Influence of myopia and aging on the optimal spherical aberration of soft contact lenses

Spherical aberration gauge for human vision

Spherical aberration affects vision in varying degrees depending on pupil size, accommodation, individual eye characteristics, and interpretations by the brain. We developed a spherical aberration gauge to help evaluate the correction potential of spherical aberration in human vision. Variable aberration levels are achieved with laterally shifted polynomial plates from which a user selects a setting that provides the best vision. The aberration is mapped into the pupil of the eye using a simple telescope. Calibration data are given.

Clinical Ocular Wavefront Analyzers

PURPOSE

To provide a summary of the methods used by clinical wavefront analyzers and their historical, current, and future applications.

METHODS

Review of the literature and authors' experience with the various devices.

RESULTS

A wide range of clinical wavefront aberrometers, which use different principles, are available to clinicians and researchers.

CONCLUSIONS

Applications of wavefront analyzers in vision sciences range from assessment of refractive error, refractive surgery planning, evaluation of outcomes, optimization of contact lenses and IOL designs, evaluation of pathology relating to optical performance of the eye, and evaluation of accommodation alterations.

Aberrations and myopia

It has been suggested that high levels of axial aberration or specific patterns of peripheral refraction could play a role in myopia development. Possible mechanisms involving high levels of retinal image blur caused by axial aberrations include form deprivation through poor retinal image quality in distance vision, enhanced accommodative lags favouring compensatory eye growth, and an absence of adequate directional cues to guide emmetropization. In addition, in initially emmetropic eyes, hyperopia in the retinal periphery may result in local compensatory eye growth, which induces axial myopia. Evidence in support of these ideas is reviewed and it is concluded that, for any fixed pupil diameter, evidence for higher levels of axial aberration in myopes in comparison with other refractive groups is weak, making involvement of axial aberrations in myopization through image degradation at the fovea unlikely. If, however, some potential myopes had unusually large pupil diameters, their effective aberration levels and associated retinal blur would be larger than those of the rest of the population. There is stronger evidence in favour of differences in patterns of peripheral refraction in both potential and existing myopes, with myopes tending to show relative hyperopia in the periphery. These differences appear to be related to a more prolate eyeball shape. Longitudinal studies are required to confirm whether the retinal defocus associated with the peripheral hyperopia can cause patterns of eyeball growth which lead to axial myopia.

Wavefront technology: Past, present and future

After outlining what is meant by wavefront aberration, the history of the field of wavefront technology is sketched and methods for measuring ocular wavefront aberration are briefly described. The variations in aberration of the normal eye with the individual and their pupil size, accommodation and age are summarised. Potential contact lens applications are outlined, including the design and on-eye performance of single-vision lenses, lenses for presbyopes and keratoconics, orthokeratology, tear film studies, and the design and performance of customised contact lenses intended to minimise residual lens-eye wavefront error.

Influence of the rate of contrast change on the quality of contrast sensitivity assessment: a comparison of three psychophysical methods

Criterion-free forced-choice procedures for measuring contrast sensitivity with a cathode ray tube (CRT) have low within-subject, intersubject, and test-retest variabilities, but a long test time compared with psychophysical methods that rely on the subject's criterion to determine threshold. Test time and variability of criterion-dependent methods depend on the rate at which the contrast changes on the CRT display. This study compared two criterion-dependent psychophysical methods for measuring contrast sensitivity (the method of increasing contrast and the von Békésy tracking method) with a criterion-free two-alternative forced-choice procedure. A range of rates of contrast change was studied: 0.1, 0.3, 0.5, 0.7 and 1.0 log unit s-1. Contrast sensitivity, within-subject variability, intersubject variability, test-retest variability, and test time of the three methods were compared. The 2-AFC procedure performed best with regard to within-subject, intersubject, and test-retest variabilities. A time-efficient alternative was the von Békésy tracking method at rates between 0.1 and 0.5 log unit s-1.

Assessment of just-noticeable differences for refractive errors and spherical aberration using visual simulation

PURPOSE

The aim of this study was to evaluate the threshold levels of aberration change that a typical reference eye is able to detect.

