- Email: [email protected]
Laser in situ keratomileusis for high astigmatism in myopic and hyperopic eyes
ARTICLE
Laser in situ keratomileusis for high astigmatism in myopic and hyperopic eyes Anders Ivarsen, MD, PhD, Kristian Næser, MD, DMSc, Jesper Hjortdal, MD, DMSc
PURPOSE: To evaluate outcomes after laser in situ keratomileusis (LASIK) in highly astigmatic myopic and hyperopic eyes. SETTING: University eye clinic. DESIGN: Retrospective case series. METHODS: Eyes with more than 2.0 diopters (D) of astigmatism were identified from patient records. The mean preoperative cylinder was 3.92 D G 0.82 (SD) in myopic eyes and 4.42 G 1.10 D in hyperopic eyes. Patients were examined preoperatively and 3 months postoperatively. Laser in situ keratomileusis was performed with a Visumax femtosecond laser and a MEL-80 excimer laser. Preoperative and postoperative refractions were converted to polar values. Induced torsion and achieved correction of sphere and cylinder were determined. RESULTS: After 3 months, the mean sphere was 0.48 G 0.68 D in myopic eyes and the mean cylinder 0.97 G 0.52 D. Spherical equivalent (SE) correction was 0.05 G 0.42 D from target. Astigmatism was 0.77 G 0.62 D undercorrected (P<.01), and the mean induced torsion was 0.18 G 0.51 D (PZ.02). Astigmatic undercorrection was 21% of the intended correction. In hyperopia, the mean 3-month refraction was 0.79 G 0.83 D in sphere and 1.38 G 0.90 D in cylinder. The SE refraction was 0.01 G 0.71 D from target. Astigmatism was 1.17 G 0.81 D undercorrected (P<.01), with no significant torsion. Intended and achieved astigmatic corrections were correlated, with astigmatism being 28% undercorrected. CONCLUSIONS: Laser in situ keratomileusis in highly astigmatic eyes precisely corrected SE refraction but led to astigmatic undercorrection, particularly in hyperopia. Little to no torsion of the cylinder axis was induced. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2013; 39:74–80 Q 2012 ASCRS and ESCRS
Laser in situ keratomileusis (LASIK) is a highly successful keratorefractive procedure for the treatment of myopia and low degrees of hyperopia. However, treatment of astigmatism is generally considered more difficult than treatment of simple spherical errors; in particular, hyperopic astigmatism has been found challenging.1–3 In general, rigorous studies of LASIK for high astigmatism are few2–5 and many studies consist of combined results of astigmatic and spherical treatments or lack proper analysis of the postoperative refractive changes with polar conversion or vector analysis.6–10 The present study evaluated the refractive outcomes of myopic and hyperopic patients treated with LASIK for high astigmatism using the MEL-80 (Carl Zeiss Meditec AG) platform. 74
Q 2012 ASCRS and ESCRS Published by Elsevier Inc.
PATIENTS AND METHODS Patients treated with LASIK for refractive astigmatism greater than 2.00 diopters (D) were retrospectively identified from patient records at the Department of Ophthalmology, Aarhus University Hospital. Patients were examined preoperatively and 3 months after treatment. Examinations included manifest refraction as well as uncorrected (UDVA) and corrected (CDVA) distance visual acuity. All clinical refractions were obtained at a vertex distance of 12.0 mm and were performed by trained optometrists.
Surgical Technique In all patients, LASIK was performed with topical anesthesia using 2 drops of oxybuprocaine 0.8% with a 2-minute interval. A corneal flap was cut using a Visumax femtosecond laser (Carl Zeiss Meditec AG). The flap diameter ranged 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2012.08.054
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from 7.9 to 8.0 mm and the flap thickness, from 110 to 120 mm; in all patients, the flap edge was undercut at a 70-degree angle. A MEL-80 flying-spot laser (Carl Zeiss Meditec AG) was used for the excimer laser treatment. In all patients, the optical zone diameter was 6.0 mm and photoablation was performed using the aspheric “smart” algorithm. During surgery, the patient fixated on a light coaxial with the excimer beam, the eye tracker locked onto the pupil, and the photoablation was, per default, centered on the pupil. Treatments were performed using minus-cylinder ablation in myopic eyes and plus-cylinder ablation in hyperopic eyes. Postoperative treatment comprised fluorometholone and chloramphenicol eyedrops 4 times a day tapered over 2 weeks.
