Scott MacRae

Causes of spherical aberration induced by laser refractive surgery

Purpose: To develop a corneal model to better explain how refractive surgery procedures induce spherical aberration. Setting: Department of Ophthalmology and Center for Visual Science, University of Rochester, Rochester, New York, USA. Methods: The preoperative cornea was modeled as a rotationally symmetric surface with various radii of curvature and asphericities. The postoperative cornea was defined as the difference between the preoperative cornea and an ablation thickness profile computed based on the Munnerlyn equation. A ray‐tracing program and Zernike polynomial fitting were used to calculate the induced amount of spherical aberration assuming a fixed ablation depth per pulse or a variable ablation depth depending on the incidence angle of each pulse on the cornea. A biological eye model of the corneal surface change after laser refractive surgery was also developed to explain the induced spherical aberrations after myopic and hyperopic treatments. Results: The clinical data showed that positive spherical aberration was induced after myopic correction and negative spherical aberration increased after hyperopic correction. In contrast, assuming a fixed ablation depth per pulse, the theoretical prediction was that negative spherical aberration with myopic treatment and positive spherical aberration with hyperopic treatment would increase. However, when assuming a variable ablation depth per pulse caused by non‐normal incidence of laser spot on the cornea, the theoretically predicted induction of spherical aberration tends to fit better with the myopic and hyperopic clinical data. The effect of a variable ablation depth accounted for approximately half the clinically observed amount of spherical aberration. The biological model of the corneal surface change used to explain this remaining discrepancy showed the magnitude of the biological response in myopic correction is 3 times smaller than in hyperopic correction and that the direction of the biological response in hyperopic treatment is opposite that in myopic treatment. Conclusions: This nontoric eye model, which separates the effects of differences in ablation efficiency and biological corneal surface change quantitatively, explains how spherical aberration is induced after myopic and hyperopic laser refractive surgery. With the corneal topographic data, this model can be incorporated into the ablation algorithm to decrease induced spherical aberrations, improving the outcomes of conventional and customized treatments.

Recurrent Erosion: Treatment by Anterior Stromal Puncture

The majority of patients with recurrent corneal erosion respond to conventional forms of therapy such as topical lubricants, patching, debridement, or bandage soft contact lenses. However, there remain a small number who do not. For the small number of patients who do not respond to this type of treatment, this report describes a procedure: multiple anterior stromal punctures are created that presumably stimulate more secure epithelial adhesion to the underlying stroma. Of 21 eyes in 18 patients treated in this manner, three eyes required retreatment of adjacent areas; otherwise, there were no recurrences in follow-up periods ranging from 5 months to 12 years. This procedure is a simple and effective method for safe office treatment of patients with recalcitrant recurrent erosion.

Effect of Beam Size on the Expected Benefit of Customized Laser Refractive Surgery

PURPOSE Customized laser surgery attempts to correct higher order aberrations, as well as defocus and astigmatism. The success of such a procedure depends on using a laser beam that is small enough to produce fine ablation profiles needed to correct higher order aberrations. METHODS Wave aberrations were obtained from a population of 109 normal eyes and 4 keratoconic eyes using a Shack-Hartmann wavefront sensor. We considered a theoretical customized ablation in each eye, performed with beams of 0.5, 1.0, 1.5, and 2.0 mm in diameter. We then calculated the residual aberrations remaining in the eye for the different beam sizes. Retinal image quality was estimated by means of the modulation transfer function (MTF), computed from the residual aberrations. Fourier analysis was used to study spatial filtering of each beam size. RESULTS The laser beam acts like a spatial filter, smoothing the finest features in the ablation profile. The quality of the correction declines steadily when the beam size increases. A beam of 2 mm is capable of correcting defocus and astigmatism. Beam diameters of 1 mm or less may effectively correct aberrations up to fifth order. CONCLUSION Large diameter laser beams decrease the ability to correct higher order aberrations. A top-hat laser beam of 1 mm (Gaussian with FWHM of 0.76 mm) is small enough to produce a customized ablation for typical human eyes.

Vitamin A is present as retinol in the tears of humans and rabbits

Vitamin A is required for the normal growth maintenance and maturation of the corneal epithelium and is effective in the treatment of xerophthalmia and experimental corneal epithelial wounds when applied topically as retinoic acid. The normal route of delivery of vitamin A to the cornea has remained undefined. We collected tears from normal and vitamin A deficient rabbits and from humans and analyzed them by high pressure liquid chromatography. A peak corresponding to a retinol standard was eluted from normal rabbit and human tears but was absent from the vitamin A deficient rabbit tears. The retinol concentration in rabbit tears was 69 ng/ml (0.2 X 10(-6)M) and in human tears was 16 ng/ml (0.5 X 10(-7)M). This demonstration that vitamin A is present in the tears as retinol establishes the rationale for treatment of corneal disease with topical vitamin A.

