Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery

PURPOSE: To quantify the cyclorotation occurring between wavefront measurement and laser refractive surgery. SETTING: LaserVue Eye Center Ophthalmic Clinic, Santa Rosa, California, USA. METHODS: The pupil camera of the Visx WaveScan wavefront device was used to obtain images of 51 eyes (26 patients) from 5 to 20 minutes before refractive laser surgery. Additionally, an infrared camera was mounted on the Visx Star S3 ActiveTrak excimer laser system to obtain another image immediately before the laser was fired. After surgery, the 2 sets of images were compared to determine the amount of cyclotorsion between the measurement and surgery. RESULTS: Cyclorotation of individual eyes was as high as 9.5 degrees. The mean was approximately 2.0 degrees for each eye. Binocular excyclotorsion was the predominant trend, affecting 19 of 24 patients. CONCLUSIONS: A low to moderate amount of cyclotorsion was observed in the transition from seated to supine position. Comparison of eye position at the time of measurement to eye position at the time of surgery can be used to adjust the laser ablation algorithm to compensate for this rotational displacement.     

Intraoperative corneal topography for image registration

PURPOSE: To present a method to measure the three-dimensional shape of the cornea and to use the data for registration purposes in order to optimize ablation pattern alignment during corneal laser surgery. METHODS: The three dimensional shape of the cornea can be measured with a modified fringe projection technique using UV laser pulses. A method to register these shape images is presented. The registration is done via established algorithms that use peripheral elevation data, which is not affected during the laser treatment. The method also provides a means to control the absolute amount of tissue removal. The three-dimensional registration method is compared with conventional two-dimensional eye tracking. RESULTS: Due to the parallax of the cornea with respect to the pupil center, considerable decentration of laser ablation patterns can occur when tracking just the pupil center. Registration using three-dimensional shape measurements provides a more accurate means to control ablation pattern application. CONCLUSIONS: A new method to register corneal shapes is discussed. It should allow monitoring the real ablation rate online during the treatment and might eventually serve as an online feedback system to control the laser ablation-induced corneal shape changes.     

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.     

Zyklorotation des Auges bei wellenfrontgeführter LASIK mit statischem Iriserkennungstracker

PURPOSE: Centration of the ablation zone decisively influences the result of wavefront-guided LASIK. Cyclorotation of the eye occurs as the patient changes from the sitting position during aberrometry to the supine position during laser surgery and may lead to induction of lower and higher order aberrations. METHODS: Twenty patients (40 eyes) underwent wavefront-guided LASIK (B&L 217z 100 excimer laser) with a static eyetracker driven by iris recognition (mean preoperative SE: -4.72+/-1.45 D; range: -1.63 to -7.00 D). The iris patterns of the patients' eyes were memorized during aberrometry and after flap creation. RESULTS: The mean absolute value of the measured cyclorotation was -1.5+/-4.2 degrees (range: -11.0 to 6.9 degrees ). The mean cyclorotation was 3.5+/-2.7 masculine (range: 0.1 to 11.0 degrees ). In 65% of all eyes cyclorotation was >2 masculine. CONCLUSIONS: A static eyetracker driven by iris recognition demonstrated that cyclorotation of up to 11 degrees may occur in myopic and myopic astigmatic eyes when changing from a sitting to a supine position. Use of static eyetrackers with iris recognition may provide a more precise positioning of the ablation profile as they detect and compensate cyclorotation.     

Ocular cyclotorsion during customized laser ablation

PURPOSE: To compare the measured difference and degree of cyclotorsion in eyes undergoing customized laser ablation between the time of wavefront measurement and laser refractive surgery. METHODS: The degree of cyclotorsion was measured in 1019 consecutive eyes of 732 patients who underwent wavefront-guided corneal ablation for the treatment of myopia with or without astigmatism with the Alcon LADARVision excimer laser. Prior to obtaining the wavefront measurement, the horizontal axis of each eye was marked. Cyclotorsional alignment was measured on the supine patient prior to beginning the laser. RESULTS: The predominant trend was for excyclotorsion. The total range of torsion in degrees was 0.5 degrees to 17.5 degrees (mean: 4.05 +/- 2.9 degrees). Cyclotorsion > 2 degrees occurred in 68% of all eyes. In the right eyes, 83% excyclotorted, 14% incyclotorted, and 3% displayed no torsion. In the left eyes, 62% excyclotorted, 34% incyclotorted, and 4% exhibited no torsion. In a subset of 112 patients undergoing simultaneous bilateral surgery, 46% displayed bilateral excyclotorsion and 1.7% displayed bilateral incyclotorsion. CONCLUSIONS: In this study, the majority of eyes had a low to moderate amount of cyclotorsion observed with the change of orientation from seated to supine position. Excyclotorsion was the predominant pattern. Proper registration prior to wavefront-guided corneal ablation is essential for proper correction of astigmatism and higher order aberrations to achieve optimal visual outcomes.     

