Monocularly, WGSLs were reported to provide better VA. However, binocularly, the patient reported an "imbalanced feeling" and preferred the SLs over WGSLs. Binocular VA at distance was 20/25 with SLs and 20/25 -2 with WGSL. To investigate, the Worth Four-Dot test was performed, and the outcomes reported fusion with SLs but suppression OS at distance with WGSLs. Stereoacuity was 160 arc seconds at near and 120 arc seconds at distance with SLs and 400 arc seconds at near and >1200 arc seconds at distance with WGSLs. Dichoptic contrast balancing showed a balance point of 0.48 with SLs and 0.17 with WGSLs, indicating a strong preference toward OD. Simulation of the patient's retinal image revealed a greater difference in image contrast between the two eyes with WGSLs. CONCLUSIONS: Wavefront-guided scleral lenses reduced HORMS and improved VA compared with SLs. However, in this case, it inadvertently caused binocular imbalance. As WGSLs become more widely available, future work should include methods to optimize binocular balance to maximize overall patient satisfaction.

Influence of rigid lens decentration and rotation on visual image quality in normal and keratoconic eyes.

PURPOSE: To investigate whether the movement of a rigid sphero-cylindrical contact lens has a greater impact on the visual image quality in highly aberrated eyes than in normal eyes. METHODS: For 20 normal and 20 keratoconic SyntEyes, a previously determined best sphero-cylindrical rigid lens was permitted to shift by up to ±1 mm from the line of sight and rotate up to ±15°. Each of the 52,111 lens locations sampled was ray-traced to determine the influence on the wavefront aberration. In turn, the logarithm of visual Strehl ratio (log10 [VSX]) was calculated for each aberration structure and was used to estimate the associated changes in logMAR visual acuity. Finally, contour surfaces of two-letter change in visual acuity were plotted in three-dimensional misalignment space, consisting of decentrations in the x and y directions and rotation, and volumes within these surfaces were calculated. RESULTS: The variations in image quality within the misalignment space were unique to each eye. A two-letter loss was generally reached with smaller misalignments in keratoconic eyes (10.5 ± 4.7° of rotation or 0.27 ± 0.13 mm of shift) than in normal eyes (13.4 ± 1.8° and 0.39 ± 0.15 mm, respectively) due to larger cylindrical errors. For keratoconic eyes, on average, 14.4 ± 14.9% of misalignment space saw VSX values above the lower normal VSX threshold, well below the values of normal eyes of 48.5 ± 18.5%. In some eyes, a specific combination of lens shift and lens rotation away from the line of sight leads to a simulated improvement in visual image quality. CONCLUSION: Variations in visual image quality due to the misalignment of rigid sphero-cylindrical contact lens corrections are larger for keratoconic eyes than for normal eyes. In some cases, a specific misalignment may improve visual image quality, which could be considered in the design of the next generation of rigid contact lenses.

Clinical applications of personalising the neural components of visual image quality metrics for individual eyes.

PURPOSE: Eyecare is evolving increasingly personalised corrections and increasingly personalised evaluations of corrections on-eye. This report describes individualising optical and neural components of the VSX (visual Strehl) metric and evaluates personalisation using two clinical applications. (1) Better understanding visual experience: While VSX tracks visual performance in typical eyes, non-individualised metrics underestimated visual performance in highly aberrated eyes - could this be understood by personalising metrics? (2) Metric-optimised objective spherocylindrical refractions in typical and atypical eyes have used neural weighting functions of typical eyes - will personalisation affect the outcome in clinical 0.25D steps? METHODS: Orientation-specific neural contrast sensitivity was measured in four typical myopic and astigmatic eyes and six eyes with keratoconus. Wavefront error was measured in all eyes while uncorrected and when the keratoconic eyes wore wavefront-guided scleral lenses. Total experiment duration was 24-28 h per subject. Two versions of VSX were calculated for each application: one weighted ocular optics with measured neural contrast sensitivity data from that eye, another weighted optics with a representative neural function of typical eyes. Wavefront-guided corrections were evaluated using the two metric values. Spherocylindrical corrections that optimised each metric were identified. RESULTS: Metric values for keratoconic eyes improved by a mean factor of 1.99 (~0.3 log units) when personalised. Applying this factor to a larger sample of eyes from a previous keratoconus study reconciled dissonances between the percentage of eyes reaching normative best-corrected metric levels and the percentages of eyes reaching normative levels of visual acuity and contrast sensitivity. Spherocylindrical corrections that optimised both versions of VSX were clinically equivalent (mean ± SD Euclidean dioptric difference 0.13 ± 0.18 D). CONCLUSIONS: Personalising visual image quality metrics is beneficial when actual metric values are used (evaluating ophthalmic corrections on-eye against norms) and when fine increments in visual quality are imparted (wavefront-guided corrections). However, partially individualised metrics appear adequate when metrics relatively rank spherocylindrical corrections in 0.25 D steps.

