From wavefront device to laser: an alignment method for complete registration of the ablation to the cornea

PURPOSE: When customized laser refractive surgery is based on a wavefront measurement, correct registration of the laser ablation to the prescribed treatment area is vital. If the treatment is not ablated exactly where it was measured, the error could negate some benefits of the procedure. Currently, treatments are aligned to the pupil center, which has been shown to shift between measurement and treatment. This article describes an alignment method that uses an image-based coordinate system to compensate for shifts of the pupil center as well as cyclorotation. METHODS: Images of patients' eyes were taken during wavefront measurements and again when patients were in position for surgery. An image-processing method, which matches multiple ocular landmarks in both image sets, was used to align the laser beam to the ablation target. RESULTS: Analyses of the data collected from both instruments suggest that alignment errors caused by movement of the pupil center and cyclorotation could be significantly reduced by using the proposed alignment method. CONCLUSIONS: Because the pupil center undergoes significant movement with changes in lighting conditions, it is unsuitable as the sole alignment point for custom ablations. Several reference points are necessary to compensate for changing pupil size as well as the cyclotorsional eye position changes that occur between wavefront measurement and laser procedure. By providing multiple points for alignment, the registration method described herein increases safety and effectiveness of wavefront-based laser vision correction.     

Changes in corneal topography after excimer laser photorefractive keratectomy for myopia

Computer-assisted analysis of corneal topography was performed in 17 normally sighted human eyes during the first year after excimer laser photorefractive keratectomy (PRK) for myopia. Laser ablation of the central cornea produced an optical zone with a smooth power transition to the peripheral cornea. Decentration of the ablation was noted in some eyes (less than 0.5 mm in 3 eyes, 0.5 to 1.0 mm in 10 eyes, 1 to 1.5 mm in 3 eyes, and 2.1 mm in 1 eye), suggesting that careful alignment of the laser beam is critical. Improved methods to align the ablation within the center of the entrance pupil are needed. In 12 of 17 eyes, the topographic pattern appeared to stabilize between 3 and 7 months after PRK. In the remaining five eyes, central ablation power changed by more than 0.5 diopters (D) between the 6- and 12-month examinations. Regression was more common and more pronounced in eyes with intended corrections more than 5 D, whereas the majority of eyes with intended corrections of 5 D or less showed good correspondence between the final change in central ablation power and the attempted correction. Two eyes had a loss of at least two lines of best spectacle-corrected visual acuity that was attributable to irregular astigmatism, decentration of the ablation, and/or corneal opacification.     

Centration on the cornea vertex normal during hyperopic refractive photoablation using videokeratoscopy

PURPOSE: To evaluate a method for centering the ablation in standard hyperopic LASIK using an excimer laser with a video-based eye tracker system. METHODS: Results of 52 consecutive hyperopic eyes treated with the ESIRIS excimer laser were retrospectively reviewed. Ablation was shifted from the pupil center to the vertex normal of the cornea using pupillary offset measured with the Keratron Scout videokeratoscope. Outcomes were assessed 3 months postoperatively. RESULTS: All eyes preoperatively had a nasally oriented vertex normal in relation to the pupil center. Three months postoperatively a refractive outcome of < 0.50 diopters of spherical equivalent was achieved in 94% (49/52) of eyes. No eye lost more than one line of best spectacle-corrected visual acuity. CONCLUSIONS: Standard hyperopic LASIK with the ESIRIS laser system leads to good predictable efficacy and safety results when the ablation center is shifted to the cornea vertex normal based on videokeratoscopy data.     

Centration analysis of ablation over the coaxial corneal light reflex for hyperopic LASIK

