The refractive outcome of astigmatism correction has been shown to depend on the accuracy of the axis treatment. Any rotational movement of the eye (cyclotorsion) during treatment may cause a shift in the treatment axis, leading to undesirable results such as undercorrection and induction of aberrations.1–3 Most of the currently available excimer laser platforms have the ability to detect and compensate for the static and dynamic cyclotorsion due to positional changes and treatment by advanced software and eye tracking.1,4
Active cyclotorsion error correction in LASIK improved the accuracy of cylinder correction.4–6 Although there are numerous studies on cyclotorsion and its compensation in LASIK, significant data do not exist for the small incision lenticule extraction (SMILE) procedure. Studies on femtosecond lenticule extraction and SMILE have shown significant undercorrection of astigmatism over time.7–9 The probable explanation of these results could be the unavailability of an active eye tracking software in the VisuMax femtosecond laser system (Carl Zeiss Meditec, Jena, Germany) used to perform these procedures. Because no definite method of cyclotorsion compensation exists for SMILE, this may also be considered a potential limitation of this procedure.
Previous studies on LASIK suggested that manual markings were equally safe and effective as the automated dynamic eye trackers for cyclotorsion compensation during surgery.10 Based on these observations, we attempted to investigate the feasibility of manual compensation for the intraoperative torsional error by using limbal markings as a guide in patients undergoing SMILE for myopic astigmatism.
We describe a simple and practical method for the manual compensation of cyclotorsion during SMILE in patients with significant myopic astigmatism (> 0.75 diopters [D]). We evaluated the safety, efficacy, and reliability of this technique in terms of cylindrical correction and postoperative refractive outcomes in a prospective, interventional study with 3-month follow-up.
Patients and Methods
This prospective, nonrandomized study was approved by our institutional ethics committee and adhered to the tenets of the Declaration of Helsinki. Written informed consent was obtained from all patients participating in the study.
Inclusion criteria were age between 21 and 40 years, myopic astigmatism with up to −10.00 D spherical equivalent (SE) with a minimum astigmatism of 0.75 D, stable refraction (< 0.50 D change in the past 12 months), corrected distance visual acuity (CDVA) of 20/30 or better, healthy ocular surface, absence of corneal ectatic diseases, corneal scars, absence of any retinal pathology likely to affect visual outcomes, and ensured follow-up visits. Eyes with thin corneas (central corneal thickness < 480 µm), diagnosed or suspicious cases of corneal ectatic conditions, severe dry eyes, and contact lens–induced allergy, patients taking systemic steroids, immunosuppressants, oral contraceptives, or antidepressants, and pregnant females were excluded from the study.
All patients underwent a thorough preoperative evaluation including anterior and posterior segment examination, cycloplegic and subjective refraction, assessment of uncorrected distance visual acuity (UDVA) and CDVA, corneal topography using the Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany) and Orbscan II (Bausch & Lomb, Rochester, NY), contrast sensitivity using the Functional Acuity Contrast Test (F.A.C.T. Stereo Optical Co., Inc., Chicago, IL), aberrometry (Hoya iTrace Surgical Workstation; Hoya Surgical Optics, Tokyo, Japan), specular microscopy (Tomey, Nagoya, Japan), and dry eye evaluation (Schirmer 1 and tear film break-up time).
Patients using soft or rigid contact lenses were instructed to discontinue their lenses at least 1 and 3 weeks, respectively, prior to the topographic evaluation.
Regardless of the degree of myopia, a 10% overcorrection nomogram was applied to both the spherical and cylindrical components of the refractive error for all eyes.
All surgeries were performed by a single experienced refractive surgeon (SG) under topical anesthesia using the VisuMax femtosecond laser, with a pulse repetition rate of 500 kHz, cut energy of 160 nJ with a spot separation of 4.5 µm, 6.5- to 7-mm optical zone, cap thickness of 120 µm, and 2-mm superior incision. The treatment was centered on the visual axis. Mean optical zone used was 6.50 ± 0.23 mm.