Data and Statistical Analysis Subjective spherocylindrical refractive data were converted into spherical equivalent (SE) power and 2 polar values as previously reported.11,12 In brief, the astigmatic polar value (AKP) was calculated at the main meridian a (equivalent to the cylinder axis). Furthermore, the astigmatic polar value AKP(C45) was calculated at an oblique meridian 45 degrees counterclockwise to the main meridian (a C 45 degrees). From these 2 polar values, any cylinder can be uniquely characterized. Surgically induced astigmatism was determined as the change in AKP from before to after surgery, whereas the change in AKP(C45) indicated the amount of torsion or rotation of the cylinder axis, with negative values indicating a clockwise change. Likewise, the errors of treatment AKPerror and AKP(C45)error were calculated by subtracting the target from the achieved astigmatism. For details, please refer to Naeser et al.11 To estimate the visual loss induced by the postoperative refraction (error of treatment), the amount of defocus was calculated as described in reference 12, equation 38. Furthermore, average polar values were converted to net cylinder format as previously detailed.11,12 Data were tested for normality using the D’AgostinoPearson test. Statistics were performed as paired t tests of univariate means and as bivariate analyses of the combined mean polar values with calculation of 2-dimensional confidence ellipses and determination of Hotelling T2, as previously detailed.11,13
hyperopic astigmatism. Table 1 shows the preoperative and postoperative characteristics by group. No perioperative or postoperative complications were observed. Myopic Astigmatism Table 2 shows the polar values of refractive data in the myopic group. The achieved and intended SE corrections were highly correlated (r2 Z 0.97, P!.001) (Figure 1, A). Thirty-one (87%) of the 36 eyes intended for plano refraction were within G0.50 D of emmetropia, and 35 eyes (98%) were within G1.00 D (Figure 2). The mean postoperative astigmatic polar value AKP was equivalent to a mean achieved flattening (or surgically induced astigmatism) at the main meridian of 3.09 G 0.76 D. Given the mean intended postoperative astigmatism (target AKP), there was a significant undercorrection of astigmatism (or error of treatment, AKPerror) (P!.001, paired t test). The achieved and intended astigmatic corrections were highly correlated (r2 Z 0.52, P!.001) (Figure 3, A), with an undercorrection of 21% per diopter (linear regression analysis). In addition to the changes at the main meridian, there was induced astigmatism at the oblique meridian. Thus, the mean AKP(C45)error indicated a minor, but significant, clockwise torsion of the cylinder axis (PZ.02, paired t test). The mean defocus was 0.64 G 0.30 D, and the astigmatic error in net cylinder format was calculated to 0.79 @ 7 relative to the preoperative stronger meridian.
Table 1. Patient demographics and preoperative and postoperative refraction by group.
Parameter
RESULTS The study comprised 46 eyes of 31 patients with myopic astigmatism and 52 eyes of 30 patients with Submitted: June 23, 2012. Final revision submitted: August 7, 2012. Accepted: August 12, 2012. From the Departments of Ophthalmology, Aarhus University Hospital (Ivarsen, Næser, Hjortdal), Aarhus, and Randers Regional Hospital (Næser), Randers, Denmark. Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, USA, May 2012. Corresponding author: Anders Ivarsen, MD, PhD, Department of Ophthalmology, Aarhus University Hospital, Noerrebrogade 44, DK-8000 Aarhus C, Denmark. E-mail: [email protected].
Myopia Group (46 eyes)
Age (y) Mean G SD 38.9 G 7.6 Range 22, 53 Sex (n) Female 20 Male 11 Preop sphere (D) Mean G SD 3.1 G 2.6 Range 10.0, 0.0 Preop cylinder (D) Mean G SD 3.9 G 0.8 Range 6.0, 2.75 3-month postop sphere (D) Mean G SD 0.5 G 0.7 Range 2.5, 1.75 3-month postop cylinder (D) Mean G SD 1.0 G 0.5 Range 2.25, 0.25
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Hyperopia Group (52 eyes) 35.6 G 9.2 21, 53 16 14 3.5 G 2.3 0.0, 8.75 4.4 G 1.1 6.5, 2.0 0.8 G 0.8 0.25, 3.0 1.4 G 0.9 4.5, 0.0
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Table 2. Polar values in myopic eyes. Mean (D) G SD Parameter SE power AKP AKP(C45)
Preop 5.12 G 2.47 3.92 G 0.82 0
Postop 0.00 G 0.63 0.83 G 0.56 0.18 G 0.43
Target 0.06 G 0.34 0.06 G 0.22 0.00 G 0.30
Error of Treatment 0.05 G 0.42 0.77 G 0.62* 0.18 G 0.51†
AKP Z astigmatic polar value or power along the main meridian; AKP(C45) Z torsional power at the reference meridian, 45 degrees counterclockwise to the main meridian; SE Z spherical equivalent *Significant undercorrection (P!.001, paired t test) † Significant torsion (PZ.02, paired t test)
Figure 4, A, shows the bivariate analysis of the polar values. Confidence ellipses are given for individual samples and for the combined mean and show a significant combined error of treatment (Hotelling’s T2 Z 100; P!.001). Of the 36 myopic eyes treated for plano refraction, 21 eyes (58%) had a UDVA of 20/25 or better (Figure 5). No myopic eye lost more than 1 line of CDVA, and 1 eye gained 2 lines (Figure 6). Hyperopic Astigmatism Table 3 shows the polar values in the hyperopic group. Three months after surgery, the mean SE power was 0.10 G 0.71 D with an insignificant mean SE power error of 0.01 G 0.17 D. The achieved and intended SE corrections were highly correlated
(r2 Z 0.93, P!.001) (Figure 1, B). Thirty-one (71%) of the 44 eyes intended for plano refraction were within G0.50 D of emmetropia, and 40 eyes (90%) were within G1.00 D (Figure 2). The mean postoperative AKP was equivalent to a mean surgically induced flattening at the main meridian of 3.17 G 1.10 D. Based on the mean intended postoperative astigmatism (target AKP), there was a significant astigmatic undercorrection (AKPerror) (P!.001, paired t test). The intended and achieved astigmatic corrections were correlated (r2 Z 0.46; P!.001) (Figure 3, B), with an undercorrection of 28% per diopter (linear regression analysis). There was a significant difference in the amount of undercorrection between myopic eyes and hyperopic eyes (PZ.01, unpaired t test). No significant torsion was
Figure 1. Intended versus achieved SE correction in myopic eyes (A) and hyperopic eyes (B). Points above and below the dashed lines indicate overcorrecton and undercorrection, respectively. The solid lines indicate the outcome of the linear regression analyses. J CATARACT REFRACT SURG - VOL 39, JANUARY 2013
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UDVA of 20/25 or better (Figure 5). No hyperopic eye lost or gained more than 1 line of CDVA (Figure 6). DISCUSSION
Figure 2. Percentage of eyes attaining the specified postoperative SE refractions. Includes only eyes targeted for plano refraction.
