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Supplementary effect of static cyclotorsion compensation with dynamic cyclotorsion compensation on the refractive and visual outcomes of laser in situ keratomileusis for myopic astigmatism
ARTICLE
Supplementary effect of static cyclotorsion compensation with dynamic cyclotorsion compensation on the refractive and visual outcomes of laser in situ keratomileusis for myopic astigmatism Minoru Tomita, MD, PhD, Miyuki Watabe, PhD, Satoshi Yukawa, MD, Nobuo Nakamura, MD, PhD, Tadayuki Nakamura, MD, Thomas Magnago, Dipl-Ing (FH)
PURPOSE: To evaluate the add-on effect of static cyclotorsion compensation (SCC) over dynamic cyclotorsion compensation (DCC) on the refractive and visual outcomes in patients having laser in situ keratomileusis (LASIK) for myopic astigmatism. SETTING: Private center, Tokyo, Japan. DESIGN: Comparative study. METHODS: Consecutive patients had LASIK with a target of emmetropia between August 2009 and June 2010. Patients had preoperative myopic astigmatism of 2.0 diopters (D) or greater and more than 3 months of follow-up. Patients had SCC plus DCC treatment (study group) or DCC treatment only (control group). RESULTS: The 2 groups were similar preoperatively in refraction, visual acuity, and higher-order aberrations (HOAs). After treatment, the refractive outcome in the study group was significantly better than in the control group, with a mean sphere of 0.13 D G 0.29 (SD) versus 0.17 G 0.30 D (PZ.009), a mean cylinder of 0.11 G 0.29 D versus 0.19 G 0.36 D (P<.001), and a mean spherical equivalent of 0.07 G 0.29 D versus 0.08 G 0.32 D (PZ.020). Astigmatism vector analysis also yielded better outcomes in the study group. However, the 2 groups were statistically similar in postoperative uncorrected and corrected visual acuities and induced HOAs. The mean static cyclotorsion value in the study group was 2.29 G 1.74 degrees (range 0 to 11.1 degrees). CONCLUSION: The combination of SCC and DCC using an aberration-free aspheric ablation profile produced a statistically significant improvement in astigmatism outcomes. Financial Disclosure: Mr. Magnago is an employee at Schwind eye-tech-solutions GmbH and Co. KG. No other author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2013; 39:752–758 Q 2013 ASCRS and ESCRS
The postoperative outcomes of the correction of astigmatic errors by excimer laser treatments do not reach the same level of efficacy as the correction of spherical errors.1,2 This is believed to be the result of misalignment of the laser ablation caused by the rotational movements of the eye around the visual axis3–7; or, in other words, by the application of the laser on an axis different from the one measured during diagnosis. This results in undercorrection or induction of astigmatism. In addition to rotation of the head and body under the laser 752
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and unmasking of cyclophoria,6 the eye makes 2 types of torsional movements. One, induced by the vestibular system when posture changes from the upright to supine position, is referred to as static cyclotorsion, and the other, which happens during the laser ablation procedure, is called dynamic cyclotorsion.1,3,8 A comparison of static cyclotorsion and dynamic cyclotorsion in the published literature shows a similar magnitude; the mean static cyclotorsion in previous studies ranges from 1.22 to 4.1 degrees4,6 and the 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2012.11.030
SUPPLEMENTARY EFFECT OF STATIC OVER DYNAMIC CYCLOTORSION COMPENSATION IN LASIK
mean dynamic cyclotorsion from 1.32 to 3.81 degrees.1,9,10 When ocular cyclotorsion of more than 2 degrees occurs and is not corrected, astigmatism correction can be influenced and significant aberrations can be induced by the excimer laser treatment.3,11 Theoretically, a 4-degree misalignment would result in an undercorrection of 14%, a 6-degree misalignment in an undercorrection of 20%, and a 16-degree misalignment in an undercorrection 50%.4,6 Therefore, it is essential that the laser ablation algorithm be adjusted to compensate for both dynamic cyclotorsion and static cyclotorsion components. Iris registration–based eye trackers have been used to manage dynamic cyclotorsion. Recently, these systems have been adapted to interact with diagnostic equipment to measure and compensate for static cyclotorsion.12,13 Although the compensation of static cyclotorsion can be performed using manual registration based on scleral marking performed at the slitlamp and subsequent alignment with the laser microscope reticule when the patient is supine,3,6 these automated ablation algorithms allow improved patient comfort, reduced surgeon time, and better standardization of static cyclotorsion compensation (SCC). This study compared the visual and refractive outcomes of aberration-free aspheric excimer laser profile with or without automated SCC in patients with myopic astigmatism of 2.0 diopters (D) or greater.
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approved by the institutional review board and fulfilled the principles of the Declaration of Helsinki. Patients in the first half of the study had treatment with dynamic cyclotorsion compensation (DCC) only and formed the control group. After the SCC protocol was implemented, patients in the second half of the study had treatment with SCC plus DCC and constituted the study group. Other than the supplementary use of SCC in the study group, there was no change in any other variable between the study group and the control group. Preoperatively, patients had a full ophthalmic examination. This included Snellen visual acuity assessment, subjective manifest and cycloplegic refractions, corneal topography, and corneal pachymetry. After LASIK, the patients were examined at 1 day, 1 week, and 3 months. Study parameters included uncorrected visual acuity (UDVA), CDVA, manifest refraction, Jackson (J)0 and J45 power vectors, and higher-order aberrations (HOAs).
