From Augenklinik, Kantonsspital, Lucerne, Switzerland (Bucher, Schipper); Moorfields Eye Hospital NHS Foundation Trust, London, United Kingdom (Zuberbuhler); South Australian Institute of Ophthalmology, University of Adelaide, Australia (Goggin); and School of Nursing and Midwifery, University of South Australia, Adelaide, Australia (Esterman).
Dr Zuberbuhler is on the advisory board of Zubisoft Corp. The remaining authors have no financial or proprietary interest in the materials presented herein.
Study concept and design (C.B., B.Z., A.E., I.S.); data collection (C.B., A.E., I.S.); analysis and interpretation of data (C.B., B.Z., M.G., A.E., I.S.); drafting of the manuscript (C.B., B.Z., A.E., I.S.); critical revision of the manuscript (C.B., B.Z., M.G., A.E., I.S.); statistical expertise (B.Z., A.E.)
Correspondence: Isaak Schipper, MD, Augenklinik, Kantonsspital, CH-6000 Lucerne 16, Switzerland. Tel: 41 412 051 111; Fax: 41 412 053 406; E-mail: email@example.com
Excimer laser treatment of myopia is widely used, and several million eyes have been treated worldwide. Whereas results of the treatment of myopia were consistently good, the outcome of myopic astigmatism has not been as reliable. It was assumed that the axial misalignment may be a major factor for the inconsistent refractive outcome.1 Reasons for a misalignment include cyclotorsion after changes in body position from seated to supine, head tilting, head rotation, autorotation, and distortion of the globe by the lid speculum.2,3
To avoid misalignment of the eye, newer generation excimer lasers offer advanced iris-tracking devices. For surgeons not having access to such equipment, a simple method of corneal limbal marking was suggested to reduce the problem of misalignment.2,4–8 This technique showed a benefit in photorefractive keratectomy (PRK) of myopic eyes with >1.25 diopters (D) of astigmatism.6
The purpose of this study was to analyze the influence of limbal marking on the refractive and visual outcomes of eyes with astigmatic myopia undergoing LASIK and laser epithelial keratomileusis (LASEK).
Patients and Methods
A total of 108 eyes were included in this retrospective study. Inclusion criteria were myopia >0.50 D and astigmatism >0.50 D. Exclusion criteria were myopia >8.00 D, astigmatism >4.00 D, and previous refractive laser treatments. The study group consisted of 47 eyes with limbal markings (marked group), and the remaining unmarked 61 eyes formed the control group.
The marking of the limbal conjunctiva was performed horizontally, at the 3 and 9 o’clock positions, with a surgical skin marker (Visimark, Bedfordshire, UK) at the slit lamp under topical anesthesia. The patient was then positioned under the laser unit, and a speculum was applied. The 0° axis of the microscope’s reticle was aligned with the 0° axis of the laser. The patient was urged to look at the blinking fixation light. Whenever the 0° axis of the reticle was not congruent with the limbal markings, the head of the patient was moved until congruency was achieved.
All laser treatments were performed with a Technolas 217 excimer laser (Bausch & Lomb, Feldkirchen, Germany) by a single laser surgeon (I.S.). The eye tracker device of the Technolas 217 laser allowed for the measurement of eye positions and movements in the x-axis and y-axis, but did not allow tracking rotations of the eye.
Twenty-three eyes (of 47 eyes) of the marked group underwent LASIK and 24 eyes underwent LASEK treatment. Within the unmarked group, 31 eyes (of 61 eyes) underwent LASIK and 30 eyes underwent LASEK treatment. For LASEK, a 9.0-mm corneal trephine (Janach J2901; E. Janach srl, Como, Italy) was used to create an epithelial incision. A 9.5-mm holding well (alcohol solution cone, Janach J2906; E. Janach srl) was placed on the cornea, filled with ethyl alcohol (20%) and left for 20 seconds. The epithelium was detached with a microhoe (Janach J2915, E. Janach srl) and folded upwards. After the ablation, the epithelium was repositioned with a blunt-tipped spatula (Janach J2920, E. Janach srl) and temporarily covered with a bandage contact lens (Acuvue; Johnson & Johnson, New Brunswick, New Jersey). LASIK was performed with a Hansatome microkeratome (Bausch & Lomb), creating a superiorly hinged flap of 160-μm thickness and 8.5- to 9.0-mm diameter.