Assessing the visual image quality provided by refractive corrections during keratoconus progression.

PURPOSE: To expand the SyntEyes keratoconus (KTC) model to assess the Visual Image Quality (VIQ) of sphero-cylindrical spectacle and rigid contact lens corrections as keratoconus progresses. METHODS: The previously published SyntEyes KTC eye model to determine best sphero-cylindrical spectacle and rigid contact lens correction in keratoconic eyes was expanded to include the natural progression of keratoconus, thus allowing the assessment of corrected VIQ with disease progression. RESULTS: As keratoconus progresses, the pattern of visual Strehl ratio (VSX) in correction space for spectacles alters from a typical hourglass into a shell pattern. The former would guide the subjective refraction towards the optimal correction while the latter is relatively insensitive to large dioptric steps. In 15 out of the 20 SyntEyes, the shell pattern eventually produces two foci on different sides of the correction space separated by a clinically significant dioptric difference with a similar, albeit lower VIQ. Wearing the best possible spectacle corrections provided an average gain of up to 3.5 lines of logMAR visual acuity compared to the uncorrected cases, which increased to 5.5 lines for the best rigid contact lens correction. Continuing to wear a spectacle correction as the disease progresses often leads to a VIQ that is almost as bad as the uncorrected case. Continuing to wear a rigid contact lens correction as the disease progresses maintains a relatively high level of VIQ, albeit in the low range for typically well-corrected normal eyes. CONCLUSIONS: The results reflect the clinical experience that subjective refraction is difficult in highly-aberrated keratoconic eyes, the benefit of spectacle correction is short lived and that rigid contact lenses provide better and more stable VIQ with disease progression. Other aspects, such as the presence and behaviour of the second focus in some cases, remain to be confirmed clinically.

Modeling refractive correction strategies in keratoconus.

This work intends to determine the optimal refractive spectacle and scleral lens corrections for keratoconus patients using the visual Strehl (VSX) visual image quality metric and the SyntEyes models with the synthetic biometry of 20 normal eyes and 20 keratoconic eyes. These included the corneal tomography and intraocular biometry. A series of virtual spherocylindrical spectacle and scleral lens corrections spanning the entire phoropter range were separately applied to each eye, followed by ray tracing to determine the residual wavefront aberrations and identify the correction with the highest possible VSX (named a "focus"). To speed up calculations, a smart scanning algorithm was used, consisting of three consecutive scans over increasingly finer dioptric grids. In the dioptric space, the VSX pattern for normal eyes considered over the correction range for either spectacle or scleral lens corrections resembled an hourglass with one distinct focus and a quick drop in VSX away from that focus. For 18 of the 20 keratoconic eyes, the spectacle-corrected VSX pattern resembled a shell that in 9 of the 20 cases showed two foci separated by a large dioptric distance (13.3 ± 4.9 diopters). In keratoconic eyes, scleral lenses also produced hourglass patterns, but with a VSX lower than in normal eyes. The hourglass pattern in dioptric space shows how, in normal eyes, the refracting process automatically funnels practitioners toward the optimal correction. The shell patterns in keratoconus, however, present far more complexity and, possibly, multiple foci. Depending on the starting point, refracting procedures may lead to a local maximum rather than the optimal correction.

