Contributions of the cornea and the lens to the aberrations of the human eye

Risk Factors for Corneal Monochromatic Aberrations and Implications for Multifocal and Extended Depth-of-Focus Intraocular Lens Implantation

PURPOSE

To discuss factors influencing corneal aberrations that might influence the optical quality after intraocular lens (IOL) implantation.

METHODS

PubMed and Scopus were the main resources used to search the medical literature. An extensive search was performed to identify relevant articles concerning factors influencing the level of corneal aberrations as of August 27, 2023. The following keywords were used in various combinations: corneal, aberrations, defocus, astigmatism, spherical aberration, coma, trefoil, quadrafoil, intraocular lens, and IOL.

RESULTS

Conclusive evidence is lacking regarding the correlation between age and changes in corneal aberrations. Patients with astigmatism have greater corneal higher-order aberrations than those with minimal astigmatism, particularly concerning trefoil and coma. Increased levels of corneal higher-order aberrations are noted following contact lens wear, in patients with dry eye disease, and with pterygium. Increased higher-order aberrations have been reported following corneal refractive surgery and for 3 months following trabeculectomy; regarding intraocular lens surgery, the results remain controversial.

CONCLUSIONS

Several factors influence the level of corneal higher-order aberrations. Multifocal and extended depth-of-focus IOLs can share similarities in their optical properties, and the main difference arises in their design and performance with respect to spherical aberration. Preoperative evaluation is critical for proper IOL choice, particularly in corneas with risk of high levels of aberrations. [J Refract Surg. 2024;40(6):e420-e434.].

Functional visual tests to evaluate the effect of small astigmatism correction with toric contact lenses

The prescription of daily contact lenses does not often include a full astigmatic correction. We question here whether this full astigmatic correction (for low to moderate astigmatism) provides a substantial improvement in the overall visual performance compared to a more conservative approach based only on the prescription of spherical contact lenses. The visual performance of 56 contact lens neophytes divided in two contact lens fitting groups (toric versus spherical lens fit) was assessed using standard visual acuity and contrast sensitivity tests. A new set of functional tests simulating everyday tasks was also used. Results showed that subjects with toric lenses had significantly better visual acuity and contrast sensitivity than those with spherical lenses. Functional tests did not render significant differences between groups, which was explained by several factors like i) the visual demand of the functional tests, ii) the dynamic blur due to misalignments and iii) small misfits between the available and measured axis of the astigmatic contact lens.

Higher order aberrations and visual function in a young Asian population of high myopes

Purpose

To examine the associations between higher order aberrations (HOAs), visual performance, demographics, and ocular characteristics in a young Asian population with high myopia.

Methods

This was a retrospective review of military pre-enlistees conducted between March 2014 to September 2018. Visual acuity and contrast sensitivity were tested under photopic, mesopic and simulated night conditions. Ocular, corneal and internal HOAs were measured with a Hartmann-Shack wavefront aberrometer (KR-1W, Topcon Co., Tokyo, Japan).

Results

522 eyes of 263 consecutive subjects with severe high myopia (defined as spherical equivalent refraction [SER] ≤ -10.00D) in at least one eye, and high myopia (SER ≤ -6.00D) in the fellow eye, [mean (SD) SER -11.85 (2.03D)] were analysed. The mean (SD) age of subjects was 18.5 (1.6) years. Chinese eyes had significantly greater internal total HOA root-mean-square (RMS) compared to Malay eyes [mean difference (SD) 0.0246 (0.007) μm, p < 0.001). More negative SER was associated with greater ocular total HOA (p = 0.038), primary coma (p = 0.003) and tetrafoil (p = 0.025) RMS, as well as more positive ocular (p = 0.003) and internal primary spherical aberration (p = 0.009). Greater ocular total HOAs was associated with reduced visual acuity in simulated night conditions and low contrast, decreased contrast sensitivity under mesopic and simulated night conditions (all p < 0.05).

Conclusions

Greater HOAs were associated with Chinese ethnicity and more negative SER in a young Asian population with high myopia. Greater HOAs were associated with poorer visual performance in low luminance and reduced contrast conditions.

Repeatability and reproducibility of corneal higher-order aberrations measurements after small incision lenticule extraction using the Scheimpflug-Placido topographer

Background

To evaluate the precision of corneal higher-order aberrations measurements after small incision lenticule extraction (SMILE) using the Sirius Scheimpflug-Placido topographer (CSO, Italy).

Methods

Seventy-five eyes from 75 postoperative subjects were included in this prospective study. Three consecutive corneal aberrometric measurements were obtained with the Scheimpflug-Placido topographer by two experienced operators to assess intra- and inter-observer reproducibility. The within-subject standard deviation (S_w), test-retest repeatability (TRT) and the intraclass correlation coefficient (ICC) were calculated.

Results

For intraobserver repeatability of anterior and total corneal aberrations, all ICCs were more than 0.922, except for trefoil (0.722 to 0.768). The ICCs of total root mean square (RMS), coma Z (3, ± 1), and spherical aberration Z (4, 0) were over 0.810 while higher-order RMS, trefoil Z (3, ± 3), and astigmatism II Z (4, ± 2) were below 0.634 for posterior corneal surface aberrations. All S_w values for all types of aberrations were equal to or below 0.07 μm. Regarding interobserver reproducibility, all TRT values were no more than 0.12 μm, 0.05 μm, and 0.11 μm for anterior, posterior, and total corneal aberrations, respectively. The ICC values ranged from 0.875 to 0.989, from 0.686 to 0.976 and over 0.834 for anterior, posterior, and total corneal aberrations, respectively.

Conclusions

The repeatability of measurements of anterior and total corneal aberrations with the Sirius system in corneas after SMILE surgery was high, except for trefoil. There was some variability in posterior corneal aberrometric measurements. High reproducibility of corneal aberrometric measurements was observed between measurements of both examiners, except for trefoil, with poor to moderate reproducibility.

Paraxial equivalent of the gradient-index lens of the human eye

The lens of the eye has a refractive index gradient that changes as the lens grows throughout life. These changes play a key role in the optics of the eye. Yet, the lens is generally simulated using a homogeneous model with an equivalent index that does not accurately represent the gradient. We present an analytical paraxial model of the gradient lens of the eye that gives the direct relation between refractive index distribution and paraxial characteristics. The model accurately simulates the changes in lens power with age and accommodation. It predicts that a decrease in equivalent index with age is associated with a flattening of the axial refractive index profile and that changes in lens power with accommodation are due primarily to changes in the axial variation of the iso-indicial curvature, consistent with Gullstrand's intracapsular theory of accommodation. The iso-indicial curvature gradient causes a shift of the principal planes compared to the homogeneous equivalent model. This shift introduces a clinically significant error in eye models that implement a homogenous lens. Our gradient lens model can be used in eye models to better predict the optics of the eye and the changes with age and accommodation.

Ocular aberrations in eyes with Primary Congenital Glaucoma

PURPOSE

To highlight the magnitude of ocular higher order aberrations (HOA) and lower order aberrations (LOA), including component contributions from corneal and internal planes in Primary Congenital Glaucoma (PCG) patients.