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 patient's 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.     

The effects of automated registration on aberration measurement with the LADARWave aberrometer

PURPOSE: To evaluate the effects of automating the registration process for wavefront measurement using the LADARWave aberrometer. Measures of interest include accuracy, repeatability, and speed of measurement. METHODS: Five separate registration images were collected for each of eight eyes of four individuals with varying iris color using the LADARWave aberrometer. Technicians manually aligned the images and the autoregistration feature was used to align the same images. Time, accuracy, and repeatability of the alignment were measured using objective techniques. The potential effect on positional error was evaluated using a retrospective reprocessing of LADARWave data from an existing sample dataset. RESULTS: Autoregistration was the fastest method of alignment, from five to seven times faster than manual registration. It was also the most consistent, with the lowest pupil/limbus offset error (45 microm). Technicians averaged 48 to 91 microm. The differences in wavefronts used to generate the composite wavefront image for surgery export were 18% lower with autoregistration. CONCLUSIONS: The autoregistration feature of the LADARWave aberrometer not only removes inter-technician variability, but reduces the time to perform image alignments and improves the consistency of the alignments. This helps reduce errors related to wavefront position, potentially improving outcomes.     

The pupil is a moving target: centration, repeatability, and registration

PURPOSE: To evaluate the ability to accurately center and register wavefront data to provide effective custom wavefront application for vision correction. METHODS: Centration images were obtained of 40 undilated pupils at 5 different illumination levels using the Alcon LADARWave aberrometer (Alcon Laboratories Inc, Fort Worth, Tex). Pupil position relative to the limbus was determined for all eyes under all levels of illumination. Variability in centration of the pupil was based on difference in magnitude of offset between pupil center and limbal center. Human operators were compared to a computerized limbus recognition system through repeatability testing using 10 different images of one eye of a single patient. RESULTS: As determined by human operators, the pupil center in undilated pupils shifted a mean of 175 microm between the lowest level and the highest level of illumination (range 34 to 335 microm [standard deviation 84 microm]). Repeatability testing suggests human measurements can accurately position the pupil to within approximately 50 microm using current technology. Computer image processing using limbus recognition software appears equal or superior to manual pupil centration. CONCLUSION: The pupil center can have a significant change in position with changing illumination. Wavefront data must be centered on a fixed eye structure rather than the pupil center to ensure that wavefront data captured in scotopic light are appropriately matched to the pupil in the photopic light used at surgery. The goal is to avoid clinically significant decentration of the wavefront between capture and excimer laser application.     

准分子激光原位角膜磨镶术中瞳孔中心移位及眼球旋转的研究

目的探讨准分子激光原位角膜磨镶术(laser insitu keratomileusis,LASIK)患者术前检查位(坐位)与术中体位(卧位)改变引起眼球旋转的规律及瞳孔中心移位的特点。方法LASIK手术患者285例(565眼),术前在暗室坐位自然瞳孔状态下,应用wavescan wavefrontTM system进行波前像差检查和虹膜纹理识别,在LASIK术中卧位应用激光机进行虹膜定位追踪手术,显示瞳孔中心移位(565眼)及眼球旋转(562眼)情况。结果术中565眼全部发生了瞳孔中心移位,双眼均以鼻上方移位明显。右眼平均移位距离为(0.33±0.25)mm,左眼平均移位距离为(0.34±0.27)mm。双眼平均移位距离差异无统计学意义(P>0.05)。556眼(98.9%)发生眼球旋转,右眼平均旋转3.80°±3.95°,左眼平均旋转2.40°±2.95°,右眼旋转度数大于左眼,差异具有统计学意义(P<0.01)。右眼外旋明显,左眼内外旋几率基本相当。结论LASIK术中眼球的旋转运动和瞳孔中心移位是普遍存在的,屈光手术医生应高度重视,在准分子激光手术中应注意眼球旋转及瞳孔中心移位所引起的偏中心切削和欠矫问...     