Orientation-specific long-term neural adaptation of the visual system in keratoconus.

Eyes with the corneal ectasia keratoconus have performed better than expected (e.g. visual acuity) given their elevated levels of higher-order aberrations that cause rotationally asymmetric retinal blur. Adapted neural processing has been suggested as an explanation but has not been measured across multiple meridional orientations. Using a custom Maxwellian-view laser interferometer to bypass ocular optics, sinusoidal grating neural contrast sensitivity was measured in six eyes (three subjects) with keratoconus and four typical eyes (two subjects) at six spatial frequencies and eight orientations using a two-interval forced-choice paradigm. Total measurement duration was 24 to 28 hours per subject. Neural contrast sensitivity functions of typical eyes agreed with literature and generally showed the oblique effect on a linear-scale and rotational symmetry on a log-scale (rotational symmetry was quantified as the ratio of the minor and major radii of an ellipse fit to all orientations within each spatial frequency; typical eye mean 0.93, median 0.93; where a circle = 1). Mean sensitivities of eyes with keratoconus were 20% to 60% lower (at lower and higher spatial frequencies respectively) than typical eyes. Orientation-specific neural contrast sensitivity functions in keratoconus showed substantial rotational asymmetry (ellipse radii ratio: mean 0.84; median 0.86) and large meridional reductions. The visual image quality metric VSX was used with a permutation test to combine the asymmetric optical aberrations of the eyes with keratoconus and their measured asymmetric neural functions, which illustrated how the neural sensitivities generally mitigated the detrimental effects of the optics.

Case Report: What Are We Doing for Our "20/20 Unhappy" Scleral Lens Patients?

SIGNIFICANCE: Scleral lenses (SLs) partially mask higher-order aberrations (HOAs) in highly aberrated eyes. Although visual acuity (VA) may show satisfactory quantitative clinical outcomes during SL wear, residual (uncorrected) HOAs can leave subjective visual quality goals unmet. PURPOSE: The purpose of this study was to report a case where a "20/20 unhappy" patient with SLs was able to meet visual goals with wavefront-guided SLs. CASE REPORT: A 40-year-old male with bilateral keratoconus, whose Snellen VA with SLs was 20/20 right eye (OD) 20/16 left eye (OS), reported halos and glare at night and perceptual smearing. When viewing a point of light, a "Ferris wheel" shadowing was observed OD and a U-shaped shadowing OS. Residual higher-order root mean square wavefront error was 0.49 µm OD and 0.39 µm OS; visual image quality measured by visual Strehl ratio was 0.067 OD and 0.092 OS (pupil size, 4.00 mm). Wavefront-guided SLs reduced residual higher-order root mean square to 0.19 µm OD and 0.25 µm OS, VA improved to 20/10 OD and 20/13 OS, and visual Strehl improved to 0.150 OD and 0.121 OS. The patient reported reduced smearing, shadowing, and night vision concerns, meeting his visual expectations and goals. CONCLUSIONS: Wavefront sensing quantifies both lower and HOAs, which can cause visual dissatisfaction in individuals with highly aberrated eyes, despite sometimes reaching typical levels of VA. As wavefront-guided SLs targeting these residual aberrations to improve visual image quality become more available, they should be considered for 20/20 unhappy patients when conventional clinical options are unsatisfactory.

The Impact of Misaligned Wavefront-guided Correction in a Scleral Lens for the Highly Aberrated Eye.