Surgical Technique All eyes had LASIK by the same surgeon (M.T.) using the Schwind Amaris aberration-free aspheric ablation profile (Schwind eye-tech-solutions GmbH and Co. KG). The corneal flaps were created using a Femto LDV femtosecond laser (Ziemer Group AG). Calculation of the static cyclotorsion was based on comparisons of the iris G limbus image obtained from the corneal wavefront analyzer diagnostic equipment (Keratron Scout, Schwind eye-tech-solutions GmbH and Co. KG) with the patient in the upright position. The image taken with the excimer laser camera was with the patient supine.
Static and Dynamic Cyclotorsion Compensation PATIENTS AND METHODS This retrospective comparative case series comprised eyes of consecutive patients who had laser in situ keratomileusis (LASIK) between August 2009 and June 2010 for preoperative myopic astigmatism of 2.0 D or greater and a corrected distance visual acuity (CDVA) of 20/20 or better and who were available for the 3-month follow-up. The study was
Submitted: July 12, 2012. Final revision submitted: November 24, 2012. Accepted: November 28, 2012. From the Shinagawa LASIK Center (Tomita, Watabe, Yukawa, N. Nakamura, T. Nakamura), Chiyoda-ku, Tokyo, Japan, and Schwind eye-tech-solutions GmbH and Co. KG (Magnago), Kleinostheim, Germany. Raman Bedi, MD, critically reviewed the manuscript. IrisARC– Analytics, Research & Consulting, Chandigarh, India, provided statistical analysis, vector analysis, and editorial assistance in the preparation of the manuscript. Presented at the 26th Congress of the Asia-Pacific Academy of Ophthalmology, Sydney, Australia, March 2011. Corresponding author: Minoru Tomita, MD, PhD, Shinagawa LASIK Center, Yurakucho ITOCiA 14F, 2-7-1 Yurakucho, Chiyoda-ku, Tokyo 100-0006, Japan. E-mail: [email protected].
The Schwind Amaris excimer laser has a 500 Hz ablation rate and compensates for 5 movements of the eye for ablation control. The eye tracker works at 1050 Hz for lateral movements as well as for pupil, iris, and limbus tracking with a latency time of 1 to 3 milliseconds depending on the pulse positioning from 1 pulse to the next pulse and other relevant parameters. It compensates for linear eye movements of the eye; that is, up, down, left, and right, which are the first and second dimensions; rolling eye movements, which are the third and fourth dimensions of the eye; and torsional movements around the visual axis, which are known as cyclotorsion (fifth-dimension eye movements).8 The term dimensions includes the degrees of freedom as well as the lateral movements. The DCC system tracks the torsional movements of the eye during ablation, whereas the SCC system compensates for the torsional rotation of the eye between the upright position (during evaluation with diagnostic equipment) and the supine position (during laser ablation). The SCC system allows the laser ablation algorithm to interact with the preoperative diagnostic equipment. Thus, it compares the reference picture of the patient’s eye in the upright position obtained during the preoperative workup with the image taken from the excimer laser camera with the patient supine to estimate the cyclotorsion offset. The laser computer algorithm searches for landmarks on the iris starting at the pupil and moves spirally outward until the image is completely scanned or the number of prerequisite important points is reached. To improve the robustness of iris-registration data, the SCC algorithm modulates the
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illumination to match the ambient levels at the time of the preoperative workup to obtain a similarly sized pupil. If enough registration points are not available on the iris, the scan automatically spirals out to the limbus and searches for additional registration points on the limbus. Before the treatment starts, the advanced SCC algorithm of the laser compares the 2 images, superimposes the important landmarks, and calculates the angle of rotation. The resulting angle is used for the correction of static cyclotorsion in the ablation algorithm up to a maximum compensation angle of G12.5 degrees. The cyclotorsion compensation rotates each excimer laser pulse according to the concurrently measured dynamic cyclotorsion angle and formerly registered static cyclotorsion angle.
Statistical Analysis Statistical analysis was performed using SPSS software (SPSS, Inc.). Snellen visual acuity was converted to logMAR units for statistical analysis. Differences between groups were assessed using the chi-square test for categorical variables and independent t test or Mann-Whitney U test for continuous variables, as appropriate, after testing for normal distributions. A statistically significant difference was based on a 2-tailed a equal to 0.05 (P!.05). Vector analysis was performed as proposed by Thibos and Horner.14 With this decomposition method, any spherocylindrical error, expressed as sphere, cylinder, and axis, can be converted into a set of 3 power vectors (M, J0, J45), represented geometrically as the (x, y, z) coordinates of a point in a 3-dimensional dioptric space in a way that each of the 3 fundamental components is mathematically independent of the others. The components are represented by the following formulas: M Z S C C/2; J0 Z ( C/2) cos(2a); J45 Z ( C/2) sin(2a).
