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Influence of blurred vision, accommodation, and target laser settings on eye movements during LASIK
ARTICLE
Influence of blurred vision, accommodation, and target laser settings on eye movements during LASIK Anna Christina Sasse, MD, Mehdi Shajari, MD, Thomas Kohnen, MD, PhD, FEBO
PURPOSE: To analyze the influence of blurred vision, accommodation, and target laser settings on eye movements during laser in situ keratomileusis (LASIK). SETTING: Department of Ophthalmology, Goethe University, Frankfurt, Germany. DESIGN: Prospective randomized study. METHODS: Participants had simulated LASIK treatment. They were instructed to focus on the fixation light; the treatment laser was blocked, and all other settings were applied according to standard LASIK treatments. To simulate blurred vision a 0.0 diopter (D) soft contact lens received a 5.0 D myopia laser treatment and was then applied to the participant’s eye. To diminish accommodation, a second lens that had a refraction of the patient’s spherical equivalent, plus 3.0 D to compensate for accommodation, was used. There were 4 treatment modalities as follows: (1) blurred lens with target laser on, (2) blurred lens with target laser turned off, (3) C3.0 D lens with target laser on, and (4) C3.0 D lens with target laser turned off. Lateral and torsional eye movements were recorded. Fourier analysis was used to obtain temporal power spectra from dynamic eye movements. The Fn criterion was set as the frequency below which n% of eye movements in the cohort occurred (n Z 95%, 80%, and 50%). RESULTS: The study comprised 11 eyes of 11 participants. There was 1 significant difference between the eye movements based on measurement modalities. In 1 variable in the y-axis, there was movement that showed a significant difference in the F80 criterion. CONCLUSION: Surgical circumstances such as blurred vision, accommodation, and target light had little influence on eye movements during LASIK. Financial Disclosure: Proprietary or commercial disclosures are listed after the references. J Cataract Refract Surg 2016; 42:1424–1430 Q 2016 ASCRS and ESCRS
Laser in situ keratomileusis (LASIK) uses 2 discrete treatment steps. The first is a corneal flap cut that is created using a femtosecond laser or microkeratome. The second step is the corneal ablation performed using an excimer laser. A laser target provides live feedback to the surgeon reassembling the excimer laser's position, and indicates that treatment is in progress as well as specifies the ablation zone.1,2 To the patient, the laser target appears as a flickering pattern of red dots during the treatment. To optimize the positioning of the excimer laser ablation spots, eye movements are minimized by showing a laser fixation to patients. The laser fixation of the Amaris 750S excimer laser (Schwind eye-tech-solutions 1424
Q 2016 ASCRS and ESCRS Published by Elsevier Inc.
GmbH and Co. KG) is a blinking light-emitting diode. However, eye movements still occur.3–9 When performing LASIK, 2 measurements are used to record eye movements. First, static eye tracking is performed before surgery after the patient is changed from the seated to supine position. The second measurement, intraoperative eye tracking or dynamic eye-tracking, provides data on the position of the eye in a 3-dimensional (3-D) system to the laser system. Eye tracking software can track eye movements and make the laser follow them or stop if the eye position is too far off. In previous studies, it was shown that torsional misalignment has an effect on visual outcomes after surgery. Cyclotorsion of the ablation
http://dx.doi.org/10.1016/j.jcrs.2016.07.030 0886-3350
EYE-MOVEMENT EFFECTS OF BLURRED VISION, ACCOMMODATION, AND LASER TARGETS DURING LASIK
zone can lead to residual postoperative astigmatism10,11 and be responsible for lower- and higherorder aberrations.3,12–18 Kim and Joo16 and Mrochen et al.19 reported that using an iris-registration system during refractive surgery leads to a better outcome. The treatment method itself can generate several problems that might reduce the efficacy of the LASIK treatment. While the flap is open, the refraction of the cornea is impaired, resulting in blurred vision. The laser fixation is not seen as a punctiform light source but appears rather as a cloud; patients might have difficulty in fixating on a certain point. Blurred vision and the target laser source's distance might stimulate accommodation and thus increase eye movements.5 The aim of this study was to examine the influence of these factors on eye movements by simulating the circumstances of LASIK surgery. We evaluated whether it is possible to minimize intraoperative eye movements by altering surrounding conditions. PARTICIPANTS AND METHODS This prospective case series included myopic eyes of volunteers. The required number of participants was determined by a previous sample-size calculation. Inclusion and exclusion criteria were chosen following the LASIK criteria of the Kommission Refraktive Chirugie (Commission of Refractive Surgery) (KRC).20 According to the KRC criteria, safe LASIK can be performed in eyes with myopia of up to 8.0 diopters (D) or with hyperopia of up to C3.0 D. The limit for astigmatism is 5.0 D. All participants were experienced contact lens wearers and signed a written consent form for the procedure. The study was approved by the Ethical Review Committee of the Medical Faculty of the Goethe University, Frankfurt, Germany. The study adhered to the tenets of the Declaration of Helsinki. The measurements were performed with the Amaris 750S excimer laser's eye tracker. Which eye to be treated (right or left) was randomly selected.
Submitted: December 30, 2014. Final revision submitted: July 19, 2016. Accepted: July 22, 2016. From the Department of Ophthalmology, Goethe-University Frankfurt am Main, Frankfurt, Germany. Data collection was performed in cooperation with Schwind eyetech-solutions GmbH & Co. KG, because evaluated data could not be extracted directly from the system used. Oliver Klaproth helped with initiating the study and helping the authors to obtain approval of the local ethics committee. Corresponding author: Thomas Kohnen, MD, PhD, FEBO, Department of Ophthalmology, Goethe-University Frankfurt, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany. E-mail: [email protected].