METHODS

The method involved simulation of the foveal vision of a typical eye in polychromatic light through optics affected by different levels of the various chosen monochromatic aberrations. The reference eye had the following monochromatic wavefront characteristics based on the aberrations of a population of young adults: no spherical defocus, astigmatism -0.37D oriented at 0 degrees celsius, coma -0.17 D/mm oriented at 270 degrees celsius, and spherical aberration -0.12 D/mm. Average amounts of longitudinal and transverse chromatic aberration were assumed, and allowance was made for the Stiles-Crawford effect. The pupil diameter of the simulated eye was kept fixed at 6 mm. Three observers each compared, 100 times, a simulated image as seen through the standard reference eye with a variant "aberrated" image. The varying parameter was the value of a chosen additional aberration affecting the variant image in the reference eye. The test was repeated for varying amounts of spherical defocus, astigmatic defocus, and spherical aberration. For each of these aberrations and each observer, the discrimination probability as a function of the aberration level in the variant image was determined. The just-noticeable difference in aberration (JNDA) was derived from each discrimination curve as the difference between the aberrations corresponding to discrimination probabilities of 75% and 25%. The JNDA values obtained were expressed in the form of root mean square (RMS) wavefront error thresholds.

RESULTS

It was found that 0.04 microm of RMS aberration should be considered as the threshold of just-noticeable image change, in good agreement with the Maréchal criterion.

CONCLUSIONS

The results imply that in normal viewing conditions (e.g., a 3-mm pupil size), optical corrections should be in the range of +/-0.15 D in sphere and cylinder from the target prescription if perceptible change in the quality of the perceived images is to be avoided. The design of conventional soft contact lenses of high negative power or positive power should aim to produce -0.07 D/mm of spherical aberration, with a tolerated interval between -0.15 to +0.01 D/mm for a 6-mm pupil size.

Correcting ocular spherical aberration with soft contact lenses

Following aberroscopy, aspheric front surface soft contact lenses (SCLs) were custom-made to correct spherical refractive error and ocular spherical aberration (SA) of 18 myopic and five hypermetropic subjects (age, 20.5 +/- 5 yr). On-eye residual aberrations, logMAR visual acuity, and contrast sensitivity were compared with the best-correcting spectacle lens, an equally powered standard SCL, and an SCL designed to be aberration free in air. Custom-made and spherical SCLs reduced SA (p < 0.001; p < 0.05) but did not change total root-mean-square (rms) wave-front aberration (WFA). Aberration-free SCLs increased SA (p < 0.05), coma (p < 0.05), and total rms WFA. Visual acuity remained unchanged with any of the SCL types compared with the spectacle lens correction. Contrast sensitivity at 6 cycles/degree improved with the custom-made SCLs (p < 0.05). Increased coma with aspheric lens designs and uncorrected astigmatism limit the small possible visual benefit from correcting ocular SA with SCLs.

Polarization-based compensation of astigmatism

One approach to aberration compensation of an imaging system is to introduce a suitable phase mask at the aperture plane of an imaging system. We utilize this principle for the compensation of astigmatism. A suitable polarization mask used on the aperture plane together with a polarizer-retarder combination at the input of the imaging system provides the compensating polarization-induced phase steps at different quadrants of the apertures masked by different polarizers. The aberrant phase can be considerably compensated by the proper choice of a polarization mask and suitable selection of the polarization parameters involved. The results presented here bear out our theoretical expectation.

The prospects for super-acuity: limits to visual performance after correction of monochromatic ocular aberration

It has recently been suggested that correction of the monochromatic aberration of the eye could lead to substantial improvements in visual acuity and contrast sensitivity function. After consideration of the best-corrected visual acuity of normal eyes, the optical and neural limits to visual performance are reviewed. It is concluded that, even if current problems with the accuracy of the suggested techniques of aberration correction, through corneal excimer laser ablation or customised contact lenses, can be overcome, changes in monochromatic ocular aberration over time, the continuing presence of chromatic aberration, errors of focus associated with lags and leads in accommodation, and other factors, are likely to result in only minor improvements in the high-contrast acuity performance of most normal eyes being produced by attempted aberration control. Significant gains in contrast sensitivity might, however, be achievable, particularly under mesopic and scotopic conditions when the pupil is large, provided that correct focus can be maintained. In the immediate future, reduction of the high levels of aberration that are currently found in eyes that have undergone refractive surgery and in some abnormal eyes should bring useful benefits.