observed in hyperopic eyes due to an insignificant change in AKP(C45)error. The mean defocus was 0.92 G 0.53 D, and the astigmatic error in net cylinder format was 1.18 @ 4 relative to the preoperative stronger meridian. In the bivariate analysis (Figure 4, B), the confidence ellipse for the combined means showed a significant error of treatment (Hotelling’s T2 Z 110; P!.001). In the hyperopic group, 44 eyes were treated for plano refraction and 19 eyes (36%) had a postoperative
In the present study, LASIK gave precise correction of SE power in myopic and hyperopic astigmatic patients. However, there was significant undercorrection of the cylinder in both groups, with a larger error of treatment in hyperopic patients. Thus, the astigmatic undercorrection amounted to 21% in the myopic group and 28% in the hyperopic group. Furthermore, the precision of the refractive treatment appeared poorer in hyperopic eyes. These observations are in concordance with results in other studies of LASIK for high astigmatism,2–10 which report varying degrees of astigmatic undercorrection using the proprietary algorithms of different laser platforms. However, the ablation profile may have a significant impact on the postoperative result. Thus, several treatment algorithms have been proposed for hyperopic, mixed, or myopic astigmatism, with different clinical success. There are some reports of very accurate astigmatic correction.4–6,10 Unfortunately, in most of these studies, the astigmatic changes were not properly evaluated using polar or vector conversion, making it difficult to determine which approach is better. In the present study, the proprietary MEL-80 monotoric ablation profile was used in all cases, with plus cylinder for hyperopic eyes and minus cylinder for myopic
Figure 3. Intended versus achieved astigmatic correction in myopic eyes (A) and hyperopic eyes (B), expressed as AKP (the astigmatic polar value at the main meridian). Points below the dashed lines indicate astigmatic undercorrection. J CATARACT REFRACT SURG - VOL 39, JANUARY 2013
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Figure 4. Bivariate representation of the error of treatment in myopic (A) and hyperopic (B) eyes. The AKPerror indicates the error of treatment at the main meridian (positive values representing the amount of undercorrection). The AKP(C45)error indicates the induced astigmatism at a 45-degree oblique meridian; thus, representing the amount of torsion or rotation of the cylinder axis. Confidence ellipses comprising 95% of the observations are given for individual samples (dashed) and for the combined mean (solid); the ellipses being equivalent to the standard variation and the standard error of mean, respectively, in a univariate analysis. The dimensions of the confidence ellipses are an indication of the relative inaccuracy of the treatments. The statistical significance may be read directly from the figure, with the combined mean errors being significantly different from zero, as the points (0,0) are not included in the confidence ellipses (solid). All values are given in diopters.
eyes. This algorithm appeared to give excellent SE correction in both myopic eyes and hyperopic eyes despite significant undercorrection of the astigmatic component. Theoretically, the observed undercorrection could also be caused by regression of the induced astigmatic
change. However, the present data do not allow us to determine whether regression played a significant role in the 3-month clinical outcomes. In this study, we found that LASIK induced a minor clockwise torsion, or rotation, of the cylinder axis in myopic eyes. A similar, although insignificant, change
Figure 5. Cumulative percentages of eyes that attained the specified UDVA in Snellen lines 3 months after surgery. Includes only eyes treated for emmetropia.
Figure 6. Percentage of eyes with the specified gain or loss in CDVA 3 months after surgery.