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All eyes had a fictional LASIK treatment of 5.0 D myopia without astigmatism. This ensured that every participant was exposed to the same target light pattern and treatment duration. The optical zone was 6.3 mm. Lateral eye movements were recorded with a frequency of 1050 Hz and torsional movements with 30 Hz. The camera has a frame rate of 1050 Hz; every 32nd frame was taken for analysis. These eye movements are referred to as EyeX, EyeY, and Torsion in this study. Imagining the bulb in the center of a 3-D coordinate system, EyeX describes a lateral movement along the x-axis, EyeY refers to lateral movement along the y-axis, and Torsion describes a rotation around the z-axis. During LASIK, eye movements are recorded by dynamic eye tracking, and the excimer laser follows them. The tracking limit for EyeX and EyeY is G1.5 mm for laser ablation. The dynamic cyclotorsion limit is G9 degrees from the beginning of the treatment. The accuracy level for EyeX and EyeY was 55 mm and for Torsion, 2 degrees. The resolution was 4.3 mm for EyeX and EyeY and 0.01 degrees for Torsion. The treatment laser was blocked with a custom barrier created so measurements could be performed without actual treatment. The barrier was made from the laser shutter of the excimer laser with a window for the target laser. To analyze the influence of blurred vision, accommodation, and target light, eye movements were recorded 4 times, every time under a different modality. To simulate blurred vision, a 0.0 D Pure Vision silicon–hydrogel lens (Bausch & Lomb, Inc.) was placed on a glass eye below the excimer laser (at this point not blocked) before the simulated treatments of participants. The lens was then treated for 5.0 D of myopia. Each participant received his or her own lens. All were 0.0 D lenses that had received the same laser treatment before the measurement. It was assumed that patients fixate on the light source as well as on the surrounding area (eg, laser arm), which has a distance of 33 cm; 3.0 D is of a reciprocate distance from a patient's eye to the fixation light. To prevent accommodation from influencing a sharp picture, a second lens that took into consideration the participant's own refraction was used. For example, if the participant's refraction were 1.75 D, a C1.25 D lens was applied. There were 4 modalities: Modality 1 was recorded with a blurred lens and the laser target on. Modality 2 used the same lens as modality 1 with the laser target off. Modality 3 was measured with a lens that caused the patient's vision to become 3.0 D with the laser target on. Modality 4 was recorded with the same lens as modality 3 but with the laser target turned off. The order of the measurement modalities was randomized ex ante. The participants were not made aware of the purpose of the study. They were only instructed to fixate on the fixation light. Before measurement, each participant had an ophthalmologic examination of both eyes. The participants were placed below the eye tracker in a supine position with a cushion holding the participant's head in the proper position. The examined eye was placed under the eye tracker while the other eye was covered. Then, a local anesthesia (oxybuprocaine hydrochloride [Conjuncain]), lid retractor, and the first lens were applied. Artificial tears were used between measurements to keep the cornea moist because the patients were not able to blink. After the first 2 measurements, the lenses were changed; then, measurements 3 and 4 were recorded. After the last measurement, the lens was removed. The participants were instructed to refrain from wearing their own contact lenses for 24 hours because the eye was still numbed by anesthetic. The procedure was finished with
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another ophthalmologic examination to guarantee intactness, especially of the cornea. Next, all premeasurement contact lens treatment data and the measurement data of each participant's eye movements were sent to Schwind eyetech-solutions GmbH and Co. KG to be transposed into a workable file. The raw data, which could not be opened with commonly available programs, was transferred into Excel software (Microsoft Corp.) and then analyzed with SPSS software (SPSS, Inc.).
Statistical Analysis According to BiAS for Windows software (version 11.01, Epsilon-Verlag), 11 subjects had to be included to obtain valid results. The measurement was deemed valid if the eye tracker detected eye movement at least 80% of the time. Fourier analysis was used to obtain power spectra of lateral and torsional eye movements. The F95 criterion was defined as the frequency below which 95% of eye movements occur; F80 was the frequency below which 80% of movements occur; and F50, the frequency below which 50% of movements occur. Fourier analysis could not analyze eye movements above 525 Hz for lateral movements and above 15 Hz for torsional ones.
RESULTS Five right eyes and 6 left eyes were examined. Seven participants were women, 4 were men. The mean age was 25.9 years G 5.9 (SD) (range 23 to 41 years). The mean spherical equivalent refraction was 2.09 G 1.55 D (range 0.5 to 5.0 D). The measurement was deemed valid in all 11 participants. Temporal power spectra showed a dominance of high-frequency, low-power eye movements. Figure 1 shows the temporal power spectra of all participants in modality 2. The Shapiro-Wilk test showed that measurement data were not normally distributed. According to the Friedman test, there was a significant difference in the F80 variable in EyeY (P Z .039). The Wilcoxon test showed that there was a significant difference between modality 3 and modality 4 in this variable (P Z .033). Except for this difference, the
remaining data showed no significant difference in standard deviation (SD) or in the power spectra (Table 1). According to the Friedman test, there was no significant difference in the F95 and F50 frequency. Figures 2 to 4 show the cumulative amplitudes of all 11 participants. DISCUSSION The aim of this study was to compare eye movements under different surgical circumstances and to build a model with the influencing factors altered. We also wanted to determine whether it was possible to construct a surgery environment that reduces eye movements. The Friedman test showed a significant difference in the F80 criterion in EyeY. The F80 frequency in modality 3 was significantly lower than in modality 4. This means that eye movements were faster in modality 4 than in modality 3. Based on the assumption that high-frequency movements have low power, it is possible that the extent of eye movements was smaller in modality 4. This can be shown for EyeY (SD 12.57 mm in modality 3 versus 6.52 mm in modality 4). We have no explanation for the significant difference for only F80 between modality 3 and modality 4; however, with a rather low significance level (P Z .039), it might be a false rejection of the null hypothesis. The Friedman test showed no significant difference, although there was an indication that turning off the laser target could reduce the extent of vertical eye movements. We suggest further studies be performed to compare lateral eye movements with and without a laser target. It might be possible to evaluate this factor in a simulated setting as well as in real surgery circumstances. In this study, 11 eyes of 11 participants were included based on a previous sample-size calculation. Because eyes do not work independently of each other, we decided to include only 1 eye per participant.
Figure 1. Eye movements for EyeX (along x-axis) (A), EyeY (along y-axis) (B), and Torsion (C) for all participants in modality 2. A dominance of high-frequency lowamplitude movements was present.