PURPOSE: To analyze postoperative topographic centration when the coaxially sighted corneal light reflex was used for laser centration in hyperopic LASIK. METHODS: Centration photographs of 21 eyes (12 patients) that underwent hyperopic LASIK with centration over the coaxially sighted corneal light reflex were reviewed to determine the distance from the entrance pupil center to the coaxially sighted corneal light reflex. Postoperative ablation centration was determined topographically at day 1 and 3 months by four different methods. The difference between the actual decentration and the decentration that would have occurred had the ablation been centered over the entrance pupil center was calculated. RESULTS: The mean deviation of the coaxially sighted corneal light reflex from the entrance pupil center preoperatively was 0.34 +/- 0.24 mm nasal or 4.5 +/- 3.0 degrees. At 1 day, the average decentration was 0.10 mm or 1.3 degrees temporal. The mean decentration that would have occurred if the ablation had been centered over the entrance pupil center was 0.44 mm or 5.5 degrees temporal. At 3 months, the average decentration was 0.07 mm or 0.25 degrees temporal. The mean decentration that would have occurred if the ablation had been centered over the entrance pupil center was 0.45 mm or 5.6 degrees temporal. Mean uncorrected visual acuity (logMAR) improved 3 lines from 0.54 +/- 0.14 (20/70) to 0.22 +/- 0.17 (20/32). No eye lost >2 lines of best spectacle-corrected visual acuity (BSCVA); 2 (10%) eyes lost 1 line of BSCVA at 3-month follow-up. CONCLUSIONS: Excellent centration in hyperopic ablation is possible even in eyes with positive angle kappa when the ablation is centered over the corneal light reflex.     

Scanning slit delivery system

We describe the structure and function of a delivery system designed for use with an argon fluoride excimer laser for corneal surgery. The basic principle of the delivery system is to shape the laser beam with a slit mask to a defined configuration. The image of the slit is moved across the cornea in a mathematically defined pattern to ablate a given amount of tissue, creating a new anterior radius of curvature for the cornea. The location of the beam on the cornea is determined by a rotating dove prism and a translating spherical lens, each of which is controlled by computer-regulated servomotors. The system is versatile and can be used for laser myopic or hyperopic keratomileusis, for correction of astigmatism by surface ablation, for creation of radial or transverse linear cuts, for excision of corneal buttons, and for uniform lamellar corneal smoothing procedures. The delivery system is currently used in the laboratory for the ablation of nonhuman primate corneas.     

Limitations of pupil tracking in refractive surgery: systematic error in determination of corneal locations

PURPOSE: The goal of this investigation was to show the theoretical limitations of pupil tracking in refractive surgery. The parallax error associated with localizing corneal positions by tracking the subjacent entrance pupil center was quantified. METHODS: Optical ray-tracing in a schematic model eye was performed to determine the geometric parallax error. The calculations required several assumptions regarding ocular geometry, eye movements, and eye tracker position. Various parameter combinations were evaluated to assess the potential range of error to be expected in clinical practice. RESULTS: Tracking error can amount to 30% (or more for eye trackers mounted closer than 500 mm to the eye) of the detected lateral shift. Thus, if the eye tracker registers a lateral shift of the entrance pupil of 0.2 mm away from the tracking reference axis, the point of interest located on the cornea would essentially be 0.26 mm away from this reference axis. A laser pulse fired at that moment would be systematically displaced by 60 microm. Our results depended on geometric parameters of the eye and the tracking device. Based on conservative assumptions regarding these geometric parameters, partial compensation could be realized by adding a certain percentage to the modulus of each eye tracker reading. CONCLUSIONS: The fact that corneal displacement was generally underestimated by up to 30% of the measured entrance pupil shift demonstrates the severity of the parallax effect.     

Holographic interferometry of excimer-laser-ablated bovine eyes: first results

PURPOSE: To investigate the effects of areal corneal ablations of different depth on the biomechanical properties of the cornea. METHOD: The qualitative change of the holographic interferometric fringe pattern of enucleated bovine corneas due to areal mechanical ablations (group A) of 35% and 70% depth of the central cornea and 15, 30, and 50% excimer laser ablations in each eye (group B) was investigated. RESULTS: Ablations of 50% (group B) and 70% (group A) showed a significant variation of the fringe pattern as an effect of the corneal weakening. 15% excimer laser ablations of 5 mm diameter did not result in noticeable changes of the fringe pattern. Half of the 30% ablated corneas in group B showed newly appearing fringes within the area of ablation, indicating a local reduction of the corneal stiffness after ablation. CONCLUSION: The thickness of the remaining tissue is very important for the biomechanical behavior of the cornea after areal ablations. There are interindividual differences regarding the change of material properties due to excimer laser ablation of 30% depth of ablation.     