SIGNIFICANCE: To achieve maximum visual benefit, wavefront-guided scleral lens corrections (WGCs) are aligned with the underlying wavefront error of each individual eye. This requirement adds complexity to the fitting process. With a view toward simplification in lens fitting, this study quantified the consequences of placing WGCs at two pre-defined locations. PURPOSE: This study aimed to quantify performance reduction accompanying the placement of the WGC at two locations: (1) the average decentered location (ADL; average decentration observed across individuals wearing scleral lenses) and (2) the geometric center (GC) of the lens. METHODS: Deidentified residual aberration and lens translation data from 36 conventional scleral lens-wearing eyes with corneal ectasia were used to simulate WGC correction in silico. The WGCs were decentered from the eye-specific pupil position to both the ADL and GC locations. The impact of these misalignments was assessed in terms of change (from the aligned, eye-specific pupil position) in higher-order root mean square (HORMS) wavefront error, change in log of the visual Strehl ratio (logVSX), and predicted change in logMAR visual acuity (VA). RESULTS: As expected, HORMS increased, logVSX decreased, and predicted VA was poorer at both ADL and GC compared with the aligned condition (P < .001). Thirty-four of 36 eyes had greater residual HORMS, and 33 of 36 eyes had worse logVSX values at the GC than at the ADL. In clinical terms, 19 of 36 eyes at the ADL and 35 of 36 eyes at the GC had a predicted loss in VA of three letters or greater. CONCLUSIONS: The placement of the WGC at either ADL or GC is predicted to lead to a noticeable reduction in VA for more than half of the eyes studied, suggesting the simplification of the fitting process is not worth the cost in performance.

Combining optical and neural components in physiological visual image quality metrics as functions of luminance and age.

Visual image quality metrics combine comprehensive descriptions of ocular optics (from wavefront error) with a measure of the neural processing of the visual system (neural contrast sensitivity). To improve the ability of these metrics to track real-world changes in visual performance and to investigate the roles and interactions of those optical and neural components in foveal visual image quality as functions of age and target luminance, models of neural contrast sensitivity were constructed from the literature as functions of (1) retinal illuminance (Trolands, td), and (2) retinal illuminance and age. These models were then incorporated into calculation of the visual Strehl ratio (VSX). Best-corrected VSX values were determined at physiological pupil sizes over target luminances of 104 to 10-3 cd/m2 for 146 eyes spanning six decades of age. Optical and neural components of the metrics interact and contribute to visual image quality in three ways. At target luminances resulting in >900 td at physiological pupil size, neural processing is constant, and only aberrations (that change as pupil size changes with luminance) affect the metric. At low mesopic luminances below where pupil size asymptotes to maximum, optics are constant (maximum pupil), and only the neural component changes with luminance. Between these two levels, both optical and neural components of the metrics are affected by changes in target luminance. The model that accounted for both retinal illuminance and age allowed VSX, termed VSX(td,a), to best track visual acuity trends (measured at 160 and 200 cd/m2) as a function of age (20s through 70s) from the literature. Best-corrected VSX(td,a) decreased by 2.24 log units between maximum and minimum target luminances in the youngest eyes and by 2.58 log units in the oldest. The decrease due to age was more gradual at high target luminances (0.70 log units) and more pronounced as target luminance decreased (1.04 log units).

Alignment of a wavefront-guided scleral lens correction in the presence of a lens capsulotomy.

PURPOSE: To demonstrate the necessity of aligning a wavefront-guided scleral lens (WGSL) optical correction to the eye's effective pupil, with misalignments leading to reduced performance. CASE REPORT: A 34 year old subject with a history of failed LASIK in the left eye, leading to penetrating keratoplasty, extracapsular extraction of the crystalline lens and neodymium:yttrium-aluminum-garnet (Nd:YAG) laser posterior capsulotomy, enrolled in a study examining WGSL performance. Habitual logMAR acuity OS (aided with a scleral lens) was +0.04. Residual higher order root mean square (HORMS) wavefront error (WFE) was 0.28 µm (Φ =4.75 mm, mean age-matched norm =0.17 µm), and objective over-refraction was -0.30 -0.54 × 008. When a WGSL (targeting aberrations up to the 5 th radial order) was manufactured with the wavefront-guided optics aligned to the center of the dilated pupil, logMAR acuity worsened to +0.15, residual HORMS WFE worsened to 0.44 µm (Φ =4.75 mm), and objective over-refraction increased to +1.19 -0.30 × 122. Slit lamp imagery revealed that the effective pupil was no longer defined by the iris of the eye, but rather the capsular opening created by the capsulotomy. When the WGSL was redesigned to align the wavefront-guided optics to the center of the capsular opening, logMAR acuity improved to -0.14, residual HORMS WFE reduced to 0.17 µm (Φ =4.75 mm) and objective over-refraction reduced to +0.20 -0.15 × 111. CONCLUSION: WGSLs are an emerging option for patients with highly aberrated, ectatic and post-surgical corneas whose visual symptoms cannot be alleviated with conventional corrections. However, alignment of the optics of the WGSL to the underlying optics of the eye over the effective pupil is critical in achieving good optical and visual performance.