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Table 1. The mean SD of eye movements in all patients; the frequencies below which 95%, 80%, and 50% of eye movements occurred; and the mean total distance the eyes moved. Parameter Mean SD x-axis (mm) y-axis (mm) Torsion (degrees) Frequency (%) F95X F80X F50X F95Y F80Y F50Y F95T F80T F50T Pupil translation (mm)
Modality 1
Modality 2
8.78 10.73 2.12
11.19 8.66 1.67
327.8 61.57 4.59 391.41 109.98 6.39 11.27 4 0.5 61675.36
339.75 62.45 3.5 399.84 121.81 7.59 11.01 3.43 0.28 57152.67
Modality 3
10.19 12.57 1.65 323.34 46.22 3.88 382.9 89.61* 4.79 11.03 3.48 0.35 57181.59
Modality 4
8.86 6.52 1.25 342.95 68.67 5.25 419.96 160.91* 13.67 11.18 3.73 0.39 55387.58
T Z torsion; X Z x-axis (EyeX); Y Z y-axis (EyeY) *Statistically significant difference
Before dynamic torsional eye tracking became available, static eye tracking was performed immediately before surgery. Today, both measurements are performed. Cyclotorsion during static eye tracking could be explained by the change from a seated to a supine position, as described by Swami et al.,10 Ciccio et al.,14 Kim and Joo,16 and Chernyak,21 independent of visual faculty during surgery. The benefits of using an eye tracker during refractive surgeries have been shown in previous studies.16,19,22 Several studies3,4,23,24 showed that ocular counter roll does not only occur before but also during surgery. Porter et al.5 assumed that the visual effects that occur
during LASIK, such as, blur, might have an effect on eye movements. They performed LASIK in 17 eyes of 9 patients. Eye movements were recorded in 10 of these eyes during surgery with a frequency of 50 Hz. After the surgery, the temporal power spectra were computed from the results (unfortunately only for horizontal and vertical eye movements, not for cyclotorsional ones). They found a mean cyclotorsion of 0.22 G 0.08 degrees with a mean torsion of 1.22 G 0.38 degrees, ranging from 0.55 to 1.96 degrees. Because of the irrelevance of such small torsional values (eg, a torsion of 1.96 degrees in a patient with 1.25 D of cylinder would result in 0.09 D of
Figure 2. Cumulative amplitudes in EyeX (comparison of all 4 modalities).
Figure 3. Cumulative amplitudes in EyeY (comparison of all 4 modalities).
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Figure 4. Cumulative amplitudes in Torsion (comparison of all 4 modalities).
postoperative cylinder), torsional data were not further calculated. As a representative value for the degree of eye movement, the SD was chosen. Small values show a rather steady fixation, whereas high values represent eyes with larger movements and less fixation. Mean values are not adequate to describe eye movements because positive and negative movements compensate each other (mean values for eye movements are expected to vary around 0). Analysis of the temporal power spectra of lateral eye movements (horizontal and vertical) in Porter et al.'s study5 showed a dominance of slow drifts with an F95 of 1.4 Hz for horizontal eye movements and 0.6 Hz for vertical eye movements. As in our study, temporal power spectra showed that eye movements with high power had low frequencies and lowpower movements had high frequencies. Porter et al.5 found that the fixation stability could be influenced by the altered viewing conditions during LASIK, such as blurred vision provoked by the flap cut; this factor might decrease the patient's accuracy in fixating on a certain point. The authors concluded that it is essential to track these eye movements and postulated that the eye movements dictate the elaborateness of the eye tracker. From his somewhat simplified analysis of mean dynamic eye movements, Chang3 concluded that dynamic intraoperative tracking of torsional eye movements is imperative to improve precise astigmatic refractive outcomes after surgery. Prakash et al.4 also showed the appearance of intraoperative cyclotorsion. In that study, only cyclotorsion was calculated. Rather than SD, they used the absolute amount of torsion, namely the difference between the
lowest and highest value in each patient. They assumed that the direction and degree of cyclotorsion recorded during surgery correlate with the presurgery static cyclotorsion values. They also supposed that cyclotorsion increases with the sum of light pulses, meaning that a long treatment might lead to a loss of fixation. In our study, the duration of the measurement was fixed; each patient received a fictional treatment of 5.0 D. Our finding that eye movements occur independent of environmental circumstances prompted the question of whether the performance of the eye tracker in terms of speed and recognition rate is sufficient. This study showed a predication about the quality of eye movements. The analysis of temporal power spectra determined that there are highpower, low-frequency eye movements and lowpower, high-frequency eye movements. It is thus important to track the high-power low-frequency movements in particular because the effect of a decentered ablation zone becomes worse the longer the misalignment between the calculated ablation zone and eye position persists. Fourier analysis showed that 95% of all eye movements in all modalities occurred below the frequency of 420 Hz (x-axis and y-axis movements) and below 12 Hz (torsional eye movements). The Schwind Amaris 750S eye tracker works at a frequency of 1050 Hz for lateral eye movements and 30 Hz for torsional eye movements. Because we found that high-frequency movements have low power, it might be assumed that movements above 420 Hz are small enough to be ignored. This could lead to the conclusion that the measurement frequency might be adequate. In particular, high-power low-frequency eye movements are covered in this frequency range. The question remains whether the extent and type of refractive error might have an influence on eye movements. Furthermore, we were unable to analyze movements along the propagation axis of the laser beam. Future analysis on these variables would be desirable. A further limitation of the study is the frequency of movements in the analyzed cohort. The limiting frequency of human eyes is unknown; therefore, other patients could have higher frequency eye movements that might lead to a different result and conclusion. Finally, our data show that factors such as blur, accommodation, and the laser target have no significant influence on intraoperative lateral and torsional eye movements. This means that even with these factors eradicated, eye movements occur in the same magnitude and frequency as they would if interference were present. Turning off the laser target might have decreased eye movements, at least in EyeY.
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EYE-MOVEMENT EFFECTS OF BLURRED VISION, ACCOMMODATION, AND LASER TARGETS DURING LASIK
WHAT WAS KNOWN Eye movements occur during corneal laser treatments and can impair the visual outcome. Measures to compensate for and reduce eye movements include asking the patient to fixate on the laser beam during LASIK. These measures have been shown to improve the visual outcome. WHAT THIS PAPER ADDS Blurred vision and accommodation, which can occur during the surgical procedure, did not lead to a significant difference in eye movements.