Preoperatively, the limbus was marked in the 0° to 180° axis with a dye permitting the transmission of infrared radiation (Viscot surgical skin marker 1436; Viscot Medical, East Hanover, NJ) using either a marker pen or Ganesh bubble marker (Epsilon Surgical, Chino, CA) (Figure A, available in the online version of this article) in the upright position. The patient was then positioned under the VisuMax femtosecond laser and instructed to look into the green flashing fixation light. Once proper centration was achieved, the eye was docked to the patient interface and suction was applied. The extent of cyclotorsion, if any, was determined using the reticule (present in the right eyepiece) and any cyclotorsion (incyclotorsion or excyclotorsion) was manually compensated for by gently rotating the contact glass to align the horizontal marks on the eye to the 0° to 180° axis of the reticule (Figure A and Video 1, available in the online version of this article). Once both were aligned, the active laser process was started to create the refractive lenticule. After lenticule creation by the femtosecond laser, the anterior and posterior lenticule planes were dissected using a blunt dissector, followed by lenticule extraction through the side cut. The interface was washed with balanced salt solution.
(A) Preoperative limbal marking with the Ganesh bubble marker (Epsilon Surgical, Chino, CA) under topical anesthesia in the upright position. This instrument uses three marks on the limbus at 0°, 90°, and 180°, extending 2 mm toward the center of the cornea, which are easy to visualize while the eye is being docked. (B) Method of manual cyclotorsion compensation by a gentle rotation of the cone while holding the same at the attachment of the tube to the cone. (C) Position of the limbal marks (red arrows) under suction ‘ON’ condition without cyclotorsion compensation before starting the laser, showing approximately 12° of cyclotorsion. (D) Final position of the limbal marks after manual compensation of the cyclotorsion error (alignment with the horizontal axis of the eyepiece reticule). Delivery of the laser follows this.
Postoperative medications included topical 0.3% ofloxacin (Exocin; Allergan, Irvine, CA) four times for 3 days, 0.1% prednisolone acetate eye drops (Pred Forte; Allergan) in tapering dosage for 4 weeks, and lubricants four times for 4 weeks or more.
All surgeries were uneventful and no complications such as suction loss, black spots, difficult dissection, or incomplete separation of lenticule due to the ink marks blocking the laser occurred in any of the eyes.
Patients were followed up at postoperative 1 day, 2 weeks, and 3 months. On all follow-up visits from 2 weeks on, assessment of UDVA, manifest refraction, CDVA, and topography were also performed.
SPSS for Windows software (version 17.0.0; IBM Corporation, Armonk, NY) was used for statistical analysis. All values were expressed as mean ± standard deviation (SD). The independent samples t test was performed for intergroup comparison and the paired t test was used for intragroup comparison of means. A P value of .05 or less was considered statistically significant. Standard refractive graphs were generated using Datagraph-med 5.20 software ( http://www.datagraph.eu).
Vector Analysis of Astigmatism
Only the left eyes were included in the analysis. Astigmatism outcomes were reported according to the standardized format.11–13 Refractive astigmatism at the spectacle plane was converted to the corneal plane using a vertex distance of 12 mm. It was then analyzed with the vector analysis of Alpins using the Assort software (ASSORT Pty. Ltd., Victoria, Australia), with consideration of the change in the astigmatic axis, measuring three vectors and relationships among them. Target induced astigmatism vector (TIA) was defined as the astigmatic change that the surgery was intended to induce, surgically induced astigmatism vector (SIA) was defined as the astigmatic change that the surgery actually induced, and the difference vector (DV) was defined as the induced astigmatic change that would enable the initial surgery to achieve its intended target or the postoperative astigmatism. Magnitude of error (ME) is the arithmetic difference between the SIA and TIA. Angle of error (AE) is the angle between the axis of the SIA and TIA. The flattening index (FI) is a measure of the impact of an astigmatic treatment at off-axis orientation on the astigmatic change at its intended axis.
A total of 81 left eyes from 81 patients were analyzed. The eyes were categorized into low (≤ 1.50 D, n = 37) and high (> 1.50 D, n = 44) cylinder groups, based on the magnitude of preoperative astigmatism. The preoperative patient characteristics are given in Table 1.
