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An objective scatter index based on double-pass retinal images of a point source to classify cataracts.

Purpose To propose a new objective scatter index (OSI) based in the analysis of double-pass images of a point source to rank and classify cataract patients. This classification scheme is compared with a current subjective system. Methods We selected a population including a group of normal young eyes as control and patients diagnosed with cataract (grades NO2, NO3 and NO4) according to the Lens Opacities Classification System (LOCS III). For each eye, we recorded double-pass retinal images of a point source. In each patient, we determined an objective scatter index (OSI) as the ratio of the intensity at an eccentric location in the image and the central part. This index provides information on the relevant forward scatter affecting vision. Since the double-pass retinal images are affected by both ocular aberrations and intraocular scattering, an analysis was performed to show the ranges of contributions of aberrations to the OSI. Results We used the OSI values to classify each eye according to the degree of scatter. The young normal eyes of the control group had OSI values below 1, while the OSI for subjects in LOCS grade II were around 1 to 2. The use of the objective index showed some of the weakness of subjective classification schemes. In particular, several subjects initially classified independently as grade NO2 or NO3 had similar OSI values, and in some cases even higher than subjects classified as grade NO4. A new classification scheme based in OSI is proposed. Conclusions We introduced an objective index based in the analysis of double-pass retinal images to classify cataract patients. The method is robust and fully based in objective measurements; i.e., not depending on subjective decisions. This procedure could be used in combination with standard current methods to improve cataract patient surgery scheduling.

Mechanism of compensation of aberrations in the human eye

We studied the mechanism of compensation of aberrations within the young human eye by using experimental data and advanced ray-tracing modeling. Corneal and ocular aberrations along with the alignment properties (angle kappa, lens tilt, and decentration) were measured in eyes with different refractive errors. Predictions from individualized ray-tracing optical models were compared with the actual measurements. Ocular spherical aberration was, in general, smaller than corneal spherical aberration without relation to refractive error. However, horizontal coma compensation was found to be significantly larger for hyperopic eyes where angle kappa tended to also be larger. We propose a simple analytical model of the relationship between the corneal coma compensation effect with the field angle and corneal and crystalline shape factors. The actual shape factors corresponded approximately to the optimum shapes that automatically provide this coma compensation. We showed that the eye behaves as an aplanatic optical system, an optimized design solution rendering stable retinal image quality for different ocular geometries.

Optical Quality of the Eye in Subjects with Normal and Excellent Visual Acuity

PURPOSE To study the relationship between visual acuity (VA) and the eyes optical quality in subjects with normal and excellent spatial vision. VA ranged from decimal values of 1.0 (20/20) to 2.0 (20/10) when defocus and astigmatism were carefully corrected. METHODS In 60 eyes of young subjects, visual and optical performance with the natural pupil were measured. A forced-choice procedure was used to measure tumbling-E high-contrast VA (HCVA) and low-contrast VA (LCVA). Wavefront aberration (WA) was measured using a Hartmann-Shack sensor. The associated point-spread function (PSF) and modulation transfer function (MTF) were also estimated. High-order aberrations (HOA) and several image quality parameters were represented as a function of VA. Subjects were classified into three groups according to their VA, and average optical parameters were calculated. RESULTS Coma and trefoil vary between 0 and 0.5 mum, and spherical aberration ranges from -0.40 mum to +0.45 mum, with an average value of approximately zero. LCVA is not correlated with any of the aberration terms. Coma and spherical aberration are not correlated with HCVA. However, eyes with trefoil equal to or higher than 0.25 mum have an HCVA less than 1.5. The average optical quality in eyes with HCVA greater than 1.4 is slightly better than in eyes with normal VA. However, some eyes had relatively poor image quality and excellent VA. CONCLUSIONS No significant correlations were found between VA measurements and the optical quality of the eye in young subjects with normal or excellent spatial vision. Some subjects with normal degrees of aberrations attained excellent VA.

Minimum amount of astigmatism that should be corrected

Purpose To evaluate how small amounts of astigmatism affect visual acuity and the minimum astigmatism values that should be corrected to achieve maximum visual performance. Setting Optics Laboratory, University of Murcia, Murcia, Spain. Design Case series. Methods A wavefront sensor was used to measure astigmatism and higher‐order aberrations (HOAs) in normal young eyes with astigmatism ranging from 0.0 to 0.5 diopter (D). Astigmatism was corrected for natural pupil diameters using a purpose‐designed cross‐cylinder device. Visual acuity was measured for high‐contrast and low‐contrast stimuli at best subjective focus with the natural and corrected astigmatism. From the aberrations, optical image‐quality metrics were calculated for 3 conditions: natural astigmatism, corrected astigmatism, and astigmatism only (with all HOAs removed). Results The study evaluated 54 eyes. There was no significant correlation between the amount of astigmatism and visual acuity. The correction of astigmatism improved visual acuity for only high‐contrast letters from 0.3 D, but with a high variability between subjects. Low‐contrast visual acuity changed randomly as astigmatism was corrected. The correction of astigmatism increased the mean image‐quality values; however, there was no significant correlation with visual performance. The deterioration in image quality given by astigmatism higher than 0.3 D was limited by HOAs. Conclusions In most subjects, astigmatism less than 0.5 D did not degrade visual acuity. This suggests that under clinical conditions, the visual benefit of precise correction of astigmatism less than 0.5 D would be limited. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Effect of corneal aberrations on intraocular lens power calculations.