METHODS

Consecutive treated PCG patients co-operative for ocular examination and aberrometry, were enrolled over two years for this cross-sectional, comparative, single centre, unmasked study. Best corrected visual acuity, refraction, IOP, wavefront aberrometry and topography (iTrace) were performed and results were compared with unaffected fellow eyes of unilateral glaucoma patients as well as age and sex matched controls with no ocular anomalies other than treatable refractive error.

RESULTS

Both eyes of 32 consecutive PCG patients (17 unilateral, 15 bilateral) and 39 controls were enrolled. The median LogMAR corrected distance visual acuity of PCG eyes was 0.68 (IQR: 0.2 - 1.8). Total ocular (Root mean square (RMS) 1.7µm vs 0.3µm, p = 0.014), corneal (RMS 1.1µm vs 0.3µm, p = 0.004) and internal (RMS 1.1µm vs 0.2µm, p = 0.013) aberrations, as well as HOAs and LOAs at each plane were significantly higher in PCG eyes than in controls. Component HOAs from corneal and internal planes were positively correlated with each other (p < 0.001; rs: 0.7). Total aberrations were greater in the affected eyes of PCG compared to the rest. The predominant subtype of HOAs in PCG were coma and trefoil. PCG with corneal opacity/Haab's striae had significantly higher astigmatism than the affected eyes with clear corneae at corneal plane (p = 0.02). The aberrations were not statistically associated with the corneal diameter or refractive error in PCG eyes.

CONCLUSIONS

Significantly greater aberrations (Total, HOAs and LOAs, at corneal as well as internal plane) were seen among eyes affected with PCG. Though the exact impact of these aberrations on the final visual outcome is difficult to determine, these could play a pertinent role in compromising visual function, thus impacting the management of visual rehabilitation in these patients.

-Intracellular hydrostatic pressure regulation in the bovine lens: a role in the regulation of lens optics?

The optical properties of the bovine lens have been shown to be actively maintained by an internal microcirculation system. In the mouse lens, this water transport through gap junction channels generates an intracellular hydrostatic pressure gradient, which is subjected to a dual feedback regulation that is mediated by the reciprocal modulation of the transient receptor potential vanilloid channels, TRPV1 and TRPV4. Here we test whether a similar feedback regulation of pressure exists in the bovine lens, and whether it regulates overall lens optics. Lens pressure was measured using a microelectrode/pico-injector-based pressure measurement system, and lens optics were monitored in organ cultured lenses using a laser ray tracing system. Like the mouse, the bovine lenses exhibited a similar pressure gradient (0 to 340 mmHg). Activation of TRPV1 with capsaicin caused a biphasic increase in surface pressure, while activation of TRPV4 with GSK1016790A caused a biphasic decrease in pressure. These biphasic responses were abolished if lenses were pre-incubated with either the TRPV1 inhibitor A-889425, or the TRPV4 inhibitor HC-067047. While modulation of lens pressure by TRPV1 and TRPV4 had minimal effects on lens power and overall vision quality, the changes in lens pressure did induce opposing changes to lens geometry and GRIN that effectively kept lens power constant. Hence, our results suggest that the TRPV1/4 mediated feedback control of lens hydrostatic pressure operates to ensure that any fluctuations in lens water transport, and consequently water content, do not result in changes in lens power and therefore overall vision quality.

Ten-year longitudinal investigation of astigmatism: The Yamagata Study (Funagata)

Despite numerous investigations into ocular or corneal astigmatism, the dynamic nature of astigmatism remains poorly understood. To reveal potential associations between age and astigmatism, 264 Japanese participants who underwent systemic and ophthalmological examinations in Funagata Town (Yamagata Prefecture, Japan) were evaluated over a 10-year period. Astigmatism was evaluated with regard to the cylinder power, cylinder axis, and vector analyses. Whereas the refractive cylinders showed age-related increases in patients in their 40s to 60s, the corneal cylinders did not change over 10 years. Nevertheless, cylindrical axis of the cornea demonstrated a continuous shift toward against-the-rule (ATR) astigmatism. Vector analyses revealed that the astigmatic shift toward ATR progressed continually after patients reached their 40s, although the shift did not accelerate with age. These novel insights may pave the way for the development of potential strategies for vision correction, including refractive surgeries, and vision-quality maintenance in the elderly.

Near Vision Tasks and Optical Quality of the Eye

Purpose

To study the effect of near-vision reading task on optical quality of the eye when performed on a computer monitor and on printed paper, and to identify which of the two results in greater changes.

Methods

Two groups of subjects performed a 30-min reading task in two different conditions: on a computer monitor and on printed paper. Ocular, corneal, and internal wavefront aberrations (Zernike coefficients up to 6 th order), root-mean-square of low- and high-order aberrations, spherical equivalent, vectoral components of ocular astigmatism (J45 and J0), and the compensation factor between internal and corneal aberrations were measured before and after the tasks. Their changes were analyzed in each group and between groups.

Results

Statistically significant changes in wavefront aberrations and in root mean square of low- and high-order aberrations were observed in both groups which was significantly greater when the task was performed on printed paper. Partial loss of compensation mechanism and variation in spherical equivalent in a negative direction occurred after both reading tasks; however, it was statistically significant only with printed paper reading task. The vectoral components of ocular astigmatism did not show statistically significant changes in either groups.

Conclusion

Near-vision reading tasks can change the optical quality of the eye, especially when the task is performed on printed paper.

Retinal image quality with multifocal, EDoF, and accommodative intraocular lenses as studied by pyramidal aberrometry

Background

To study and compare the clinical optical image quality following implantation with different premium IOLs by analysing the point spread function (PSF) Strehl ratio using a pyramidal wavefront sensor (PWS)-based aberrometer.

Methods

This study included 194 eyes implanted with: (a) 19 AcrySof SA60AT (control group); (b) 19 Miniwell; (c) 24 LENTIS Mplus LS-313 MF30; d) 33 LENTIS Mplus LS-313 MF15; (e) 17 AkkoLens Lumina; (f) 31 AT LISA Tri 839MP; (g) 20 Precizon Presbyopic; (h) 20 AcrySof IQ PanOptix; (i) 11 Tecnis Eyhance. Main outcome measures were PSF Strehl ratio, PSF Strehl ratio excluding second-order aberrations (PSFw2), total root mean square (RMS), low-order aberration (LOA) and high-order aberration (HOA) RMS measured by PWS aberrometer.

Results

AT LISA Tri had the highest PSFw2 Strehl ratio at both 3.0- and 4.0-mm pupil size (0.52 ± 0.14 and 0.31 ± 0.10; P < 0.05), followed by SA60AT (0.41 ± 0.11 and 0.28 ± 0.07) and PanOptix (0.4 ± 0.07 and 0.26 ± 0.04). AT LISA Tri was found to provide a significantly better retinal image quality than PanOptix at both 3.0 mm (P < 0.0001) and 4.0 mm (P = 0.004). Mplus MF15 was found to be significantly better than Mplus MF30 at both 3.0 mm (P < 0.0001) and 4.0 mm (P = 0.002). Total RMS, LOA RMS, HOA RMS, PSF Strehl ratio and PSFw2 varied significantly between the studied groups (P < 0.001).