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Table 3. Polar values in hyperopic eyes. Mean (D) G SD Parameter
Preop
SE power AKP AKP(C45)
1.31 G 2.31 4.42 G 1.10 0
Postop
Target
0.10 G 0.71 1.25 G 0.89 0.15 G 0.59
0.11 G 0.51 0.08 G 0.42 0.02 G 0.74
Error of Treatment 0.01 G 0.17 1.17 G 0.81*,† 0.17 G 0.69
AKP Z astigmatic polar value or power along the main meridian; AKP(C45) Z torsional power at the reference meridian, 45 degrees counterclockwise to the main meridian; SE Z spherical equivalent *Significant undercorrection (P!.001, paired t test) † Significant difference in undercorrection between hyperopic eyes and myopic eyes (PZ.01, unpaired t test)
was observed in hyperopic eyes. This induced torsion may have several causes, with cyclotorsion and head tilt being the 2 most obvious.14,A In the present study, no specific measures were used to prevent head tilt or cyclotorsion. Yet, the induced torsion in both groups was small. Thus, the net cylinder values were 0.79 @ 7 and 1.18 @ 4 in myopic eyes and hyperopic eyes, respectively. With the postoperative astigmatic power at the main meridian being 0.77 D and 1.17 D in myopic eyes and hyperopic eyes, respectively, the induced torsion contributed little to the observed astigmatic undercorrection. Nevertheless, technologies such as iris registration may allow the induced torsion to be further reduced, and several lasers, including the excimer laser used in the present study, offer such solutions. In both myopic eyes and hyperopic eyes, LASIK for high astigmatism was as safe as LASIK procedures in general. Thus, no eye lost more than 1 line of CDVA. Considering UDVA, the outcomes in myopic patients appeared to be better than those in hyperopic patients. This difference may in part be attributed to the poorer astigmatic correction in hyperopia, as suggested by the estimated defocus of 0.64 D in myopic eyes and 0.92 D in hyperopic eyes. However, mild amblyopia in the hyperopic group could also have contributed to the observed difference. Thus, the myopic group had a mean preoperative CDVA of 0.04 G 0.07 logMAR (range 0.1 to 0.2 logMAR) and the hyperopic group, of 0.08 G 0.09 logMAR (range 0.1 to 0.4 logMAR). In summary, LASIK for high astigmatism was safe in myopic eyes and in hyperopic eyes. The treatment induced only minor torsion of the cylinder axis, which was considered to be of little importance for the visual outcomes. However, we observed considerable astigmatic undercorrection that was more pronounced in hyperopic eyes. Thus, high degrees of astigmatism can be treated with LASIK using the MEL-80 excimer laser, although with less precision and accuracy in hyperopic eyes than in myopic eyes.
WHAT WAS KNOWN Correction of high astigmatism with LASIK is more difficult than correction of spherical refractive errors, such as myopia or low hyperopia. Results in the published literature vary greatly; however, many studies lack proper analysis of induced astigmatic changes (vector or polar analysis), including determination of induced torsion of the cylinder axis. Few studies have examined the correction of high astigmatism in both myopic eyes and hyperopic eyes using the same laser platform. WHAT THIS PAPER ADDS Laser in situ keratomileusis with the MEL-80 platform was found safe in myopic eyes and hyperopic eyes, with excellent correction of SE refraction. However, polar-value analysis showed slight torsion of the cylinder axis and significant undercorrection of the astigmatic component. This should be taken into account when counseling patients for surgery.
REFERENCES 1. Varley GA, Huang D, Rapuano CJ, Schallhorn S, Boxer Wachler BS, Sugar A. LASIK for hyperopia, hyperopic astigmatism, and mixed astigmatism; a report by the American Academy of Ophthalmology (Ophthalmic Technology Assessment). Ophthalmology 2004; 111:1604–1617 2. Salz JJ, Stevens CA, for the LADARVision LASIK Hyperopia Study Group. LASIK correction of spherical hyperopia, hyperopic astigmatism, and mixed astigmatism with the LADARVision excimer laser system. Ophthalmology 2002; 109:1647–1657; discussion by RR Krueger, 1657 1658 3. Ismail MM. LASIK using the NIDEK EC-5000 for the correction of hyperopic astigmatism. J Refract Surg 2006; 22:S1069–S1072 4. Arbelaez MC, Vidal C, Arba-Mosquera S. Excimer laser correction of moderate to high astigmatism with a non-wavefrontguided aberration-free ablation profile: six-month results. J Cataract Refract Surg 2009; 35:1789–1798 5. Abolhassani A, Shojaei A, Baradaran-Rafiee AR, Eslani M, Elahi B, Noorizadeh F. Vector analysis of cross cylinder LASIK
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6.