PURPOSE: To use ray tracing to determine the influence of corneal aberrations on the prediction of the optimum intraocular lens (IOL) power for implantation in normal eyes and eyes with previous laser in situ keratomileusis (LASIK). SETTING: Hospital Universitario Virgen de la Arrixaca, Murcia, Spain. DESIGN: Case series. METHODS: The optimum IOL power was calculated by ray tracing using a patient‐customized eye model in cataract surgery cases. The calculation can be performed with or without inclusion of the patients corneal aberrations. Standard predictions were also generated using current state‐of‐the‐art IOL power calculation techniques. The results for all predictions were compared with the optimum IOL power after cataract surgery. RESULTS: For patients without previous LASIK (n = 18), the standard approaches and the ray‐tracing procedure gave a similar mean absolute residual error and variance. The incorporation of corneal aberrations did not improve the accuracy of the ray‐tracing prediction in these cases. For post‐LASIK patients (n = 10), the ray‐tracing prediction incorporating corneal aberrations generated the most accurate results. The difference between the prediction with and without considering corneal aberrations correlated with the amount of corneal spherical aberration (r2 = 0.82), resulting in a difference of up to 3.00 diopters in IOL power in some cases. CONCLUSIONS: Ray tracing using patient‐customized eye models was a robust procedure for IOL power calculation. The incorporation of corneal aberrations is crucial in post‐LASIK eyes, primarily because of the elevated corneal spherical aberration. Financial Disclosure: Mrs. Canovas and Dr. Artal hold a provisional patent application on the ray‐tracing procedure. Mrs. Canovas is an employee of Abbott Medical Optics Groningen B.V. No other author has a financial or proprietary interest in any material or method mentioned.

Refractive accuracy with light-adjustable intraocular lenses

Purpose To evaluate efficacy, predictability, and stability of refractive treatments using light‐adjustable intraocular lenses (IOLs). Setting University Hospital Virgen de la Arrixaca, Murcia, Spain. Design Prospective nonrandomized clinical trial. Methods Eyes with a light‐adjustable IOL (LAL) were treated with spatial intensity profiles to correct refractive errors. The effective changes in refraction in the light‐adjustable IOL after every treatment were estimated by subtracting those in the whole eye and the cornea, which were measured with a Hartmann‐Shack sensor and a corneal topographer, respectively. The refractive changes in the whole eye and light‐adjustable IOL, manifest refraction, and visual acuity were obtained after every light treatment and at the 3‐, 6‐, and 12‐month follow‐ups. Results The study enrolled 53 eyes (49 patients). Each tested light spatial pattern (5 spherical; 3 astigmatic) produced a different refractive change (P<.01). The combination of 2 light adjustments induced a maximum change in spherical power of the light‐adjustable IOL of between −1.98 diopters (D) and +2.30 D and in astigmatism of up to −2.68 D with axis errors below 9 degrees. Intersubject variability (standard deviation) ranged between 0.10 D and 0.40 D. The 2 required lock‐in procedures induced a small myopic shift (range +0.01 to +0.57 D) that depended on previous adjustments. Conclusions Light‐adjustable IOL implantation achieved accurate refractive outcomes (around emmetropia) with good uncorrected distance visual acuity, which remained stable over time. Further refinements in nomograms and in the treatment’s protocol would improve the predictability of refractive and visual outcomes with these IOLs. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Impact of positive coupling of the eye’s trefoil and coma in retinal image quality and visual acuity

When the eyes higher-order aberrations are measured and reported, as important as the magnitude of each individual term are the possible combinations between them, which may change the overall retinal image quality and therefore visual performance. We have evaluated the relationships among different aberration terms in the human eye-coma, trefoil, and spherical aberration-and their effects on both retinal image quality and visual acuity (VA). In a group of normal young subjects with normal to excellent vision, we measured the eyes aberrations and high contrast VA under natural conditions after carefully correcting defocus and astigmatism. Among the different combinations of aberration terms, we only found a significant negative correlation (r2=0.30) between the vertical coefficients of trefoil C(3,-3) and coma C(3,-1). This is a positive coupling that produces a better retinal image quality than any of the other possible combinations of these terms. However, this improvement in image quality is limited by the presence of other aberrations. Only in a few eyes that presented the larger values of coupled vertical trefoil and coma appeared a significant improvement of image quality. Although we did not find a clear correction between the coma-trefoil vertical coupling and VA, most eyes with large amounts of aberrations (RMS>0.4 μm) have these terms coupled, keeping decimal acuity around 1.2 or higher.