Technical Note: Comparison of the wavefront aberrations between natural and pharmacological pupil dilations

Ocular wavefront aberrations were tested with the wavefront-supported corneal ablation (WASCA) aberrometer when the pupil was dilated naturally under scotopic illumination levels, and pharmacologically using tropicamide. Wavefront aberrations were measured across a 5-mm pupil in one eye of each of 58 subjects. There was a significant difference in the pupil diameter between the scotopic and cycloplegic conditions (p < 0.001). For higher order aberrations (HOAs), no significant difference was found in Zernike coefficients between the scotopic and cycloplegic conditions. There was no significant difference in the spherical equivalent refraction. Accommodation can be neglected during the wavefront aberration measurement with the WASCA aberrometer. In view of the shift of the pupil centre caused by pharmacological pupil dilation, obtaining measurements under natural conditions rather than using mydriatics is recommended for customized laser in situ keratomileusis (LASIK) surgery.     

Eye movement during laser in situ keratomileusis

PURPOSE: To measure eye motion in patients having laser in situ keratomileusis (LASIK) using a video technique and determine centration and variance of the eye position during surgery. SETTING: Laser refractive surgery center. METHODS: The procedure was videotaped in 5 consecutive eyes having LASIK performed by a single surgeon with the VISX Star S2 excimer laser. Following surgery, video images of the eyes were digitized and stored in a computer for processing. Digitized images were obtained at a rate of 25 images per second during the laser procedure. The pupil margin and a visual landmark, such as a scleral blood vessel, were identified in the initial image of each eye. Custom software was used to track the location of the landmark and the pupil center in subsequent images. RESULTS: Three of the 5 eyes were well centered on average. The remaining 2 eyes were decentered inferiorly by approximately 0.25 mm. The standard deviation in all eyes was approximately 0.10 mm. CONCLUSIONS: With these techniques, the position of the entrance pupil center relative to the excimer laser axis could be determined. Although the system is not fast enough to be used during surgery, it does allow quantification of centration and intraoperative motion after surgery.     

Repeatability of ocular wavefront measurement.

PURPOSE: To assess the repeatability of measurements of ocular aberrations using wavefront sensing in a small group of observers and to assess the potential effect of measurement error on custom corneal correction due to this variability. METHOD: A Shack-Hartmann wavefront sensor was used to measure the ocular wavefront in nine eyes. Head position was stabilized using a dental bite bar, and the pupil was centred using a cathode ray tube monitor and circular grating. Twenty Shack-Hartmann images were collected for each measurement. Each observer had three sets of measurements taken; the first and the second after careful alignment and the final after regrasping the bite bar in the same position as for the second measurement, but without pupil realignment. The modulation transfer functions for each set were calculated, and the effect of best-aligned custom treatments on the modulation transfer function was estimated. RESULTS: There were highly statistically significant differences in a large number of Zernike modes between the three sets of measurements. The modulation transfer functions calculated for the residual wavefronts after aligned custom treatment were below the diffraction limit. The root mean square wavefront errors were consistently better for the residual wavefronts obtained using the realigned data than using data taken without pupil realignment. CONCLUSIONS: Sequential measurement of ocular aberrations shows statistically significant differences in a large number of Zernike modes. If aberrations determined by a single measurement are to be used in a custom correction, the resulting modulation transfer function is likely to remain below the diffraction limit. Pupil realignment is critical in reduction of the residual root mean square wavefront values to a minimum.     