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with the NIDEK EC-5000 excimer laser for high astigmatism. J Refract Surg 2009; 25:1075–1082 mez L, Rodrıguez X, Robledo N, Chayet AS, Montes M, Go MacRae S. Bitoric laser in situ keratomileusis for the correction of simple myopic and mixed astigmatism. Ophthalmology 2001; 108:303–308 Lee VW. The LADAR6000: results in highly myopic and highly astigmatic eyes. J Refract Surg 2006; 22:S980–S982 Kermani O, Schmeidt K, Oberheide U, Gerten G. Hyperopic laser in situ keratomileusis with 5.5-, 6.5-, and 7.0-mm optical zones. J Refract Surg 2005; 21:52–58 Lui MM, Silas MAG, Apelbaum B, Oshima A. Laser in situ keratomileusis with the Nidek EC-5000 excimer laser for astigmatism greater than 4.00 D. J Refract Surg 2002; 18:S321–S322 Gamaly T. LASIK with the optimized aspheric transition zone and cross-cylinder technique for the treatment of astigmatism from 1.00 to 4.25 diopters. J Refract Surg 2009; 25:S927–S930 Naeser K, Hjortdal J. Polar value analysis of refractive data. J Cataract Refract Surg 2001; 27:86–94
12. Næser K. Assessment and statistics of surgically induced astigmatism. Acta Ophthalmol 2008; 86(issue thesis 1):5–28. Available at: http://onlinelibrary.wiley.com/doi/10.1111/j.1755 -3768.2008.01234.x/pdf. Accessed September 5, 2012 13. Naeser K, Hjortdal J. Multivariate analysis of refractive data: mathematics and statistics of spherocylinders. J Cataract Refract Surg 2001; 27:129–142 14. Chang J. Cyclotorsion during laser in situ keratomileusis. J Cataract Refract Surg 2008; 34:1720–1726
OTHER CITED MATERIAL A. Prickett AL, Chamon W, Bui KM, Hallak J, Bakhtiari P, Azar DT. A novel approach to determine theoretical head tilt effect on ocular cyclotorsion measurements during laser refractive surgery. IOVS 2012; 53:ARVO E-Abstract 5570. Available at: http://abstracts.iovs.org//cgi/content/abstract/53/6/5570?sidZ 0e0ca9e2-de04-49cc-b528-501626f0aa19. Accessed September 5, 2012
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Laser in situ keratomileusis for high astigmatism in myopic and hyperopic eyes Anders Ivarsen, MD, PhD, Kristian Næser, MD, DMSc, Jesper Hjortdal, MD, DMSc
PURPOSE: To evaluate outcomes after laser in situ keratomileusis (LASIK) in highly astigmatic myopic and hyperopic eyes. SETTING: University eye clinic. DESIGN: Retrospective case series. METHODS: Eyes with more than 2.0 diopters (D) of astigmatism were identified from patient records. The mean preoperative cylinder was 3.92 D G 0.82 (SD) in myopic eyes and 4.42 G 1.10 D in hyperopic eyes. Patients were examined preoperatively and 3 months postoperatively. Laser in situ keratomileusis was performed with a Visumax femtosecond laser and a MEL-80 excimer laser. Preoperative and postoperative refractions were converted to polar values. Induced torsion and achieved correction of sphere and cylinder were determined. RESULTS: After 3 months, the mean sphere was 0.48 G 0.68 D in myopic eyes and the mean cylinder 0.97 G 0.52 D. Spherical equivalent (SE) correction was 0.05 G 0.42 D from target. Astigmatism was 0.77 G 0.62 D undercorrected (P<.01), and the mean induced torsion was 0.18 G 0.51 D (PZ.02). Astigmatic undercorrection was 21% of the intended correction. In hyperopia, the mean 3-month refraction was 0.79 G 0.83 D in sphere and 1.38 G 0.90 D in cylinder. The SE refraction was 0.01 G 0.71 D from target. Astigmatism was 1.17 G 0.81 D undercorrected (P<.01), with no significant torsion. Intended and achieved astigmatic corrections were correlated, with astigmatism being 28% undercorrected. CONCLUSIONS: Laser in situ keratomileusis in highly astigmatic eyes precisely corrected SE refraction but led to astigmatic undercorrection, particularly in hyperopia. Little to no torsion of the cylinder axis was induced. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. J Cataract Refract Surg 2013; 39:74–80 Q 2012 ASCRS and ESCRS
Laser in situ keratomileusis (LASIK) is a highly successful keratorefractive procedure for the treatment of myopia and low degrees of hyperopia. However, treatment of astigmatism is generally considered more difficult than treatment of simple spherical errors; in particular, hyperopic astigmatism has been found challenging.1–3 In general, rigorous studies of LASIK for high astigmatism are few2–5 and many studies consist of combined results of astigmatic and spherical treatments or lack proper analysis of the postoperative refractive changes with polar conversion or vector analysis.6–10 The present study evaluated the refractive outcomes of myopic and hyperopic patients treated with LASIK for high astigmatism using the MEL-80 (Carl Zeiss Meditec AG) platform. 74
Q 2012 ASCRS and ESCRS Published by Elsevier Inc.
PATIENTS AND METHODS Patients treated with LASIK for refractive astigmatism greater than 2.00 diopters (D) were retrospectively identified from patient records at the Department of Ophthalmology, Aarhus University Hospital. Patients were examined preoperatively and 3 months after treatment. Examinations included manifest refraction as well as uncorrected (UDVA) and corrected (CDVA) distance visual acuity. All clinical refractions were obtained at a vertex distance of 12.0 mm and were performed by trained optometrists.
