Standardized Vector Analysis of Correction for different types ofastigmatism after FS-LASIK

Objective: To evaluate by standardized vector analysis the correction of different types of astigmatism after femtosecond laser-assisted in situ keratomileusis (FS-LASIK). Methods: In this retrospective case control study, 106 myopic eyes of 53 cases from June 2012 to June 2013 at Shandong Province Guangming Eye Hospital with astigmatism were treated with FS-LASIK. The cases were divided into three groups according to the type of astigmatism: corneal (n=34), intraocular (n=36), and mixed (n=36).Uncorrected visual acuity (UCVA), proportion of UCVA higher than the preoperative best corrected visual acuity (BCVA), index of surface variance (ISV), index of vertical asymmetry (IVA), and the proportion of smooth cornea were measured six months postoperatively. The astigmatism status was evaluated based on standardized vector analysis. The data were analyzed by analysis of variance and Chi-squared tests. Results: Six months postoperatively, the UCVA differences among the three groups were significant (F=4.80, P=0.013). UCVA and the proportion of UCVA≥BCVA in the corneal astigmatism group and in the mixed astigmatism group were greater than in the intraocular astigmatism group (P<0.05 for each). The postoperative remaining uncorrected astigmatism of the corneal astigmatism group was the lowest, while that of the intraocular astigmatism group was the highest, and the difference between them was significant (P=0.015). The correction ratio (CR) in the corneal astigmatism group was higher than in the intraocular astigmatism and mixed astigmatism groups (P<0.01). The absolute surgically induced refractive correction (|SIRC|) of the corneal astigmatism group was higher than the intraocular astigmatism and mixed astigmatism groups (P<0.01 each). Differences in CR and |SIRC| between the intraocular astigmatism and mixed astigmatism groups were not statistically significant. The error of angle, error of magnitude, error ratio, and absolute error vector (|EV|) of the corneal astigmatism group were lower than for the intraocular astigmatism and mixed astigmatism groups (P<0.01 each). The |EV| of the mixed astigmatism group was lower than the intraocular astigmatism group (P=0.030). ISV and IVA of the corneal astigmatism group were lower than the mixed astigmatism group (P<0.01 for both), and the ISV and IVA of the mixed astigmatism group were lower than the intraocular astigmatism group (P<0.05 for both). The proportions of smooth cornea of the corneal astigmatism, intraocular astigmatism, and mixed astigmatism groups were 71%, 39%, and 56%. Conclusions: The efficacy of astigmatism correction after FS-LASIK is closely correlated with the type of astigmatism. Corrective efficacy was best for corneal astigmatism, and worst for intraocular astigmatism. Key words: vector analysis; astigmatism; femtosecond laser; laser in situ keratomileusis