Conclusions

Far distance clinical image quality parameters measured by PWS aberrometer differed significantly according to the technology of the implanted lens. AT LISA Tri, SA60AT and PanOptix showed the highest values of far distance retinal image quality, while the lowest PSFw2 Strehl ratios were displayed by Miniwell, Mplus MF30 and Precizon Presbyopic.

Effects of varying illumination on ocular aberrations and aberration compensation before and after small incision Lenticule extraction: a prospective cohort study

BACKGROUND

There are few reports regarding the influence of varying illumination on the compensation effect before and after corneal refractive surgery. We aimed to evaluate the changes in refraction, higher-order aberrations, and aberration compensation between mesopic and photopic illumination before and after small incision lenticule extraction.

METHODS

In this prospective cohort study, only the right eyes of patients who underwent small incision lenticule extraction for the correction of myopia and myopic astigmatism at the Tianjin Eye Hospital were included. Wavefront refraction and higher-order aberrations were measured preoperatively and 3 months postoperatively under mesopic and photopic illumination. Compensation factors were calculated as 1 - (aberration of the whole eye/aberration of the anterior corneal surface).

RESULTS

Forty patients undergoing small incision lenticule extraction were enrolled. All surgeries were completed without postoperative complications. Preoperatively, the eyes only had a statistically significantly higher (t = - 4.589, p < .001) spherical refractive error under mesopic vs. photopic illumination (median [interquartile range], - 6.146 [2.356] vs. - 6.030 [2.619] diopters [D]), whereas postoperatively, the eyes also exhibited statistically significantly higher (t = - 3.013, p = .005) astigmatism (- 0.608 [0.414] vs. - 0.382 [0.319] D). Differences in spherical refraction between the two illuminations were the highest in postoperative eyes (Δ > 0.5 D). Only postoperative eyes exhibited statistically significant elevations (t ≥ 4.081, p < .001) in higher-order aberrations under mesopic illumination, and only preoperative eyes exhibited statistically significantly enhanced (χ² = 6.373, p = .01 for fourth-order and χ² = 11.850, p = .001 for primary spherical aberrations) and decreased (χ² = 13.653, p = .001 for horizontal trefoil) compensation factors under mesopic illumination.

CONCLUSIONS

Exaggerations in higher-order aberrations and myopic shift after small incision lenticule extraction became apparent under mesopic illumination. Slight undercorrection may have an enhanced effect under low illumination and may reduce night vision. The specific changes in compensation effects in preoperative eyes may improve optical quality under mesopic illumination. Postoperative eyes have reduced compensation ability, specifically for spherical aberrations, under mesopic illumination, which may diminish night vision. Further studies that include the measurement of subjective night vision parameters should be conducted.

Short-term effect of atropine on higher-order aberrations in myopic children

BACKGROUND

This study aimed to investigate the short-term effect of cycloplegia on higher-order aberrations (HOAs) in school-age myopic children who received 0.25% atropine for cycloplegic refraction.

METHODS

We performed a retrospective chart review of 24 myopic children between the ages of 5 and 15 years, who had received one topical drop of 0.25% atropine for three consecutive nights before undergoing cycloplegic refraction. Auto-refraction, visual acuity, and HOAs measured with the iTrace aberrometer were compared before and after atropine use. To account for the effect of cycloplegia, the amount of HOAs under matching scanning sizes was compared.

RESULTS

There were statistically significant differences in the spherical equivalent, with a hyperopic shift after atropine use (p < 0.001). Corrected visual acuity and spherical aberrations showed no significant change under the respective pupil and scanning sizes before and after atropine use. Under identical scanning sizes, there was a significant change in total spherical aberration (from 0.03 to 0.06 μm, p = 0.044) and internal spherical aberration (from -0.10 to -0.05 μm, p = 0.049) after atropine use. Differences in corneal spherical aberration were insignificant.

CONCLUSION

The positive shift of spherical aberration induced by the inhibition of accommodation in myopic children may have a possible effect against myopic progression. Future studies can focus on the long-term effect on HOAs and impact on visual quality with lower concentrations of atropine.

Distorted optical input affects human perception

Collinear facilitation, the mechanism for grouping contour elements, is a process involving lateral interactions that improve the detectability of a target by the presence of collinear flankers. It was shown that the development of collinear facilitation is experience dependent and that it may be impaired when the visual input is distorted in one meridian (meridional amblyopia). In oblique astigmatism, the blurring is on the opposite oblique meridian in both eyes, resulting in two conflicting images, which may affect the development of binocular vision. We hypothesized that the collinear facilitation of adults with oblique astigmatism is reminiscent of the abnormal development of the lateral facilitation of meridional amblyopia. We explored the perception of binocular vision and collinear facilitation in cases of both distorted and non-distorted vision. Fully corrected participants that tested for the target contrast detection of Gabor patches and two collinear flankers, presented for 80 ms, were positioned at different orientations (0° (180°), 45°, 90°, and 135°) and for different eyes (monocular, binocular). The results show a significant anisotropy for monocular collinear facilitation between the blured and the clear meridians, being lower in the blurriest meridian than in the clearest meridian, resembling the meridional amblyopia results. Collinear facilitation results in poor binocular summation between the monocular channels. Our results indicate that the perceptual behavior was similar to that of meridional amblyopic subjects having an anisotropy of collinear facilitation between cardinal meridians in oblique astigmatic subjects.

Analysis of corneal real astigmatism and high order aberration changes that cause visual disturbances after lower eyelid epiblepharon repair surgery

This retrospective, cross-sectional study investigated changes in corneal low and high order aberrations (LOAs and HOAs) after lower eyelid epiblepharon repair surgery. In total, 108 eyes from 54 patients were evaluated. Wavefront analyses for calibrated LOAs and HOAs were performed using a Galilei G4 Dual Scheimpflug Analyzer before surgery and during the first and second follow-ups (f/u), adjusting for several risk factors. Flat keratometry (K) and axis values decreased significantly from baseline at the first f/u. At the second f/u, mean K and axis values decreased. Coma and trefoil increased from baseline at the first f/u and normalized by the second f/u. Spherical aberrations (SA) only decreased at the second f/u. After correction for risk factors, cylinder, coma, trefoil, and SA significantly increased at the first f/u; axis and flat K values decreased. At the second f/u, cylinder increased while axis and mean K values significantly decreased. Epiblepharon repair surgery may result in a shift from 'with-the-rule' to 'against-the-rule' axis change. Flat K, coma, and trefoil may be affected by mechanical force changes immediately post-surgery, while mean K values and SA may also change with corneal state changes including corneal erosion healing after the second f/u during the postoperative period.