RESULTS The study comprised 735 eyes of 603 patients, 363 eyes of 303 patients in the study group and 372 eyes of 300 patients in the control group. Table 1 compares the preoperative refractive and visual acuity variables; there was no statistically significant difference in any variable. Refraction and Visual Acuity After treatment, the study group had a significantly better refractive status (sphere, cylinder, SE, and vector J0) than the control group (Table 1). However, the 2 groups were statistically similar in postoperative UDVA, CDVA, and vector J45 (Table 1). All eyes in both groups had a postoperative UDVA of 20/20 or better. Three months postoperatively, there was no statistically significant difference in induced corneal or ocular HOAs at a 4.0 or 6.0 mm diameter between the 2 groups. Figure 1 shows the predictability of the treatment in the 2 groups. The astigmatism correction was better in the study group, which was confirmed by vector analysis of the achieved versus attempted correction of power vectors J0 and J45 (Figure 2). The refractive
Table 1. Comparison of visual and refractive outcomes between groups. Mean G SD Parameter Preoperative Age UDVA (logMAR) CDVA (logMAR) Sphere (D) Cylinder (D) SE (D) Vector J0 (D) Vector J45 (D) 3 mo postoperative UDVA (logMAR) CDVA (logMAR) Sphere (D) Cylinder (D) SE (D) Vector J0 (D) Vector J45 (D)
Study Group Control Group (n Z 363) (n Z 372) P Value 34.28 G 7.95 1.17 G 0.26 0.16 G 0.06 3.82 G 1.79 2.52 G 0.59 5.09 G 1.76 1.12 G 0.56 0.02 G 0.32
34.44 G 7.66 1.17 G 0.27 0.16 G 0.06 3.85 G 1.81 2.50 G 0.57 5.10 G 1.78 1.11 G 0.55 0.02 G 0.34
.89 .848 .193 .932 .796 .962 .443 .561
0.16 G 0.08 0.18 G 0.06 0.13 G 0.29 0.11 G 0.29 0.07 G 0.29 0.04 G 0.13 0.00 G 0.08
0.15 G 0.08 0.18 G 0.06 0.17 G 0.30 0.19 G 0.36 0.08 G 0.32 0.06 G 0.16 0.01 G 0.11
.16 .901 .009 !.001 .02 .033 .358
CDVA Z corrected distance visual acuity; J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SE Z spherical equivalent; UDVA Z uncorrected distance visual acuity
outcomes within the attempted correction for SE were better in the study group (Figure 3, top). Similarly, the analysis of frequency distribution of residual cylinder in the 2 groups showed better outcomes in the study group (Figure 3, bottom). The study group also had a higher percentage of eyes with a postoperative CDVA of 20/20 or better (Figure 4). Distribution of the Compensated Cyclotorsion The mean static cyclotorsion values recorded and compensated for by the system in the SCC group was 2.29 G 1.74 degrees (range 0 to 11.1 degrees). The SCC values were greater than 3 degrees in 104 eyes (28.7%) and greater than 4 degrees in 47 eyes (13%) (Figure 5). Two hundred two eyes (55.6%) had excyclotorsion (mean 2.46 G 1.75 degrees), 157 eyes (43.3%) had incyclotorsion (mean 2.13 G 1.69 degrees), and 4 eyes (1.1%) had no static cyclotorsion. Axis Rotation in Eyes with Manifest Residual Cylinder of More Than 0.25 Diopters Three hundred six eyes (84.3%) in the study group and 276 eyes (74.2%) in the control group had no residual cylinder. Of the eyes with residual cylinder, there was significantly lower axis rotation in the study group (mean 2.64 G 9.54 degrees) than in the control
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Figure 1. Achieved versus attempted correction of SE (left) and astigmatism (right) demonstrating the add-on effect of SCC over the control group (SCC Z static cyclotorsion compensation).
group (mean 4.88 G 12.37 degrees) (PZ.006) (Figure 6, top). Comparison of the frequency distribution of axis rotation in eyes with residual cylinder more than 0.25 D also showed better outcomes in the study group (Figure 6, bottom). DISCUSSION The precise alignment of the ablation to the intended location on the cornea is vital to the accuracy of refractive outcomes.1 In addition to other influences, cyclotorsion of the eye is an important factor in misalignment. It has been reported that as many as 68% of eyes rotate more than 2 degrees from the seated position until the end of laser treatment (static cyclotorsion and dynamic cyclotorsion, respectively).4 When ocular cyclotorsion of more than 2 degrees occurs and is not corrected, astigmatism correction can be influenced and significant aberrations can be induced by the excimer laser treatment.3,11 It is, therefore, important to measure cyclotorsion movements and compensate for them using the laser ablation algorithm. Independent measurements of dynamic cyclotorsion and static cyclotorsion show that correction of static
cyclotorsion is as important as correction of dynamic cyclotorsion. For example, Febbraro et al.9 report dynamic cyclotorsion of more than 5 degrees in 34% of eyes, while Neuhann et al.1 and Chernyak15 report static cyclotorsion of more than 5 degrees in 30% of eyes and 21% of eyes, respectively. In fact, the maximum static cyclotorsion is reported to be as high as 17.5 degrees in a previous study.4 It is therefore imperative to measure and compensate for static cyclotorsion in addition to dynamic cyclotorsion. The aim of this study was to evaluate the effect of SCC plus DCC on refractive outcomes in eyes with moderate to high astigmatism using an aspheric excimer laser ablation profile. The 2 groups in this study (SCC plus DCC [study] and DCC alone [control]) were comparable preoperatively in visual acuity and manifest refraction, with no statistical difference in SE or astigmatism (cylinder, J0, and J45). Both groups received comparable optimized aspheric ablation profiles according to the manifest refraction and (with incorporation of cycloplegic refraction where appropriate) any dynamic cyclotorsion during the procedure was automatically compensated for by the eye tracker. Thus, any differences in outcomes could be attributed to the use of SCC.
Figure 2. Achieved versus attempted correction of vector J0 (left) and vector J45 (right) (J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SCC Z static cyclotorsion compensation). J CATARACT REFRACT SURG - VOL 39, MAY 2013
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Figure 4. Comparison of preoperative CDVA and postoperative UDVA between the study group and the control group (CDVA Z corrected distance visual acuity; SCC Z static cyclotorsion compensation; UDVA Z uncorrected distance visual acuity).
Figure 3. Comparison of the refractive outcomes between the SCC group (SCC plus DCC treatment) versus the control group (DCC only treatment). Top: Percentage of eyes within attempted SE. Bottom: Frequency distribution of eyes with residual cylinder of 0.25 D or more.