Analysis was performed on the data from the 12-month follow-up. Patients were refracted by in-house optometrists and uncorrected and best-corrected distance Snellen visual acuity was converted into decimal. Corneal astigmatism was assessed by Orbscan topography (Bausch & Lomb).
Refractive and topographic data were analyzed using vector analysis as described by Alpins.9,10 Briefly, the target induced astigmatism vector is the astigmatic change the surgery was intended to induce. The actual induced change in amount and axis of astigmatism following surgery is represented by the surgically induced astigmatism vector. The angle of error is the angle between the targeted induced astigmatism and the surgically induced astigmatism. It is designated as negative if the surgically induced astigmatism vector is clockwise to the target induced astigmatism vector and positive if counterclockwise and is the principle measure of misalignment of astigmatism treatment. The difference vector is the required astigmatic change a second surgery would need to achieve to reach the initial target. The magnitude of error is the arithmetic difference between the magnitude of the surgically induced astigmatism and target induced astigmatism. Magnitude of error is positive for overcorrections and negative for undercorrections. The index of success represents the relationship of the difference vector to the target induced astigmatism (difference vector/target induced astigmatism). It provides a relative measure of surgical success and is preferably zero. Refractive measures of error alone were used to direct treatment in this study. Topographic data were not used to modify the refractive plan. In any eye thus treated, if a difference exists between topographic and refractive cylinder (a status present in most eyes, at least to a small degree) then the topographic cylinder will be altered by the treatment and not fully corrected. How much it is reduced depends on how closely it coincides with the refractive cylinder, and if a wide disparity exists, the topographic cylinder may even increase. Ocular residual astigmatism11,12 is the vectorial difference between the preoperative corneal plane refractive and topographic cylinders and is a measure of the minimum total cylinder (refractive, topographic, or combined) that can be expected in any individual eye for a given refraction or topographic surgical plan for that eye. The ocular residual astigmatism for a surgical plan fully directed at refractive cylinder correction is reported herein.
The Internet Based Refractive Analysis Software (IBRA; Zubisoft Corp, Oberhasli, Switzerland)13 was used for refractive analysis along with SPSS software (SPSS Inc, Chicago, Illinois) for statistical purposes. Vector analysis was carried out using Microsoft Excel 2003 (Microsoft Corp, Redmond, Washington). For comparative statistics, the Wilcoxon matched pairs test was used, and for independent samples, the Mann-Whitney U test was used. P values <.05 were considered statistically significant.
The marked study group comprised 47 eyes (mean patient age 34±7 years [range: 22 to 47 years]). This group consisted of 24 eyes from female patients and 23 eyes from male patients. The unmarked control group comprised 61 eyes (mean patient age 37±9 years [range: 20 to 67 years]). This group consisted of 25 eyes from female patients and 36 eyes from male patients. Statistically, no difference was noted between the marked and unmarked group regarding patient demographics, preoperative uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA), preoperative refractive astigmatism, and spherical equivalent refraction (Table 1).
Table 1: Visual Outcome (Decimal) and Refractive Outcome (D) at 12-Month Follow-Up for Eyes that Underwent LASIK and LASEK with and Without Limbal Marking
Mean distance UCVA (decimal) increased from 0.07 in both groups to 0.81 in the marked group and 0.77 in the unmarked group at 12 months (Table 1). The UCVA was 1.0 or better in 53% of eyes in the marked group and 38% of eyes in the unmarked group (Fig 1). The UCVA was 0.5 or better in 96% of eyes in the marked group and 97% of eyes in the unmarked group. Statistically, no significant difference in UCVA was noted between groups.