Optische Online-Pachymetrie bei Laser-in-situ-Keratomileusis

PURPOSE: Currently the microkeratome incision and the ablation depth are unpredictable in laser in situ keratomileusis (LASIK). Online optical coherence pachymetry is a high-resolution and non-contact method, which enables the corneal thickness changes to be monitored intraoperatively. METHOD. In 12 patients undergoing myopic LASIK, online optical coherence pachymetry with a wavelength of 1310 nm and a measurement frequency of 74 Hz was studied. The central corneal thickness changes were determined continuously. RESULTS: Online optical coherence pachymetry enabled intraoperative visualization and assessment of the central corneal thickness, the flap thickness after the microkeratome pass, the time-resolved ablation and the residual stromal thickness. Intraoperatively the mean flap thickness was 113+/-31 microm and the residual stromal thickness was 277+/-49 microm. The optically determined ablation depth was 116+/-30 micro m, which corresponded to 33 microm higher mean values than the nominal ablation depth. CONCLUSIONS: Online optical coherence pachymetry enabled the flap and residual stromal thickness to be measured intraoperatively. Also the individual ablation depth and possible dehydration effects of the cornea were monitored continuously. Thus, online optical coherence pachymetry could contribute to improve the safety standards during LASIK.     

Corneal perforation during laser in situ keratomileusis.

Two cases of corneal perforation that occurred during laser ablation but not during flap formation in laser in situ keratomileusis (LASIK) are reported. In the first case, no problem in flap formation occurred, and stromal bed thickness was assumed to be about 239 microns after laser application. However, a stromal bed perforation was found when 608 pulses were applied (theoretical ablation depth was 140 microns). Excessive dehydration due to prolonged exposure to the operating microscope light may change the ablation rate and cause corneal perforation. In the second case, LASIK retreatment was performed using the original flap for correction of regressed myopia (-4.0 diopters). Although the stromal bed thickness was assumed to be about 175 microns after the laser application, a stromal bed perforation was found when 151 pulses were applied (theoretical ablation depth was 37 microns). Unexpected corneal ectasia after the first LASIK treatment may cause a corneal perforation in LASIK retreatment. In conclusion, excessive dehydration of the cornea should be avoided, and careful examination of the corneal shape is necessary to prevent corneal perforation during laser ablation in LASIK.     

Theoretical analysis of peripheral imaging after excimer laser corneal refractive surgery for myopia

PURPOSE: To explore theoretically the retinal point images in the peripheral fields of eyes that have had excimer laser refractive surgery.University research laboratory. METHODS: Model eyes were based on Navarro's finite schematic eye, the eyes being made myopic by an increase in axial length. To simulate photorefractive keratectomy (PRK), the anterior shape and thickness of the cornea were modified. Variables included pupil size, ablation zone size, preexisting refractive error, and the addition of a blending zone. Image-quality criteria for each retinal point image were its size and the angular separation of the centroids of those parts of the image produced by rays passing through ablated and unablated corneal zones. RESULTS: In the peripheral visual field, the boundary between the ablated and unablated cornea caused a separation of the retinal image of a single point into 2 parts. The separation increased with the preexisting refractive error. Image quality was correspondingly reduced by ablation. As pupil size increased, the field angle at which the retinal image doubling first occurred decreased. Increasing the diameter of the ablation zone or using a blending zone increased the angle at which the doubling first occurred, and the blending zone improved image quality considerably. Chromatic effects appeared to be relatively unimportant. CONCLUSIONS: This analysis provides further evidence of the disadvantages of small central ablation zones in excimer laser refractive surgery and of the advantages of well-designed blending zones in improving postsurgical peripheral image quality. Image quality in the peripheral field of the pseudoemmetropic post-PRK eye is generally worse than in a naturally emmetropic eye, even though the axial image quality may be similar.     