Bifocal and Multifocal Contact Lenses for Presbyopia and Myopia Control

Bifocal and multifocal optical devices are intended to get images into focus from objects placed at different distances from the observer. Spectacles, contact lenses, and intraocular lenses can meet the requirements to provide such a solution. Contact lenses provide unique characteristics as a platform for implementing bifocality and multifocality. Compared to spectacles, they are closer to the eye, providing a wider field of view, less distortion, and their use is more consistent as they are not so easily removed along the day. In addition, contact lenses are also minimally invasive, can be easily exchangeable, and, therefore, suitable for conditions in which surgical procedures are not indicated. Contact lenses can remain centered with the eye despite eye movements, providing the possibility for simultaneous imaging from different object distances. The main current indications for bifocal and multifocal contact lenses include presbyopia correction in adult population and myopia control in children. Considering the large numbers of potential candidates for optical correction of presbyopia and the demographic trends in myopia, the potential impact of contact lenses for presbyopia and myopia applications is undoubtedly tremendous. However, the ocular characteristics and expectations vary significantly between young and older candidates and impose different challenges in fitting bifocal and multifocal contact lenses for the correction of presbyopia and myopia control. This review presents the recent developments in material platforms, optical designs, simulated visual performance, and the clinical performance assessment of bifocal and multifocal contact lenses for presbyopia correction and/or myopia progression control.

Posterior corneal shape: Comparison of height data from 3 corneal topographers

PURPOSE

To compare the ability of 3 clinical corneal topographers to describe the posterior corneal shape.

SETTING

University Medical Center Groningen, the Netherlands.

DESIGN

Prospective observational study.

METHODS

Corneas of healthy participants were measured twice with a dual Scheimpflug instrument (Galilei G2), a scanning-slit system (Orbscan IIz), and a single Scheimpflug instrument (Pentacam HR). Height data describing the posterior corneal shape were fit with Zernike polynomials. Mean values with standard deviations (SD), test-retest variability (coefficient of repeatability [CoR]), and interdevice variability were determined for the defocus Z(2,0), astigmatism Z(2,-2) and Z(2,2), and higher-order terms coma Z(3,-1) and Z(3,1), trefoil Z(3,-3) and Z(3,3), and spherical aberration Z(4,0) coefficients for 5.5 mm and 8.0 mm diameters.

RESULTS

For the 5.5 mm diameter, CoRs ranged from 0.3 to 4.3 μm with the dual Scheimpflug instrument, 1.6 to 5.2 μm with the scanning-slit system, and 0.3 to 2.0 μm with the single Scheimpflug instrument. The CoR was similar for the Scheimpflug instruments (P = .43) but poorer for the scanning-slit system (P < .001). The CoRs of the Scheimpflug instruments were smaller than the corresponding population SD for defocus, cardinal astigmatism, coma, and spherical aberration. The scanning-slit system failed to provide 8.0 mm diameter data. There was a significant bias (interdevice variability) between the Scheimpflug instruments in the higher-order coefficients at both diameters.

CONCLUSIONS

Repeatability in assessing the posterior corneal shape was generally good for the Scheimpflug instruments but poor for the scanning-slit system. Interdevice variability between the Scheimpflug instruments compromised the interchangeability of higher-order coefficients. For astigmatism, CoR and 95% limits of agreement of the Scheimpflug instruments typically corresponded to 0.1 diopter per astigmatism term.

Adaptive optics imaging of the human retina

Adaptive Optics (AO) retinal imaging has provided revolutionary tools to scientists and clinicians for studying retinal structure and function in the living eye. From animal models to clinical patients, AO imaging is changing the way scientists are approaching the study of the retina. By providing cellular and subcellular details without the need for histology, it is now possible to perform large scale studies as well as to understand how an individual retina changes over time. Because AO retinal imaging is non-invasive and when performed with near-IR wavelengths both safe and easily tolerated by patients, it holds promise for being incorporated into clinical trials providing cell specific approaches to monitoring diseases and therapeutic interventions. AO is being used to enhance the ability of OCT, fluorescence imaging, and reflectance imaging. By incorporating imaging that is sensitive to differences in the scattering properties of retinal tissue, it is especially sensitive to disease, which can drastically impact retinal tissue properties. This review examines human AO retinal imaging with a concentration on the use of the Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO). It first covers the background and the overall approaches to human AO retinal imaging, and the technology involved, and then concentrates on using AO retinal imaging to study the structure and function of the retina.

Age-related variations in corneal geometry and their association with astigmatism: The Yamagata Study (Funagata)

To investigate how aging affects corneal geometry in Japanese adults, and the association between corneal geometry and astigmatism.We included 421 participants who had undergone systemic and ophthalmological examinations in 2015 in Funagata town, Yamagata, Japan. Corneal topographic data were obtained using anterior-segment optical coherence tomography (CASIA SS-1000). Astigmatism was evaluated using power vector analyses where J0 represents the power of the orthogonal astigmatism. Positive values of J0 indicate with-the-rule astigmatism, while negative values indicate against-the-rule (ATR) astigmatism.Regarding age-related variations in corneal geometry, the anterior elevations at axis 0° and 180° decreased, and those at axis 90° and 270° increased with increasing age in linear regression analyses, demonstrating horizontal steepening and vertical flattening of the corneal surface. There were no significant age-related variations in posterior elevations and pachymetry findings, including central corneal thickness. Regarding age-related variations in orthogonal astigmatism, the mean values of J0 and corneal J0 (cJ0) decreased by -0.014 and -0.015 per year of increase in age, indicating astigmatic shift toward ATR. Regarding the correlation between corneal geometry and astigmatism, the shift toward ATR was positively correlated with horizontal steepening and vertical flattening, in accordance with the age-related corneal variations. In addition, the posterior surface of the cornea also has an association with this shift to some extent.The results of our population-based study demonstrated that the age-related variation in astigmatism is associated with geometrical changes in the cornea, especially those in the anterior surface of the cornea.

Variation of axial and oblique astigmatism with accommodation across the visual field

In this study we investigated the impact of accommodation on axial and oblique astigmatism along 12 meridians of the central 30° of visual field and explored the compensation of corneal first-surface astigmatism by the remainder of the eye's optical system. Our experimental evidence revealed no systematic effect of accommodation on either axial or oblique astigmatism for two adult populations (myopic and emmetropic eyes). Although a few subjects exhibited systematic changes in axial astigmatism during accommodation, the dioptric value of these changes was much smaller than the amount of accommodation. For most subjects, axial and oblique astigmatism of the whole eye are both less than for the cornea alone, which indicates a compensatory role for internal optics at all accommodative states in both central and peripheral vision. A new method for determining the eye's optical axis based on visual field maps of oblique astigmatism revealed that, on average, the optical axis is 4.8° temporal and 0.39° superior to the foveal line-of-sight in object space, which agrees with previous results obtained by different methodologies and implies that foveal astigmatism includes a small amount of oblique astigmatism (0.06 D on average). Customized optical models of each eye revealed that oblique astigmatism of the corneal first surface is negligible along the pupillary axis for emmetropic and myopic eyes. Individual variation in the eye's optical axis is due in part to misalignment of the corneal and internal components that is consistent with tilting of the crystalline lens relative to the pupillary axis.