REFERENCES 1. Kohnen T, Strenger A, Klaproth OK. Basic knowledge of refractive surgery; correction of refractive errors using modern surgical procedures. Dtsch Arztebl Int 2008; 105:163–170. quiz 170–172. Available at: http://www.ncbi.nlm.nih.gov/pmc/ar ticles/PMC2696744/pdf/Dtsch_Arztebl_Int-105-0163.pdf. Accessed July 30, 2016 2. Sakimoto T, Rosenblatt MI, Azar DT. Laser eye surgery for refractive errors. Lancet 2006; 367:1432–1447 3. Chang J. Cyclotorsion during laser in situ keratomileusis. J Cataract Refract Surg 2008; 34:1720–1726 4. Prakash G, Ashok Kumar D, Agarwal A, Jacob S, Sarvanan Y, Agarwal A. Predictive factor analysis for successful performance of iris recognition-assisted dynamic rotational eye tracking during laser in situ keratomileusis. Am J Ophthalmol 2010; 149:229–237 5. Porter J, Yoon G, MacRae S, Pan G, Twietmeyer T, Cox IG, Williams DR. Surgeon offsets and dynamic eye movements in laser refractive surgery. J Cataract Refract Surg 2005; 31:2058–2066; erratum 2006; 32:378 6. Martinez-Conde S, Macknik SL, Troncoso XG, Dyar TA. Microsaccades counteract visual fading during fixation. Neuron 2006; 49:297–305. Available at: http://www.cell.com/neuron/pdf/ S0896-6273(05)01056-1.pdf. Accessed July 30, 2016 7. Ditchburn RW, Fender DH, Mayne S. Vision with controlled movements of the retinal image. J Physiol 1959; 145:98–107. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1356943/pdf/ jphysiol01308-0119.pdf. Accessed July 30, 2016 8. Nachmias J. Determiners of the drift of the eye during monocular fixation. J Opt Soc Am 1961; 51:761–766 9. Steinman RM, Cunitz RJ, Timberlake GT, Herman M. Voluntary control of microsaccades during maintained monocular fixation. Science 1967; 155:1577–1579. Available at: http:// www3.psych.purdue.edu/wzpizlo/rsteinma/Bob-FOR%20CV/ Steinman%20etal%201967%20Voluntary%20micros%20SCI ENCE%20report.pdf. Accessed July 30, 2016 10. Swami AU, Steinert RF, Osborne WE, White AA. Rotational malposition during laser in situ keratomileusis. Am J Ophthalmol 2002; 133:561–562
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11. Tjon-Fo-Sang MJ, de Faber J-THN, Kingma C, Beekhuis WH. Cyclotorsion: a possible cause of residual astigmatism in refractive surgery. J Cataract Refract Surg 2002; 28:599–602 €hne C, Cichocki M, Strenger A. Zyklorotation des 12. Kohnen T, Ku € hrter LASIK mit statischem IriserkenAuges bei wellenfrontgefu nungstracker [Cyclorotation of the eye in wavefront-guided LASIK using a static eyetracker with iris recognition]. Ophthalmologe 2007; 104:60–65 € hren J, Yoon G, Kenner S, MacRae S, Huxlin K. The effect of 13. Bu optical zone decentration on lower- and higher-order aberrations after photorefractive keratectomy in a cat model. Invest Ophthalmol Vis Sci 2007; 48:5806–5814. Available at: http://iovs.arvo journals.org/article.aspx?articleidZ2125624. Accessed July 30, 2016 14. Ciccio AE, Durrie DS, Stahl JE, Schwendeman F. Ocular cyclotorsion during customized laser ablation. J Refract Surg 2005; 21:S772–S774 15. Guirao A, Williams DR, Cox IG. Effect of rotation and translation on the expected benefit of an ideal method to correct the eye’s higher-order aberrations. J Opt Soc Am A Opt Image Sci Vis 2001; 18:1003–1015 16. Kim H, Joo C-K. Ocular cyclotorsion according to body position and flap creation before laser in situ keratomileusis. J Cataract Refract Surg 2008; 34:557–561 17. Arba-Mosquera S, Merayo-Lloves J, de Ortueta D. Clinical effects of pure cyclotorsional errors during refractive surgery. Invest Ophthalmol Vis Sci 2008; 49:4828–4836. Available at: http://iovs.arvojournals.org/article.aspx?articleidZ2125382. Accessed July 30, 2016 18. Moreno-Barriuso E, Merayo Lloves J, Marcos S, Navarro R, Llorente L, Barbero S. Ocular aberrations before and after myopic corneal refractive surgery: LASIK-induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci 2001; 42:1396–1403. Available at: http://iovs.arvojournals.org/ article.aspx?articleidZ2162653. Accessed July 30, 2016 €tz W. 19. Mrochen M, Eldine MS, Kaemmerer M, Seiler T, Hu Improvement in photorefractive corneal laser surgery results using an active eye-tracking system. J Cataract Refract Surg 2001; 27:1000–1006 €tssi20. Kohnen T, Neuhann T, Knorz MC. Bewertung und Qualita cherung refraktiv-chirurgischer Eingriffe durch die Deutsche Ophthalmologische Gesellschaft und den Berufsverband der €rzte Deutschlands; Stand January 2014 [Assessment Augena and quality assurance of refractive surgical interventions by the DOG (German Society of Ophthalmology) and the BVA (Professional Association of German Ophthalmologists): Update January 2014]. Ophthalmologe 2014; 111:320–329 21. Chernyak DA. Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery. J Cataract Refract Surg 2004; 30:633–638 22. Ghosh S, Couper TA, Lamoureux E, Jhanji V, Taylor HR, Vajpayee RB. Evaluation of iris recognition system for wavefront-guided laser in situ keratomileusis for myopic astigmatism. J Cataract Refract Surg 2008; 34:215–221 ez J, Brucks M, Zimmerman G, Bekendam P, Bacon G, 23. Narva Schmid K. Intraoperative cyclorotation and pupil centroid shift during LASIK and PRK. J Refract Surg 2012; 28:353–357 24. Febbraro J-L, Koch DD, Khan HN, Saad A, Gatinel D. Detection of static cyclotorsion and compensation for dynamic cyclotorsion in laser in situ keratomileusis. J Cataract Refract Surg 2010; 36:1718–1723
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Financial Disclosure Dr. Kohnen is a consultant to Abbott Medical Optics, Inc., Alcon Laboratories, Inc., Carl Zeiss Meditec AG, Geuder AG, Oculus Optikger€ ate GmbH, Rayner Intraocular Lenses Ltd., Schwind eye-tech-solutions GmbH and Co. KG, Tearlab Corp., Thieme Compliance
GmbH, and Ziemer Ophthalmic Systems AG. He has received grants from Abbott Medical Optics, Inc., Alcon Laboratories, Inc., Avedro, Inc., Carl Zeiss Meditec AG, Oculus Optikger€ate GmbH, and Schwind eyetech-solutions GmbH and Co. KG. No other author has a financial or proprietary interest in any material or method mentioned.