Surgical Technique In all patients, LASIK was performed with topical anesthesia using 2 drops of oxybuprocaine 0.8% with a 2-minute interval. A corneal flap was cut using a Visumax femtosecond laser (Carl Zeiss Meditec AG). The flap diameter ranged 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2012.08.054
75
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from 7.9 to 8.0 mm and the flap thickness, from 110 to 120 mm; in all patients, the flap edge was undercut at a 70-degree angle. A MEL-80 flying-spot laser (Carl Zeiss Meditec AG) was used for the excimer laser treatment. In all patients, the optical zone diameter was 6.0 mm and photoablation was performed using the aspheric “smart” algorithm. During surgery, the patient fixated on a light coaxial with the excimer beam, the eye tracker locked onto the pupil, and the photoablation was, per default, centered on the pupil. Treatments were performed using minus-cylinder ablation in myopic eyes and plus-cylinder ablation in hyperopic eyes. Postoperative treatment comprised fluorometholone and chloramphenicol eyedrops 4 times a day tapered over 2 weeks.
Data and Statistical Analysis Subjective spherocylindrical refractive data were converted into spherical equivalent (SE) power and 2 polar values as previously reported.11,12 In brief, the astigmatic polar value (AKP) was calculated at the main meridian a (equivalent to the cylinder axis). Furthermore, the astigmatic polar value AKP(C45) was calculated at an oblique meridian 45 degrees counterclockwise to the main meridian (a C 45 degrees). From these 2 polar values, any cylinder can be uniquely characterized. Surgically induced astigmatism was determined as the change in AKP from before to after surgery, whereas the change in AKP(C45) indicated the amount of torsion or rotation of the cylinder axis, with negative values indicating a clockwise change. Likewise, the errors of treatment AKPerror and AKP(C45)error were calculated by subtracting the target from the achieved astigmatism. For details, please refer to Naeser et al.11 To estimate the visual loss induced by the postoperative refraction (error of treatment), the amount of defocus was calculated as described in reference 12, equation 38. Furthermore, average polar values were converted to net cylinder format as previously detailed.11,12 Data were tested for normality using the D’AgostinoPearson test. Statistics were performed as paired t tests of univariate means and as bivariate analyses of the combined mean polar values with calculation of 2-dimensional confidence ellipses and determination of Hotelling T2, as previously detailed.11,13
hyperopic astigmatism. Table 1 shows the preoperative and postoperative characteristics by group. No perioperative or postoperative complications were observed. Myopic Astigmatism Table 2 shows the polar values of refractive data in the myopic group. The achieved and intended SE corrections were highly correlated (r2 Z 0.97, P!.001) (Figure 1, A). Thirty-one (87%) of the 36 eyes intended for plano refraction were within G0.50 D of emmetropia, and 35 eyes (98%) were within G1.00 D (Figure 2). The mean postoperative astigmatic polar value AKP was equivalent to a mean achieved flattening (or surgically induced astigmatism) at the main meridian of 3.09 G 0.76 D. Given the mean intended postoperative astigmatism (target AKP), there was a significant undercorrection of astigmatism (or error of treatment, AKPerror) (P!.001, paired t test). The achieved and intended astigmatic corrections were highly correlated (r2 Z 0.52, P!.001) (Figure 3, A), with an undercorrection of 21% per diopter (linear regression analysis). In addition to the changes at the main meridian, there was induced astigmatism at the oblique meridian. Thus, the mean AKP(C45)error indicated a minor, but significant, clockwise torsion of the cylinder axis (PZ.02, paired t test). The mean defocus was 0.64 G 0.30 D, and the astigmatic error in net cylinder format was calculated to 0.79 @ 7 relative to the preoperative stronger meridian.
Table 1. Patient demographics and preoperative and postoperative refraction by group.
Parameter
RESULTS The study comprised 46 eyes of 31 patients with myopic astigmatism and 52 eyes of 30 patients with Submitted: June 23, 2012. Final revision submitted: August 7, 2012. Accepted: August 12, 2012. From the Departments of Ophthalmology, Aarhus University Hospital (Ivarsen, Næser, Hjortdal), Aarhus, and Randers Regional Hospital (Næser), Randers, Denmark. Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, USA, May 2012. Corresponding author: Anders Ivarsen, MD, PhD, Department of Ophthalmology, Aarhus University Hospital, Noerrebrogade 44, DK-8000 Aarhus C, Denmark. E-mail: [email protected].