Preoperative Baseline Characteristics of Patients (N = 81)
Incyclotorsion was more commonly observed (44% of eyes) compared to excyclotorsion (38% of eyes), whereas 18% of eyes did not show any cyclotorsion. However, for convenience of analysis, both incyclotorsion and excyclotorsion were considered broadly as “cyclotorsion” and separate results were not analyzed for incyclotorsion and excyclotorsion.
Overall, the average cyclotorsion observed was 5.64° ± 2.55° (range: 2° to 12°). The magnitude of cyclotorsion was 5° or less in 81%, between 6° and 10° in 17.6%, and 10° or greater in 1.2% of eyes. The mean cyclotorsion was comparable in both the low (5.73° ± 2.8°) and high (5.53° ± 2.29°) cylinder groups, with no statistically significant difference between their values (P = .822) (Table 1).
The mean UDVA showed significant improvement in both groups from 2 weeks to 3 months postoperatively (P < .05 for both groups). However, there was no statistically significant improvement in CDVA in either group over time (Table A, available in the online version of this article).
Visual and Refractive Results
Efficacy (Postoperative UDVA/Preoperative CDVA)
All eyes had UDVA of 20/32 or better at 3 months, with 84% eyes with UDVA of 20/20 or better (Figure 1). However, the percentage of eyes achieving UDVA of 20/20 or better was higher in the low (95%) versus the high (75%) cylinder group (Figures B–C, available in the online version of this article).
Standard graphs for visual outcomes for total eyes. UDVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity; D = diopters; SEQ = spherical equivalent refraction
Standard graphs for visual outcomes for high cylinder eyes. UCVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity; D = diopters; SEQ = spherical equivalent refraction
Standard graphs for visual outcomes for low cylinder eyes. UDVA = uncorrected distance visual acuity; CDVA = corrected distance visual acuity; D = diopters; SEQ = spherical equivalent refraction
Safety (Postoperative CDVA/Preoperative CDVA)
Overall, 56% eyes gained one or more lines of CDVA at the end of the 3-month follow-up (Figure 1). In the high cylinder group, the gain in CDVA was greater (60%) compared to the low cylinder group (54%) (Figures B–C). No eye in either group had loss of CDVA compared to preoperative corrected vision.
The mean SE reduced from −5.00 ± 2.18 D preoperatively to −0.20 ± 0.24 D at the end of the 3-month follow-up (P = .000). A total of 95% eyes were within ±0.50 D, whereas all eyes were within ±1.50 D of SE correction (Figure 2, Table A).
Predictability scatter (attempted vs achieved) of cylindrical refraction with small incision lenticule extraction (SMILE) at 3 months. Data for (A) total (N = 81 eyes), (B) low cylinder (n = 37 eyes), and (C) high cylinder (n = 44 eyes) groups. Above the white line in the middle is overcorrection and below is undercorrection. The red solid line indicates the outcome of linear regression analysis.
Overall, the predictability of cylinder correction was excellent, with all eyes within ±1.00 D of astigmatism correction at 3 months (Figure 2). However, the predictability was slightly better in the low (average deviation: −0.04 D) versus the high (average deviation: −0.21 D) cylinder group, indicating more undercorrection of astigmatism treatment in the latter group (Figure 2).
The postoperative SE and cylinder remained fairly stable, with no significant difference between 2 weeks and 3 months (P > .05) (Figure 1, Figures B–C).
The vector analysis results of the 81 eyes using 2-week and 3-month refractive data are shown in Tables B–C (available in the online version of this article). No significant differences were found for SIA, DV, AE, correction index (CI), ME, IOS, and FI between 2 weeks and 3 months (P > .05).
Vector Analysis of Total (N = 81) Eyes Undergoing SMILE With Manual Cyclotorsion Compensation Postoperatively
Subgroup Analysis of Astigmatic Correction Based on Degree of Target-Induced Astigmatism After SMILE
Intergroup analysis at 3 months showed that there was no significant difference between the low and high cylinder groups in terms of the DV, CI, ME, and FI. However, the absolute AE and IOS was significantly higher in the low cylinder compared to the high cylinder group (P = .032 and .024 for AE and IOS, respectively) (Figure 3). On the other hand, the ME was higher in the high cylinder compared to the low cylinder group (P = .032) (Tables B–C).