Figure 1. Uncorrected Visual Acuity for the Unmarked and Marked Groups 12 Months After Excimer Laser Treatment for Myopic Astigmatism.
Mean preoperative distance BSCVA (decimal) was 0.95 in both groups. Mean BSCVA was 0.94 in the marked group and 1.00 in the unmarked group at 12 months (Table 1). In the marked group, no eye lost more than 2 Snellen lines of BSCVA. One (2%) eye lost 2 lines; 8 (17%) eyes lost 1 line; 29 (62%) eyes remained unchanged; and 9 (19%) eyes gained 1 line (Fig 2). In the unmarked group, no eye lost more than 2 Snellen lines of BSCVA. One (2%) eye lost 2 Snellen lines; 6 (10%) eyes lost 1 line; 37 (60%) eyes remained unchanged; 16 (26%) eyes gained 1 line; and 1 (2%) eye gained more than 2 Snellen lines in BSCVA (see Fig 2). Statistically, no difference could be found between groups regarding BSCVA.
Figure 2. Safety of the Unmarked and Marked Groups 12 Months After Excimer Laser Treatment for Myopic Astigmatism.
Postoperative mean refractive astigmatism improved from 1.30 D to 0.31 D in the marked group and from 1.09 D to 0.41 D in the unmarked group at final follow-up (P=.333, Table 1). At 12 months, the refractive astigmatism was <0.50 D in 62% of eyes in the marked group and 52% of eyes in the unmarked group (P=.120, Fig 3). Statistically, no difference was noted in postoperative refractive astigmatism between groups.
Figure 3. Subjective Astigmatism of the Unmarked and Marked Groups 12 Months After Excimer Laser Treatment for Myopic Astigmatism.
Postoperative mean corneal astigmatism was 0.60 D for the marked group and 0.57 D for the unmarked group (P=.952, Table 1). At 12 months, corneal astigmatism was <0.50 D in 43% of eyes in the marked group and 39% of eyes in the unmarked control group.
Mean spherical equivalent refraction changed from −3.86 to −0.04 D in the marked group and from −3.99 to 0.05 D in the unmarked group after laser surgery (Table 1). At 12 months, 92% of eyes in the marked group and 75% of eyes in the unmarked group were within ±0.50 D of emmetropia (Fig 4).
Figure 4. Spherical Equivalent Refraction of the Unmarked and Marked Groups 12 Months After Excimer Laser Treatment for Myopic Astigmatism.
The results of vector analysis are summarized in Table 1. Angle of error did not differ significantly (either in signed or absolute terms) between groups. Statistically, no difference could be seen between the refractive target induced astigmatism, surgically induced astigmatism, difference vector, magnitude of error, index of success, or ocular residual astigmatism between the marked and unmarked groups.
Subgroup analysis showed no influence of the limbal marking on eyes treated either with LASIK or LASEK (Table 2). The limbal marking did not show a benefit in eyes of a third subgroup with a preoperative refractive astigmatism >1.25 D (Table 2).
Table 2: Visual Outcome (Decimal) and Refractive Outcome (D) for the LASIK, LASEK, and Higher Astigmatism Subgroups at Final Follow-Up
No intraoperative complications were encountered. In two eyes of the marked group (4.2% of all marked eyes) and three eyes of the unmarked group (4.9% of all unmarked eyes), a retreatment was necessary to improve the refractive and uncorrected visual outcome (all 0.63 decimal). Following retreatment, the UCVA remained unchanged at 0.63 (decimal) for one eye of the unmarked group, improved to 0.8 in two eyes of the marked group, and improved to 1.0 in two eyes of the unmarked group. The results following retreatment were not used for analysis in this study.