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Influence of blurred vision, accommodation, and target laser settings on eye movements during LASIK Anna Christina Sasse, MD, Mehdi Shajari, MD, Thomas Kohnen, MD, PhD, FEBO
PURPOSE: To analyze the influence of blurred vision, accommodation, and target laser settings on eye movements during laser in situ keratomileusis (LASIK). SETTING: Department of Ophthalmology, Goethe University, Frankfurt, Germany. DESIGN: Prospective randomized study. METHODS: Participants had simulated LASIK treatment. They were instructed to focus on the fixation light; the treatment laser was blocked, and all other settings were applied according to standard LASIK treatments. To simulate blurred vision a 0.0 diopter (D) soft contact lens received a 5.0 D myopia laser treatment and was then applied to the participant’s eye. To diminish accommodation, a second lens that had a refraction of the patient’s spherical equivalent, plus 3.0 D to compensate for accommodation, was used. There were 4 treatment modalities as follows: (1) blurred lens with target laser on, (2) blurred lens with target laser turned off, (3) C3.0 D lens with target laser on, and (4) C3.0 D lens with target laser turned off. Lateral and torsional eye movements were recorded. Fourier analysis was used to obtain temporal power spectra from dynamic eye movements. The Fn criterion was set as the frequency below which n% of eye movements in the cohort occurred (n Z 95%, 80%, and 50%). RESULTS: The study comprised 11 eyes of 11 participants. There was 1 significant difference between the eye movements based on measurement modalities. In 1 variable in the y-axis, there was movement that showed a significant difference in the F80 criterion. CONCLUSION: Surgical circumstances such as blurred vision, accommodation, and target light had little influence on eye movements during LASIK. Financial Disclosure: Proprietary or commercial disclosures are listed after the references. J Cataract Refract Surg 2016; 42:1424–1430 Q 2016 ASCRS and ESCRS
Laser in situ keratomileusis (LASIK) uses 2 discrete treatment steps. The first is a corneal flap cut that is created using a femtosecond laser or microkeratome. The second step is the corneal ablation performed using an excimer laser. A laser target provides live feedback to the surgeon reassembling the excimer laser's position, and indicates that treatment is in progress as well as specifies the ablation zone.1,2 To the patient, the laser target appears as a flickering pattern of red dots during the treatment. To optimize the positioning of the excimer laser ablation spots, eye movements are minimized by showing a laser fixation to patients. The laser fixation of the Amaris 750S excimer laser (Schwind eye-tech-solutions 1424
Q 2016 ASCRS and ESCRS Published by Elsevier Inc.
GmbH and Co. KG) is a blinking light-emitting diode. However, eye movements still occur.3–9 When performing LASIK, 2 measurements are used to record eye movements. First, static eye tracking is performed before surgery after the patient is changed from the seated to supine position. The second measurement, intraoperative eye tracking or dynamic eye-tracking, provides data on the position of the eye in a 3-dimensional (3-D) system to the laser system. Eye tracking software can track eye movements and make the laser follow them or stop if the eye position is too far off. In previous studies, it was shown that torsional misalignment has an effect on visual outcomes after surgery. Cyclotorsion of the ablation
http://dx.doi.org/10.1016/j.jcrs.2016.07.030 0886-3350
EYE-MOVEMENT EFFECTS OF BLURRED VISION, ACCOMMODATION, AND LASER TARGETS DURING LASIK
zone can lead to residual postoperative astigmatism10,11 and be responsible for lower- and higherorder aberrations.3,12–18 Kim and Joo16 and Mrochen et al.19 reported that using an iris-registration system during refractive surgery leads to a better outcome. The treatment method itself can generate several problems that might reduce the efficacy of the LASIK treatment. While the flap is open, the refraction of the cornea is impaired, resulting in blurred vision. The laser fixation is not seen as a punctiform light source but appears rather as a cloud; patients might have difficulty in fixating on a certain point. Blurred vision and the target laser source's distance might stimulate accommodation and thus increase eye movements.5 The aim of this study was to examine the influence of these factors on eye movements by simulating the circumstances of LASIK surgery. We evaluated whether it is possible to minimize intraoperative eye movements by altering surrounding conditions. PARTICIPANTS AND METHODS This prospective case series included myopic eyes of volunteers. The required number of participants was determined by a previous sample-size calculation. Inclusion and exclusion criteria were chosen following the LASIK criteria of the Kommission Refraktive Chirugie (Commission of Refractive Surgery) (KRC).20 According to the KRC criteria, safe LASIK can be performed in eyes with myopia of up to 8.0 diopters (D) or with hyperopia of up to C3.0 D. The limit for astigmatism is 5.0 D. All participants were experienced contact lens wearers and signed a written consent form for the procedure. The study was approved by the Ethical Review Committee of the Medical Faculty of the Goethe University, Frankfurt, Germany. The study adhered to the tenets of the Declaration of Helsinki. The measurements were performed with the Amaris 750S excimer laser's eye tracker. Which eye to be treated (right or left) was randomly selected.
Submitted: December 30, 2014. Final revision submitted: July 19, 2016. Accepted: July 22, 2016. From the Department of Ophthalmology, Goethe-University Frankfurt am Main, Frankfurt, Germany. Data collection was performed in cooperation with Schwind eyetech-solutions GmbH & Co. KG, because evaluated data could not be extracted directly from the system used. Oliver Klaproth helped with initiating the study and helping the authors to obtain approval of the local ethics committee. Corresponding author: Thomas Kohnen, MD, PhD, FEBO, Department of Ophthalmology, Goethe-University Frankfurt, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany. E-mail: [email protected].
