Rosario G. Anera

Changes in corneal asphericity after laser in situ keratomileusis

Purpose: To analyze the origin of the changes in corneal asphericity (p‐factor) after laser in situ keratomileusis (LASIK) and the effect of postsurgery asphericity on contrast‐sensitivity function (CSF) under photopic conditions. Setting: Department of Optics, University of Granada, Granada, Spain. Methods: The p‐factor and CSF (best corrected before surgery and 1, 3, and 6 months after surgery) were measured in 24 eyes. Results: An increase in the p‐factor after LASIK was noted; there was an 87.2% change in the asphericity using the paraxial formula of Munnerlyn and coauthors. Other factors such as decentration, type of laser, optical role of the flap, wound healing, biomechanical effects, technical procedures, and reflection losses of the laser on the cornea could account for the greater than expected increase (12.8%) in the p‐factor. The CSF measurements deteriorated after LASIK; the change was significant (P<.05) in patients with myopia worse than −4.0 diopters at frequencies of 9.2, 12, 15, and 20 cycles per degree. Conclusion: The increase in corneal asphericity after surgery, greater with a higher degree of myopia, and the deterioration in CSF with high myopia justify new ablation algorithms and further study of the variables that could modify the ablation unpredictably.

Effect on laser-ablation algorithms of reflection losses and nonnormal incidence on the anterior cornea

In this paper, we provide an analytic expression for an adjustment factor in ablation algorithms for photorefractive laser surgery which take into account reflection losses and the nonnormal incidence on the cornea. We evaluate the influence of this factor on certain ocular parameters, calculating alterations in the radius of anterior cornea and corneal asphericity when we use the algorithm given by Munnerlyn’s paraxial formula. Our data indicate that this adjustment factor should be considered in the ablation algorithms that are currently being proposed in customized corneal ablation and that need great accuracy for the correction of eye aberrations.

Changes in corneal asphericity after laser refractive surgery, including reflection losses and nonnormal incidence upon the anterior cornea.

We tested the effects on laser ablation of reflection losses and nonnormal incidence on the anterior cornea. We measured presurgical and postsurgical corneal asphericity in 94 myopic eyes, comparing it with the corneal asphericity predicted by the Munnerlyn formula, modified or not, with an adjustment factor that takes into account the two effects mentioned above. The predictive power of the experimental results was stronger when we considered the adjustment factor. We propose to modify ablation algorithms by taking this adjustment factor into account, especially in customized corneal ablation, which needs high accuracy for correcting eye aberrations.

Corneal asphericity after refractive surgery when the Munnerlyn formula is applied

We deduce a mathematical equation for corneal asphericity after refractive surgery when the Munnerlyn formula is used. For this, an analytical least-squares procedure is used. The equation explains the discrepancies found by different authors when the Munnerlyn formula or its paraxial approximation is used. Equations for corneal asphericity deduced here may be of clinical relevance, for example, in studying quantitatively the role of different factors (decentration, type of laser, optical role of the flap, wound healing, biomechanical effects, technical procedures) during corneal ablation.

Prevalence of refractive errors in school-age children in Morocco

Background:  The prevalence of refractive errors in school‐age children in Morocco was assessed.

Simple device for quantifying the influence of halos after lasik surgery

We present a simple device (called a halometer) to detect and quantify the phenomenon of halos after certain surgical procedures, such as refractive surgery. The task of the subject consists basically of discriminating, after dark adaptation, a small light source around a central high-luminance stimulus. The device, which is connected to a computer, provides a disturbance index to measure the effect of halos on the observers vision. Tested with normal subjects and patients after LASIK (laser in situ keratomileusis) surgery, this apparatus proved sufficiently sensitive to quantify the halos in subjects who had undergone surgery.

Correction factor for ablation algorithms used in corneal refractive surgery with gaussian-profile beams.