Axial misalignment secondary to head rotation was found to be as high as 14° (mean 4.4°) in LASIK, and cyclotorsional movements ranged between −13° and +17° during PRK.2,14 Misalignment due to eye rotation has been implicated in the lack of predictability in the outcome of treatment of higher corneal astigmatism.2 In fact, a misalignment of only 5° between the laser axis and the axis of the corneal astigmatism can result in 17% of undercorrection.1 Using a simple technique with limbal marking, Farah et al6 were able to improve the refractive outcome in a high astigmatism (≥1.25 D) subgroup.
The present study did not show any benefit from limbal corneal marking on refractive outcome. The achieved cylinder reduction, spherical reduction, and refractive predictability were similar for the marked and unmarked groups in the overall study collective (Table 1), in the LASIK and LASEK subgroups (Table 2), and in a higher astigmatism subgroup (>1.25 D). Further, the limbal marking showed no influence on visual acuity. The final distance UCVA and the change in distance BSCVA were statistically similar between the marked and unmarked groups for the overall collective and the three subgroups. An improvement of the visual outcome could be noticed during the first year for the marked and unmarked group, likely attributable to wound healing mechanisms and decreasing dry eye problems (reinnervation of the cornea).
Vector analysis of misalignment of the astigmatic treatment with the intended axis (the preoperative refractive cylinder axis in this study) is expressed as the angle of error. This is the angle between the target induced astigmatism (amount and direction of astigmatic change to achieve the targeted postoperative cylinder value) and the surgically induced astigmatism (achieved astigmatic change). The angle of error was calculated both in arithmetic terms (negative when the surgically induced astigmatism is clockwise from the target induced astigmatism and positive when counterclockwise) and absolute terms. A mean of the former will be close to zero if the spread of axes is equal in both clockwise and counterclockwise directions, even if the magnitude is large. The absolute mean overcomes this problem. In both groups, absolute angle of error is highly skewed with the bulk of observations close to zero. An exact Mann-Whitney U test of this variable shows the two groups to be similar (P=.299).
The difference vector is the vector value of the astigmatic change that would be required to reach the intended target cylinder in an eye that has not reached its target postoperatively. It is influenced both by errors in the power of the cylinder change and misalignment. The mean difference vector is approximately the same between the two groups but a summated vector mean is a factor of approximately 3 larger for the unmarked group (0.20 D vs 0.07 D), approaching a clinically significant magnitude. A summated vector mean is a derived vector value arrived at by adding vector values tail to head and dividing the resultant vector by the total number of observations. It is a measure of systematic error in treatment but is not amenable to significance testing. In both groups, the difference vector is highly skewed with the bulk of observations close to zero. Median regression modeling shows that as the group moves from unmarked to marked, the median value of difference vector decreases by 0.0012 D. However, the true value of this difference could be anywhere between −0.256 and 0.253, the 95% confidence interval of the difference. If 0.25 D is taken as the equivalence limit, then the two groups could be considered to be equivalent with respect to difference vector. Because the summated vector mean is derived from this difference vector data, which seem equivalent, it would be unsafe to conclude that the difference between groups, although approaching clinical significance, is in fact different.
The index of success (difference vector magnitude divided by target induced astigmatism magnitude) is nearer the ideal zero in the marked group (0.31 D vs 0.62 D). It is contributed to both by errors of power correction and errors of alignment. If a treatment is misaligned, a difference vector is derivable. However, if a treatment is perfectly aligned but off-target in terms of its dioptric power, a difference vector will also be derivable. This is also highly skewed with the bulk of observations close to zero (the ideal value) and an exact Mann-Whitney U test shows the two groups to be similar (P=.165).
Compared to the refractive and visual outcomes published by other authors, the present results of the marked and unmarked (control) group meet the refractive standards for excimer laser treatments (LASIK and LASEK) in astigmatic myopia.15–22 The additional step of limbal marking was a safe procedure and did not change the complication rate in laser surgery.