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All eyes had a fictional LASIK treatment of 5.0 D myopia without astigmatism. This ensured that every participant was exposed to the same target light pattern and treatment duration. The optical zone was 6.3 mm. Lateral eye movements were recorded with a frequency of 1050 Hz and torsional movements with 30 Hz. The camera has a frame rate of 1050 Hz; every 32nd frame was taken for analysis. These eye movements are referred to as EyeX, EyeY, and Torsion in this study. Imagining the bulb in the center of a 3-D coordinate system, EyeX describes a lateral movement along the x-axis, EyeY refers to lateral movement along the y-axis, and Torsion describes a rotation around the z-axis. During LASIK, eye movements are recorded by dynamic eye tracking, and the excimer laser follows them. The tracking limit for EyeX and EyeY is G1.5 mm for laser ablation. The dynamic cyclotorsion limit is G9 degrees from the beginning of the treatment. The accuracy level for EyeX and EyeY was 55 mm and for Torsion, 2 degrees. The resolution was 4.3 mm for EyeX and EyeY and 0.01 degrees for Torsion. The treatment laser was blocked with a custom barrier created so measurements could be performed without actual treatment. The barrier was made from the laser shutter of the excimer laser with a window for the target laser. To analyze the influence of blurred vision, accommodation, and target light, eye movements were recorded 4 times, every time under a different modality. To simulate blurred vision, a 0.0 D Pure Vision silicon–hydrogel lens (Bausch & Lomb, Inc.) was placed on a glass eye below the excimer laser (at this point not blocked) before the simulated treatments of participants. The lens was then treated for 5.0 D of myopia. Each participant received his or her own lens. All were 0.0 D lenses that had received the same laser treatment before the measurement. It was assumed that patients fixate on the light source as well as on the surrounding area (eg, laser arm), which has a distance of 33 cm; 3.0 D is of a reciprocate distance from a patient's eye to the fixation light. To prevent accommodation from influencing a sharp picture, a second lens that took into consideration the participant's own refraction was used. For example, if the participant's refraction were 1.75 D, a C1.25 D lens was applied. There were 4 modalities: Modality 1 was recorded with a blurred lens and the laser target on. Modality 2 used the same lens as modality 1 with the laser target off. Modality 3 was measured with a lens that caused the patient's vision to become 3.0 D with the laser target on. Modality 4 was recorded with the same lens as modality 3 but with the laser target turned off. The order of the measurement modalities was randomized ex ante. The participants were not made aware of the purpose of the study. They were only instructed to fixate on the fixation light. Before measurement, each participant had an ophthalmologic examination of both eyes. The participants were placed below the eye tracker in a supine position with a cushion holding the participant's head in the proper position. The examined eye was placed under the eye tracker while the other eye was covered. Then, a local anesthesia (oxybuprocaine hydrochloride [Conjuncain]), lid retractor, and the first lens were applied. Artificial tears were used between measurements to keep the cornea moist because the patients were not able to blink. After the first 2 measurements, the lenses were changed; then, measurements 3 and 4 were recorded. After the last measurement, the lens was removed. The participants were instructed to refrain from wearing their own contact lenses for 24 hours because the eye was still numbed by anesthetic. The procedure was finished with
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another ophthalmologic examination to guarantee intactness, especially of the cornea. Next, all premeasurement contact lens treatment data and the measurement data of each participant's eye movements were sent to Schwind eyetech-solutions GmbH and Co. KG to be transposed into a workable file. The raw data, which could not be opened with commonly available programs, was transferred into Excel software (Microsoft Corp.) and then analyzed with SPSS software (SPSS, Inc.).
Statistical Analysis According to BiAS for Windows software (version 11.01, Epsilon-Verlag), 11 subjects had to be included to obtain valid results. The measurement was deemed valid if the eye tracker detected eye movement at least 80% of the time. Fourier analysis was used to obtain power spectra of lateral and torsional eye movements. The F95 criterion was defined as the frequency below which 95% of eye movements occur; F80 was the frequency below which 80% of movements occur; and F50, the frequency below which 50% of movements occur. Fourier analysis could not analyze eye movements above 525 Hz for lateral movements and above 15 Hz for torsional ones.
RESULTS Five right eyes and 6 left eyes were examined. Seven participants were women, 4 were men. The mean age was 25.9 years G 5.9 (SD) (range 23 to 41 years). The mean spherical equivalent refraction was 2.09 G 1.55 D (range 0.5 to 5.0 D). The measurement was deemed valid in all 11 participants. Temporal power spectra showed a dominance of high-frequency, low-power eye movements. Figure 1 shows the temporal power spectra of all participants in modality 2. The Shapiro-Wilk test showed that measurement data were not normally distributed. According to the Friedman test, there was a significant difference in the F80 variable in EyeY (P Z .039). The Wilcoxon test showed that there was a significant difference between modality 3 and modality 4 in this variable (P Z .033). Except for this difference, the
remaining data showed no significant difference in standard deviation (SD) or in the power spectra (Table 1). According to the Friedman test, there was no significant difference in the F95 and F50 frequency. Figures 2 to 4 show the cumulative amplitudes of all 11 participants. DISCUSSION The aim of this study was to compare eye movements under different surgical circumstances and to build a model with the influencing factors altered. We also wanted to determine whether it was possible to construct a surgery environment that reduces eye movements. The Friedman test showed a significant difference in the F80 criterion in EyeY. The F80 frequency in modality 3 was significantly lower than in modality 4. This means that eye movements were faster in modality 4 than in modality 3. Based on the assumption that high-frequency movements have low power, it is possible that the extent of eye movements was smaller in modality 4. This can be shown for EyeY (SD 12.57 mm in modality 3 versus 6.52 mm in modality 4). We have no explanation for the significant difference for only F80 between modality 3 and modality 4; however, with a rather low significance level (P Z .039), it might be a false rejection of the null hypothesis. The Friedman test showed no significant difference, although there was an indication that turning off the laser target could reduce the extent of vertical eye movements. We suggest further studies be performed to compare lateral eye movements with and without a laser target. It might be possible to evaluate this factor in a simulated setting as well as in real surgery circumstances. In this study, 11 eyes of 11 participants were included based on a previous sample-size calculation. Because eyes do not work independently of each other, we decided to include only 1 eye per participant.
Figure 1. Eye movements for EyeX (along x-axis) (A), EyeY (along y-axis) (B), and Torsion (C) for all participants in modality 2. A dominance of high-frequency lowamplitude movements was present.