Myopia Group (46 eyes)
Age (y) Mean G SD 38.9 G 7.6 Range 22, 53 Sex (n) Female 20 Male 11 Preop sphere (D) Mean G SD 3.1 G 2.6 Range 10.0, 0.0 Preop cylinder (D) Mean G SD 3.9 G 0.8 Range 6.0, 2.75 3-month postop sphere (D) Mean G SD 0.5 G 0.7 Range 2.5, 1.75 3-month postop cylinder (D) Mean G SD 1.0 G 0.5 Range 2.25, 0.25
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Hyperopia Group (52 eyes) 35.6 G 9.2 21, 53 16 14 3.5 G 2.3 0.0, 8.75 4.4 G 1.1 6.5, 2.0 0.8 G 0.8 0.25, 3.0 1.4 G 0.9 4.5, 0.0
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Table 2. Polar values in myopic eyes. Mean (D) G SD Parameter SE power AKP AKP(C45)
Preop 5.12 G 2.47 3.92 G 0.82 0
Postop 0.00 G 0.63 0.83 G 0.56 0.18 G 0.43
Target 0.06 G 0.34 0.06 G 0.22 0.00 G 0.30
Error of Treatment 0.05 G 0.42 0.77 G 0.62* 0.18 G 0.51†
AKP Z astigmatic polar value or power along the main meridian; AKP(C45) Z torsional power at the reference meridian, 45 degrees counterclockwise to the main meridian; SE Z spherical equivalent *Significant undercorrection (P!.001, paired t test) † Significant torsion (PZ.02, paired t test)
Figure 4, A, shows the bivariate analysis of the polar values. Confidence ellipses are given for individual samples and for the combined mean and show a significant combined error of treatment (Hotelling’s T2 Z 100; P!.001). Of the 36 myopic eyes treated for plano refraction, 21 eyes (58%) had a UDVA of 20/25 or better (Figure 5). No myopic eye lost more than 1 line of CDVA, and 1 eye gained 2 lines (Figure 6). Hyperopic Astigmatism Table 3 shows the polar values in the hyperopic group. Three months after surgery, the mean SE power was 0.10 G 0.71 D with an insignificant mean SE power error of 0.01 G 0.17 D. The achieved and intended SE corrections were highly correlated
(r2 Z 0.93, P!.001) (Figure 1, B). Thirty-one (71%) of the 44 eyes intended for plano refraction were within G0.50 D of emmetropia, and 40 eyes (90%) were within G1.00 D (Figure 2). The mean postoperative AKP was equivalent to a mean surgically induced flattening at the main meridian of 3.17 G 1.10 D. Based on the mean intended postoperative astigmatism (target AKP), there was a significant astigmatic undercorrection (AKPerror) (P!.001, paired t test). The intended and achieved astigmatic corrections were correlated (r2 Z 0.46; P!.001) (Figure 3, B), with an undercorrection of 28% per diopter (linear regression analysis). There was a significant difference in the amount of undercorrection between myopic eyes and hyperopic eyes (PZ.01, unpaired t test). No significant torsion was
Figure 1. Intended versus achieved SE correction in myopic eyes (A) and hyperopic eyes (B). Points above and below the dashed lines indicate overcorrecton and undercorrection, respectively. The solid lines indicate the outcome of the linear regression analyses. J CATARACT REFRACT SURG - VOL 39, JANUARY 2013
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UDVA of 20/25 or better (Figure 5). No hyperopic eye lost or gained more than 1 line of CDVA (Figure 6). DISCUSSION
Figure 2. Percentage of eyes attaining the specified postoperative SE refractions. Includes only eyes targeted for plano refraction.
observed in hyperopic eyes due to an insignificant change in AKP(C45)error. The mean defocus was 0.92 G 0.53 D, and the astigmatic error in net cylinder format was 1.18 @ 4 relative to the preoperative stronger meridian. In the bivariate analysis (Figure 4, B), the confidence ellipse for the combined means showed a significant error of treatment (Hotelling’s T2 Z 110; P!.001). In the hyperopic group, 44 eyes were treated for plano refraction and 19 eyes (36%) had a postoperative
In the present study, LASIK gave precise correction of SE power in myopic and hyperopic astigmatic patients. However, there was significant undercorrection of the cylinder in both groups, with a larger error of treatment in hyperopic patients. Thus, the astigmatic undercorrection amounted to 21% in the myopic group and 28% in the hyperopic group. Furthermore, the precision of the refractive treatment appeared poorer in hyperopic eyes. These observations are in concordance with results in other studies of LASIK for high astigmatism,2–10 which report varying degrees of astigmatic undercorrection using the proprietary algorithms of different laser platforms. However, the ablation profile may have a significant impact on the postoperative result. Thus, several treatment algorithms have been proposed for hyperopic, mixed, or myopic astigmatism, with different clinical success. There are some reports of very accurate astigmatic correction.4–6,10 Unfortunately, in most of these studies, the astigmatic changes were not properly evaluated using polar or vector conversion, making it difficult to determine which approach is better. In the present study, the proprietary MEL-80 monotoric ablation profile was used in all cases, with plus cylinder for hyperopic eyes and minus cylinder for myopic
Figure 3. Intended versus achieved astigmatic correction in myopic eyes (A) and hyperopic eyes (B), expressed as AKP (the astigmatic polar value at the main meridian). Points below the dashed lines indicate astigmatic undercorrection. J CATARACT REFRACT SURG - VOL 39, JANUARY 2013
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Figure 4. Bivariate representation of the error of treatment in myopic (A) and hyperopic (B) eyes. The AKPerror indicates the error of treatment at the main meridian (positive values representing the amount of undercorrection). The AKP(C45)error indicates the induced astigmatism at a 45-degree oblique meridian; thus, representing the amount of torsion or rotation of the cylinder axis. Confidence ellipses comprising 95% of the observations are given for individual samples (dashed) and for the combined mean (solid); the ellipses being equivalent to the standard variation and the standard error of mean, respectively, in a univariate analysis. The dimensions of the confidence ellipses are an indication of the relative inaccuracy of the treatments. The statistical significance may be read directly from the figure, with the combined mean errors being significantly different from zero, as the points (0,0) are not included in the confidence ellipses (solid). All values are given in diopters.