The marking technique failed due to three main reasons. First, the eye rotates during excimer laser procedures. A continuous rotation as high as 2.33° was detected with a torsion error detector system during LASIK.23 As permanent rotations cannot be addressed by marking techniques, the resulting increase in bias can mask the potential benefit of limbal marking. Second, no consensus exists among surgeons as to whether the laser should be aligned with the axis of the refractive astigmatism or the axis of the corneal astigmatism. Unavoidably in the eye’s optical system, a significant difference in amount (>0.50 D) and axis (>10°) often exists between refractive and corneal astigmatism, which is generally considered to be a bad prognostic sign for the treatment of myopic astigmatic eyes with LASIK.12,24 As suggested by Alpins et al,25 the cornea may be less disturbed when the axis of treatment is aligned with the topographical axis. In this study, whenever a difference between the astigmatism was noted, we chose to treat the refractive cylinder and axis. Third, the positioning of the marks at the slit lamp is relatively inaccurate. However, this might not have a major influence on the outcome, as a study by Burka et al26 compared horizontal marking with and without the use of a slit lamp, resulting in no differences in the refractive outcome between the two methods.
Recently, the technology developed from simple eye tracking based on pupil or limbus recognition, to tracking of specific iris patterns. Chernyak27,28 described an automated alignment method based on iris patterns. Bharti and Bains29 compared prospectively the effectiveness and safety of eyes treated with and without active cyclotorsion compensation using the torsion error correction system. During the operation, the torsion error correction continuously scans the iris for preoperatively acquired landmarks and adjusts the laser beam so the landmarks match. Although this is a powerful tool, it did not lead to a difference in visual acuity and safety between the study and control groups.29 Similarly, no significant visual or refractive benefit could be achieved using wavefront-guided treatment with iris registration in comparison to a treatment without iris registration with the VISX STAR S4 system (AMO, Ettlingen, Germany).30
In our study, the corneal limbal marking technique failed to improve the visual and refractive results for the treatment of myopic astigmatism with LASIK and LASEK, independent of the amount of preoperative astigmatism. Further investigation in automated alignment methods, using dynamic iris pattern recognition, offers hope for better results.
- Stevens JD. Astigmatic excimer laser treatment: theoretical effects of axis misalignment. European Journal of Implantation and Refractive Surgery. 1994;6:310–318.
- Suzuki A, Maeda N, Watanabe H, Kiritoshi A, Shimomura Y, Tano Y. Using a reference point and videokeratography for intraoperative identification of astigmatism axis. J Cataract Refract Surg. 1997;23:1491–1495.
- Ciccio AE, Durrie DS, Stahl JE, Schwendeman F. Ocular cyclotorsion during customized laser ablation. J Refract Surg. 2005;21:S772–S774.
- Swami AU, Steinert RF, Osborne WE, White AA. Rotational mal-position during laser in situ keratomileusis. Am J Ophthalmol. 2002;133:561–562. doi:10.1016/S0002-9394(01)01401-5 [CrossRef]
- Huppertz M, Schmidt E, Teiwes W. Eye tracking and refractive surgery. In: MacRae SM, Krueger RR, Applegate RA, eds. Customized Corneal Ablation: The Quest for SuperVision. Thorofare, NJ: SLACK Incorporated; 2001:158.
- Farah SG, Olafsson E, Gwynn DG, Azar DT, Brightbill FS. Outcome of corneal and laser astigmatic axis alignment in photoastigmatic refractive keratectomy. J Cataract Refract Surg. 2000;26:1722–1728. doi:10.1016/S0886-3350(00)00695-7 [CrossRef]
- Smith EM Jr, Talamo JH, Assil KK, Petashnick DE. Comparison of astigmatic axis in the seated and supine positions. J Refract Corneal Surg. 1994;10:615–620.