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Table 1. The mean SD of eye movements in all patients; the frequencies below which 95%, 80%, and 50% of eye movements occurred; and the mean total distance the eyes moved. Parameter Mean SD x-axis (mm) y-axis (mm) Torsion (degrees) Frequency (%) F95X F80X F50X F95Y F80Y F50Y F95T F80T F50T Pupil translation (mm)
Modality 1
Modality 2
8.78 10.73 2.12
11.19 8.66 1.67
327.8 61.57 4.59 391.41 109.98 6.39 11.27 4 0.5 61675.36
339.75 62.45 3.5 399.84 121.81 7.59 11.01 3.43 0.28 57152.67
Modality 3
10.19 12.57 1.65 323.34 46.22 3.88 382.9 89.61* 4.79 11.03 3.48 0.35 57181.59
Modality 4
8.86 6.52 1.25 342.95 68.67 5.25 419.96 160.91* 13.67 11.18 3.73 0.39 55387.58
T Z torsion; X Z x-axis (EyeX); Y Z y-axis (EyeY) *Statistically significant difference
Before dynamic torsional eye tracking became available, static eye tracking was performed immediately before surgery. Today, both measurements are performed. Cyclotorsion during static eye tracking could be explained by the change from a seated to a supine position, as described by Swami et al.,10 Ciccio et al.,14 Kim and Joo,16 and Chernyak,21 independent of visual faculty during surgery. The benefits of using an eye tracker during refractive surgeries have been shown in previous studies.16,19,22 Several studies3,4,23,24 showed that ocular counter roll does not only occur before but also during surgery. Porter et al.5 assumed that the visual effects that occur
during LASIK, such as, blur, might have an effect on eye movements. They performed LASIK in 17 eyes of 9 patients. Eye movements were recorded in 10 of these eyes during surgery with a frequency of 50 Hz. After the surgery, the temporal power spectra were computed from the results (unfortunately only for horizontal and vertical eye movements, not for cyclotorsional ones). They found a mean cyclotorsion of 0.22 G 0.08 degrees with a mean torsion of 1.22 G 0.38 degrees, ranging from 0.55 to 1.96 degrees. Because of the irrelevance of such small torsional values (eg, a torsion of 1.96 degrees in a patient with 1.25 D of cylinder would result in 0.09 D of
Figure 2. Cumulative amplitudes in EyeX (comparison of all 4 modalities).
Figure 3. Cumulative amplitudes in EyeY (comparison of all 4 modalities).
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Figure 4. Cumulative amplitudes in Torsion (comparison of all 4 modalities).
postoperative cylinder), torsional data were not further calculated. As a representative value for the degree of eye movement, the SD was chosen. Small values show a rather steady fixation, whereas high values represent eyes with larger movements and less fixation. Mean values are not adequate to describe eye movements because positive and negative movements compensate each other (mean values for eye movements are expected to vary around 0). Analysis of the temporal power spectra of lateral eye movements (horizontal and vertical) in Porter et al.'s study5 showed a dominance of slow drifts with an F95 of 1.4 Hz for horizontal eye movements and 0.6 Hz for vertical eye movements. As in our study, temporal power spectra showed that eye movements with high power had low frequencies and lowpower movements had high frequencies. Porter et al.5 found that the fixation stability could be influenced by the altered viewing conditions during LASIK, such as blurred vision provoked by the flap cut; this factor might decrease the patient's accuracy in fixating on a certain point. The authors concluded that it is essential to track these eye movements and postulated that the eye movements dictate the elaborateness of the eye tracker. From his somewhat simplified analysis of mean dynamic eye movements, Chang3 concluded that dynamic intraoperative tracking of torsional eye movements is imperative to improve precise astigmatic refractive outcomes after surgery. Prakash et al.4 also showed the appearance of intraoperative cyclotorsion. In that study, only cyclotorsion was calculated. Rather than SD, they used the absolute amount of torsion, namely the difference between the
lowest and highest value in each patient. They assumed that the direction and degree of cyclotorsion recorded during surgery correlate with the presurgery static cyclotorsion values. They also supposed that cyclotorsion increases with the sum of light pulses, meaning that a long treatment might lead to a loss of fixation. In our study, the duration of the measurement was fixed; each patient received a fictional treatment of 5.0 D. Our finding that eye movements occur independent of environmental circumstances prompted the question of whether the performance of the eye tracker in terms of speed and recognition rate is sufficient. This study showed a predication about the quality of eye movements. The analysis of temporal power spectra determined that there are highpower, low-frequency eye movements and lowpower, high-frequency eye movements. It is thus important to track the high-power low-frequency movements in particular because the effect of a decentered ablation zone becomes worse the longer the misalignment between the calculated ablation zone and eye position persists. Fourier analysis showed that 95% of all eye movements in all modalities occurred below the frequency of 420 Hz (x-axis and y-axis movements) and below 12 Hz (torsional eye movements). The Schwind Amaris 750S eye tracker works at a frequency of 1050 Hz for lateral eye movements and 30 Hz for torsional eye movements. Because we found that high-frequency movements have low power, it might be assumed that movements above 420 Hz are small enough to be ignored. This could lead to the conclusion that the measurement frequency might be adequate. In particular, high-power low-frequency eye movements are covered in this frequency range. The question remains whether the extent and type of refractive error might have an influence on eye movements. Furthermore, we were unable to analyze movements along the propagation axis of the laser beam. Future analysis on these variables would be desirable. A further limitation of the study is the frequency of movements in the analyzed cohort. The limiting frequency of human eyes is unknown; therefore, other patients could have higher frequency eye movements that might lead to a different result and conclusion. Finally, our data show that factors such as blur, accommodation, and the laser target have no significant influence on intraoperative lateral and torsional eye movements. This means that even with these factors eradicated, eye movements occur in the same magnitude and frequency as they would if interference were present. Turning off the laser target might have decreased eye movements, at least in EyeY.
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EYE-MOVEMENT EFFECTS OF BLURRED VISION, ACCOMMODATION, AND LASER TARGETS DURING LASIK
WHAT WAS KNOWN Eye movements occur during corneal laser treatments and can impair the visual outcome. Measures to compensate for and reduce eye movements include asking the patient to fixate on the laser beam during LASIK. These measures have been shown to improve the visual outcome. WHAT THIS PAPER ADDS Blurred vision and accommodation, which can occur during the surgical procedure, did not lead to a significant difference in eye movements.