Aberrations induced in wavefront-guided laser refractive surgery due to shifts between natural and dilated pupil center locations

PURPOSE: To determine the aberrations induced in wavefront‐guided laser refractive surgery due to shifts in pupil center location from when aberrations are measured preoperatively (over a dilated pupil) to when they are corrected surgically (over a natural pupil). SETTING: Center for Visual Science and Department of Ophthalmology, University of Rochester, Rochester, New York, USA. METHODS: Shifts in pupil center were measured between dilated phenylephrine hydrochloride (Neo‐Synephrine [2.5%]) and nonpharmacological mesopic conditions in 65 myopic eyes treated with wavefront‐guided laser in situ keratomileusis (Technolas 217z, Bausch & Lomb). Each patients preoperative and 6‐month postoperative wave aberrations were measured over the dilated pupil. Aberrations theoretically induced by decentration of a wavefront‐guided ablation were calculated and compared with those measured 6 months postoperatively (6.0 mm pupil). RESULTS: The mean magnitude of pupil center shift was 0.29 mm ± 0.141 (SD) and usually occurred in the inferonasal direction as the pupil dilated. Depending on the magnitude of shift, the fraction of the higher‐order postoperative root‐mean‐square wavefront error that could be due theoretically to pupil center decentrations was highly variable (mean 0.26 ± 0.20 mm). There was little correlation between the calculated and 6‐month postoperative wavefronts, most likely because pupil center decentrations are only 1 of several potential sources of postoperative aberrations. CONCLUSIONS: Measuring aberrations over a Neo‐Synephrine‐dilated pupil and treating them over an undilated pupil typically resulted in a shift of the wavefront‐guided ablation in the superotemporal direction and an induction of higher‐order aberrations. Methods referencing the aberration measurement and treatment with respect to a fixed feature on the eye will reduce the potential for inducing aberrations due to shifts in pupil center.

Corneal Ulcer and Adverse Reaction Rates in Premarket Contact Lens Studies

We analyzed clinical data on 22,739 contact lens wearers who were studied and whose lenses were approved under 48 manufacturer-sponsored studies for the Food and Drug Administration between 1980 and 1988. The incidence of corneal ulcers was low in the cosmetic (nontherapeutic) daily-wear soft and rigid gas-permeable lens wearers (1/1,923 and 1/1,471 patient-years, respectively). Corneal ulcers and severe adverse reactions occurred two to four times more frequently in extended-wear cosmetic soft and rigid gas-permeable lens wearers than in cosmetic daily-wear lens wearers. Aphakic extended-wear soft lens users were nine times more likely to develop a corneal ulcer when compared to the soft daily-wear cosmetic group. Corneal abrasions and keratitis accounted for 81 of 159 severe adverse reactions, whereas corneal ulcers accounted for 28 of 159 adverse reactions. The data indicate that overnight extended wear of contact lenses is associated with a greater risk of serious, sight-threatening complications than daily wear.

Slit skiascopic-guided ablation using the Nidek laser.

PURPOSE To present the approach of using a scanning slit refractometer (the ARK 10000) in conjunction with a corneal topography system to guide customized corneal ablation. This diagnostic system is coupled with the Nidek EC-5000 system which combines scanning slit and a scanning small area ablation (1.0 mm) to perform a customized ablation. METHODS The ARK 10000 diagnostic system which contains a scanning slit refractometer is described. Information generated from the ARK 10000 wavefront sensor and corneal topography system can be coupled to the new Nidek EC-5000 excimer laser system, which combines the larger area of scanning slit ablation with the small area (1.0 mm) ablation. RESULTS The Nidek ARK 10000 diagnostic system captures wavefront information using a retinoscopic system which is converted into a refractive power map. This is different from other autorefraction systems in that it has four sensors at different diameters of the cornea and captures 1440 points in 0.4 seconds. This map is used in conjunction with corneal topography-captured simultaneously. This information is then combined to perform a customized ablation using the new Nidek EC-5000 system. CONCLUSIONS The ARK 10000 diagnostic system represents a different approach to customized ablation in that it combines a corneal topography system with a wavefront system and a larger treatment area of the traditional scanning slit ablation with a new small area ablation treatment for greater efficiency.