- Tjon-Fo-Sang MJ, de Faber JT, Kingma C, Beekhuis WH. Cyclotorsion: a possible cause of residual astigmatism in refractive surgery. J Cataract Refract Surg. 2002;28:599–602. doi:10.1016/S0886-3350(01)01279-2 [CrossRef]
- Alpins NA, Goggin M. Practical astigmatism analysis for refractive outcomes in cataract and refractive surgery. Surv Ophthalmol. 2004;49:109–122. doi:10.1016/j.survophthal.2003.10.010 [CrossRef]
- Alpins N. Astigmatism analysis by the Alpins method. J Cataract Refract Surg. 2001;27:31–49. doi:10.1016/S0886-3350(00)00798-7 [CrossRef]
- Duke-Elder S, ed. Ophthalmic optics and refraction. In: System of Ophthalmology. Vol 5. St Louis, MO: Mosby; 1970:275–278.
- Alpins NA. New method of targeting vectors to treat astigmatism. J Cataract Refract Surg. 1997;23:65–75.
- Zuberbuhler B, Galloway P, Reddy A, Saldana M, Gale R. A web-based information system for management and analysis of patient data after refractive eye surgery. Comput Methods Programs Biomed. 2007;88:210–216. doi:10.1016/j.cmpb.2007.09.003 [CrossRef]
- Fea AM, Sciandra L, Annetta F, Musso M, Dal Vecchio M, Grignolo FM. Cyclotorsional eye movements during a simulated PRK procedure. Eye. 2006;20:764–768. doi:10.1038/sj.eye.6701994 [CrossRef]
- Balazsi G, Mullie M, Lasswell L, Lee PA, Duh YJ. Laser in situ keratomileusis with a scanning excimer laser for the correction of low to moderate myopia with and without astigmatism. J Cataract Refract Surg. 2001;27:1942–1951. doi:10.1016/S0886-3350(01)01017-3 [CrossRef]
- Stojanovic A, Nitter TA. 200 Hz flying-spot technology of the LaserSight LSX excimer laser in the treatment of myopic astigmatism: six and 12 month outcomes of laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg. 2001;27:1263–1277. doi:10.1016/S0886-3350(01)00996-8 [CrossRef]
- O’Doherty M, O’Keeffe M, Kelleher C. Five year follow up of laser in situ keratomileusis for all levels of myopia. Br J Ophthalmol. 2006;90:20–23. doi:10.1136/bjo.2005.075127 [CrossRef]
- Payvar S, Hashemi H. Laser in situ keratomileusis for myopic astigmatism with the Nidek EC-5000 laser. J Refract Surg. 2002;18:225–233.
- Shaikh NM, Manche EE. Laser in situ keratomileusis for myopia and compound myopic astigmatism using the Technolas 217 scanning-spot laser. J Cataract Refract Surg. 2002;28:485–490. doi:10.1016/S0886-3350(01)01287-1 [CrossRef]
- Partal AE, Rojas MC, Manche EE. Analysis of the efficacy, predictability, and safety of LASEK for myopia and myopic astigmatism using the Technolas 217 excimer laser. J Cataract Refract Surg. 2004;30:2138–2144. doi:10.1016/j.jcrs.2004.02.083 [CrossRef]
- Shahinian L Jr, . Laser-assisted subepithelial keratectomy for low to high myopia and astigmatism. J Cataract Refract Surg. 2002;28:1334–1342. doi:10.1016/S0886-3350(02)01444-X [CrossRef]
- Taneri S, Feit R, Azar DT. Safety, efficacy, and stability indices of LASEK correction in moderate myopia and astigmatism. J Cataract Refract Surg. 2004;30:2130–2137. doi:10.1016/j.jcrs.2004.02.070 [CrossRef]
- 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.
- Bragheeth MA, Dua HS. Effect of refractive and topographic astigmatic axis on LASIK correction of myopic astigmatism. J Refract Surg. 2005;21:269–275.
- Alpins NA, Tabin GC, Adams LM, Aldred GF, Kent DG, Taylor HR. Refractive versus corneal changes after photorefractive keratectomy for astigmatism. J Refract Surg. 1998;14:386–396.
