Vector analysis and comprison of small incision lenticule extraction versus laser in-situ keratomileusis for low to moderate myopic astigmatism

Abstract

Background Small incision lenticule extraction (SMILE) is lack of eye-tracking system and there is controversy in the treatment of astigmatism.It is very important to understand its correction effctiveness for myopic astigmatism. Objective This study was to evaluate the clinical outcomes in the correction of myopic astigmatism between SMILE and femtosecond laser assisted laser in situ keratomileusis (FS-LASIK). Methods In this retrospective study, 95 eyes of 51 patients and 69 eyes of 40 patients with myopic astigmatism underwent the SMILE surgery and FS-LASIK surgery respectively in Tianjin Eye Hospital from December 2013 to July 2014 under the informed consent.No significant differences were found in spherical power, astigmatic power and spherical equivalent (SE) between the SMILE group and FS-LASIK group before surgery (all at P>0.05). The uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), manifest refraction, slit-lamp microscopy, intraocular pressure and corneal topography were measured before surgery and 1 day, 1 week, 1 month, 3 months after surgery.Vector analysis was used to calculate the components of astigmatism before and after surgery.Correction index (CI), index of success (IOS), angle of error (AofE) and flattening index (FI) were used to evaluate the correction effectiveness of astigmatism.Safety index and efficacy index were used to evaluate the recovery status of visual acuity.The corrected effectiveness was compared between the two groups, and the correlations of kappa angle with measured parameter were assessed by Spearman rank correlation analysis. Results The mean spherical power, astigmatic power and SE were (0.003±0.106), (-0.063 ± 0.126) and (-0.029 ± 0.101)D in the SMILE group, which were significantly lower than (0.112 ± 0.212), (-0.091 ± 0.142) and (0.067 ± 0.198)D in the FS-LASIK group (Z=-4.328, -3.197, both at P 0.05). Negative correlations were found between safety index or efficacy index and preoperative kappa angle in the SMILE group (r=-0.258, -0.257, both at P<0.05). Conclusions Both SMILE and FS-LASIK surgeries are effective and safe in correcting myopic astigmatism with good postoperative visual acuity.SMILE surgery shows less axis rotation and higher accuracy than FS-LASIK because of small incision and flapless procedure.The correction of cylinder can adjust based on experience of surgeon. Key words: Cornea/surgery; Femtosecond laser; Myopia; Astigmatism; Vector analysis; Small incision lenticule extraction