Index of success (IOS) for (A) low cylinder and (B) high cylinder groups at 3 months.
Previous studies evaluating astigmatism correction showed superior results with femtosecond laser–assisted LASIK compared to SMILE.14,15 Chan et al.14 showed that the alignment of treatment was more variable in SMILE, leading to a lower efficacy compared to LASIK at 3 months in eyes with low to moderate myopic astigmatism.
Ivarsen et al.16 demonstrated a significant undercorrection of astigmatism and an increased error of treatment with higher attempted cylinder correction. The reported undercorrection was 13% per diopter of attempted cylinder correction in low astigmatic and 16% per diopter in highly astigmatic eyes. This was mainly attributed to noncompensation of errors of cyclotorsion and nonapplication of nomograms during the procedure.
However, in contrast to the study by Ivarsen et al., improved results for astigmatism treatment were achieved in the current study with SMILE. Because we attempted the compensation of cyclotorsion manually and also applied a 10% nomogram (based on the postoperative results of our initial cases of SMILE, which showed undercorrection of up to 1.00 D), the accuracy of astigmatism correction observed was far better compared to the results reported in previous studies.14–16 This was indicated by the high CIs of 0.97 and 0.93 for the low and high cylinder groups, respectively, suggesting an undercorrection of 3% and 7%, respectively, which is approximately 0.25 D, at 3 months. One reason for more undercorrection in the high cylinder group may be a lower mean age compared to the low cylinder group; hence patients in this group were younger with different corneal biomechanics and epithelial healing patterns, which could have given rise to the undercorrection.17 Also, higher degrees of cylinders were shown to have a tendency for greater undercorrection over time.16 However, from a clinical point of view, slight undercorrection would be preferred to overcorrection because a change in the direction of the cylinder axis would probably be poorly accepted by patients.
Although the CI was close to 1 (0.97), the IOS had a higher and thus less favorable numerical value than the high cylinder group, indicating greater misalignment in the low cylinder group. Although the absolute AE and IOS were significantly higher in the low cylinder group compared to the high cylinder group, indicating a suboptimal correction of astigmatism, the visual outcomes in terms of UDVA were similar in both groups at 3 months. This may suggest that it may not be compulsory to manually correct for cyclotorsion in lower degrees of astigmatism in SMILE. The accuracy of treatment of lower degrees of astigmatism would depend on various factors, such as the reliability of preoperative measurement of the magnitude and axis of the cylinder and manual marking, and also assessment of the extent of intraoperative cyclotorsion. Therefore, the advantages of manual compensation in low cylinders may not be ascertained, especially if the intraoperative cyclotorsion is less (< 5°). However, it may have a definite advantage in moderate to high degrees of astigmatism because even minimal meridional errors may have significant negative refractive consequences.18
In this study, the magnitude of intraoperative cyclotorsion was 5° or less in most eyes (81.6%). However, rotation of greater than 5° was found in 18.4% of eyes and 10° or greater in 1.2% of eyes. Studies have shown that rotation of 5° or greater can induce significant undercorrection of the astigmatic component of the refraction and that undercorrection is more noticeable as the degree of astigmatism increases.19,20 Hence, based on the results of this study, we recommend manual compensation of cyclotorsion error for all eyes with high astigmatism (> 1.50 D) with any degree of cyclotorsion and low astigmatism (≤ 1.50 D) with 5° or greater, using the technique described.
In terms of flattening effect achieved, our study showed better FI (0.93) at 3 months compared to the study by Kobashi et al.,21 which showed insufficient flattening with both femtosecond lenticule extraction and SMILE (median flattening indices of 0.79 to 0.80).