Previous studies found a higher magnitude of static cyclotorsion (eg, Ciccio et al.,4 4.05 G 2.9 degrees; Neuhann et al.,1 3.96 G 2.96 degrees; Chang,3 2.7 G 2.4 degrees; Park et al.,16 2.58 G 1.56 degrees for incyclotorsion and 2.94 G 1.87 degrees for excyclotorsion). This may be due to the ethnic factors10 or a different workflow. Theoretical analyses suggest that this mean amount of static cyclotorsion would account
We found a statistically significant improvement in the refractive parameters (sphere, cylinder, SE) in the study group. The scatterplot between the attempted versus the achieved cylinder and the comparison of distribution of residual cylinder also showed better outcomes with the use of the SCC protocol. Although the mean postoperative UDVA and CDVA were better in the study group, the difference was not statistically significant. It could possibly be due to the lower magnitude of static cyclotorsion in our study population (mean 2.29 G 1.74 degrees; range 0 to 11.1 degrees), which decreased the difference between the study group and the control group.
Figure 5. Distribution of static cyclotorsion in the SCC group.
Figure 6. Uncorrected cyclotorsion astigmatism analysis. Top: Mean axis rotation from preoperative to 3-month postoperative visit. Bottom: Absolute rotation of manifest axis for eyes with residual manifest cylinder more than or equal to 0.25 D (SCC Z static cyclotorsion compensation).
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Figure 7. Comparison of preoperative and postoperative mean J0 (left) and mean J45 (right) vectors between the SCC and the control group (J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SCC Z static cyclotorsion compensation).
for a 14% undercorrection of astigmatism, which would increase further with larger angles of static cyclotorsion.4,6 It is therefore desirable to use cyclotorsion-compensation protocols, dynamic as well as static, to achieve the best astigmatic refractive outcomes, especially because both components cannot be calculated for an individual patient in advance. Precise torsional alignment gains more importance in wavefront-guided treatments, which must be performed with torsional precision of approximately 1 degree or better17 because higher cyclotorsion can induce substantial HOAs and reduce retinal image quality.15,18,19 Vector analysis of the J0 and J45 components of astigmatism showed that the respective components were comparable preoperatively between the study group and the control group. In the comparison of postoperative values, although there was significantly lower residual postoperative J0 vector in the study group than in the control group, the postoperative J45 vector was statistically similar in the 2 groups (Figure 7). Although significantly less than in the control group, residual astigmatism (cylinder, J0, and J45) in the study group indicates that a few cyclotorsion errors were uncompensated for in the study group as well. This could be attributed to the resolution and accuracy of the diagnosis and laser image, possible misalignment of the scanner to the camera, possible misalignment of the manifest astigmatism to the topography analysis, or other, yet to be studied factors affecting excimer laser refractive outcomes. The retrospective design and short duration of follow-up may be viewed as the limitations of our study. Future prospective studies should evaluate the effect of cyclotorsion compensation in wavefrontguided ablation profiles. We conclude that the supplementary use of SCC with DCC using an aberration-free aspheric ablation
profile produces a statistically significant improvement in astigmatism outcomes. WHAT WAS KNOWN Uncompensated for static cyclotorsion and dynamic cyclotorsion are important factors contributing to less than optimum results in the correction of astigmatism. Automated dynamic cyclotorsion compensation modules significantly improve the outcomes of astigmatism correction. Recent technological advancements allow laser platforms to interact with diagnostic equipment to automatically measure and compensate for static cyclotorsion. WHAT THIS PAPER ADDS Supplementary use of automated SCC in addition to the use of DCC statistically improves astigmatism correction outcomes. This shows that static cyclotorsion should be compensated for, especially in patients with higher amount of astigmatism, to achieve better refractive outcomes.
REFERENCES 1. Neuhann IM, Lege BAM, Bauer M, Hassel JM, Hilger A, Neuhann TF. Static and dynamic rotational eye tracking during LASIK treatment of myopic astigmatism with the Zyoptix laser platform and Advanced Control Eye Tracker. J Refract Surg 2010; 26:17–27 2. Uc‚akhan OO. Laser in situ keratomileusis for compound myopic astigmatism using the Meditec MEL 70 G-Scan excimer laser. J Refract Surg 2003; 19:124–130 3. Chang J. Cyclotorsion during laser in situ keratomileusis. J Cataract Refract Surg 2008; 34:1720–1726 4. Ciccio AE, Durrie DS, Stahl JE, Schwendeman F. Ocular cyclotorsion during customized laser ablation. J Refract Surg 2005; 21:S772–S774
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5. Hori-Komai Y, Sakai C, Toda I, Ito M, Yamamoto T, Tsubota K. Detection of cyclotorsional rotation during excimer laser ablation in LASIK. J Refract Surg 2007; 23:911–915 6. Swami AU, Steinert RF, Osborne WE, White AA. Rotational malposition during laser in situ keratomileusis. Am J Ophthalmol 2002; 133:561–562 7. 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 8. Aslanides IM, Toliou G, Padroni S, Arba Mosquera S, Kolli S. The effect of static cyclotorsion compensation on refractive and visual outcomes using the Schwind Amaris laser platform for the correction of high astigmatism. Cont Lens Anterior Eye 2011; 34:114–120 9. 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 10. 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 11. 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://www.iovs.org/cgi/reprint/49/11/4828. Accessed December 15, 2012 12. Arba Mosquera S, Arbelaez MC. Use of a six-dimensional eyetracker in corneal laser refractive surgery with the SCHWIND AMARIS TotalTech laser. J Refract Surg 2011; 27:582–590
