Antonio Benito

Ocular wave-front aberration statistics in a normal young population.

Monochromatic ocular aberrations in 108 eyes of a normal young population (n=59) were studied. The wave-front aberration were obtained under natural conditions using a near-infrared Shack-Hartmann wave-front sensor. For this population and a 5 mm pupil, more than 99% of the root-mean square wave-front error is contained in the first four orders of a Zernike expansion and about 91% corresponds only to the second order. Comparison of wave-fronts aberrations from right and left eye in 35 subjects, showed a good correlation between most of the second- and third-order terms and a slight (but not clear) tendency for mirror symmetry between eyes.

The human eye is an example of robust optical design.

In most eyes, in the fovea and at best focus, the resolution capabilities of the eyes optics and the retinal mosaic are remarkably well adapted. Although there is a large individual variability, the average magnitude of the high order aberrations is similar in groups of eyes with different refractive errors. This is surprising because these eyes are comparatively different in shape: Myopic eyes are longer whereas hyperopic eyes are shorter. In most young eyes, the amount of aberrations for the isolated cornea is larger than for the complete eye, indicating that the internal ocular optics (mainly the crystalline lens) play a significant role in compensating for the corneal aberrations, thereby producing an improved retinal image. In this paper, we show that this compensation is larger in the less optically centered eyes that mostly correspond to hyperopic eyes. This suggests a type of mechanism in the eyes design that is the most likely responsible for this compensation. Spherical aberration of the cornea is partially compensated by that of the lens in most eyes. Lateral coma is also compensated mainly in hyperopic eyes. We found that the distribution of aberrations between the cornea and lens appears to allow the optical properties of the eye to be relatively insensitive to variations arising from eye growth or exact centration and alignment of the eyes optics relative to the fovea. These results may suggest the presence of an auto-compensation mechanism that renders the eyes optics robust despite large variation in the ocular shape and geometry.

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.

Instrument for measuring the misalignments of ocular surfaces

A compact and robust instrument for measuring the alignment of ocular surfaces has been designed and used in living eyes. It is based on recording Purkinje images (reflections of light at the ocular surfaces) at nine different angular fixations. A complete analysis on the causes of misalignments of Purkinje images and its relations with those physical variables to be measured (global eye tilt, lens decentration and lens tilt) is presented. A research prototype based on these ideas was built and tested in normal and pseudophakic eyes (after cataract surgery). The new analysis techniques, together with the semicircular extended source and multiple fixation tests that we used, are significant improvements towards a robust approach to measuring the misalignments of the ocular surfaces in vivo. This instrument will be of use in both basic studies of the eyes optics and clinical ophthalmology.

Aberration generation by contact lenses with aspheric and asymmetric surfaces

PURPOSE We explored the potential of aberration correction in the human eye by using a new generation of soft contact lenses with aspheric and asymmetric surfaces. METHODS Soft contact lens samples were designed with one asymmetrical surface (front) and one spherical (back) to produce predetermined amounts of desired pure defocus, astigmatism, trefoil, coma, and spherical aberration. Contact lens wavefront aberrations were measured ex vivo using a Fizeau-Tolanski interferometer and compared with the in vivo wavefronts obtained by subtracting the aberrations of the eye with and without the contact lenses. These second set of measurements were obtained using a Shack-Hartmann sensor. RESULTS We found that an aberration-free contact lens sample induced in the eye a small amount of residual aberration. We obtained a good match between the ex vivo and in vivo wavefront measurements for most of the samples of the contact lenses. CONCLUSIONS The aberrations generated by soft contact lenses on the eye were predictable. Rotations and translations of the contact lenses with respect to correct position on the eye were, however, the main limitation for precise correction of the ocular aberrations.

Laser in situ keratomileusis disrupts the aberration compensation mechanism of the human eye.