Do Polymer Coatings Change the Aberrations of Conventional and Wavefront-guided Scleral Lenses?

SIGNIFICANCE: The findings of this study indicate that patients could simultaneously be offered the individualized optical correction of wavefront-guided (WFG) lenses and the superior comfort afforded by polymer coatings. This could be helpful to patients with ectasia suffering ocular dryness or dependent on scleral lenses for lengthy periods of wear. PURPOSE: Wavefront-guided scleral lenses target lower- and higher-order aberrations of individual eyes using submicrometer-level contours in the anterior lens surface. Hydrophilic polyethylene glycol (PEG) polymer coatings applied to lens surfaces improve comfort and wettability. This study aimed to quantify aberration changes (e.g., masking) when applying polymer coatings to WFG and conventional scleral lenses. METHODS: Two control lenses (remained uncoated) and 14 experimental lenses (two repeated builds of seven aberration designs: one spherical, two coma, four full WFG [second- to fifth-order aberrations]) were manufactured, and aberrations were measured (mean of three) by two operators before and after coating. Root mean square (RMS) and visual image quality (logVSX) differences were calculated for 6-mm diameters. RESULTS: Median RMS aberration change due to coating was 0.012 µm (range, 0.008 to 0.057 µm). Maximum logVSX change due to coating was 0.073, predicting an approximately one letter change in acuity. Instrument sensitivity was 0.002 µm. Acute instrument and operator variabilities (standard deviations of individual [second- to fifth-order Zernikes] were all <0.027 µm). Longitudinal variability (control lenses) was low: all less than 0.017 µm. Although RMS of differences between repeated builds of all lenses was less than 0.25 D and not statistically significant, relatively, manufacture constituted the major variability, and RMS difference between repeated builds was at least four times greater than the effect of coating (median, 0.167 µm; range, 0.088 to 0.312 µm). CONCLUSIONS: Application of polymer coatings caused measurable changes in aberrations of WFG and conventional scleral lenses; however, these were clinically and statistically insignificant and within variability of repeated lens manufacture. In their current states, WFG lenses and polymer coatings could be used simultaneously.

Comparison of Wavefront-guided and Best Conventional Scleral Lenses after Habituation in Eyes with Corneal Ectasia.

SIGNIFICANCE: Visual performance with wavefront-guided (WFG) contact lenses has only been reported immediately after manufacture without time for habituation, and comparison has only been made with clinically unrefined predicate conventional lenses. We present comparisons of habitual corrections, best conventional scleral lenses, and WFG scleral lenses after habituation to all corrections. PURPOSE: The purpose of this study was to compare, in a crossover design, optical and visual performance of eyes with corneal ectasias wearing dispensed best conventional scleral lens corrections and dispensed individualized WFG scleral lens corrections. METHODS: Ten subjects (20 eyes) participated in a randomized crossover study where best conventional scleral lenses and WFG scleral lenses (customized through the fifth radial order) were worn for 8 weeks each. These corrections, as well as each subject's habitual correction and normative data for normal eyes, were compared using (1) residual higher-order aberrations (HORMS), (2) visual acuity (VA), (3) letter contrast sensitivity (CS), and (4) visual image quality (logarithm of the visual Strehl ratio, or logVSX). Correlations were performed between Pentacam biometric measures and gains provided by WFG lenses. RESULTS: Mean HORMS was reduced by 48% from habitual to conventional and 43% from conventional to WFG. Mean logMAR VA improved from habitual (+0.12) to conventional (-0.03) and further with WFG (-0.09); six eyes gained greater than one line with WFG over conventional. Area under the CS curve improved by 26% from habitual to conventional and 14% from conventional to WFG. The percentage of the eyes achieving normal levels were as follows: HORMS, 40% for conventional and 85% for WFG; VA, 50% for conventional and 85% for WFG; and CS, 60% for conventional and 90% for WFG. logVSX improved by 16% from habitual to conventional and 25% further with WFG. Reduction in aberrations with WFG lenses best correlated with posterior cornea radius of curvature. CONCLUSIONS: Visual performance was superior to that reported with nonhabituated WFG lens wear. With WFG lenses, HORMS and logVSX significantly improved, allowing more eyes to reach normal levels of optical and visual performance compared with conventional lenses.

