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 observer's 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.
Abstract: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.
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 signifi cantly (PϽ.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. [J Refract Surg. 2006;22:679-688.] V arious studies have demonstrated that corneal and eye aberrations 1-4 as well as night vision disturbances 5,6 can increase after refractive surgery, while contrast sensitivity function can diminish after refractive surgery. [7][8][9] In general, studies on postoperative deterioration have concerned only monocular function, analyzing the results for each eye, with postoperative binocular function rarely being considered. 10 Interocular differences are known to affect visual performance. [11][12][13][14][15][16] Jimenez et al 16 demonstrated binocular summation measured by contrast sensitivity function in emmetropic subjects is reduced when the asphericities of the cornea differ. After surgery, pronounced changes such as an increase in aberrations and differences in corneal shape 1,2,5,8,17,18 occur in the eye; these changes partially explain the monocular deterioration in contrast sensitivity. 7 In the binocular case, a reduction in postoperative binocular summation can be expected when large interocular differences appear in eye aberrations and corneal shape.In this study, we analyzed the role of interocular differences in aberrations and corneal shape on binocular visual function. Although many different visual functions can be used to evaluate binocular vision performance, this study examined contrast sensitivity function and the disturbance index for halos (a measure for quantifying night vision disturbances). Constrast sensitivity is a common and useful function to JRS0906JIMENEZ.indd 679 JRS0906JIMENEZ.indd 679
We calculate whether deviations of Lambert-Beer's 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-Beer's law). Differences were also significant for corneal aberrations. These results show that corneal-ablation algorithms should include analytical information on deviations from Lambert-Beer's law for achieving an accurate eye correction.
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 observer's visual function.
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