Comparison of Objective and Subjective Changes Induced by Multiple-Pinhole Glasses and Single-Pinhole Glasses

Multiple-pinhole (MPH) glasses are currently sold in many countries with unproven advertisements; however, their objective and subjective effects have not been investigated. Therefore, to investigate the effects of MPH glasses excluding the single-pinhole (SPH) effect, we compared the visual functional changes, reading speed, and ocular discomfort after reading caused by MPH and SPH glasses. Healthy 36 participants with a mean age of 33.1 years underwent examinations of pupil size, visual acuity (VA), depth of focus (DOF), and near point accommodation (NPA); tests for visual field (VF), contrast sensitivity (CS), stereopsis, and reading speed; and a survey of ocular discomfort after reading. Both types of pinhole glasses enlarged pupil diameter and improved VA, DOF, and NPA. However, CS, stereopsis, and VF parameters deteriorated. In comparison with SPH glasses, MPH glasses induced smaller pupil dilation (5.3 and 5.9 mm, P < 0.001) and showed better VF parameters with preserved peripheral VF. However, no significant difference was observed for VA, DOF, NPA, stereopsis, and CS. Reading speed using pinhole glasses was significantly slower than baseline; SPH glasses showed the slowest reading speed. Both types of glasses caused significant ocular discomfort after reading compared with baseline, and symptoms were worst with MPH glasses. In conclusion, both types of pinhole glasses had positive effects due to the pinhole effect; however, they had negative effects on VF, CS, stereopsis, reading speed, and ocular discomfort. In spite of the increased luminance and preserved peripheral VF with MPHs, these glasses caused more severe ocular discomfort than SPH glasses.

The Significance of TRPV4 Channels and Hemichannels in the Lens and Ciliary Epithelium

To function normally, all cells must maintain ion homeostasis, establish a membrane potential, and regulate water content. These actions require active Na-K transport provided by Na,K-ATPase. The lens, however, is made up almost entirely of fiber cells that have little or no Na,K-ATPase activity. Lens ion and water homeostasis rely on Na,K-ATPase activity in a small number of cells at the periphery of epithelium monolayer. Therefore, the function of the epithelium must be integrated with the needs of the fiber mass. This suggests that a remote control mechanism may adjust Na,K-ATPase activity to match increases or decreases of ion leakage, which may occur a considerable distance away. Here, we review evidence that TRPV4 channels in the epithelium become activated when the lens is subjected to osmotic- or damage-induced swelling. This triggers a chain of events in the lens epithelium that opens connexin hemichannels, allowing ATP release that stimulates purinergic receptors, activates Src family tyrosine kinases, and increases Na,K-ATPase activity. Recent studies also revealed functional connexin hemichannels along with TRPV4 channels in nonpigmented ciliary epithelial (NPE) cells that secrete aqueous humor into the eye. Because TRPV4 channels are mechanosensitive, we speculate they might enable the NPE to respond to stimuli such as mechanical distortion associated with volume homeostasis during fluid transfer across the ciliary epithelium or changes in intraocular pressure.

Correlation between apical protrusion in the Scheimflug imaging and Corneal Hysteresis and Corneal Resistance factor by Ocular Response Analyzer, among refractive non-keratoconic Egyptian patients

Introduction

Apical protrusion in the central 4-mm ring in the Scheimflug imaging (Pentacam), both for the anterior and posterior floats as well as Corneal Hysteresis and Corneal Resistance Factor by Ocular Response Analyzer (ORA), generally are considered important predictors for post-Lasik ectasia. The aim of this work was to find out if there is a statistically significant correlation between these different predictors and their correlation with the central corneal thickness for refractive non-keratoconic Egyptian patients trying to achieve a better decision and avoiding ectasia.

Methods

This case-control study involved 142 eyes (of 77 patients with various refractive errors) arriving at the refractive surgery unit in the Research Institute of Ophthalmology in Giza (Egypt) in 2014–2015 seeking excimer laser ablation. The flattest, steepest keratometry readings, central corneal thickness as well as the apical protrusion in the central 4-mm ring, both for the anterior and posterior floats, in microns were measured by Scheimflug imaging. The Corneal Hysteresis and Corneal Resistance Factor were measured by the ocular response analyzer. Statistical analysis was performed by SPSS, using the Pearson correlation test.

Results

The spherical refractive error ranged from +7.00 to −13.00 diopters (−3.80 ± 2.89). The central pachymetry ranged from 494 to 634 μm (550.35 ± 32.13). For the central 4-mm ring, the apical protrusion ranged from 0 to +15 μ (6.93 ± 2.99) for the anterior float and from −3 to +20 μ (9.33 ± 4.55) for the posterior float. The Corneal Hysterisis (CH) ranged from 7 to 14.8 mmHg (10.18±1.44), while the Corneal Resistance Factor (CRF) ranged from 7.5 to 14.9 mmHg (10.58 ± 1.67). There was a strong positive correlation between the central corneal thickness and both Corneal Hysteresis (CH: r = 0.56, P ≤ 0.01) and Corneal Resistance Factor (r = 0.46, P ≤ 0.01). A significant correlation (P < 0.05, r = 0.15) existed between apical protrusion in the posterior float and the central corneal thickness. Also, significant negative correlation (P < 0.05, r = −0.12) existed between apical protrusion in the anterior float by pentacam and the Corneal Resistance Factor by ocular response analyzer.

Conclusions

Our finding of a strong positive correlation between both Corneal Hysteresis and Corneal Resistance Factor and the Central corneal thickness being important for biomechanical corneal stability. The findings of this study also support using both machines preoperatively to decrease the risk of post-Lasik ectasia.

Aberration compensation between anterior and posterior corneal surfaces after Small incision lenticule extraction and Femtosecond laser-assisted laser in-situ keratomileusis

PURPOSE

To investigate the aberration compensation between anterior and posterior corneal surfaces after SMILE and FS-LASIK.

METHODS

Fifty-five subjects (55 eyes) undergoing SMILE and 51 subjects (51 eyes) undergoing FS-LASIK were enrolled in this study. Wavefront aberrations of anterior and posterior corneal surfaces and the whole cornea at 6 mm in diameter were measured using a Scheimpflug Camera preoperatively and one, three and 6 months postoperatively. The compensation factor (CF), where CF = 1 - (aberration of the whole cornea/aberration of anterior corneal surface), was calculated.

RESULTS

Spherical aberration of the posterior surface and the whole cornea remained stable after SMILE. However, spherical aberration of posterior surface increased significantly at 6 months in the FS-LASIK group. The total higher-order aberration (tHOA) of the anterior surface and the whole cornea was lower at 6 months than at one and 3 months (p = 0.001 and 0.001, respectively) in the FS-LASIK group. Meanwhile, in the SMILE group, no significant difference in tHOA was found between various postoperative time points. There were significant decreases in the CF of tHOA compared with preoperative values in both groups. The CF of spherical aberration reduced significantly in both groups at 3 and 6 mm in diameter one, three and 6 months postoperatively. Significant decreases in the CF of vertical coma were found at three and 6 months postoperatively in the FS-LASIK group compared with preoperative values at 6 mm in diameter (p = 0.021 and 0.008, respectively). The change in CF (ΔCF) of spherical aberration was smaller in the SMILE group than in the FS-LASIK group at one and 3 months postoperatively (p = 0.003 and p < 0.0001, respectively). The ΔCF of spherical aberration was significantly lower in moderately myopic subjects than in subjects with high myopia at 1 month in the SMILE group (p = 0.041) and at one, three and 6 months in the FS-LASIK group (p = 0.014, 0.020, and 0.004, respectively).