13. Arba-Mosquera S, Arbelaez MC. Three-month clinical outcomes with static and dynamic cyclotorsion correction using the SCHWIND AMARIS. Cornea 2011; 30:951–957 14. Thibos LN, Horner D. Power vector analysis of the optical outcome of refractive surgery. J Cataract Refract Surg 2001; 27:80–85 15. Chernyak DA. Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery. J Cataract Refract Surg 2004; 30:633–638 16. Park SH, Kim M, Joo C-K. Measurement of pupil centroid shift and cyclotorsional displacement using iris registration. Ophthalmologica 2009; 223:166–171 17. Bueeler M, Mrochen M, Seiler T. Maximum permissible torsional misalignment in aberration-sensing and wavefront-guided corneal ablation. J Cataract Refract Surg 2004; 30:17–25 18. 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 19. Venter J. Outcomes of myopic LASIK with and without NIDEK active torsion error correction. J Refract Surg 2009; 25:985–990
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First author: Minoru Tomita, MD, PhD Private practice, Chiyoda-ku, Tokyo, Japan
Supplementary effect of static cyclotorsion compensation with dynamic cyclotorsion compensation on the refractive and visual outcomes of laser in situ keratomileusis for myopic astigmatism Minoru Tomita, MD, PhD, Miyuki Watabe, PhD, Satoshi Yukawa, MD, Nobuo Nakamura, MD, PhD, Tadayuki Nakamura, MD, Thomas Magnago, Dipl-Ing (FH)
PURPOSE: To evaluate the add-on effect of static cyclotorsion compensation (SCC) over dynamic cyclotorsion compensation (DCC) on the refractive and visual outcomes in patients having laser in situ keratomileusis (LASIK) for myopic astigmatism. SETTING: Private center, Tokyo, Japan. DESIGN: Comparative study. METHODS: Consecutive patients had LASIK with a target of emmetropia between August 2009 and June 2010. Patients had preoperative myopic astigmatism of 2.0 diopters (D) or greater and more than 3 months of follow-up. Patients had SCC plus DCC treatment (study group) or DCC treatment only (control group). RESULTS: The 2 groups were similar preoperatively in refraction, visual acuity, and higher-order aberrations (HOAs). After treatment, the refractive outcome in the study group was significantly better than in the control group, with a mean sphere of 0.13 D G 0.29 (SD) versus 0.17 G 0.30 D (PZ.009), a mean cylinder of 0.11 G 0.29 D versus 0.19 G 0.36 D (P<.001), and a mean spherical equivalent of 0.07 G 0.29 D versus 0.08 G 0.32 D (PZ.020). Astigmatism vector analysis also yielded better outcomes in the study group. However, the 2 groups were statistically similar in postoperative uncorrected and corrected visual acuities and induced HOAs. The mean static cyclotorsion value in the study group was 2.29 G 1.74 degrees (range 0 to 11.1 degrees). CONCLUSION: The combination of SCC and DCC using an aberration-free aspheric ablation profile produced a statistically significant improvement in astigmatism outcomes. Financial Disclosure: Mr. Magnago is an employee at Schwind eye-tech-solutions GmbH and Co. KG. No other author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2013; 39:752–758 Q 2013 ASCRS and ESCRS
The postoperative outcomes of the correction of astigmatic errors by excimer laser treatments do not reach the same level of efficacy as the correction of spherical errors.1,2 This is believed to be the result of misalignment of the laser ablation caused by the rotational movements of the eye around the visual axis3–7; or, in other words, by the application of the laser on an axis different from the one measured during diagnosis. This results in undercorrection or induction of astigmatism. In addition to rotation of the head and body under the laser 752
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and unmasking of cyclophoria,6 the eye makes 2 types of torsional movements. One, induced by the vestibular system when posture changes from the upright to supine position, is referred to as static cyclotorsion, and the other, which happens during the laser ablation procedure, is called dynamic cyclotorsion.1,3,8 A comparison of static cyclotorsion and dynamic cyclotorsion in the published literature shows a similar magnitude; the mean static cyclotorsion in previous studies ranges from 1.22 to 4.1 degrees4,6 and the 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2012.11.030
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mean dynamic cyclotorsion from 1.32 to 3.81 degrees.1,9,10 When ocular cyclotorsion of more than 2 degrees occurs and is not corrected, astigmatism correction can be influenced and significant aberrations can be induced by the excimer laser treatment.3,11 Theoretically, a 4-degree misalignment would result in an undercorrection of 14%, a 6-degree misalignment in an undercorrection of 20%, and a 16-degree misalignment in an undercorrection 50%.4,6 Therefore, it is essential that the laser ablation algorithm be adjusted to compensate for both dynamic cyclotorsion and static cyclotorsion components. Iris registration–based eye trackers have been used to manage dynamic cyclotorsion. Recently, these systems have been adapted to interact with diagnostic equipment to measure and compensate for static cyclotorsion.12,13 Although the compensation of static cyclotorsion can be performed using manual registration based on scleral marking performed at the slitlamp and subsequent alignment with the laser microscope reticule when the patient is supine,3,6 these automated ablation algorithms allow improved patient comfort, reduced surgeon time, and better standardization of static cyclotorsion compensation (SCC). This study compared the visual and refractive outcomes of aberration-free aspheric excimer laser profile with or without automated SCC in patients with myopic astigmatism of 2.0 diopters (D) or greater.