Objective evaluation of through-focus optical performance of presbyopia-correcting intraocular lenses using an optical bench system

PURPOSE: TO evaluate spherical aberration and through‐focus optical performances of 5 presbyopia‐correcting and 2 monofocal intraocular lenses (IOLs). SETTING: Flaum Eye Institute, University of Rochester, Rochester, New York, USA. DESIGN: Experimental study. METHODS: Five presbyopia‐correcting IOLs (Restor +4D SN6AD3, Restor +3D SN6AD1, Rezoom NXG1, Tecnis multifocal ZM900, Crystalens HD500) were tested using an optical bench system consisting of a model eye, a high‐resolution Hartmann‐Shack wavefront sensor, and an image‐capturing device. Two monofocal IOLs (Sofport AO LI60AOV, Acrysof SN60AT) were measured for comparison. No accommodation was simulated. The spherical aberration profiles of each IOL were measured using the wavefront sensor. Through‐focus performance was evaluated by calculating cross‐correlation coefficients and comparing the likenesses of captured images of a resolution target and a perfect reference image. RESULTS: With a 5.0 mm entrance pupil, the SN6AD3, SN6AD1, ZM900, NXG1, and HD500 IOLs had spherical aberration of −0.18 μm, −0.14 μm, −0.15 μm, −0.07 μm, and −0.01 μm, respectively. Distance image quality was poorer with multifocal and accommodating IOLs than with monofocal IOLs. All multifocal IOLs had effective distance and near image quality but had a loss in intermediate image quality. The HD 500 accommodating IOL had decreased distance image quality and slightly increased depth of focus compared with the monofocal IOLs because of the bispheric design. CONCLUSIONS: The presbyopia‐correcting IOLs had different optical characteristics, including spherical aberration profile and through‐focus performance. An accurate understanding of the optical characteristics of individual IOLs is essential to selecting the best presbyopia‐correcting IOL and thus improving cataract surgery outcomes. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned.

Corneal inlays for presbyopia correction.

Purpose of review This study provides an overview of the three types of corneal inlays now in use for the correction of presbyopia and reviews recently published evidence of the inlays’ safety and efficacy. Recent findings Results for corneal reshaping and refractive inlays are promising, but very limited. Small-aperture inlays are already in widespread use and have been shown to improve uncorrected near and intermediate vision without a significant loss in distance acuity or an unacceptable increase in visual symptoms. Complications have been minimal, but the inlays may be removed if necessary. They do not prevent visualization or imaging of the retina and may be retained during subsequent cataract surgery. Summary The presbyopic demographic is large and growing, with a high level of interest in spectacle independence. There is currently no other effective solutions for presbyopes who desire good uncorrected vision at all distances without the risks of intraocular surgery or the visual compromises of monovision. Additional research is needed, but the future for corneal inlay technology is bright.

Wavefront guided ablation

A DAPTIVE OPTICS WAS FIRST SUGGESTED IN 1953 BY astronomer Horace Babcock to remove the blurring effects of turbulence in the atmosphere on telescopic images of stars.1 The U.S. Defense Department later invested heavily in the development of adaptive optics technology to improve the effectiveness of laser weapons as part of its Star Wars Program. This information would eventually allow vision scientists to apply this technology to better understand the eye’s optic and retinal image quality. In 1994, Liang and associates used a Shack–Hartmann wavefront sensor to describe higher order aberrations in the human eye.2 In 1997, Liang, Williams, and Miller used the Shack–Hartmann wavefront sensor to detect the eye’s aberrations and then applied an adaptive optics deformable mirror to correct the eye’s lower and higher order aberrations.3 With this system, they noted that adaptive optics provided a sixfold increase in contrast sensitivity to high spatial frequencies when the pupil was large. This study was the first to demonstrate that the correction of higher-order aberrations can lead to supernormal visual performance in normal eyes. The Liang, Williams, and Miller study used monochromatic light.4 Normal viewing conditions usually involve broadband light, and retinal images formed in broadband (white) light are blurred by chromatic aberration, as well as the monochromatic aberrations that adaptive optics can correct. Yoon and Williams showed that, in broadband light which characterizes normal viewing conditions, adaptive optics still provides a twofold increase in contrast sensitivity at high spatial frequencies in typical eyes, even when chromatic aberration is present.5 These findings spurred a ground swell of interest in wavefront sensing and the possibility of coupling it with wavefront correction in the form of customized corneal ablation. In this editorial, we will look at the visual benefit of correcting higher-order aberrations, the limits of the human visual system, and some of the future challenges of the ambitious and sometimes misunderstood world of customized corneal ablation. The wavefront sensor allows the clinician not only to measure the defocus and astigmatism that are the most important determinants of refractive error, but also “higher-order aberrations” as well. Defocus and astigmatism are referred to as second-order aberrations. Higher-order aberrations, such as coma and spherical aberration, refer to aberrations other than defocus and astigmatism. The wavefront sensor, such as that constructed by Liang and Williams,3 can reliably detect as many as 64 higher-order aberrations. Some of these higher-order aberrations had not been previously measured in human eyes and all were usually lumped by clinicians into a single category misleadingly called “irregular astigmatism.” They are better referred to as higher-order aberrations since most have nothing to do with astigmatism. The spectacle correction that provides the best subjective refraction depends not only on defocus and astigmatism but also, to a lesser extent, on higher-order aberrations.6 For this reason, the description of the eye’s wave aberration provided by a wave-front sensor, when properly processed, can provide an especially accurate objective estimate of subjective refraction.