There were two potential limitations to this study. First, we did not compare our results with a control group without cyclotorsion compensation. This was done on ethical grounds because we had observed some cases with suboptimal outcomes with high cylinders, and after using this technique our outcomes with astigmatism improved significantly. Second, we determined the postoperative astigmatism at 3 months, when the corneal shape was considered to have been stabilized, taking into account the wound healing responses of the cornea. However, a longer follow-up may be desirable to evaluate the long-term stability of the results obtained with this technique. Also, we did not study the corneal aberrations and their relationship with cylinder undercorrection. The main purpose of the study was to report the outcomes with the technique of manual compensation for cyclotorsion in SMILE because, to the authors' knowledge, this has not been reported earlier. In addition, we wanted to analyze the results of vector analysis of astigmatism with this technique for low cylinder (≤ 1.50 D) and high cylinder (> 1.50 D) eyes, and provide future recommendations for using this technique based on our experience.
Nevertheless, to the best of our knowledge, this is the first preliminary report on the outcomes of astigmatism correction in SMILE using manual compensation. In our experience, gentle rotation of the cone does not lead to loss of suction and no complications occur due to limbal marking. Hence, it may be a safe, simple, and effective approach to improve results of astigmatism with SMILE in the absence of an active eye tracker in the current version of the VisuMax femtosecond laser. However, prospective randomized controlled studies with a longer follow-up may be necessary to confirm the validity of our results.
- Febbraro JL, Koch D, 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. doi:10.1016/j.jcrs.2010.05.019 [CrossRef]
- Swami AU, Steinert RF, Osborne WE, White AA. Rotational malposition during laser in situ keratomileusis. Am J Ophthalmol. 2002;133:561–562. doi:10.1016/S0002-9394(01)01401-5 [CrossRef]
- Arba-Mosquera S, Merayo-Lloves J, de Ortueta D. Clinical effects of pure cyclotorsional errors during refractive surgery. Invest Ophthalmol Vis Sci. 2008;49:4828–4236. doi:10.1167/iovs.08-1766 [CrossRef]
- Reinstein DZ, Gobbe M, Gobbe L, Archer TJ, Carp GI. Optical zone centration accuracy using corneal fixation-based SMILE compared to eye tracker-based femtosecond laser-assisted LASIK for myopia. J Refract Surg. 2015;31:586–592. doi:10.3928/1081597X-20150820-03 [CrossRef]
- 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. doi:10.1016/j.clae.2011.02.012 [CrossRef]
- Lazaridis A, Droutsas K, Sekundo W. Topographic analysis of the centration of the treatment zone after SMILE for myopia and comparison to FS-LASIK: subjective versus objective alignment. J Refract Surg. 2014;30:680–686. doi:10.3928/1081597X-20140903-04 [CrossRef]
- Kunert KS, Russmann C, Blum M, Sluyterman WG. Vector analysis of myopic astigmatism corrected by femtosecond refractive lenticule extraction. J Cataract Refract Surg. 2013;39:759–769. doi:10.1016/j.jcrs.2012.11.033 [CrossRef]