Feasibility of intraoperative corneal topography monitoring during photorefractive keratectomy

PURPOSE: We propose a feasibility study of new corneal topography technology with the aim of monitoring intraoperative corneal topography during excimer laser photorefractive keratectomy. The PAR system measures corneal topography with single grid projection and triangulation but requires fluorescent fluid to be deposited on the corneal surface for shape extraction. We propose and demonstrate a novel corneal topography system based on structured incoherent visible light projection and triangulation that does not require addition of fluorescent fluid. METHODS: We used a binary liquid crystal spatial light modulator to display multiple fringe patterns onto the cornea. The depth accuracy of the corneal topography system was measured using a white reflected test sphere mounted on a micrometer translation stage. The performance of the corneal topography system was tested on 5 de-epithelialized swine eyes in vitro ablated with a VISX excimer laser. RESULTS: Depth accuracy on the test sphere was 0.5+/-0.75 microm over an area of 17.6 mm2. On de-epithelialized swine corneas, ablation at the apex of the cornea treated with an excimer laser was measured without addition of fluorescein. CONCLUSIONS: This new corneal topography system achieved an adequate level of accuracy on a test sphere.     

The Alcon/Summit/Autonomous perspective on fixed vs. variable spot ablation

The LADARVision system utilizes a small fixed diameter excimer laser beam coupled with a high fidelity eye tracking system to perform wavefront-guided refractive surgery. The fixed small beam provides a consistent ablation per pulse. By delivering many identical pulses in a predetermined pattern, which includes an optimized custom blend zone, the laser is able to ablate complex (higher order) corneal shapes accurately. The closed loop eye tracking system ensures that accurate ablation is delivered to the right place on the eye.     

瞳孔大小和瞳孔中心位移曲线随照明度变化的研究

背景准分子激光角膜屈光手术已成为矫治屈光不正的主要方式之一。术中切削中心的精确定位对术后视觉质量有重要影响。瞳孔中心位置的变化是影响切削中心定位的重要因素，因此有必要对其进行深入研究。目的研究不同照明条件下，人眼瞳孔中心随瞳孔大小改变发生位移过程中的动态规律，探讨瞳孔中心位移曲线的规律在准分子激光角膜屈光手术中的临床应用。方法在Astra Max3D角膜地形图测量系统基础上，对其照明亮度及图像拍摄模式进行自定义修改。利用该系统对57例114眼中度近视患者由照明亮度改变所产生的瞳孔大小和瞳孔中心位置的改变进行测量。照明亮度分为0．8、4．4、18．9、82．3、355．0lx5个光照度等级。使用向量和相对值分析的方法对瞳孔中心相对角膜共轴反光点的位置变化进行统计。不同光照度状态下瞳孔大小的相对变化幅度（AP）以占总变化幅度的百分数（最暗时瞳孔大小的变化设为0％，最亮时变化幅度为100％）表示；瞳孔中心的相对位移量（AC）以占总位移量的百分数（最暗时为0％，最亮时设为100％，以角膜中央顶点为原点）表示，并分析瞳孔中心在随光照度不同而发生变化过程中的位移曲线特点。结果5个光照度水平下瞳孔平均变化量分别为：0、（1．28±0．40）、（2．34±0．53）、（3．34±0．54）、（4．03±0．56）mm。AP分别为0％、32％、58％、83％、100％。随着光照度水平的增加，瞳孔由大变小时，瞳孔的中心位置由角膜顶点的颞侧向鼻侧发生位移。5个光照度情况下瞳孔中心平均位移量分别为：0、（78±33）、（116±60）、（143-±66）、（170±71）μm。AC分别为0％、46％、68％、84％、100％。AC与AP之间存在近似线性关系（r=0．980，P=0．025）；瞳孔中心位移的幅度和瞳孔大小变化幅度呈正相关（r=0．480，P〈0．01）。结论当光照度水平发生变化时，瞳孔中心位置随瞳孔大小变化发生一致性改变，可以用于指导个性化准分子激光角膜屈光手术切削中心的精确定位。     

In situ collagen gel mold as an aid in excimer laser superficial keratectomy.