CONCLUSIONS

The posterior corneal surface plays an important role in compensating for spherical aberration of the anterior corneal surface. The compensation mechanisms of spherical aberration and higher-order aberration between anterior and posterior corneal surfaces were disrupted by the SMILE and the FS-LASIK procedures. The change in the CF of spherical aberration was smaller in the SMILE group compared with the FS-LASIK group, especially in subjects with high myopia.

Evaluation of actual retinal images produced by misaligned aspheric intraocular lenses in a model eye

Purpose

To examine the effect of misalignment (decentration and tilt) of intraocular lenses (IOLs) on retinal image quality using a water-immersed model eye with corneal spherical aberration adjusted to the values found in normal human eyes (spherical aberration 0.25 μm; pupil diameter 6 mm).

Methods

Three types of IOL holders were prepared. The first was without decentration or tilt, the second had a decentration of 0.5 mm, and the third had a tilt of 5.0°. One spherical IOL and three aspherical IOLs, each with a power of +20 D, were set in the holders and their optical properties (wave front aberration, defocused modulation transfer function, defocused point spread function, and Landolt ring simulations) were compared.

Results

Coma aberrations generated by misaligned IOLs were related to the spherical aberration corrective power of the IOLs. Landolt ring simulations show that the depth of focus increased as spherical aberration increased and that the retinal image quality was degraded by increases in coma aberration.

Conclusion

Coma aberration was generated by IOLs with a large degree of spherical aberration correction, leading to reduced retinal image quality when the IOL was misaligned. This suggests that, in a clinical setting, the quality of vision might be improved by reducing the degree of coma aberration using IOLs that retain, or minimally correct, spherical aberration.

An analytical method for predicting the geometrical and optical properties of the human lens under accommodation

We present an analytical method to describe the accommodative changes in the human crystalline lens. The method is based on the geometry-invariant lens model, in which the gradient-index (GRIN) iso-indicial contours are coupled to the external shape. This feature ensures that any given number of iso-indicial contours does not change with accommodation, which preserves the optical integrity of the GRIN structure. The coupling also enables us to define the GRIN structure if the radii and asphericities of the external lens surfaces are known. As an example, the accommodative changes in lenticular radii and central thickness were taken from the literature, while the asphericities of the external surfaces were derived analytically by adhering to the basic physical conditions of constant lens volume and its axial position. The resulting changes in lens geometry are consistent with experimental data, and the optical properties are in line with expected values for optical power and spherical aberration. The aim of the paper is to provide an anatomically and optically accurate lens model that is valid for 3 mm pupils and can be used as a new tool for better understanding of accommodation.

Influence of cycloplegia with topical cyclopentolate on higher-order aberrations in myopic children

PURPOSE

To investigate the influence of cycloplegia with topical cyclopentolate on wavefront aberrations in myopic children.

DESIGN

This is a prospective, comparative study.

METHODS

Twenty-eight myopic children with a mean age of 7.25 ± 2.55 were enrolled in this study. We evaluated refraction and wavefront aberrations before and after cycloplegia with 1% cyclopentolate hydrochloride. Ocular and corneal aberrations were simultaneously measured and compared with each other. Individual Zernike components were also analyzed up to the sixth order. All these parameters were compared before and after cycloplegia.

RESULTS

Ocular higher-order aberrations (HOAs) significantly increased after cycloplegia (P=0.012 for spherical-like and P=0.015 for total HOAs). Corneal HOAs did not change after cycloplegia. When corneal and ocular HOAs were compared, the ocular HOAs were significantly smaller than the corneal HOAs in spherical-like aberrations (P<0.001) and total HOAs (P=0.006). As for individual Zernike components, ocular aberration generally showed smaller or equivalent values in comparison with corneal aberration. In addition, each Zernike component showed a large standard deviation.

CONCLUSIONS

Internal optics compensates for corneal HOAs in myopic children, and paralysis of tonic accommodation with cyclopentolate considerably affects ocular HOAs. However, inter-individual variation in each Zernike component is quite large in myopic children.

Optical quality comparison between spherical and aspheric toric intraocular lenses

PURPOSE

To measure and compare the optical quality of spherical and aspheric toric intraocular lenses (IOLs).

METHODS

Wavefront aberrations of AcrySof Toric and IQ Toric IOLs (Alcon Laboratories) for different powers were measured at 3- and 5-mm pupils by Nimo TR0805 instrument. The Zernike coefficients of trefoil, coma, tetrafoil, secondary astigmatism, and spherical aberration were evaluated. The point spread functions (PSFs) of each IOL evaluated were calculated from the wavefront aberrations. The PSF images also were calculated from the IOL wavefront aberrations, adding the cornea's aberrations to simulate the optical quality after their implantation.

RESULTS

Spherical toric IOLs showed positive and aspheric toric IOLs negative spherical aberrations. Statistically significant differences were found in spherical aberration root mean square (RMS) values between spherical and aspheric IOLs for both pupil sizes (p<0.05). Aspheric toric IOLs showed higher spherical aberration RMS values. We found differences in PSF images between both IOL designs at 5-mm pupil. The PSFs corresponding to the aspheric toric IOLs showed more spread out than the PSFs corresponding to the spherical toric IOLs. However, when corneal aberrations were added, aspheric toric IOLs showed better optical quality than spherical toric IOLs.

CONCLUSIONS

At 3-mm pupil, the optical quality between the IOL designs was similar, but at 5-mm pupil the optical quality was higher for spherical IOLs than aspheric IOLs. However, when theoretical corneal aberrations were added, aspheric toric IOLs showed better optical quality than spherical toric IOLs, due to the compensation between aspheric toric IOL negative spherical aberration and corneal positive spherical aberration.

Ocular and optical coherence tomography-based corneal aberrometry in keratoconic eyes treated by intracorneal ring segments

PURPOSE

To analyze corneal and total aberrations using custom-developed anterior segment spectral optical coherence tomography (OCT) and laser ray tracing in keratoconic eyes implanted with intracorneal ring segments (ICRS).

DESIGN

Evaluation of technology. Prospective study. Case series.

METHODS

Nineteen keratoconic eyes were measured before and after ICRS surgery. Anterior and posterior corneal topographic and pachymetric maps were obtained pre- and postoperatively from 3-dimensional OCT images of the anterior segment, following automatic image analysis and distortion correction. The pupil center coordinates were used as reference for estimation of corneal aberrations. Corneal aberrations were estimated by computational ray tracing on the anterior and posterior corneal surfaces. Total aberrations were measured using a custom-developed laser ray tracing aberrometer. Corneal and total aberrations were compared in 8 eyes pre- and postoperatively for 4-mm pupils.