眼位对主动追踪效果的实验影响

目的研究眼位对主动追踪效果的影响。方法以金属环模拟虹膜及瞳孔,其上置3.5 mm厚透明PMMA板模拟角膜,将该模拟装置置于MEL-70准分子激光主动追踪系统下,分别标记模拟装置在中央位置及位置偏移5 mm及10 mm后主动追踪系统所指示切削中心位置,观测两种位置偏移引起的追踪效果偏差量。结果偏差量分别为0.15mm及0.25 mm。结论眼位对主动追踪有影响,这种影响对常规屈光手术是可接受的,对高阶像差的影响则不容忽视。     

Relative contribution of central and peripheral aberrations to overall high order corneal wavefront aberration

PURPOSE: To analyze the influence of specific combinations of corneal high order aberrations on the optical image quality of the cornea before and after photorefractive keratectomy (PRK) for low to high myopia and myopic astigmatism. METHODS: Corneal topography was obtained for 80 eyes that underwent PRK using a scanning-spot excimer laser. The eyes were subdivided into three groups according to the preoperative refraction. The topographical data were imported into a custom software program that combined the Zernike high order terms having the same sign and angular frequency up to seventh order for 4-mm and 6-mm pupils, ie, coma and spherical aberrations, and midperipheral and peripheral high order aberrations. RESULTS: Photorefractive keratectomy induced a significant amount of the root-mean-square (RMS) values of the combinations of midperipheral and peripheral high order aberrations over the smaller pupil size for deeper myopic ablations (P<.05). Over the larger pupil, spherical myopic ablations showed a significant increase (P<.05) of the RMS values of coma and spherical aberrations. In the astigmatism group, the combination of terms having higher angular frequency increased significantly (P<.05) after surgery both over 4-mm and 6-mm pupils. CONCLUSIONS: After surface ablation, ablation depth and profile significantly influence the distribution and contribution of determined combinations of high order aberrations to the overall high order corneal wavefront aberration. Terms having high angular frequency were increased following large myopic correction and wide treatment zone. Quality of the whole corneal optics will be enhanced by designing future ablation profiles to compensate for peripheral high order optical aberrations.     

Changes in wavefront aberration with pharmaceutical dilating agents

PURPOSE: Finding the best way to capture the wavefronts of small pupils for refractive surgery has become a more pressing issue as the general population ages. This study explores whether pharmaceutical dilation impacts wavefront measurement and pupil centroid. METHODS: Baseline measurements were performed on 32 eyes using the VISX WaveScan Wavefront system. Pupils were dark adapted. One drop of 0.05% tropicamide was placed in each eye, and wavefront measurements were conducted at 10, 20, and 30 minutes. One drop each of 0.5% tropicamide and 2.5% phenylephrine were administered after the measurements. Wavefronts were captured again 30 minutes after the last eye drop. All patients returned for repeated procedures. Wavefront analysis was performed using the same pupillary area as the smallest capture from the same visit. Pupil center shift was taken into account. RESULTS: Mean patient age was 40 +/- 12 years (range: 20 to 59 years). Mean dark-adapted pupil was 6.40 +/- 1.17 mm. Pupil centers shifted randomly after pharmacological dilation compared to the dark-adapted condition. Pupil centers of 45% of the population shifted by > or = 0.2 mm. Repeatability coefficients were established for the wavefront measurements. After controlling for pupil diameter and pupil center, total high order aberrations root-mean-square (RMS) had changed significantly in 18% of the population. The diluted tropicamide formula, which caused less dilation effect, also induced less high order aberration RMS change. CONCLUSIONS: Pharmaceutical dilation agents cause random shifts of the pupil centroid from the dark-adapted pupil condition and could induce changes in wavefront measurements. Caution is required when resorting to dilation to obtain a wavefront measurement of smaller pupils.     

Separate effects of the microkeratome incision and laser ablation on the eye's wave aberration

PURPOSE: To study the optical changes induced by the microkeratome cut, the subsequent laser ablation, and the biomechanical healing response of the cornea in normal laser in situ keratomileusis (LASIK) eyes. DESIGN: Prospective randomized clinical trial. METHODS: A Hansatome microkeratome was used to cut a corneal flap in one eye (study eye) of 17 normal myopic patients and a subsequent laser ablation was performed 2 months after this initial microkeratome incision. Control eyes received conventional LASIK treatments at the latter time point. The wave aberration of both the study and contralateral control eyes were measured over a 6-mm pupil with a Shack-Hartmann wavefront sensor for all preoperative, postflap cut, and postablation visits. RESULTS: The eye's higher order aberrations had a small, but significant increase (P =.03) of approximately 30% 2 months after cutting a flap. No systematic changes were observed in nearly all Zernike coefficients from their preoperative levels at 2 months postflap cut. A significant difference between the study and control eyes was observed for one trefoil mode, Z(3)(3) (P =.04). CONCLUSIONS: There was a wide variation in the response of individual Zernike modes across patients after cutting a flap. The majority of spherical aberration induced by the LASIK procedure seems to be due to the laser ablation and not the microkeratome cut. In addition, the total and higher order root mean square of wavefront errors were nearly identical for both the study and control eyes 3-months after the laser ablation, indicating that a procedure in which the incision and the ablation are separated in time to better control aberrations does not compromise the outcome of a conventional LASIK treatment.     

