Accuracy Of Astigmatism Correction In Femtosecond Laser Small Incision Lenticule Extraction

Purpose: This study explored the accuracy of astigmatism correction by femtosecond laser small incision lenticule extraction(SMILE)surgery,through exploring the deviation between the actual ablation central corneal thickness and the predicted ablation central corneal thickness in SMILE surgery.Methods: A prospective cohort study.This study had a total of 67 eyes of 34 patients with myopic astigmatism who underwent SMILE surgery in myopic center of Shengjing Hospital affiliated to China Medical University from January 2018 to October 2019 were selected.All subjects were divided into two groups according to the degree of astigmatism.One grouping had 53 eyes with low astigmatism and ≤-1D with the columnar diopters.The other grouping had 14 eyes with high astigmatism and >-1D with columnar diopters.Follow-up time is pre-operation and 6 months after operation.The central corneal thickness(CCT)was measured by Pachymetry mode of Cirrus anterior segment analysis system within OCT.The actual ablation central corneal thickness is the preoperative central corneal thickness minus the postoperative central corneal thickness,and the predicted ablation central corneal thickness is the thickness of the lens given in the surgical equipment.The central corneal ablation deviation is the actual ablation central corneal thickness minus the predicted ablation central corneal thickness.Results: 1.Six months after operation,there was significant difference in postoperative uncorrected visual acuity between the low astigmatism group and the high astigmatism group(t =-1.869,P<0.05).Only one patient in the low astigmatism group did not reach the preoperative best corrected visual acuity,losing one line compared with the preoperative best corrected visual acuity,and the other eyes achieved the preoperative best corrected visual acuity.The postoperative best corrected visual acuity of all eyes in the high astigmatism group reached the preoperative best corrected visual acuity.In the low astigmatism group,the postoperative uncorrected visual acuity was higher than the preoperative best corrected visual acuity,and the percentage was 84.75%.In the high astigmatism group,the postoperative uncorrected visual acuity was higher than the preoperative best corrected visual acuity,and the percentage was 78.57%.2.Six months after operation,the residual astigmatism was-0.085 ±0.557 D in low astigmatism group and-0.196 ±0.827 D in high astigmatism group.There was no significant difference in postoperative residual astigmatism between the two groups(t=0.510,P=0.617).3.Six months after operation,the actual ablation central corneal thickness(A-CCT)in the low astigmatism group was 93.190 ±22.015 μm,and the surgical predicted ablation central corneal thickness(P-CCT)was 110.080 ±22.165 μm and there was significant difference between A-CCT and P-CCT(t=-21.232,P<0.05).In the high astigmatism group,A-CCT was 102.000 ±18.115 μm,and P-CCT was 120.930 ±17.013 μm,and there was significant difference between A-CCT and P-CCT(t=-14.482,P<0.05).4.Six months after operation,the difference between the actual ablation central corneal thickness and the predicted ablation central corneal thickness was-16.890 ±5.790 μm in the low astigmatism group,and that was-18.930 ±4.891 μm in the high astigmatism group.There was no significant difference in the ablation error between the two groups(t=1.209,P=0.231).Conclusion: When SMILE surgery is used to correct myopic astigmatism,the actual ablation central corneal thickness is less than the predicted ablation central corneal thickness,which indicates that SMILE surgery is safe,but there is a possibility of undercorrection.The calculation of operation degree needs appropriately incremental spherical equivalent to improve the accuracy of surgery.