RESULTS

Total and corneal aberrations were highly correlated. Average root mean square of corneal and total high-order aberrations (HOAs) were 0.78 ± 0.35 μm and 0.57 ± 0.39 μm preoperatively, and 0.88 ± 0.36 μm and 0.53 ± 0.24 μm postoperatively (4-mm pupils). The anterior corneal surface aberrations were partially compensated by the posterior corneal surface aberrations (by 8.3% preoperatively and 4.1% postoperatively). Astigmatism was 2.03 ± 1.11 μm preoperatively and 1.60 ± 0.94 μm postoperatively. The dominant HOA aberrations both pre- and postoperatively were vertical coma (Z3(-1)), vertical trefoil (Z3(-3)), and secondary astigmatism (Z4(4)). ICRS decreased corneal astigmatism by 27% and corneal coma by 5%, but on average, the overall amount of HOA did not decrease significantly with ICRS treatment.

CONCLUSIONS

OCT is a reproducible technique to evaluate corneal aberrations. OCT-based corneal aberrations and ocular aberrations show a high correspondence in keratoconic patients before and after ICRS implantation. ICRS produced a decrease in astigmatism, but on average did not produce a consistent decrease of HOAs.

Diurnal variations in ocular aberrations of human eyes

PURPOSE

To investigate the diurnal variations in ocular wavefront aberrations over two consecutive days in young adult subjects.

MATERIALS AND METHODS

Measurements of both lower-order (sphero-cylindrical refractive powers) and higher-order (third and fourth-order aberration terms) ocular aberrations were collected for 30 young adult subjects at 10 different times over two consecutive days using a Hartmann-Shack aberrometer. Seventeen subjects were myopic and 13 were emmetropic. Five sets of measurements were collected each day at approximately 3 hourly intervals, with the first measurement taken at ∼9 am and the final measurement at ∼9 pm.

RESULTS

Spherical equivalent refraction (p = 0.029) and spherical aberration (p = 0.043) were both found to undergo significant diurnal variation over the two measurement days. The spherical equivalent was typically found to be at a maximum (i.e. most hyperopic) at the morning measurement, with a small myopic shift of 0.37 ± 0.15 D observed over the course of the day. The mean spherical aberration of all subjects (0.038 ± 0.048 μm) was found to be positive during the day and gradually became more negative into the evening, with a mean amplitude of change of 0.036 ± 0.02 μm. None of the other considered sphero-cylindrical refractive power components or higher-order aberrations exhibited significant diurnal variation over the two days of the experiment (p > 0.05). Except for the lower-order astigmatism at 90/180 degree (p = 0.040), there were no significant differences between myopes and emmetropes in the magnitude and timing of the observed diurnal variations (p > 0.05).

CONCLUSIONS

Significant diurnal variations in spherical equivalent and spherical aberration were consistently observed over two consecutive days of measurement. Research and clinical applications requiring precise refractive error and wavefront measurements should take these diurnal changes into account when interpreting wavefront data.

Predicting ocular residual astigmatism using corneal and refractive parameters: a myopic eye study

PURPOSE

To investigate the nature of ocular residual astigmatism (ORA) in myopic patients and to identify indicators to predict a patient's ORA from corneal and ocular refractive data.

METHODS

In total, 356 myopic eyes from 178 patients (M:F = 41:137) were examined using a Scheimpflug keratometer (Pentacam) and a wavefront analyzer (WaveScan). Vertex distance-adjusted total ocular astigmatism and corneal astigmatism [anterior corneal power (ACP) and true net power (TNP)] were used to calculate ORA, respectively. A power vector system (J0 and J45) was adopted for the astigmatism analysis.

RESULTS

The mean age of the study eyes was 27.6 ± 5.0 years, and mean refractive spherical equivalent, sphere and cylinder was -4.94 ± 1.90 diopter, -4.42 ± 1.87 diopter, and -1.05 ± 0.82, respectively. Corneal astigmatism measured by TNP was more representative of total ocular astigmatism than ACP (p < 0.001). Both the J0 and J45 components of ORA showed significant negative correlation with corneal astigmatism measured by ACP and TNP (p < 0.001). ORA (J0 and J45) was negatively correlated with total ocular spherical equivalent and positively correlated with total ocular astigmatism.

CONCLUSIONS

We verified the general compensatory effect of corneal astigmatism by internal optics in myopic patients. TNP was closer to refractive astigmatism than ACP was. Total ocular spherical equivalent and total ocular astigmatism showed significant correlation with ORA. Therefore these can be possible indicators to predict ORA.

Change in efficiency of aspheric intraocular lenses based on pupil diameter

PURPOSE

To measure the effect of spherical aberration correction by aspheric intraocular lenses (IOLs) based on pupil diameter, and to determine the minimum pupil diameter for each aspheric IOL.

DESIGN

Retrospective cross-sectional study.

METHODS

Eight-six patients (169 eyes) who were implanted with a HOYA AF-1 NY-60 (HOYA Corporation) or Tecnis ZCB00 1-piece IOL (Abbott Medical Optics Inc) were enrolled. Ocular, corneal, and internal spherical aberrations were measured at the 1-month postoperative visit using the Wavefront Analyzer KR-1W (Topcon). Minimum pupil diameter, which is required for each aspheric IOL to be effective, was calculated using a regression equation.

RESULTS

The mean value of internal spherical aberration of the Tecnis ZCB00 group (-0.09 ± 0.094 μm) was lower than that of the HOYA NY-60 group (-0.05 ± 0.072 μm) (P = .005). The original negative spherical aberrations of the HOYA NY-60 (-0.18 μm) were measured at a pupil diameter of 5.6 mm, and for the Tecnis ZCB00 (-0.27 μm) at a pupil diameter of 6.1 mm. The aspheric IOL efficiency dropped to 0% when the pupil diameter was 3.47 mm for the Tecnis ZCB00 group and 3.71 mm for the HOYA NY-60 group.

CONCLUSIONS

When the pupil diameters of patients are smaller than 3.4 mm for the Tecnis ZCB00 and 3.7 mm for the HOYA NY-60, the spherical aberration correction using these aspheric IOLs seems to be ineffective. Approximately 10% of the eyes showed smaller pupil size than the minimum effective diameter under mesopic conditions.

Adaptive Optics for Visual Simulation

A revision of the current state-of-the-art adaptive optics technology for visual sciences is provided. The human eye, as an optical system able to generate images onto the retina, exhibits optical aberrations. Those are continuously changing with time, and they are different for every subject. Adaptive optics is the technology permitting the manipulation of the aberrations, and eventually their correction. Across the different applications of adaptive optics, the current paper focuses on visual simulation. These systems are capable of manipulating the ocular aberrations and simultaneous visual testing though the modified aberrations on real eyes. Some applications of the visual simulators presented in this work are the study of the neural adaptation to the aberrations, the influence of aberrations on accommodation, and the recent development of binocular adaptive optics visual simulators allowing the study of stereopsis.

Effect of age on components of peripheral ocular aberrations

PURPOSE

To investigate the effect of age on the contributions of the anterior cornea and internal components to ocular aberrations in the peripheral visual field.