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approved by the institutional review board and fulfilled the principles of the Declaration of Helsinki. Patients in the first half of the study had treatment with dynamic cyclotorsion compensation (DCC) only and formed the control group. After the SCC protocol was implemented, patients in the second half of the study had treatment with SCC plus DCC and constituted the study group. Other than the supplementary use of SCC in the study group, there was no change in any other variable between the study group and the control group. Preoperatively, patients had a full ophthalmic examination. This included Snellen visual acuity assessment, subjective manifest and cycloplegic refractions, corneal topography, and corneal pachymetry. After LASIK, the patients were examined at 1 day, 1 week, and 3 months. Study parameters included uncorrected visual acuity (UDVA), CDVA, manifest refraction, Jackson (J)0 and J45 power vectors, and higher-order aberrations (HOAs).
Surgical Technique All eyes had LASIK by the same surgeon (M.T.) using the Schwind Amaris aberration-free aspheric ablation profile (Schwind eye-tech-solutions GmbH and Co. KG). The corneal flaps were created using a Femto LDV femtosecond laser (Ziemer Group AG). Calculation of the static cyclotorsion was based on comparisons of the iris G limbus image obtained from the corneal wavefront analyzer diagnostic equipment (Keratron Scout, Schwind eye-tech-solutions GmbH and Co. KG) with the patient in the upright position. The image taken with the excimer laser camera was with the patient supine.
Static and Dynamic Cyclotorsion Compensation PATIENTS AND METHODS This retrospective comparative case series comprised eyes of consecutive patients who had laser in situ keratomileusis (LASIK) between August 2009 and June 2010 for preoperative myopic astigmatism of 2.0 D or greater and a corrected distance visual acuity (CDVA) of 20/20 or better and who were available for the 3-month follow-up. The study was
Submitted: July 12, 2012. Final revision submitted: November 24, 2012. Accepted: November 28, 2012. From the Shinagawa LASIK Center (Tomita, Watabe, Yukawa, N. Nakamura, T. Nakamura), Chiyoda-ku, Tokyo, Japan, and Schwind eye-tech-solutions GmbH and Co. KG (Magnago), Kleinostheim, Germany. Raman Bedi, MD, critically reviewed the manuscript. IrisARC– Analytics, Research & Consulting, Chandigarh, India, provided statistical analysis, vector analysis, and editorial assistance in the preparation of the manuscript. Presented at the 26th Congress of the Asia-Pacific Academy of Ophthalmology, Sydney, Australia, March 2011. Corresponding author: Minoru Tomita, MD, PhD, Shinagawa LASIK Center, Yurakucho ITOCiA 14F, 2-7-1 Yurakucho, Chiyoda-ku, Tokyo 100-0006, Japan. E-mail: [email protected].
The Schwind Amaris excimer laser has a 500 Hz ablation rate and compensates for 5 movements of the eye for ablation control. The eye tracker works at 1050 Hz for lateral movements as well as for pupil, iris, and limbus tracking with a latency time of 1 to 3 milliseconds depending on the pulse positioning from 1 pulse to the next pulse and other relevant parameters. It compensates for linear eye movements of the eye; that is, up, down, left, and right, which are the first and second dimensions; rolling eye movements, which are the third and fourth dimensions of the eye; and torsional movements around the visual axis, which are known as cyclotorsion (fifth-dimension eye movements).8 The term dimensions includes the degrees of freedom as well as the lateral movements. The DCC system tracks the torsional movements of the eye during ablation, whereas the SCC system compensates for the torsional rotation of the eye between the upright position (during evaluation with diagnostic equipment) and the supine position (during laser ablation). The SCC system allows the laser ablation algorithm to interact with the preoperative diagnostic equipment. Thus, it compares the reference picture of the patient’s eye in the upright position obtained during the preoperative workup with the image taken from the excimer laser camera with the patient supine to estimate the cyclotorsion offset. The laser computer algorithm searches for landmarks on the iris starting at the pupil and moves spirally outward until the image is completely scanned or the number of prerequisite important points is reached. To improve the robustness of iris-registration data, the SCC algorithm modulates the
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illumination to match the ambient levels at the time of the preoperative workup to obtain a similarly sized pupil. If enough registration points are not available on the iris, the scan automatically spirals out to the limbus and searches for additional registration points on the limbus. Before the treatment starts, the advanced SCC algorithm of the laser compares the 2 images, superimposes the important landmarks, and calculates the angle of rotation. The resulting angle is used for the correction of static cyclotorsion in the ablation algorithm up to a maximum compensation angle of G12.5 degrees. The cyclotorsion compensation rotates each excimer laser pulse according to the concurrently measured dynamic cyclotorsion angle and formerly registered static cyclotorsion angle.
Statistical Analysis Statistical analysis was performed using SPSS software (SPSS, Inc.). Snellen visual acuity was converted to logMAR units for statistical analysis. Differences between groups were assessed using the chi-square test for categorical variables and independent t test or Mann-Whitney U test for continuous variables, as appropriate, after testing for normal distributions. A statistically significant difference was based on a 2-tailed a equal to 0.05 (P!.05). Vector analysis was performed as proposed by Thibos and Horner.14 With this decomposition method, any spherocylindrical error, expressed as sphere, cylinder, and axis, can be converted into a set of 3 power vectors (M, J0, J45), represented geometrically as the (x, y, z) coordinates of a point in a 3-dimensional dioptric space in a way that each of the 3 fundamental components is mathematically independent of the others. The components are represented by the following formulas: M Z S C C/2; J0 Z ( C/2) cos(2a); J45 Z ( C/2) sin(2a).