On- and off-eye spherical aberration of soft contact lenses and consequent changes of effective lens power

BACKGROUND

Soft contact lenses produce a significant level of spherical aberration affecting their power on-eye. A simple model assuming that a thin soft contact lens aligns to the cornea predicts that these effects are similar on-eye and off-eye.

METHODS

The wavefront aberration for 17 eyes and 33 soft contact lenses on-eye was measured with a Shack-Hartmann wavefront sensor. The Zernike coefficients describing the on-eye spherical aberration of the soft contact lens were compared with off-eye ray-tracing results. Paraxial and effective lens power changes were determined.

RESULTS

The model predicts the on-eye spherical aberration of soft contact lenses closely. The resulting power change for a +/- 7.00 D spherical soft contact lens is +/- 0.5 D for a 6-mm pupil diameter and +/- 0.1 D for a 3-mm pupil diameter. Power change is negligible for soft contact lenses corrected for off-eye spherical aberration.

CONCLUSIONS

For thin soft contact lenses, the level of spherical aberration and the consequent power change is similar on-eye and off-eye. Soft contact lenses corrected for spherical aberration in air will be expected to be aberration-free on-eye and produce only negligibly small power changes. For soft contact lenses without aberration correction, for higher levels of ametropia and large pupils, the soft contact lens power should be determined with trial lenses with their power and p value similar to the prescribed lens. The benefit of soft contact lenses corrected for spherical aberration depends on the level of ocular spherical aberration.

Effect of rotation and translation on the expected benefit of an ideal method to correct the eye's higher-order aberrations

An ideal correcting method, such as a customized contact lens, laser refractive surgery, or adaptive optics, that corrects higher-order aberrations as well as defocus and astigmatism could improve vision. The benefit achieved with this ideal method will be limited by decentration. To estimate the significance of this potential limitation we studied the effect on image quality expected when an ideal correcting method translates or rotates with respect to the eye's pupil. Actual wave aberrations were obtained from ten human eyes for a 7.3-mm pupil with a Shack-Hartmann sensor. We computed the residual aberrations that appear as a result of translation or rotation of an otherwise ideal correction. The model is valid for adaptive optics, contact lenses, and phase plates, but it constitutes only a first approximation to the laser refractive surgery case where tissue removal occurs. Calculations suggest that the typical decentrations will reduce only slightly the optical benefits expected from an ideal correcting method. For typical decentrations the ideal correcting method offers a benefit in modulation 2-4 times higher (1.5-2 times in white light) than with a standard correction of defocus and astigmatism. We obtained analytical expressions that show the impact of translation and rotation on individual Zernike terms. These calculations also reveal which aberrations are most beneficial to correct. We provided practical rules to implement a selective correction depending on the amount of decentration. An experimental study was performed with an aberrated artificial eye corrected with an adaptive optics system, validating the theoretical predictions. The results in a keratoconic subject, also corrected with adaptive optics, showed that important benefits are obtained despite decentrations in highly aberrated eyes.

Positioning tolerances for phase plates compensating aberrations of the human eye

The positioning tolerances for phase plates used to compensate human eye aberrations are analyzed. Lateral displacements, in-plane rotations, and axial translations are considered, describing analytic and numerical procedures to compute the maximum degree of compensation achievable in each case. The compensation loss is found to be dependent both on the kind and the amount of misalignment and on the particular composition of the aberration pattern of each subject in terms of Zernike polynomials. We applied these procedures to a set of human eye aberrations measured with the laser ray-tracing method. The general trend of results suggests that lateral positioning, followed by angular positioning, are the key factors affecting compensation performance in practical setups, whereas axial positioning has far less stringent requirements.

Phase plates for wave-aberration compensation in the human eye

We present a method for manufacturing phase plates to compensate for the wave aberration in the human eye. The wave aberration of the eye is measured in vivo by a new laser ray-tracing method and then compensated for by a phase plate placed in front of the eye. This plate is made from a gray-level single-mask photosculpture in photoresist. Two experiments were carried out, first with an artificial eye and then with a human eye: 80% compensation for the wave aberration was achieved in both cases.