Luis Jiménez del Barco

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.

Binocular Visual Performance After LASIK

PURPOSE To analyze binocular visual function after LASIK. METHODS Eye aberrometry and corneal topography was obtained for both eyes in 68 patients (136 eyes). To evaluate visual performance, monocular and binocular contrast sensitivity function and disturbance index for quantifying halos were measured. Tests were performed under mesopic conditions. RESULTS Binocular summation and disturbance index diminished significantly (P<.0001) after LASIK with increasing interocular differences in corneal and eye aberrations. Binocular visual deterioration was greater than monocular deterioration for contrast sensitivity function and disturbance index. CONCLUSIONS Binocular function deteriorates more than monocular function after LASIK. This deterioration increases as the interocular differences in aberrations and corneal shape increase. Improvements in ablation algorithms should minimize these interocular differences.

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.

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.

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.

Effects on visual function of approximations of the corneal-ablation profile during refractive surgery

We offer an analysis that shows that the approximations made for the ablation depth during practical refractive surgery, in which the square-root terms are replaced by the first two terms of the series expansion, can limit the visual function of the observer by reducing the modulation transfer function (MTF). To simulate the refractive-surgical operation, we considered two groups of myopic patients with different ametropia who were emmetropized with different ablation profiles. We made the MTF calculations by taking the spherical aberration into account. In addition, a fuller analysis showed that these approximations limit the possibility of considering surfaces that are aspherical for reshaping the anterior cornea to optimize the observers visual function.

Optimizing the constant-channel chromaticity and color gamut of CRT color displays by control of brightness and contrast levels

Before a CRT color display is calibrated, the levels of the brightness and contrast controls should be considered. These levels have different effects on the relationship between the chromaticity coordinates of the red-green-blue (RGB) channels and the voltage applied to the gun as well as on the color gamut. Using three different CRT color displays, we measured both these relationships as well as the color gamut for different combinations of brightness and contrast levels. With respect to the luminance range of the white point provided by each display, the results show that the assumption of constant-channel chromaticity of the RGB channels is more valid at medium- to low-brightness levels in combination with high-contrast levels. In addition, this assumption holds at high contrast and medium- to low-brightness levels, the color gamut being optimized at these combinations of levels, with regard to the size of the triangle of primaries-the maximum luminance of the display white point.

Evaluation of color-discrimination ellipsoids in two-color spaces

We obtained the color-differential thresholds for 20 stimuli and 3 observers within 2 color-representation spaces. First, in the CIE-1931 color space we compare the results of our experiments, which followed chromatic-discrimination criteria, with those obtained by other authors that followed color-matching criteria. Second, the representation of our data in the cone-excitation space shows that all the discrimination ellipses are nearly oriented along the S-cone-variation axis. In our experiments the variations in the threshold of each color channel according to its level of excitation are similar to those found by other authors using different experimental conditions and methods, but certain discrepancies appear when the influence that one color channel may exert on the other is analyzed. The characteristics of luminance discrimination are shown to be independent of the chromaticity of the stimuli.

Differences between real and predicted corneal shapes after aspherical corneal ablation

We study the differences between real and expected corneal shapes, using an aspherical ablation algorithm with a known equation and avoiding the limitation imposed by most studies of refractive surgery in which the ablation equations are not known. We have calculated the theoretical corneal shape predicted by this algorithm, comparing this shape with the real corneal topography. The results indicate that the deviations that appear in the corneal shape are significant for visual performance and for the correction of eye aberrations. If we include in this analysis the effect of reflection losses and nonnormal incidence on the cornea, we can reduce corneal differences, but they will remain significant. These results confirm that it is essential to minimize corneal differences to achieve effective correction in refractive surgery.