RESULTS The study comprised 735 eyes of 603 patients, 363 eyes of 303 patients in the study group and 372 eyes of 300 patients in the control group. Table 1 compares the preoperative refractive and visual acuity variables; there was no statistically significant difference in any variable. Refraction and Visual Acuity After treatment, the study group had a significantly better refractive status (sphere, cylinder, SE, and vector J0) than the control group (Table 1). However, the 2 groups were statistically similar in postoperative UDVA, CDVA, and vector J45 (Table 1). All eyes in both groups had a postoperative UDVA of 20/20 or better. Three months postoperatively, there was no statistically significant difference in induced corneal or ocular HOAs at a 4.0 or 6.0 mm diameter between the 2 groups. Figure 1 shows the predictability of the treatment in the 2 groups. The astigmatism correction was better in the study group, which was confirmed by vector analysis of the achieved versus attempted correction of power vectors J0 and J45 (Figure 2). The refractive
Table 1. Comparison of visual and refractive outcomes between groups. Mean G SD Parameter Preoperative Age UDVA (logMAR) CDVA (logMAR) Sphere (D) Cylinder (D) SE (D) Vector J0 (D) Vector J45 (D) 3 mo postoperative UDVA (logMAR) CDVA (logMAR) Sphere (D) Cylinder (D) SE (D) Vector J0 (D) Vector J45 (D)
Study Group Control Group (n Z 363) (n Z 372) P Value 34.28 G 7.95 1.17 G 0.26 0.16 G 0.06 3.82 G 1.79 2.52 G 0.59 5.09 G 1.76 1.12 G 0.56 0.02 G 0.32
34.44 G 7.66 1.17 G 0.27 0.16 G 0.06 3.85 G 1.81 2.50 G 0.57 5.10 G 1.78 1.11 G 0.55 0.02 G 0.34
.89 .848 .193 .932 .796 .962 .443 .561
0.16 G 0.08 0.18 G 0.06 0.13 G 0.29 0.11 G 0.29 0.07 G 0.29 0.04 G 0.13 0.00 G 0.08
0.15 G 0.08 0.18 G 0.06 0.17 G 0.30 0.19 G 0.36 0.08 G 0.32 0.06 G 0.16 0.01 G 0.11
.16 .901 .009 !.001 .02 .033 .358
CDVA Z corrected distance visual acuity; J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SE Z spherical equivalent; UDVA Z uncorrected distance visual acuity
outcomes within the attempted correction for SE were better in the study group (Figure 3, top). Similarly, the analysis of frequency distribution of residual cylinder in the 2 groups showed better outcomes in the study group (Figure 3, bottom). The study group also had a higher percentage of eyes with a postoperative CDVA of 20/20 or better (Figure 4). Distribution of the Compensated Cyclotorsion The mean static cyclotorsion values recorded and compensated for by the system in the SCC group was 2.29 G 1.74 degrees (range 0 to 11.1 degrees). The SCC values were greater than 3 degrees in 104 eyes (28.7%) and greater than 4 degrees in 47 eyes (13%) (Figure 5). Two hundred two eyes (55.6%) had excyclotorsion (mean 2.46 G 1.75 degrees), 157 eyes (43.3%) had incyclotorsion (mean 2.13 G 1.69 degrees), and 4 eyes (1.1%) had no static cyclotorsion. Axis Rotation in Eyes with Manifest Residual Cylinder of More Than 0.25 Diopters Three hundred six eyes (84.3%) in the study group and 276 eyes (74.2%) in the control group had no residual cylinder. Of the eyes with residual cylinder, there was significantly lower axis rotation in the study group (mean 2.64 G 9.54 degrees) than in the control
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Figure 1. Achieved versus attempted correction of SE (left) and astigmatism (right) demonstrating the add-on effect of SCC over the control group (SCC Z static cyclotorsion compensation).
group (mean 4.88 G 12.37 degrees) (PZ.006) (Figure 6, top). Comparison of the frequency distribution of axis rotation in eyes with residual cylinder more than 0.25 D also showed better outcomes in the study group (Figure 6, bottom). DISCUSSION The precise alignment of the ablation to the intended location on the cornea is vital to the accuracy of refractive outcomes.1 In addition to other influences, cyclotorsion of the eye is an important factor in misalignment. It has been reported that as many as 68% of eyes rotate more than 2 degrees from the seated position until the end of laser treatment (static cyclotorsion and dynamic cyclotorsion, respectively).4 When ocular cyclotorsion of more than 2 degrees occurs and is not corrected, astigmatism correction can be influenced and significant aberrations can be induced by the excimer laser treatment.3,11 It is, therefore, important to measure cyclotorsion movements and compensate for them using the laser ablation algorithm. Independent measurements of dynamic cyclotorsion and static cyclotorsion show that correction of static
cyclotorsion is as important as correction of dynamic cyclotorsion. For example, Febbraro et al.9 report dynamic cyclotorsion of more than 5 degrees in 34% of eyes, while Neuhann et al.1 and Chernyak15 report static cyclotorsion of more than 5 degrees in 30% of eyes and 21% of eyes, respectively. In fact, the maximum static cyclotorsion is reported to be as high as 17.5 degrees in a previous study.4 It is therefore imperative to measure and compensate for static cyclotorsion in addition to dynamic cyclotorsion. The aim of this study was to evaluate the effect of SCC plus DCC on refractive outcomes in eyes with moderate to high astigmatism using an aspheric excimer laser ablation profile. The 2 groups in this study (SCC plus DCC [study] and DCC alone [control]) were comparable preoperatively in visual acuity and manifest refraction, with no statistical difference in SE or astigmatism (cylinder, J0, and J45). Both groups received comparable optimized aspheric ablation profiles according to the manifest refraction and (with incorporation of cycloplegic refraction where appropriate) any dynamic cyclotorsion during the procedure was automatically compensated for by the eye tracker. Thus, any differences in outcomes could be attributed to the use of SCC.
