Abstract: It is well known that the aberrations of the cornea are partially compensated by the aberrations of the internal optics of the eye (primarily the crystalline lens) in young subjects. This effect has been found not only for the spherical aberration, but also for horizontal coma. It has been debated whether the compensation of horizontal coma is the result of passive mechanism [Artal, P., Benito, A., & Tabernero, J. (2006). The human eye is an example of robust optical design. Journal of Vision, 6 (1), 1-7] or through an active developmental feedback process [Kelly, J. E., Mihashi, T., & Howland, H. C. (2004). Compensation of corneal horizontal/vertical astigmatism, lateral coma, and spherical aberration by internal optics of the eye. Journal of Vision, 4 (4), 262-271]. In this study we investigate the active or passive nature of the horizontal coma compensation using eyes with artificial lenses, where no active developmental process can be present. We measured total and corneal aberrations, and lens tilt and decentration in a group of 38 eyes implanted with two types of intraocular lenses designed to compensate the corneal spherical aberration of the average population. We found that spherical aberration was compensated by 66%, and horizontal coma by 87% on average. The spherical aberration is not compensated at an individual level, but horizontal coma is compensated individually (coefficients of correlation corneal/internal aberration: -0.946, p < 0.0001). The fact that corneal (but not total) horizontal coma is highly correlated with angle lamda (computed from the shift of the 1st Purkinje image from the pupil center, for foveal fixation) indicates that the compensation arises primarily from the geometrical configuration of the eye (which generates horizontal coma of opposite signs in the cornea and internal optics). The amount and direction of tilts and misalignments of the lens are comparable to those found in young eyes, and on average tend to compensate (rather than increase) horizontal coma. Computer simulations using customized model eyes and different designs of intraocular lenses show that, while not all designs produce a compensation of horizontal coma, a wide range of aspheric biconvex designs may produce comparable compensation to that found in young eyes with crystalline lenses, over a relatively large field of view. These findings suggest that the lens shape, gradient index or foveal location do not need to be fine-tuned to achieve a compensation of horizontal coma. Our results cannot exclude a fine-tuning for the orientation of the crystalline lens, since cataract surgery seems to preserve the position of the capsule. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: The minimum number of samples necessary to fully characterize the aberration pattern of the eye is a question under debate in the clinical as well as the scientific community. We performed repeated measurements of ocular aberrations in 12 healthy nonsurgical human eyes and in 3 artificial eyes, using different sampling patterns (hexagonal, circular, and rectangular with 19 to 177 samples, and 3 radial patterns with 49 sample coordinates corresponding to zeros of the Albrecht, Jacobi, and Legendre functions). For each measurement set we computed two different metrics based on the root-mean-square (RMS) of difference maps (RMS_Diff) and the proportional change in the wavefront (W%). These metrics are used to compare wavefront estimates as well as to summarize results across eyes. We used computer simulations to extend our results to "abnormal eyes" (keratoconic, post-LASIK, and post-radial keratotomy eyes). We found that the spatial distribution of the samples can be more important than the number of samples for both our measured as well as our simulated "abnormal" eyes. Experimentally, we did not find large differences across patterns except, as expected, for undersampled patterns.
Abstract: Purpose: The mouse eye is a widely used model for retinal disease and has potential to become a model for myopia. Studies of retinal disease will benefit from imaging the fundus in vivo. Experimental models of myopia often rely on manipulation of the visual experience. In both cases, knowledge of the optical quality of the eye, and in particular, the retinal image quality degradation imposed by the ocular aberrations is essential. In this study, we measured the ocular aberrations in the wild type mouse. Methods: Twelve eyes from six four-week old black C57BL/6 wild type mice were studied. Measurements were done on awake animals, one being also measured under anesthesia for comparative purposes. Ocular aberrations were measured using a custom-built Hartmann-Shack system (using 680-nm illumination). Wave aberrations are reported up to fourth order Zernike polynomials. Spherical equivalent and astigmatism were obtained from the 2nd order Zernike terms. Modulation Transfer Functions (MTF) were estimated for the best focus, and through-focus, to estimate depth-of-focus. All reported data were for 1.5-mm pupils. Results: Hartmann-Shack refractions were consistently hyperopic (10.12 +/- 1.41 D, mean and standard deviation) and astigmatism was present in many of the eyes (3.64 +/- 3.70 D, on average). Spherical aberration was positive in all eyes (0.15 +/- 0.07 Pin) and coma terms RMS were significantly high compared to other Zernike terms (0.10 +/- 0.03 mu m). MTFs estimated from wave aberrations show a modulation of 0.4 at 2 c/deg, for best focus (and 0.15 without cancelling the measured defocus). For that spatial frequency, depth-of-focus estimated from through-focus modulation data using the Rayleigh criterion was 6 D. Aberrations in the eye of one anesthetized mouse were higher than in the same eye of the awake animal. Conclusions: Hyperopic refractions in the mouse eye are consistent with previous retinoscopic data. The optics of the mouse eye is far from being diffraction-limited at 1.5-mm pupil, with significant amounts of spherical aberration and coma. However, estimates of MTFs from wave aberrations are higher than previously reported using a double-pass technique, resulting in smaller depth-of-field predictions. Despite the large degradation imposed by the aberrations these are lower than the amount of aberrations typically corrected by available correction techniques (i.e., adaptive optics). On the other hand, aberrations do not seem to be the limiting factor in the mouse spatial resolution. While the mouse optics are much more degraded than in other experimental models of myopia, its tolerance to large amounts of defocus does not seem to be determined entirely by the ocular aberrations. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: This study investigated differences in geometrical properties and optical aberrations between a group of hyperopes and myopes (age-matched 30.3+/-5.2 and 30.5+/-3.8 years old, respectively, and with similar absolute refractive error 3.0+/-2.0 and -3.3+/-2.0, respectively). Axial length (AL) was measured by means of optical biometry, and corneal apical radius of curvature (CR) and asphericity (Q) were measured by fitting corneal topography data to biconic surfaces. Corneal aberrations were estimated from corneal topography by means of virtual ray tracing, and total aberrations were measured using a laser ray tracing technique. Internal aberrations were estimated by subtracting corneal from total aberrations. AL was significantly higher in myopes than in hyperopes and AL/CR was highly correlated with spherical equivalent. Hyperopic eyes tended to have higher (less negative) Q and higher total and corneal spherical aberration than myopic eyes. RMS for third-order aberrations was also significantly higher for the hyperopic eyes. Internal aberrations were not significantly different between the myopic and hyperopic groups, although internal spherical aberration showed a significant age-related shift toward less negative values in the hyperopic group. For these age and refraction ranges, our cross-sectional results do not support evidence of relationships between emmetropization and ocular aberrations. Our results may be indicative of presbyopic changes occurring earlier in hyperopes than in myopes.
Abstract: PURPOSE: To compare wave aberrations obtained using different sampling patterns xD;and densities on the same eyes (artificial and human) in a configurable wavefront sensor, xD; and to assess the most appropriate sampling for measuring ocular aberrations. xD;METHODS:We used a laser-ray-tracing system programmed xD;with different sampling pattern configurations (hexagonal, rectangular and circular) xD;and densities (19, 37, 49, 91 & 177 spots on a 6-mm pupil) on three artificial xD;and two human eyes.Wave aberrations were fit by 7th-order Zernike polynomials. xD;Differences across patterns were assessed by the RMS of the difference xD;maps across sampling patterns, in both artificial and real eyes. Hexagonal-91 xD;pattern was used as a reference. In real eyes probability maps were obtained xD;by computing local p-values for repeated wave aberration estimates across different xD;sampling patterns. The percentage of the significantly different regions xD;within each wave aberration map was obtained for each sampling configuration. xD;RESULTS: The standard-deviation (averaged across Zernike coefficients) across xD;sampling patterns was <0.03mm in artificial eyes and <0.07mm in real eyes. xD;The RMS standard-deviation was <0.07 mm and <0.1 mm respectively, and xD;was reduced when the 19-sample patterns were excluded. The RMS difference xD;across different samplings tended to be within RMS difference across similar xD;sampling patterns, except for the 19-sample and the Jacobi and Legendre patterns. xD;The percentage of pupil areas showing statistical aberration differences xD;in real eyes ranged from 0.6% to 16%, to 24 % when Jacobi and Legendre were xD;included and up to 36% when 19-sample patterns were included. CONCLUSIONS: xD;Patterns with very small number of samples failed at reproducing the xD;wave aberration. Sample distribution can be more relevant than sample density. xD;Hexagonal-37 and Albrecht-49 show good compromise between reproducibility xD;and sample efficiency.
