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Vector analysis of Contoura Vision for the correction of myopia and myopic astigmatism

Objective: To evaluate the visual outcomes of Contoura Vision with automatic eye tracking system in eyes with myopia and myopic astigmatism. Methods: In this prospective study, 40 consecutive patients (80 eyes) with moderate myopia and irregular astigmatism were included from January to August 2018 at Liuzhou Worker’s Hospital. Subjects were randomly divided into an experimental group (40 eyes) that underwent Contoura Vision FS-LASIK and a control group (40 eyes) that underwent wavefrontoptimized FS-LASIK. Target refractions of all subjects were plano. Visual outcomes and astigmatic vector analysis were evaluated and compared between preoperatively and 3 months post-operatively. Results: The preoperative spherical and cylindrical refractive errors were similar in both groups (P>0.05). At 3 months postoperatively, cumulative uncorrected distance visual acuity (UDVA) was 20/16, 20/20, or 20/25 in 30 %, 95%, 100 % of patients in the experimental group, respectively. The experimental group was better than the control group in predictability of astigmatism correction at 3 months after operation. In the experimental group, 80% of eyes had deviation of astigmatic axis within 15° and 35% of eyes had deviation of astigmatic axis within 5°, both were better than those in the control group. The number of eyes with residual astigmatism within 0.5D were higher in the experimental group (60%, 24 eyes) than the control group (50%, 20 eyes). Compared with the preoperative, C7 significantly reduced to 0.056±0.030 in the experimental group at 3 months after the procedure (P<0.05), and were significantly lower than those in the control group (P<0.05). Conclusion: Contoura Vision with automatic eye tracking system was safe and effective for the correction of myopia and myopic astigmatism.

3 Femtosecond laser-assisted in situ keratomileusis (FS-LASIK) has become the current mainstream corneal refractive surgery because of its predictability and stability [1][2] .
However, many subjects with uncorrected distance visual acuity (UDVA) greater than 1.0 after FS-LASIK complain about poor night vision, glare, and double vision. Excimer laser treatment of spherical myopia is more predictable than that of myopic astigmatism [3] .
Studies have shown that every 1° deviation of the astigmatic axis results in a loss of correction of 3.3% [4] . Residual astigmatism less than 0.50D has an impact on visual quality [5] . With the development of technology, wavefront-guided [6][7] and topographyguided [8][9] had been used in LASIK. The conventional treatment method is wavefrontguided LASIK, which aims to correct the aberration of the entire eye and is gradually less effective with age due to changes in eye adjustment [6][7] . Topography-guided LASIK have been reported to correct irregular cornea. It has showed advantages for improved visual quality with less induction of high order aberrations (HOAs) [8][9] . In recent years, Alcon presented the innovative Contoura Vision technique based on the WaveLight refractive Suite. The studies by Manoj et al. [10][11] have demonstrated the efficacy of Contoura Vision in correcting low-order aberrations and HOAs caused by corneal asymmetry. This study aimed to observe changes of astigmatism and corneal irregularity after correction of low-to-moderate myopia with asymmetrical corneal astigmatism using automatic iris tracking and Contoura vision technology and to investigate the safety and effectiveness of this surgical procedure.
Commonly used parameters to assess efficiency of corneal refractive surgery including residual astigmatism, spherical equivalent (SE) and cylindrical lens power were used as the quantitative indices. Moreover, vector analysis that added factors impacting axial direction and lens power can be used to comprehensively evaluate the change of 4 astigmatism after corneal refractive surgery and assess surgery efficacy [12][13] . In the vector analysis based on the Alpins method [14] , accurate Cartesian coordinates provide accurate magnitude and axial direction of surgical-induced astigmatism (SIA), preoperative astigmatism, target astigmatism and postoperative astigmatism.