Figure 2. Achieved versus attempted correction of vector J0 (left) and vector J45 (right) (J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SCC Z static cyclotorsion compensation). J CATARACT REFRACT SURG - VOL 39, MAY 2013
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Figure 4. Comparison of preoperative CDVA and postoperative UDVA between the study group and the control group (CDVA Z corrected distance visual acuity; SCC Z static cyclotorsion compensation; UDVA Z uncorrected distance visual acuity).
Figure 3. Comparison of the refractive outcomes between the SCC group (SCC plus DCC treatment) versus the control group (DCC only treatment). Top: Percentage of eyes within attempted SE. Bottom: Frequency distribution of eyes with residual cylinder of 0.25 D or more.
Previous studies found a higher magnitude of static cyclotorsion (eg, Ciccio et al.,4 4.05 G 2.9 degrees; Neuhann et al.,1 3.96 G 2.96 degrees; Chang,3 2.7 G 2.4 degrees; Park et al.,16 2.58 G 1.56 degrees for incyclotorsion and 2.94 G 1.87 degrees for excyclotorsion). This may be due to the ethnic factors10 or a different workflow. Theoretical analyses suggest that this mean amount of static cyclotorsion would account
We found a statistically significant improvement in the refractive parameters (sphere, cylinder, SE) in the study group. The scatterplot between the attempted versus the achieved cylinder and the comparison of distribution of residual cylinder also showed better outcomes with the use of the SCC protocol. Although the mean postoperative UDVA and CDVA were better in the study group, the difference was not statistically significant. It could possibly be due to the lower magnitude of static cyclotorsion in our study population (mean 2.29 G 1.74 degrees; range 0 to 11.1 degrees), which decreased the difference between the study group and the control group.
Figure 5. Distribution of static cyclotorsion in the SCC group.
Figure 6. Uncorrected cyclotorsion astigmatism analysis. Top: Mean axis rotation from preoperative to 3-month postoperative visit. Bottom: Absolute rotation of manifest axis for eyes with residual manifest cylinder more than or equal to 0.25 D (SCC Z static cyclotorsion compensation).
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Figure 7. Comparison of preoperative and postoperative mean J0 (left) and mean J45 (right) vectors between the SCC and the control group (J0 Z Jackson cross-cylinder, axes at 180 degrees and 90 degrees; J45 Z Jackson cross-cylinder, axes at 45 degrees and 135 degrees; SCC Z static cyclotorsion compensation).
for a 14% undercorrection of astigmatism, which would increase further with larger angles of static cyclotorsion.4,6 It is therefore desirable to use cyclotorsion-compensation protocols, dynamic as well as static, to achieve the best astigmatic refractive outcomes, especially because both components cannot be calculated for an individual patient in advance. Precise torsional alignment gains more importance in wavefront-guided treatments, which must be performed with torsional precision of approximately 1 degree or better17 because higher cyclotorsion can induce substantial HOAs and reduce retinal image quality.15,18,19 Vector analysis of the J0 and J45 components of astigmatism showed that the respective components were comparable preoperatively between the study group and the control group. In the comparison of postoperative values, although there was significantly lower residual postoperative J0 vector in the study group than in the control group, the postoperative J45 vector was statistically similar in the 2 groups (Figure 7). Although significantly less than in the control group, residual astigmatism (cylinder, J0, and J45) in the study group indicates that a few cyclotorsion errors were uncompensated for in the study group as well. This could be attributed to the resolution and accuracy of the diagnosis and laser image, possible misalignment of the scanner to the camera, possible misalignment of the manifest astigmatism to the topography analysis, or other, yet to be studied factors affecting excimer laser refractive outcomes. The retrospective design and short duration of follow-up may be viewed as the limitations of our study. Future prospective studies should evaluate the effect of cyclotorsion compensation in wavefrontguided ablation profiles. We conclude that the supplementary use of SCC with DCC using an aberration-free aspheric ablation
profile produces a statistically significant improvement in astigmatism outcomes. WHAT WAS KNOWN Uncompensated for static cyclotorsion and dynamic cyclotorsion are important factors contributing to less than optimum results in the correction of astigmatism. Automated dynamic cyclotorsion compensation modules significantly improve the outcomes of astigmatism correction. Recent technological advancements allow laser platforms to interact with diagnostic equipment to automatically measure and compensate for static cyclotorsion. WHAT THIS PAPER ADDS Supplementary use of automated SCC in addition to the use of DCC statistically improves astigmatism correction outcomes. This shows that static cyclotorsion should be compensated for, especially in patients with higher amount of astigmatism, to achieve better refractive outcomes.
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13. Arba-Mosquera S, Arbelaez MC. Three-month clinical outcomes with static and dynamic cyclotorsion correction using the SCHWIND AMARIS. Cornea 2011; 30:951–957 14. Thibos LN, Horner D. Power vector analysis of the optical outcome of refractive surgery. J Cataract Refract Surg 2001; 27:80–85 15. Chernyak DA. Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery. J Cataract Refract Surg 2004; 30:633–638 16. Park SH, Kim M, Joo C-K. Measurement of pupil centroid shift and cyclotorsional displacement using iris registration. Ophthalmologica 2009; 223:166–171 17. Bueeler M, Mrochen M, Seiler T. Maximum permissible torsional misalignment in aberration-sensing and wavefront-guided corneal ablation. J Cataract Refract Surg 2004; 30:17–25 18. 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 19. Venter J. Outcomes of myopic LASIK with and without NIDEK active torsion error correction. J Refract Surg 2009; 25:985–990
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First author: Minoru Tomita, MD, PhD Private practice, Chiyoda-ku, Tokyo, Japan