- Burka JM, Bower KS, Cute DL, Stutzman RD, Subramanian PS, Rabin JC. Comparison of two techniques of marking the horizontal axis during excimer laser keratorefractive surgery for myopic astigmatism. Am J Ophthalmol. 2005;139:735–737. doi:10.1016/j.ajo.2004.09.080 [CrossRef]
- Chernyak DA. Cyclotorsional eye motion occurring between wavefront measurement and refractive surgery. J Cataract Refract Surg. 2004;30:633–638. doi:10.1016/j.jcrs.2003.08.022 [CrossRef]
- Chernyak DA. Iris-based cyclotorsional image alignment method for wavefront registration. IEEE Trans Biomed Eng. 2005;52:2032–2040. doi:10.1109/TBME.2005.857674 [CrossRef]
- Bharti S, Bains HS. Active cyclotorsion error correction during LASIK for myopia and myopic astigmatism with the NIDEK EC-5000 CX III laser. J Refract Surg. 2007;23:1041–1045.
- Moshirfar M, Chen MC, Espandar L, Meyer JJ, Christensen D, Christiansen SM, Dave SB, Bedke B, Kurz C. Effect of iris registration on outcomes of LASIK for myopia with the VISX Custom-Vue platform. J Refract Surg. 2009;25:493–502.
Visual Outcome (Decimal) and Refractive Outcome (D) at 12-Month Follow-Up for Eyes that Underwent LASIK and LASEK with and Without Limbal Marking
|Mean±Standard Deviation (Range)|
|UCVA||0.07±0.21 (0.05 to 0.40)||0.07±0.19 (0.05 to 0.50)||.251||0.81±0.12 (0.32 to 1.25)||0.77±0.14 (0.32 to 1.25)||.405|
|BSCVA||0.95±0.08 (0.50 to 1.25)||0.95±0.07 (0.50 to 1.25)||.880||0.94±0.07 (0.50 to 1.25)||1.00±0.07 (0.63 to 1.25)||.083|
|SEQ||−3.86±1.80 (−7.38 to −0.25)||−3.99±1.65 (−6.75 to −1.00)||.706||−0.04±0.32 (−0.75 to 0.75)||0.05±0.57 (−1.25 to 1.88)||.213|
|Refractive astigmatism||1.30±0.70 (0.75 to 3.25)||1.09±0.61 (0.75 to 2.75)||.091||0.31±0.33 (0.00 to 1.25)||0.41±0.44 (0.00 to 2.85)||.333|
|Corneal astigmatism||1.48±0.84 (0.00 to 3.75)||1.22±0.71 (0.00 to 3.00)||.158||0.60±0.55 (0.00 to 2.00)||0.57±0.51 (0.00 to 2.00)||.952|
|Refractive vector analysis|
| Target induced astigmatism||1.14±0.62||0.95±0.58||.110|
| Surgically induced astigmatism||1.16±0.73||1.06±0.71||.666|
| Difference vector||0.36±0.42||0.41±0.42||.519|
| Angle of error||−0.14±10.40||2.05±20.98||.485|
| Absolute value of angle of error||6.20±8.36||10.90±18.04||.299|
| Index of success||0.32±0.32||0.61±0.96||.165|
| Magnitude of error||0.01±0.35||0.11±0.48||.237|
| Ocular residual astigmatism||0.81±0.57||0.85±0.58||.764|
Visual Outcome (Decimal) and Refractive Outcome (D) for the LASIK, LASEK, and Higher Astigmatism Subgroups at Final Follow-Up
| Eyes (n)||23||31|
| Refractive astigmatism||0.32±0.31||0.31±0.33||.994|
| Eyes (n)||24||30|
| Refractive astigmatism||0.30±0.35||0.50±0.52||.120|
|Higher astigmatism (>1.25 D*)|
| Eyes (n)||25||23|
| Refractive astigmatism||0.43±0.37||0.40±0.32||.708|