We provide a correction factor to be added in ablation algorithms when a Gaussian beam is used in photorefractive laser surgery. This factor, which quantifies the effect of pulse overlapping, depends on beam radius and spot size. We also deduce the expected post-surgical corneal radius and asphericity when considering this factor. Data on 141 eyes operated on LASIK (laser in situ keratomileusis) with a Gaussian profile show that the discrepancy between experimental and expected data on corneal power is significantly lower when using the correction factor. For an effective improvement of post-surgical visual quality, this factor should be applied in ablation algorithms that do not consider the effects of pulse overlapping with a Gaussian beam.

Visual quality after monovision correction by laser in situ keratomileusis in presbyopic patients

PURPOSE: To evaluate visual quality after laser in situ keratomileusis (LASIK) performed to achieve monovision in presbyopic patients. SETTING: Laboratory of Vision Sciences and Applications, Department of Optics, University of Granada, Granada, and Clínica Novovisión, Madrid, Spain. DESIGN: Cohort study. METHODS: Laser in situ keratomileusis was performed with the Allegretto Wave Eye‐Q 400 Hz laser. The dominant eye was corrected for far vision and the nondominant eye for near vision by targeting −1.25 diopters of myopia. The F‐CAT algorithm was programmed targeting a postsurgical corneal asphericity of −0.80 in the dominant eye and −1.00 in the nondominant eye. Visual acuity, contrast sensitivity function, ocular aberrations, stereoacuity, the scattering index, and the visual‐discrimination capacity were analyzed preoperatively and 3 months postoperatively. RESULTS: The study enrolled 25 patients (50 eyes) with a mean age of 49.3 years ± 4.5 (SD). Postoperatively, more than 90% of patients had a binocular uncorrected distance and near visual acuity of 0.0 logMAR or better, although the contrast sensitivity function diminished, especially in the nondominant eye and with binocular vision. Stereoacuity was significantly worse in all patients (P<.001). The visual discrimination capacity declined in nondominant eyes and under binocular conditions (P<.005); no significant changes occurred in dominant eyes (P=.614). In all eyes, the mean objective scatter index value increased postoperatively, but not significantly (P>.05). CONCLUSIONS: Monovision correction by LASIK improved functional near vision in presbyopic patients. Although visual acuity was good for far vision, contrast sensitivity and stereoacuity diminished significantly. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

New testing software for quantifying discrimination capacity in subjects with ocular pathologies

We develop a new visual test, designed as software for quantifying discrimination capacity under low-illumination conditions. This is an important task in the presence of visual disturbances, such as those perceived by subjects with some ocular pathologies. For this purpose, we propose a visual-disturbance index, checking the test with two groups of observers having different ocular pathologies: a group with unilateral keratitis and another group affected with age-related macular degeneration (ARMD). To compare the test results to objective data, we use a double-pass device to measure the Strehl ratio, a parameter that quantifies the retinal-image quality, taking into account aberrations, retinal reflection, and intraocular scattering working jointly. Diseased eyes present higher disturbance indexes and a lower Strehl ratio compared to their healthy fellow eyes, registering a significant descending correlation between the disturbance index and the Strehl ratio. The lower the Strehl ratio is, the higher the disturbance index for the eyes studied. Therefore, in keratitis and ARMD eyes, our results demonstrate a deterioration in the retinal-image quality and a lower discrimination capacity to peripheral stimuli, reducing visual performance. The test presented here could be useful for the study and time course in different eye diseases, especially those involving an increase in scattered light or alterations in the ocular media, as shown in this work.

Deviations of Lambert-Beer's law affect corneal refractive parameters after refractive surgery.

We calculate whether deviations of Lambert-Beers law, which regulates depth ablation during corneal ablation, significantly influence corneal refractive parameters after refractive surgery and whether they influence visual performance. For this, we compute a point-to-point correction on the cornea while assuming a non-linear (including a quadratic term) fit for depth ablation. Post-surgical equations for refractive parameters using a non-linear fit show significant differences with respect to parameters obtained from a linear fit (Lambert-Beers law). Differences were also significant for corneal aberrations. These results show that corneal-ablation algorithms should include analytical information on deviations from Lambert-Beers law for achieving an accurate eye correction.