PURPOSE: The aim of this study is to evaluate the potential use of bovine type I collagen as an adjuvant to excimer laser keratectomy. METHOD: A suspension of collagen with the capability to polymerize into a gel was applied to the anterior corneal surface of freshly enucleated porcine eyes, using 35.0 diopter (D), 45.5 D, or 52.0 D contact lenses as molds. Keratometry, photokeratoscopy, slit-lamp photography, scanning electron microscopy, and light microscopy were performed on the new surfaces. Furthermore, an irregular corneal surface was created and a suspension of collagen was applied to mask protruding irregularities, therefore creating a smooth surface that was subjected to excimer laser keratectomy. Ablation rates for both collagen and cornea were measured. RESULTS: Collagen suspension placed on a cornea and molded with contact lenses created a smooth-surfaced gel that conformed to the shape of the contact lens and adhered to the anterior cornea; it was optically smooth and regular as shown by photokeratoscopy, keratometry, and scanning electron microscopy. The corneal curvature was altered in accordance with the base curvature of the contact lens used. Results of keratometry showed resolution of pre-existing astigmatism without induction of new astigmatism. The ablation rate of the gel was not measurably different than that of cornea; hence, when applied to an irregular corneal surface, a smooth surface was created after excimer laser ablation. CONCLUSION: This study supports the potential value of collagen gel as an adjuvant to excimer laser keratectomy for removal of corneal irregularities as well as for correction of myopia or hyperopia with or without astigmatism.     

Corneal asphericity and retinal image quality: a case study and simulations

PURPOSE: The optical quality of retinal images is dependent on the refracting elements of the eye including the nominally aspheric cornea and crystalline lens. This paper presents a retrospective theoretical analysis of the impact of corneal asphericity on the quality of retinal images. Clinical data are from the VISX Incorporated CustomVue IDE. METHOD: Topography, contrast sensitivity, and visual acuity data were collected from 278 myopic eyes before and after wavefront-guided laser surgery. The measured corneal surface of each eye was fitted to a conic, and a Q-value was computed for a 5.5-mm pupil. A model eye was used to simulate various amounts of optical asphericity. RESULTS: Preoperatively, most corneas exhibited negative conic shape constants. Postoperatively, corneas were about equally divided between positive and negative conics. There was no statistically significant correlation between the shape of the cornea and the subjects' perceptions of image quality including contrast sensitivity and visual acuity. Simulations showed that the corneal Q-value can vary from more to less prolate depending upon the shape of the internal surface. CONCLUSION: Following wavefront-guided laser in situ keratomileusis (LASIK), contrast sensitivity is usually good and is not dependent upon the corneal conic shape. Better visual outcomes are more likely with a customized shape than a standard best conic shape.     

Influence of corneal curvature on calculation of ablation patterns used in photorefractive laser surgery

PURPOSE: The aim of this work was to clarify the influence of the effective illumination area and possible reflection losses that occur during laser-tissue interaction on the modeling of profiles for customized corneal ablation, such as wavefront-guided treatments. METHODS: The changes of the ablation depth per laser pulse due to the projection of a laser spot onto the corneal front surface and reflection losses at the air-tissue interface were calculated. RESULTS: Moving with a scanning-spot from the center of the cornea toward the limbus resulted in an increase of the effective illumination area and reflection losses, which led to a decrease in the ablation depth per laser pulse. The decrease of the ablation depth was strongly related to the initial radiant exposure and the corneal curvature radius. CONCLUSIONS: The corneal front surface must be taken into consideration for ablation profile calculations, especially in customized treatments, due to the strong dependence of the ablation depth on the corneal curvature.     

Comparison of corneal aberration changes after laser in situ keratomileusis performed with mechanical microkeratome and IntraLase femtosecond laser: 1-year follow-up

PURPOSE: To compare corneal aberration changes 1 year after myopic laser in situ keratomileusis (LASIK) performed with a mechanical microkeratome and IntraLase femtosecond laser. METHODS: Twenty four eyes of 15 patients underwent LASIK with the Hansatome microkeratome, and 23 eyes of 13 patients underwent LASIK with the IntraLase femtosecond laser. A standard ablation was performed with the Bausch & Lomb Technolas 217 excimer laser. Topography data were used to calculate corneal aberrations with a 3.0 mm and 5.00 mm pupil, before and 12 months after surgery. The increasing factor (IF), defined as the ratio between the postoperative and preoperative mean value of the optical aberration, was calculated. The method of Mulhern et al was used to evaluate the centration of ablation. The comalike aberration was correlated with the decentration of ablation. The Student t test was used for the statistical anaylsis. RESULTS: The postoperative mean decentration of ablation was <0.5 mm. The comalike aberration appeared to be positively correlated with the decentration of ablation in both groups with a 5.0-mm pupil (P < 0.05). With a 3.00-mm pupil, the comalike aberration changed in the Hansatome group, whereas with a 5.00-mm pupil, all aberrations statistically significantly changed in both groups (P < 0.05). The IF similarly increased in 2 groups for spherical-like aberration, whereas IF greatly increased for total and comalike aberrations in the Hansatome group. CONCLUSIONS: Wavefront corneal aberrations change significantly 1 year after myopic LASIK performed with the Hansatome microkeratome as well as with IntraLase femtosecond lasers. Both of the procedures induce higher-order aberrations in the anterior corneal surface, but the amount of comalike aberration increases more with the Hansatome mechanical microkeratome.     