Normative best-corrected values of the visual image quality metric VSX as a function of age and pupil size.

The visual image quality metric the visual Strehl ratio (VSX) combines a comprehensive description of the optics of an eye (wavefront error) with an estimate of the photopic neural processing of the visual system, and has been shown to be predictive of subjective best focus and well correlated with change in visual performance. Best-corrected visual image quality was determined for 146 eyes, and the quantitative relation of VSX, age, and pupil size is presented, including 95% confidence interval norms for age groups between 20 and 80 years and pupil diameters from 3 to 7 mm. These norms were validated using an independently collected population of wavefront error measurements. The best visual image quality was found in young eyes at smaller pupil sizes. Increasing pupil size caused a more rapid decrease in VSX than increasing age. These objectively determined benchmarks represent the best theoretical levels of visual image quality achievable with a sphere, cylinder, and axis correction in normal eyes and can be used to evaluate both traditional and wavefront-guided optical corrections provided by refractive surgery, contact lenses, and spectacles.

Is an objective refraction optimised using the visual Strehl ratio better than a subjective refraction?

PURPOSE: To prospectively examine whether using the visual image quality metric, visual Strehl (VSX), to optimise objective refraction from wavefront error measurements can provide equivalent or better visual performance than subjective refraction and which refraction is preferred in free viewing. METHODS: Subjective refractions and wavefront aberrations were measured on 40 visually-normal eyes of 20 subjects, through natural and dilated pupils. For each eye a sphere, cylinder, and axis prescription was also objectively determined that optimised visual image quality (VSX) for the measured wavefront error. High contrast (HC) and low contrast (LC) logMAR visual acuity (VA) and short-term monocular distance vision preference were recorded and compared between the VSX-objective and subjective prescriptions both undilated and dilated. RESULTS: For 36 myopic eyes, clinically equivalent (and not statistically different) HC VA was provided with both the objective and subjective refractions (undilated mean ± S.D. was -0.06 ± 0.04 with both refractions; dilated was -0.05 ± 0.04 with the objective, and -0.05 ± 0.05 with the subjective refraction). LC logMAR VA provided by the objective refraction was also clinically equivalent and not statistically different to that provided by the subjective refraction through both natural and dilated pupils for myopic eyes. In free viewing the objective prescription was preferred over the subjective by 72% of myopic eyes when not dilated. For four habitually undercorrected high hyperopic eyes, the VSX-objective refraction was more positive in spherical power and VA poorer than with the subjective refraction. CONCLUSIONS: A method of simultaneously optimising sphere, cylinder, and axis from wavefront error measurements, using the visual image quality metric VSX, is described. In myopic subjects, visual performance, as measured by HC and LC VA, with this VSX-objective refraction was found equivalent to that provided by subjective refraction, and was typically preferred over subjective refraction. Subjective refraction was preferred by habitually undercorrected hyperopic eyes.

A Comparison of Three Methods to Increase Scleral Contact Lens On-Eye Stability.