散瞳前后瞳孔中心偏移对波前引导准分子激光原位角膜磨镶术手术的影响

目的探讨根据散瞳后的瞳孔中心调整切削区对波前引导的个体化准分子激光原位角膜磨镶术(LASIK)疗效的影响。方法瞳孔直径为6mm时高阶像差均方根(RMS)>035μm的近视患者32例(63只眼),随机分为A、B两组,接受Zyoptix个体化LASIK手术。A组以瞳孔中心为切削区中心,B组则根据测得散瞳后瞳孔中心的偏移量对切削区中心作相应调整。结果散瞳后瞳孔中心平均发生(0173±0114)mm的偏移,最大达051mm。术后3个月两组患者视力、屈光度数差异无统计学意义(P>005),波前像差检查显示手术前后彗差的差值分别为(0025±0137)和(0097±0118)μm(t=2131,P=0037),而球差差异无统计学意义(P>005)。B组彗差的矫正效果优于A组。结论散瞳后瞳孔中心普遍发生偏移,根据其偏移量在术中对切削区中心作相应调整可提高彗差的矫正效果。(中华眼科杂志,2005,412426)     

Determining the accuracy of an eye tracking system for laser refractive surgery

PURPOSE: Patient eye and head movements during laser refractive surgery may result in errors between the surgical beam position and the desired location for optimum correction. This, in turn, may lead to reduced postoperative vision, including increased higher order aberrations of the eye. Active eye tracking systems are often incorporated into laser delivery systems, which aim to reduce the effect of patient eye movement. METHODS: In this study, the accuracy of an eye tracking system designed for laser refractive surgery was determined. An enucleated porcine eye was attached to a scanning device and the movement measured using the eye tracking system. The recorded position is compared to the preprogrammed position of the scanning device. RESULTS: The system demonstrated an accuracy of 0.06 m for an intact cornea and 0.1 mm for a cornea with a thin flap removed. This compares to an average decentration of ablation of 0.4 mm for patients relying on passive fixation, as measured by previous clinical trials. CONCLUSION: implementation of this eye tracker would lead to improved alignment between the laser and eye during laser refractive surgery.     

近视患者优势眼与非优势眼的Kappa角、瞳孔大小及中心位置的动态变化

目的 探讨行飞秒激光制瓣准分子激光原位角膜磨镶术的近视患者优势眼与非优势眼的Kappa角、瞳孔大小及中心位置的动态变化.方法 选取2018年12月至2020年8月在延边大学附属医院眼科行飞秒激光制瓣准分子激光原位角膜磨镶术的近视患者117例(234眼)为研究对象.采用ALLEGRO Topolyzer角膜地形图仪(德国...     

Alcon LADARWave customcornea retreatments

PURPOSE: To present our experience performing wavefront-guided ablations for the treatment of residual refractive error following previous refractive surgery. METHODS: Four different cases are presented-1) primary wavefront-guided LASIK; 2) wavefront-guided lift-flap LASIK retreatment; 3) wavefront-guided photorefractive keratectomy (PRK) retreatment; and 4) wavefront-guided PRK over radial keratotomy retreatment. All procedures were performed with the Alcon CustomCornea laser platform. Uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), manifest refraction, and wavefront analysis (6.5-mm pupil) were performed preoperatively and 3 months after CustomCornea retreatment. The Alcon LADARWave device was used for wavefront measurements. RESULTS: Total lower order and higher order aberrations were decreased following wavefront-guided retreatment, resulting in improved quality of vision. The maximum target offset (+ 0.75 diopters) can be used to prevent an overcorrection of defocus (myopia) when treating a significant amount of spherical aberration. CONCLUSIONS: Wavefront-guided ablation was an effective treatment for residual lower and higher order aberrations following previous refractive surgery. Custom ablation treatment algorithms need to be developed specifically for retreatment procedures.