eyes. This algorithm appeared to give excellent SE correction in both myopic eyes and hyperopic eyes despite significant undercorrection of the astigmatic component. Theoretically, the observed undercorrection could also be caused by regression of the induced astigmatic
change. However, the present data do not allow us to determine whether regression played a significant role in the 3-month clinical outcomes. In this study, we found that LASIK induced a minor clockwise torsion, or rotation, of the cylinder axis in myopic eyes. A similar, although insignificant, change
Figure 5. Cumulative percentages of eyes that attained the specified UDVA in Snellen lines 3 months after surgery. Includes only eyes treated for emmetropia.
Figure 6. Percentage of eyes with the specified gain or loss in CDVA 3 months after surgery.
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Table 3. Polar values in hyperopic eyes. Mean (D) G SD Parameter
Preop
SE power AKP AKP(C45)
1.31 G 2.31 4.42 G 1.10 0
Postop
Target
0.10 G 0.71 1.25 G 0.89 0.15 G 0.59
0.11 G 0.51 0.08 G 0.42 0.02 G 0.74
Error of Treatment 0.01 G 0.17 1.17 G 0.81*,† 0.17 G 0.69
AKP Z astigmatic polar value or power along the main meridian; AKP(C45) Z torsional power at the reference meridian, 45 degrees counterclockwise to the main meridian; SE Z spherical equivalent *Significant undercorrection (P!.001, paired t test) † Significant difference in undercorrection between hyperopic eyes and myopic eyes (PZ.01, unpaired t test)
was observed in hyperopic eyes. This induced torsion may have several causes, with cyclotorsion and head tilt being the 2 most obvious.14,A In the present study, no specific measures were used to prevent head tilt or cyclotorsion. Yet, the induced torsion in both groups was small. Thus, the net cylinder values were 0.79 @ 7 and 1.18 @ 4 in myopic eyes and hyperopic eyes, respectively. With the postoperative astigmatic power at the main meridian being 0.77 D and 1.17 D in myopic eyes and hyperopic eyes, respectively, the induced torsion contributed little to the observed astigmatic undercorrection. Nevertheless, technologies such as iris registration may allow the induced torsion to be further reduced, and several lasers, including the excimer laser used in the present study, offer such solutions. In both myopic eyes and hyperopic eyes, LASIK for high astigmatism was as safe as LASIK procedures in general. Thus, no eye lost more than 1 line of CDVA. Considering UDVA, the outcomes in myopic patients appeared to be better than those in hyperopic patients. This difference may in part be attributed to the poorer astigmatic correction in hyperopia, as suggested by the estimated defocus of 0.64 D in myopic eyes and 0.92 D in hyperopic eyes. However, mild amblyopia in the hyperopic group could also have contributed to the observed difference. Thus, the myopic group had a mean preoperative CDVA of 0.04 G 0.07 logMAR (range 0.1 to 0.2 logMAR) and the hyperopic group, of 0.08 G 0.09 logMAR (range 0.1 to 0.4 logMAR). In summary, LASIK for high astigmatism was safe in myopic eyes and in hyperopic eyes. The treatment induced only minor torsion of the cylinder axis, which was considered to be of little importance for the visual outcomes. However, we observed considerable astigmatic undercorrection that was more pronounced in hyperopic eyes. Thus, high degrees of astigmatism can be treated with LASIK using the MEL-80 excimer laser, although with less precision and accuracy in hyperopic eyes than in myopic eyes.
WHAT WAS KNOWN Correction of high astigmatism with LASIK is more difficult than correction of spherical refractive errors, such as myopia or low hyperopia. Results in the published literature vary greatly; however, many studies lack proper analysis of induced astigmatic changes (vector or polar analysis), including determination of induced torsion of the cylinder axis. Few studies have examined the correction of high astigmatism in both myopic eyes and hyperopic eyes using the same laser platform. WHAT THIS PAPER ADDS Laser in situ keratomileusis with the MEL-80 platform was found safe in myopic eyes and hyperopic eyes, with excellent correction of SE refraction. However, polar-value analysis showed slight torsion of the cylinder axis and significant undercorrection of the astigmatic component. This should be taken into account when counseling patients for surgery.
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