REFERENCES 1. Kohnen T, Strenger A, Klaproth OK. Basic knowledge of refractive surgery; correction of refractive errors using modern surgical procedures. Dtsch Arztebl Int 2008; 105:163–170. quiz 170–172. Available at: http://www.ncbi.nlm.nih.gov/pmc/ar ticles/PMC2696744/pdf/Dtsch_Arztebl_Int-105-0163.pdf. Accessed July 30, 2016 2. Sakimoto T, Rosenblatt MI, Azar DT. Laser eye surgery for refractive errors. Lancet 2006; 367:1432–1447 3. Chang J. Cyclotorsion during laser in situ keratomileusis. J Cataract Refract Surg 2008; 34:1720–1726 4. Prakash G, Ashok Kumar D, Agarwal A, Jacob S, Sarvanan Y, Agarwal A. Predictive factor analysis for successful performance of iris recognition-assisted dynamic rotational eye tracking during laser in situ keratomileusis. Am J Ophthalmol 2010; 149:229–237 5. Porter J, Yoon G, MacRae S, Pan G, Twietmeyer T, Cox IG, Williams DR. Surgeon offsets and dynamic eye movements in laser refractive surgery. J Cataract Refract Surg 2005; 31:2058–2066; erratum 2006; 32:378 6. Martinez-Conde S, Macknik SL, Troncoso XG, Dyar TA. Microsaccades counteract visual fading during fixation. Neuron 2006; 49:297–305. Available at: http://www.cell.com/neuron/pdf/ S0896-6273(05)01056-1.pdf. Accessed July 30, 2016 7. Ditchburn RW, Fender DH, Mayne S. Vision with controlled movements of the retinal image. J Physiol 1959; 145:98–107. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1356943/pdf/ jphysiol01308-0119.pdf. Accessed July 30, 2016 8. Nachmias J. Determiners of the drift of the eye during monocular fixation. J Opt Soc Am 1961; 51:761–766 9. Steinman RM, Cunitz RJ, Timberlake GT, Herman M. Voluntary control of microsaccades during maintained monocular fixation. Science 1967; 155:1577–1579. Available at: http:// www3.psych.purdue.edu/wzpizlo/rsteinma/Bob-FOR%20CV/ Steinman%20etal%201967%20Voluntary%20micros%20SCI ENCE%20report.pdf. Accessed July 30, 2016 10. Swami AU, Steinert RF, Osborne WE, White AA. Rotational malposition during laser in situ keratomileusis. Am J Ophthalmol 2002; 133:561–562
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11. Tjon-Fo-Sang MJ, de Faber J-THN, Kingma C, Beekhuis WH. Cyclotorsion: a possible cause of residual astigmatism in refractive surgery. J Cataract Refract Surg 2002; 28:599–602 €hne C, Cichocki M, Strenger A. Zyklorotation des 12. Kohnen T, Ku € hrter LASIK mit statischem IriserkenAuges bei wellenfrontgefu nungstracker [Cyclorotation of the eye in wavefront-guided LASIK using a static eyetracker with iris recognition]. Ophthalmologe 2007; 104:60–65 € hren J, Yoon G, Kenner S, MacRae S, Huxlin K. The effect of 13. Bu optical zone decentration on lower- and higher-order aberrations after photorefractive keratectomy in a cat model. Invest Ophthalmol Vis Sci 2007; 48:5806–5814. Available at: http://iovs.arvo journals.org/article.aspx?articleidZ2125624. Accessed July 30, 2016 14. Ciccio AE, Durrie DS, Stahl JE, Schwendeman F. Ocular cyclotorsion during customized laser ablation. J Refract Surg 2005; 21:S772–S774 15. Guirao A, Williams DR, Cox IG. Effect of rotation and translation on the expected benefit of an ideal method to correct the eye’s higher-order aberrations. J Opt Soc Am A Opt Image Sci Vis 2001; 18:1003–1015 16. Kim H, Joo C-K. Ocular cyclotorsion according to body position and flap creation before laser in situ keratomileusis. J Cataract Refract Surg 2008; 34:557–561 17. Arba-Mosquera S, Merayo-Lloves J, de Ortueta D. Clinical effects of pure cyclotorsional errors during refractive surgery. Invest Ophthalmol Vis Sci 2008; 49:4828–4836. Available at: http://iovs.arvojournals.org/article.aspx?articleidZ2125382. Accessed July 30, 2016 18. Moreno-Barriuso E, Merayo Lloves J, Marcos S, Navarro R, Llorente L, Barbero S. Ocular aberrations before and after myopic corneal refractive surgery: LASIK-induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci 2001; 42:1396–1403. Available at: http://iovs.arvojournals.org/ article.aspx?articleidZ2162653. Accessed July 30, 2016 €tz W. 19. Mrochen M, Eldine MS, Kaemmerer M, Seiler T, Hu Improvement in photorefractive corneal laser surgery results using an active eye-tracking system. J Cataract Refract Surg 2001; 27:1000–1006 €tssi20. Kohnen T, Neuhann T, Knorz MC. Bewertung und Qualita cherung refraktiv-chirurgischer Eingriffe durch die Deutsche Ophthalmologische Gesellschaft und den Berufsverband der €rzte Deutschlands; Stand January 2014 [Assessment Augena and quality assurance of refractive surgical interventions by the DOG (German Society of Ophthalmology) and the BVA (Professional Association of German Ophthalmologists): Update January 2014]. Ophthalmologe 2014; 111:320–329 21. Chernyak DA. Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery. J Cataract Refract Surg 2004; 30:633–638 22. Ghosh S, Couper TA, Lamoureux E, Jhanji V, Taylor HR, Vajpayee RB. Evaluation of iris recognition system for wavefront-guided laser in situ keratomileusis for myopic astigmatism. J Cataract Refract Surg 2008; 34:215–221 ez J, Brucks M, Zimmerman G, Bekendam P, Bacon G, 23. Narva Schmid K. Intraoperative cyclorotation and pupil centroid shift during LASIK and PRK. J Refract Surg 2012; 28:353–357 24. Febbraro J-L, Koch DD, Khan HN, Saad A, Gatinel D. Detection of static cyclotorsion and compensation for dynamic cyclotorsion in laser in situ keratomileusis. J Cataract Refract Surg 2010; 36:1718–1723
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Financial Disclosure Dr. Kohnen is a consultant to Abbott Medical Optics, Inc., Alcon Laboratories, Inc., Carl Zeiss Meditec AG, Geuder AG, Oculus Optikger€ ate GmbH, Rayner Intraocular Lenses Ltd., Schwind eye-tech-solutions GmbH and Co. KG, Tearlab Corp., Thieme Compliance
GmbH, and Ziemer Ophthalmic Systems AG. He has received grants from Abbott Medical Optics, Inc., Alcon Laboratories, Inc., Avedro, Inc., Carl Zeiss Meditec AG, Oculus Optikger€ate GmbH, and Schwind eyetech-solutions GmbH and Co. KG. No other author has a financial or proprietary interest in any material or method mentioned.
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