General data
This study was designed as a prospective cohort study. Forty subjects (80 eyes) who underwent myopia laser treatment in the department of ophthalmology of Liuzhou Worker's Hospital from January to August 2018 were selected and randomly divided into two groups: the experimental group included 20 subjects who underwent automatic iristracking and topography-guided FS-LASIK (Contoura Vision ), and the control group included 20 subjects who underwent automatic iris-tracking LASIK. The inclusion criteria were as follows: ① subjects aged between 18 and 35 years; ② preoperative corneal topography showed a difference in refraction greater than 1.25D in the anterior corneal surface between the upper and lower portions (a 5-mm area in the central corneal area was selected); ③ subjects who stopped wearing soft contact lenses for at least 15 days or oxygen-permeable hard contact lens for over 3 months; and ④ subjects with a refraction less than -6.00D in sphere or -4.00D in cylinder.⑤the preoperative central corneal thickness(CCT) was more than 480μm. ⑥estimated postoperative residual stromal bed thickness was no less than 280µm or the corneal ablation depth was no more than 50% of the thinnest corneal thickness. Patients who had anterior seg ment abnormalities such as keratoconus or corneal ectasia, recurring eye disease such as iritis or herpetic keratitis, severe dry eye, or systemic disease such as diabetes or hyperthyroidism, were not included.

.2 Surgical procedure
All procedures were performed by the same experienced ophthalmologist (Ying Lin). The WaveLight FS200 femtosecond laser (Alcon, USA) was used to create the corneal flap with a depth of 120 μm and diameter of 8.5 mm. The corneal flap hinge were located 90°s uperiorly. The diameter of optical zone ablation was 6.5 mm. The subjects in the experimental group underwent topography-guided keratomileusis in the EX500. The topographic neutralizing treatment (TNT) method [15] which includes a comprehensive analysis of the results of manifest refraction and Topolyzer examination to adjust the actual laser correction degree was used during surgery. In the surgical design, appropriate diopter compensation should be considered for the spherical aberration that may be caused by elimination of HOAs. After the surgical design was completed, corneal ablation 6 was performed using the automatic iris tracking system. The subjects in the control group underwent automatic iris-tracking LASIK in the EX500.

Postoperative pharmacotherapy
Both groups of subjects were treated with levofloxacin ophthalmic solution (Santen, Japan) 4 times a day for a week and tobramycin dexamethasone ophthalmic solution (Alcon, USA) 4 times a day for a week. Then, the treatment continued with 0.1% fluorometholone ophthalmic solution (Santen, Japan) 3 times a day for 3 weeks followed by polyethylene glycol ophthalmic solution (Alcon, USA) 4 times a day for 4 weeks.

Observation parameters
The following parameters were observed before and 3 months after surgery: UDVA, CDVA, autorefraction to verify spherical and cylinder power, corneal curvature, C7 and C8 by Topolyzer and ISV, IVA by Oculyzer.

Statistical analysis
SPSS 15.0 statistical software was used for statistical analysis. The χ2 test was used to compare the quantitative data. The differences between C7, C8, ISV and IVA within the same group were compared by repeated measures analysis of variance followed by least significant difference (LSD) t-tests for comparison of two means between different time points. Independent sample t-tests were used to compare the differences between the two groups. The astigmatism vector analysis between the eyes with different degrees of astigmatism before and 3 months after surgery was performed with the Alpins method [14] to calculate SIA, target corrected astigmatism (TIA), and spherical equivalent. Differences in preoperative and postoperative data were analyzed using the Wilcoxon rank sum test (P<0.05 was considered statistically significant).

General data
The demographic data of the 2 subject groups before surgery are shown in Table 1. There were no significant differences in age, sex, CCT, maximum keratometry (Kmax); minimum keratometry (Kmin), sphere, cylinder and CDVA between the two groups before surgery (P>0.05).

Vision and refraction
The UDVA in the experimental group had 95% of the eyes better than 20/20 after 1 month, which was maintained at the 3 months visit( Figure 1) and 75% of eyes in control group (P<0.001). CDVA of 20/16 or better was measured for 16% of control group eyes, compared to 30% of experimental group eyes (statistically significant at P<0.001). lines. In the control group, 26 eyes (65%) had an unchanged CDVA, 2 (5%) gained 1 line.

Changes in corneal parameters
The ISV, IVA, and C7 in the experimental group were significantly lower than those in the control group 3 months after surgery (see Table 2 and 3). At 3 months postoperative, the ISV and IVA in the experimental group were significantly lower than those in the control group (t was 13.19 and 10.14 respectively, P=0.01 and 0.01).  In the experimental group, 80% (32 eyes) of subjects had a deviation of astigmatism in the axial direction less than 15°, and 35% (14 eyes) of subjects had an axial deviation of less than 5°. These results were better than those in the control group ( Figure 4). Figure 5 shows the percentage of eyes within ±0.25 D, ±0.50 D, and±1.00 D of the intended plano cylinder after surgery. The difference was significant between the two groups (P < 0.05).