METHODS

Ocular aberrations were measured in 10 young emmetropes and 7 older emmetropes using a modified commercial Hartmann-Shack aberrometer across 42° × 32° of central visual field. Anterior corneal aberrations were estimated from anterior corneal topography using theoretical ray-tracing. Internal aberrations were calculated by subtracting anterior corneal aberrations from ocular aberrations.

RESULTS

Anterior corneal aberrations of young subjects were reasonably compensated by the internal aberrations, except for astigmatism for which the internal contribution was small out to the 21° field limit. The internal coma and spherical aberration of the older subjects were considerably smaller in magnitude than those of the young subjects such that the compensation for anterior corneal aberrations was poorer. This can be explained by age-related changes in the lens shape and refractive index distribution.

CONCLUSIONS

Loss of balance between anterior cornea and internal components of higher order aberrations with increasing age, found previously for on-axis vision, applies also to the peripheral visual field.

Role of the human lens gradient-index profile in the compensation of third-order ocular aberrations

The open question regarding the compensation of the ocular aberrations between the cornea and the lens is currently being investigated. We report additional insights considering the role of the lens gradient-index (GRIN) profile in third-order ocular aberrations, since this profile changes through life. Thus, we have calculated the contribution of that profile to the ocular aberrations with aging by applying the Seidel third-order theory of tilted and decentered elements, and by using a schematic-eye model. The results show the GRIN profile is needed to account for the decoupling of the aberrations between the cornea and the lens because the geometrical changes of the ocular surfaces with aging are not enough. Therefore, the current developments of aging human-eye models, as well as the experimental studies, cannot neglect the changes of the lens GRIN structure through life when modelling mechanisms of the compensation of ocular aberrations.

Theoretical analysis for spherical aberration induction with low-order correction in refractive surgery

A theoretical foundation for the analysis of ocular aberration correction is developed. It enables a comparative study for two different refractive surgical approaches, namely, the conventional and the Q-preserved treatment modalities. A refractive surgical factor is identified that leads to a simple cubic function for the postoperative asphericity factor for the conventional treatment. A formulation is developed that paves the way for the calculation of the induction of spherical aberration for low-order aberration correction in refractive surgery. Opposite to the general belief, the Munnerlyn shape makes myopic LASIK more prolate, not oblate. A Monte Carlo simulation was conducted for 1000 eyes for these two refractive surgical modalities. It was found that, although the postoperative spherical aberration is similar for these surgical modalities, for the induction of spherical aberration from the ablation target shape, the conventional modality appears to be slightly more predictable.

Total ocular, anterior corneal and lenticular higher order aberrations in hyperopic, myopic and emmetropic eyes

Total ocular higher order aberrations and corneal topography of myopic, emmetropic and hyperopic eyes of 675 adolescents (16.9 ± 0.7 years) were measured after cycloplegia using COAS aberrometer and Medmont videokeratoscope. Corneal higher order aberrations were computed from the corneal topography maps and lenticular (internal) higher order aberrations derived by subtraction of corneal aberrations from total ocular aberrations. Aberrations were measured for a pupil diameter of 5mm. Multivariate analysis of variance followed by multiple regression analysis found significant difference in the fourth order aberrations (SA RMS, primary spherical aberration coefficient) between the refractive error groups. Hyperopic eyes (+0.083 ± 0.05 μm) had more positive total ocular primary spherical aberration compared to emmetropic (+0.036 ± 0.04 μm) and myopic eyes (low myopia=+0.038 ± 0.05 μm, moderate myopia=+0.026 ± 0.06 μm) (p<0.05). No difference was observed for the anterior corneal spherical aberration. Significantly less negative lenticular spherical aberration was observed for the hyperopic eyes (-0.038 ± 0.05 μm) than myopic (low myopia=-0.088 ± 0.04 μm, moderate myopia=-0.095 ± 0.05 μm) and emmetropic eyes (-0.081 ± 0.04 μm) (p<0.05). These findings suggest the existence of differences in the characteristics of the crystalline lens (asphericity, curvature and gradient refractive index) of hyperopic eyes versus other eyes.

Visualizing ocular lens fluid dynamics using MRI: manipulation of steady state water content and water fluxes

Studies using various MRI techniques have shown that a water-protein concentration gradient exists in the ocular lens. Because this concentration is higher in the core relative to the lens periphery, a gradient in refractive index is established in the lens. To investigate how the water-protein concentration profile is maintained, bovine lenses were incubated in different solutions, and changes in water-protein concentration ratio monitored using proton density weighted (PD-weighted) imaging in the absence and presence of heavy water (D2O). Lenses incubated in artificial aqueous humor (AAH) maintained the steady state water-protein concentration gradient, but incubating lenses in high extracellular potassium (KCl-AAH) or low temperature (Low T-AAH) caused a collapse of the gradient due to a rise in water content in the core of the lens. To visualize water fluxes, lenses were incubated in D2O, which acts as a contrast agent. Incubation in KCl-AAH and low T-AAH dramatically slowed the movement of D2O into the core but did not affect the movement of D2O into the outer cortex. D2O seemed to preferentially enter the lens cortex at the anterior and posterior poles before moving circumferentially toward the equatorial regions. This directionality of D2O influx into the lens cortex was abolished by incubating lenses in high KCl-AAH or low T-AAH, and resulted in homogenous influx of D2O into the outer cortex. Taken together, our results show that the water-protein concentration ratio is actively maintained in the core of the lens and that water fluxes preferentially enter the lens at the poles.

Optical and biometric characteristics of anisomyopia in human adults

PURPOSE

To investigate the role of higher order optical aberrations and thus retinal image degradation in the development of myopia, through the characterization of anisomyopia in human adults in terms of their optical and biometric characteristics.

METHODS

The following data were collected from both eyes of 15 young adult anisometropic myopes and 16 isometropic myopes: subjective and objective refractive errors, corneal power and shape, monochromatic optical aberrations, anterior chamber depth, lens thickness, vitreous chamber depth, and best corrected visual acuity. Monochromatic aberrations were analyzed in terms of their higher order components, and further analyzed in terms of 31 optical quality metrics. Interocular differences for the two groups (anisomyopes vs isomyopes) were compared and the relationship between measured ocular parameters and refractive errors also analyzed across all eyes.

RESULTS

As expected, anisomyopes and isomyopes differed significantly in terms of interocular differences in vitreous chamber depth, axial length and refractive error. However, interocular differences in other optical properties showed no significant intergroup differences. Overall, higher myopia was associated with deeper anterior and vitreous chambers, higher astigmatism, more prolate corneas, and more positive spherical aberration. Other measured optical and biometric parameters were not significantly correlated with spherical refractive error, although some optical quality metrics and corneal astigmatism were significantly correlated with refractive astigmatism.

CONCLUSIONS

An optical cause for anisomyopia related to increased higher order aberrations is not supported by our data. Corneal shape changes and increased astigmatism in more myopic eyes may be a by-product of the increased anterior chamber growth in these eyes; likewise, the increased positive spherical aberration in more myopic eyes may be a product of myopic eye growth.