PURPOSE: To quantify on-eye rotational and translational stability of three scleral contact lens stabilization methods and to model the variation in visual acuity when these movements occur in a wavefront-guided correction for highly aberrated eyes. METHODS: Three lens stabilization methods were integrated into the posterior periphery of a scleral contact lens designed at the Visual Optics Institute. For comparison, a lens with no stabilization method (rotationally symmetric posterior periphery) was designed. The lenses were manufactured and lens movements were quantified on 8 eyes as the average SD of the observed translations and rotations over 60 min of wear. In addition, the predicted changes in acuity for five eyes with keratoconus wearing a simulated wavefront-guided correction (full correction through the fifth order) were modeled using the measured movements. RESULTS: For each lens design, no significant differences in the translation and rotation were found between left and right eyes, and lenses behaved similarly on all subjects. All three designs with peripheral stability modifications exhibited no statistically significant differences in translation and rotation distributions of lens movement and were statistically more stable than the spherical lens in rotation. When the measured movements were used to simulate variation in visual performance, the 3 lenses with integrated stability methods showed a predicted average loss in acuity from the perfectly aligned condition of approximately 0.06 logMAR (3 letters), compared with the loss of over 0.14 logMAR (7 letters) for the lens with the spherical periphery. CONCLUSION: All three stabilization methods provided superior stability, as compared with the spherical lens design. Simulations of the optical and visual performance suggest that all three stabilization designs can provide desirable results when used in the delivery of a wavefront-guided correction for a highly aberrated eye.

Wavefront-guided scleral lens correction in keratoconus.

PURPOSE: To examine the performance of state-of-the-art wavefront-guided scleral contact lenses (wfgSCLs) on a sample of keratoconic eyes, with emphasis on performance quantified with visual quality metrics, and to provide a detailed discussion of the process used to design, manufacture, and evaluate wfgSCLs. METHODS: Fourteen eyes of seven subjects with keratoconus were enrolled and a wfgSCL was designed for each eye. High-contrast visual acuity and visual quality metrics were used to assess the on-eye performance of the lenses. RESULTS: The wfgSCL provided statistically lower levels of both lower-order root mean square (RMS) (p < 0.001) and higher-order RMS (HORMS) (p < 0.02) than an intermediate spherical equivalent scleral contact lens. The wfgSCL provided lower levels of lower-order RMS than a normal group of well-corrected observers (p << 0.001). However, the wfgSCL does not provide less HORMS than the normal group (p = 0.41). Of the 14 eyes studied, 10 successfully reached the exit criteria, achieving residual HORMS wavefront error less than or within 1 SD of the levels experienced by normal, age-matched subjects. In addition, measures of visual image quality (logVSX, logNS, and logLIB) for the 10 eyes were well distributed within the range of values seen in normal eyes. However, visual performance as measured by high-contrast acuity did not reach normal, age-matched levels, which is in agreement with prior results associated with the acute application of wavefront correction to keratoconic eyes. CONCLUSIONS: Wavefront-guided scleral contact lenses are capable of optically compensating for the deleterious effects of higher-order aberration concomitant with the disease and can provide visual image quality equivalent to that seen in normal eyes. Longer-duration studies are needed to assess whether the visual system of the highly aberrated eye wearing a wfgSCL is capable of producing visual performance levels typical of the normal population.

Change in visual acuity is well correlated with change in image-quality metrics for both normal and keratoconic wavefront errors.

We determined the degree to which change in visual acuity (VA) correlates with change in optical quality using image-quality (IQ) metrics for both normal and keratoconic wavefront errors (WFEs). VA was recorded for five normal subjects reading simulated, logMAR acuity charts generated from the scaled WFEs of 15 normal and seven keratoconic eyes. We examined the correlations over a large range of acuity loss (up to 11 lines) and a smaller, more clinically relevant range (up to four lines). Nine IQ metrics were well correlated for both ranges. Over the smaller range of primary interest, eight were also accurate and precise in estimating the variations in logMAR acuity in both normal and keratoconic WFEs. The accuracy for these eight best metrics in estimating the mean change in logMAR acuity ranged between ±0.0065 to ±0.017 logMAR (all less than one letter), and the precision ranged between ±0.10 to ±0.14 logMAR (all less than seven letters).