Discussion
Some subjects undergoing LASIK still have visual problems such as poor night vision, glare, and blurred vision. Studies have shown that the root mean square (RMS) after LASIK is 1.9-fold higher after surgery than before surgery [15] . Both spherical aberration and coma increased after surgery, and the spherical aberration increased 4-fold after surgery compared to before surgery. For corneal asymmetrical astigmatism, the refractive power of the cornea on the same meridian or on different meridians is different. Conventional LASIK may produce more optic aberrations which can seriously impair visual quality of subjects. At present, the individualized surgical methods mainly include wavefront aberration-guided or topographic-guided LASIK. The wavefront aberration-guided approach is focused on optic aberration of the whole eye, but it ignores the influence of tear film, pupils and lens adjustment on aberrations [16][17] . Corneal topography-guided LASIK is more commonly used in subjects with ocular trauma or severe irregular corneal astigmatism [18] and is rarely reported in subjects with mild to moderate corneal irregular astigmatism in primary eyes. In this study, the automatic iris tracking system is designed according to the iris texture. The three-dimensional tracking mode effectively reduces the eye rotation caused by the position change of the subject. Moreover, the system accurately adjusts the kappa angle to reduce the introduction of HOAs [19][20] .
Our study has shown that the UDVA was higher in the experimental group than in the control group 3 months after surgery. In the experimental group, 24 eyes (60%) had an unchanged CDVA, 10 (25%) gained 1 line, and 4 (10%) gained 2 lines. This is superior to improvements in the control group. This suggests that the automatic iris tracking system combined with Contoura technology is superior to conventional surgery in terms of postoperative visual acuity. The findings in this study are consistent with the findings of Elahe et al. [21][22] . An automatic iris tracking system combined with myopia laser surgery significantly improves postoperative UDVA [23] . Ciccio [24] suggested that 68% of subjects had an eye rotation greater than 2° when transitioning from the sitting position to the supine position. Rotation greater than 2° during LASIK will affect astigmatism and aberrations if not corrected. Our study has shown that the experimental group was superior to the control group in astigmatism correction, and the experimental group was superior to the control group in the prediction of astigmatism treatment. All subjects in experimental groups had a deviation of astigmatism axial direction less than 15°. This indicates that active rotation tracking eyeball shifting reduce the positional deviation of LASIK on the cornea and avoid irregular SIA [25] . The automatic iris tracking system in ISV and IVA are parameters that reflect the regularity of the corneal surface. Our results have shown that ISV and IVA in the experimental group decreased significantly 3 months after surgery, and there were no significant differences in these parameters in the control group after surgery. In the experimental group, Contoura-assisted LASIK greatly improved the regularity of the corneal surface. Additionally, C7 decreased significantly in the experimental group 3 months after surgery in our study. In contrast, they did not change significantly in the control group. In contrast, they did not change significantly in the control group. We believe that the TNT technique that was used in the Contoura surgical design to neutralize irregular astigmatism while incorporating astigmatism and spherical changes that may occur in LASIK surgery into surgical design [26] can improve surgical safety and effectiveness. In this study, there was no significant difference in C8 between the experimental group and the control group after operation. Intraoperative iris tracking and positioning technology is used to reduce coma difference caused by inaccurate adjustment of kappa angle and off-center ablation. Contoura is a new concept that applies corneal topography-guided customized ablations to subjects with primary eyes (normal cornea). On one hand, it treats low-order aberrations (such as myopia and astigmatism).
On the other hand, it treats the subject's own HOAs. Contoura software accurately provides the subject's astigmatism and its axis direction, although astigmatism and its axis may differ from manifest refection results. In the control group, C7 and C8 have no significantly difference compare with preoperative. Symmetric ablation will not eliminate HOAs. Manoj et al. [10][11] found that the HOAs were directly modifying the lower-order astigmatism. So effective elimination of higher-order phase difference can improve visual 12 quality. Their results are consistent with our results. Based on the morphology of the anterior surface of the cornea, Contoura Vision topography-guided LASIK is designed to eliminate aberrations on the anterior surface of the cornea, and it effectively treats refractive errors to achieve stable BCVA in subjects [27] .
In summary, automatic iris tracking combined with Contoura technology is a safe and effective procedure to treat mild and moderate corneal irregular astigmatism. However,