PURPOSE To study how changes induced on corneal optics by myopic and hyperopic laser in situ keratomileusis (LASIK) refractive surgery affect the aberration compensation mechanism. DESIGN Interventional case series and modeling theory. METHODS We measured ocular, corneal, and internal aberrations for a 6-mm pupil in 15 myopic and 6 hyperopic eyes with similar age range before and 6 months after standard LASIK. Ocular aberrations were measured using our own developed Hartmann-Shack wavefront sensor, whereas corneal aberrations were calculated by using the elevation data obtained by corneal topography. Ocular, corneal, and internal root mean square (RMS), spherical aberration (SA), coma, and compensation factor were compared for each patient. RESULTS After myopic LASIK, we obtained an average 1.6-fold increase in ocular RMS, mainly positive SA, and coma, associated with a similar increase in corneal aberrations. In the hyperopes, we found a higher (2.3-fold) induction of ocular aberrations after surgery, mainly negative SA and coma, but without net increases of corneal aberrations. Aberration compensation clearly decreased or even inverted after hyperopic LASIK, decreasing the ocular optical quality in a higher level than myopic LASIK. CONCLUSIONS Although ocular aberrations after myopic LASIK usually were smaller than corneal aberrations because of partial compensation of SA, after hyperopic LASIK, because of induction of negative SA and change in coma, disruption of the compensation mechanism lead to a larger increase of ocular aberrations. Customized procedures should maintain the natural compensation to achieve improved visual outcomes.

A randomized comparison of pupil-centered versus vertex-centered ablation in LASIK correction of hyperopia.

PURPOSE To compare visual and optical outcomes of pupil-centered vs vertex-centered ablation in patients undergoing laser-assisted in situ keratomileusis (LASIK) for hyperopia. DESIGN Randomized, double-masked, prospective, single-center trial. METHODS SETTING Institutional practice. STUDY POPULATION Sixty eyes of 30 patients with low and moderate hyperopia. Intervention procedure: Eyes underwent LASIK (Allegretto excimer laser). In 30 eyes, the ablation was centered on the pupil, while in the 30 other eyes the ablation was centered on the corneal reflex. MAIN OUTCOME MEASURES Primary outcome measure was the safety index. Main secondary outcome measures were efficacy index, manifest refraction, uncorrected visual acuity, best spectacle-corrected visual acuity (BCVA), and ocular high-order aberrations for a 6-mm pupil size. RESULTS At 3 months postoperatively, the safety index was 0.99 ± 0.04 in the pupil-centered group and 0.99 ± 0.08 in the vertex-centered group (P = .97). The efficacy index was also similar for both groups: 0.96 ± 0.05 in pupil-centered eyes and 0.93 ± 0.09 in vertex-centered eyes (P = .31). Optical aberrations were similar for pupil-centered and vertex-centered eyes. Considering only eyes showing large pupil decentration, we found a tendency for better visual results in favor of pupil-centered eyes in terms of safety index and a slight but significant increase of coma in vertex-centered eyes. CONCLUSION LASIK is an effective procedure for treatment of hyperopia. Pupil-centered and vertex-centered treatments provide similar visual and optical outcomes. However, in eyes showing large temporal pupil decentration, pupil-centered ablation seemed to produce a lower amount of coma and, as a consequence, a reduced loss of BCVA compared with vertex-centered patients.

Temporal evolution of ocular aberrations following laser in situ keratomileusis

Citation information: Benito A, Redondo M & Artal P. Temporal evolution of ocular aberrations following laser in situ keratomileusis. Ophthalmic Physiol Opt 2011, 31, 421–428. doi: 10.1111/j.1475‐1313.2011.00854.x

Location of achromatizing pupil position and first Purkinje reflection in a normal population.

PURPOSE Quality of vision in patients who have undergone corneal refractive surgery depends upon the optimal centration of the procedures used. The center of the pupil is used as a reference point in some corneal ablation procedures. The achromatic axis would be a more sensible option from an optical point of view, but it is not as readily detectable. As an alternative, other refractive techniques, like the small aperture corneal inlay for presbyopia correction, use the corneal reflex (first Purkinje image). To assess the relative position of these two marks, we developed a new instrument to simultaneously measure both the first Purkinje image (PI) and the intersection of the achromatic axis with the pupil plane. METHODS The apparatus records images of the pupil and the PI when illuminated with a circle of infrared light-emitting diodes. A second optical path allows determination of the achromatic axis by using a subjective method. Both the positions of the PI and the achromatic axis intersection are determined simultaneously. RESULTS A series of data were obtained in 48 eyes. The mean location of the achromatic point relative to the PI was [x = -0.05 ± 0.15 mm; y = 0.09 ± 0.18 mm]. Considered individually, in 55% of eyes, the distance between locations is less than 0.2 mm, and in 95% of eyes, distances are less than 0.4 mm. CONCLUSIONS On average, achromatic axis crossing of the pupil and PI locations coincides within measurement errors. Although there was some intersubject variability, differences in location were less than 0.6 mm in all measured eyes.