Surgeon offsets and dynamic eye movements in laser refractive surgery

PURPOSE: To determine the amount of static and dynamic pupil decentrations that occur during laser refractive surgery. SETTING: The Center of Visual Science and the Department of Ophthalmology, University of Rochester, Rochester, New York, USA. METHODS: The surgeon's accuracy in aligning the pupil center with the laser center axis was measured when engaging the eye-tracker in 17 eyes receiving conventional laser in situ keratomileusis (LASIK) procedures (Technolas 217z; Bausch & Lomb). Eye movements were measured subsequently during the treatment in 10 eyes using a pupil camera operating at 50 Hz. Temporal power spectra were calculated from the eye movement measurements. RESULTS: The mean pupil misalignment by the surgeon at the beginning of the procedure was 206.1 microm +/- 80.99 (SD) (with respect to the laser center). The laser center was typically misaligned below (inferiorly) and to the left (nasally and temporally in left and right eyes, respectively) of the pupil [corrected] center. Small amounts of cyclotorsion were observed during the ablation (<2 degrees). The mean magnitude of dynamic pupil decentration from the laser center during treatment was 227.0 +/- 44.07 microm. The mean standard deviation of eye movements was 65.7 +/- 25.64 microm. Temporal power spectra calculated from the horizontal and vertical changes in eye position during the ablation were similar. Ninety-five percent of the total power of the eye movements was contained in temporal frequencies up to 1 Hz, on average, in both directions. CONCLUSIONS: Most eye movements during LASIK are slow drifts in fixation. An eye-tracker with a 1.4 Hz closed-loop bandwidth could compensate for most eye movements in conventional or customized ablations.     

The use of the femtosecond laser in the customization of corneal flaps in laser in situ keratomileusis

PURPOSE OF REVIEW: With a growing body of work that examines the biomechanical response of the cornea following flap creation, a case has developed for creation of flaps that take advantage of the inherent strength of the stromal layers of the cornea. These flaps are thinner (+/-100 microm) and the diameter is based on the maximum ablation zone of the excimer laser. The critical component in this approach is the use of the IntraLase Femtosecond Laser (IntraLase, Irvine, California, USA) as it is the only keratome currently capable of achieving a customized flap. RECENT FINDINGS: Concern over the thickness of the corneal flap has led to a resurgence of surface ablation in order to preserve as much corneal tissue as possible. Although surface ablation does offer a higher degree of safety versus laser in situ keratomileusis (LASIK), it does not achieve the same visual results during early postoperative recovery or a high degree of patient satisfaction. New biomechanical studies comparing corneal response to various types of laser refractive procedures demonstrate that corneal flaps created with a femtosecond laser are the most stable. SUMMARY: This review is intended to summarize the primary advantages of a customized corneal flap in LASIK, as well as to review the biomechanics that support this approach.     

准分子激光角膜屈光手术的发展趋势

随着飞秒激光定制薄角膜瓣技术、改良表层切削技术、波阵面像差与角膜地形图引导技术、眼球定位及联机跟踪技术,以及老视补偿技术的不断改进和广泛应用,未来5～10年,准分子激光角膜屈光手术仍将是屈光手术的主流.术前筛选出亚临床圆锥角膜,是预防屈光手术后角膜扩张及继发性圆锥角膜的关键.为保持角膜生物力学的完整性和更好的视觉质量,可选择有晶状体眼人工晶状体植入术矫治超高度近视.