- Blum M, Kunert KS, Engelbrecht C, Dawczynski J, Sekundo W. Femtosecond lenticule extraction (FLEx): results after 12 months in myopic astigmatism [article in German]. Klin Monbl Augenheilkd. 2010;227:961–965. doi:10.1055/s-0029-1245894 [CrossRef]
- Sekundo W, Kunert KS, Blum M. Small incision corneal refractive surgery using the small incision lenticule extraction (SMILE) procedure for the correction of myopia and myopic astigmatism: results of a 6-month prospective study. Br J Ophthalmol. 2011;95:335–339. doi:10.1136/bjo.2009.174284 [CrossRef]
- Shen EP, Chen WL, Hu FR. Manual limbal markings versus iris-registration software for correction of myopic astigmatism by laser in situ keratomileusis. J Cataract Refract Surg. 2010;36:431–436. doi:10.1016/j.jcrs.2009.10.030 [CrossRef]
- Alpins NA. A new method of analyzing vectors for changes in astigmatism. J Cataract Refract Surg. 1993;19:524–533. doi:10.1016/S0886-3350(13)80617-7 [CrossRef]
- Alpins NA. New method of targeting vectors to treat astigmatism. J Cataract Refract Surg. 1997;23:65–75. doi:10.1016/S0886-3350(97)80153-8 [CrossRef]
- Alpins NA. Vector analysis of astigmatism changes by flattening, steepening, and torque. J Cataract Refract Surg. 1997;23:1503–1514. doi:10.1016/S0886-3350(97)80021-1 [CrossRef]
- Chan TC, Ng AL, Cheng GP, et al. Vector analysis of astigmatic correction after small-incision lenticule extraction and femtosecond-assisted LASIK for low to moderate myopic astigmatism. Br J Ophthalmol. 2016;100:553–559. doi:10.1136/bjophthalmol-2015-307238 [CrossRef]
- Zhang J, Wang Y, Wu W, Xu L, Li X, Dou R. Vector analysis of low to moderate astigmatism with small incision lenticule extraction (SMILE): results of a 1-year follow-up. BMC Ophthalmol. 2015;15:8. doi:10.1186/1471-2415-15-8 [CrossRef]
- Ivarsen A, Asp S, Hjortdal J. Safety and complications of more than 1500 small-incision lenticule extraction procedures. Ophthalmology. 2014;121:822–828. doi:10.1016/j.ophtha.2013.11.006 [CrossRef]
- Sharifipour F, Panahi-Bazaz M, Bidar R, Idani A, Cheraghian B. Age-related variations in corneal biomechanical properties. J Curr Ophthalmol. 2016;28:117–122. doi:10.1016/j.joco.2016.05.004 [CrossRef]
- Febbraro J-L, Aron-Rosa D, Gross M, Aron B, Brémond-Gignac D. One year clinical results of photoastigmatic refractive keratectomy for compound myopic astigmatism. J Cataract Refract Surg. 1999;25:911–920. doi:10.1016/S0886-3350(99)00072-3 [CrossRef]
- Tjon-Fo-Sang MJ, de Faber JT, Kingma C, Beekhuis WH. Cyclotorsion: a possible cause of residual astigmatism in refractive surgery eye motion occurring between wavefront measurement and refractive surgery. J Cataract Refract Surg. 2002;28:599–602. doi:10.1016/S0886-3350(01)01279-2 [CrossRef]
- Chernyak DA. Cyclotorsion 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]
- Kobashi H, Kamiya K, Ali MA, Igarashi A, Elewa MEM, Shimizu K. Comparison of astigmatic correction after femtosecond lenticule extraction and small incision lenticule extraction for myopic astigmatism. PloS One. 2015;10:e0123408. doi:10.1371/journal.pone.0123408 [CrossRef]