Abstract: Purpose: To compare the optical features of the components of myopic and hyperopic eyes. xD;Methods: Axial length, corneal topographic data (radius and asphericity) and corneal and total aberrations were obtained for 10 myopic (age 28.9±4.1 years) and 11 hyperopic eyes (age 29.9±4.8 years). The spherical equivalent ranged from –1.2 to –7.6 D (-3.8±2.4 D) for the myopic, and from 0.6 to 7.4 D (3.4±2.3 D) for the hyperopic group. Cylinder was <2.50 D. Axial length was measured with optical biometry. Corneal topographic data were obtained from a videokeratographic system and fit to a biconic function to obtain corneal radius and asphericity. Corneal aberrations were estimated from virtual ray tracing on corneal elevation maps, using custom algorithms. Total aberrations were measured with Laser Ray Tracing (sequentially scanning the pupil with a 786 nm diode laser and recording the corresponding retinal image on CCD). Pupil size was 6.5 mm. xD;Results: 1) Axial length was significantly longer (p<0.0001) in the myopic group (25.30±1.32 mm vs 22.62±0.53), and statistically significantly correlated with spherical equivalent (p=0.007), 2) Corneal radius was not significantly different between both groups. Asphericity was significantly (p=0.004) more negative in the myopic group (-0.19±0.10 vs –0.05±0.09 in hyperopes) Corneal spherical aberration was significantly lower (p<0.001) in the myopic (0.21±0.16micron) than in the hyperopic group (0.44±0.08 micron) for the group), 3) Total spherical aberration was significantly (p=0.02) lower for the myopic (0.09±0.16micron) than for the hyperopic group (0.28±0.20micron). 4) Internal aberrations were not significantly different between both groups (-0.14±0.17 and -0.12±0.10micron respectively). However, we observed a marked age-related change in the hyperopic eyes (with nearly null internal spherical aberration in eyes >30 years). xD;Conclusions: Hyperopic eyes are shorter than myopic eyes and show higher spherical aberration, mainly due to the cornea. Hyperopic eyes may show earlier loss of corneal/internal spherical aberration balance
Abstract: xD;Purpose:1) To determine the sources of aberration in myopic eyes, by measuring in vivo the optical quality of the cornea and of the internal ocular components as a function of myopia. 2) To relate measured aberrations to physical properties of myopic eyes, using eye models. xD;Methods:Experiments were done on 49 young eyes with spherical error ranging from -0.25 to -15 D. Total aberrations were measured with Laser Ray Tracing. The pupil was sampled sequentially and the corresponding images collected on a CCD camera. Ray aberrations were computed from the image deviations from a reference. Corneal aberrations were obtained from videokeratographic corneal elevation maps. Pupil size was 6.5 mm. Wave aberrations were described as Zernike polynomial expansions. Root Mean Square wavefront error (RMS) was used as optical quality metric. The wave aberration for the internal optics was the difference of total and corneal wave aberrations. xD;Results:1) Total RMS (for 3rd order and higher aberrations) increased significantly with myopia (p<0.001, slope=-0.085 mm/D, respectively). Corneal and internal aberrations also increased significantly (p<0.049 and p=0.0022), but with lower rates (slope=-0.036 and -0.058 mm/D). 2) Third order aberrations increased significantly for both components. For lower myopes (<6 D) some balance of corneal by internal aberration occurs, whereas for higher myopes individual aberrations add up. 3) As myopia increased, corneal spherical aberration increased significantly (p=0.001) toward more positive values and internal spherical aberration (p=0.009) changed toward more negative values, keeping total spherical aberration constant and low along the entire range. 4) The increase of corneal spherical aberration is due to increased corneal asphericity. Simulations using eye models show that the increase of negative internal spherical aberration is consistent with flattening of the crystalline lens posterior surface. Both properties have been reported for myopes. 5) Pupil decentration could cause a common increase of corneal and internal coma, whereas crystalline lens tilt may produce changes in the degree of balance. xD;Conclusion: 1) Degraded retinal image quality occurs in high myopia. Could image degradation imposed by aberrations contribute to disrupt emmetropization?. 2) The increase of aberrations in myopia can be explained by physical properties of the ocular components. 3) While some compensatory interactions of ocular components are lost in high myopia, balance still occurs for spherical aberration.
Abstract: Purpose:1)To assess the optical changes induced by LASIK surgery for hyperopia, by measuring total and corneal aberrations. 2)To compare the outcomes for hyperopic to those of myopic LASIK. xD;Methods:Seven hyperopic eyes (mean age: 40±12 years; mean cycloplegic spherical equivalent: +3.63±1.28 D) were measured before and after (85±54 days) LASIK. Total ocular aberrations were measured with Laser Ray Tracing, In this technique aberrations are estimated from deviations of captured aerial images corresponding to different entry pupil, which is scanned by an IR laser beam. Corneal aberrations were obtained from videokeratographic corneal elevation maps. Pupil size was 6.51 mm. Wavefront aberrations were described as Zernike polynomial expansions. Root Mean Square wavefront error (RMS) was used as an optical quality metric. xD;Results:1) Total RMS (3rd order and higher) increased after hyperopic LASIK by a factor of 2.10 (std=0.86) on average, and corneal RMS by 1.85 (std=0.77). 2) Both corneal and total pre-op spherical aberration were positive in all eyes and changed toward negative values with surgery. Increments are: -0.68±0.23 µm for the whole eye, and –0.99±0.48 µm for the anterior corneal surface. The total and corneal spherical aberration induced by surgery were correlated to intended correction (r=0.64 and r=0.79 respectively) 3) Pre-op internal spherical aberration was ~0 in five eyes, and increased significantly to positive values in the two most hyperopic eyes, indicating changes in the posterior corneal surface. 4) Third order RMS (coma-like) increased after LASIK by a factor of ~2 for both total and corneal aberrations. 5) The increase of total RMS with hyperopic LASIK (by a factor of ~2) is similar to the increase following myopic LASIK, for the same negative amount of spherical error (results from a previous study). However, average post-op spherical aberration was 0.23 µm for myopic and -0.68 µm for hyperopic LASIK. xD;Conclusion:1) Aberrations increase after hyperopic LASIK. 2) Hyperopic LASIK induces negative spherical aberration, while myopic LASIK induces positive spherical aberration. 3) Though the main change occurs on the anterior cornea, final ocular aberrations depend on individual interactions between cornea and lens (differing in hyperopes and myopes) and possible changes on the posterior cornea. 4) Knowledge of total and corneal aberrations helps to evaluate hyperopic surgery outcomes, and will contribute to the design of optimized ablation algorithms.