- Miao H, Tian M, Xu Y, Chen Y, Zhou X. Visual outcomes and optical quality after femtosecond laser small incision lenticule extraction: an 18-month prospective study. J Refract Surg. 2015;31:726–731. doi:10.3928/1081597X-20151021-01 [CrossRef]
- Ganesh S, Gupta R. Comparison of visual and refractive outcomes following femtosecond laser-assisted LASIK with SMILE in patients with myopia or myopic astigmatism. J Refract Surg. 2014;30:590–596. doi:10.3928/1081597X-20140814-02 [CrossRef]
Preoperative Baseline Characteristics of Patients (N = 81)a
|Characteristic||Total (N = 81)||Low Cylinder (n = 37)||High Cylinder (n = 44)||P|
|Age (y)||27.01 ± 5.81||28.37 ± 6.34||25.86 ± 5.11||.057|
|SE (D)||−5.00 ± 2.18||−4.73 ± 1.94||−5.23 ± 2.36||.299|
|Cylinder (D)||−1.85 ± 0.86||−1.11 ± 0.28||−2.48 ± 0.66||.000|
|UDVA (logMAR)||0.76 ± 0.22||0.73 ± 0.16||0.78 ± 0.26||.265|
|CDVA (logMAR)||0.017 ± 0.07||0.016 ± 0.06||0.018 ± 0.07||.905|
| ≤ 5º||81.6%||75.67%||86.36%||–|
| 6° to 9º||17.2%||21.62%||13.63%||–|
| ≥ 10º||1.2%||2.7%||0%||–|
| Mean ± SD||5.64 ± 2.55||5.73 ± 2.8||5.53 ± 2.29||.822|
| Range||2° to 12°||2° to 12°||2° to 10°||–|
Visual and Refractive Resultsa
|Group||Preoperative||2 Weeks Postoperative||3 Months Postoperative|
|Total (N = 81)|
| UDVA (logMAR)||0.76 ±0.22||0.00 ± 0.08||−0.014 ± 0.08|
| CDVA (logMAR)||0.017 ± 0.07||−0.048 ± 0.06||−0.058 ± 0.07|
| SE (D)||−5.00 ± 2.18||−0.20 ±0.35||−0.20 ± 0.24|
| Range||−9.50 to 1.25||−1.50 to 0.88||−1.25 to 0.50|
| Cylinder (D)||−1.85 ± 0.86||−0.28 ± 0.33||−0.28 ± 0.30|
| Range||−5.00 to −0.75||−1.00 to 0.50||−1.00 to 0.50|
|Low cylinder (n = 37)|
| UDVA (logMAR)||0.73 ± 0.16||−0.016 ± 0.07||−0.027 ± 0.07|
| CDVA (logMAR)||0.016 ± 0.06||−0.045 ± 0.08||−0.062 ± 0.08|
| SE (D)||−4.73 ± 1.94||−0.17 ± 0.39||−0.19 ± 0.25|
| Range||−9.50 to −1.90||−0.88 to 0.88||−0.75 to 0.50|
| Cylinder (D)||−1.11 ± 0.28||−0.20 ± 0.36||−0.24 ± 0.30|
| Range||−1.50 to −0.75||−1.00 to 0.50||−0.75 to 0.50|
|High cylinder (n = 44)|
| UDVA (logMAR)||0.78 ± 0.26||0.013 ± 0.09||−0.004 ± 0.08|
| CDVA (logMAR)||0.018 ± 0.07||−0.050 ± 0.05||−0.054 ± 0.06|
| SE (D)||−5.23 ± 2.36||−0.23 ± 0.32||−0.21 ± 0.24|
| Range||−9.38 to −1.25||−1.50 to 0.50||−1.25 to 0.00|
| Cylinder (D)||−2.48 ± 0.66||−0.34 ± 0.29||−0.31 ± 0.31|
| Range||−5.00 to −1.75||−1.00 to 0.00||−1.00 to 0.00|
| UDVA (P)c||–||.120||.215|
| CDVA (P)c||–||.800||.664|
Vector Analysis of Total (N = 81) Eyes Undergoing SMILE With Manual Cyclotorsion Compensation Postoperatively
|Parameter||2 Weeks Postoperative||3 Months Postoperative||P|
|SIA||1.53 ± 0.74||1.55 ± 0.78||.364|
|DV||0.33 ± 0.28||0.29 ± 0.28||.071|
|CI||0.95 ± 0.24||0.96 ± 0.21||.683|
|AE (absolute)||4.09 ± 6.05||3.66 ± 5.88||.383|
|ME||−0.108 ± 0.31||−0.08 ± 0.29||.368|
|IOS||0.24 ± 0.25||0.20 ± 0.23||.071|
|FI||0.92 ± 0.24||0.93 ± 0.21||.460|
Subgroup Analysis of Astigmatic Correction Based on Degree of Target-Induced Astigmatism After SMILE
|Parameter||Low Cylinder (n = 37)||High Cylinder (n = 44)||P|
|TIA (preoperative)||0.99 ± 0.25||2.19 ± 0.65||.000|
|SIA||0.98 ± 0.30||2.04 ± 0.72||.000|
|DV||0.25 ± 0.25||0.33 ± 0.30||.201|
|CI||0.97 ± 0.26||0.93 ± 0.15||.148|
|AE (absolute)||5.27 ± 7.36||2.31 ± 3.85||.032|
|ME||−0.012 ± 0.23||−0.149 ± 0.32||.032|
|IOS||0.27 ± 0.29||0.14 ± 0.15||.024|
|FI||0.96 ± 0.26||0.92 ± 0.16||.421|