Vector Analysis Of Surgically-induced Astigmatism After SMILE In Patients With Simple Myopia

Objective:To investigate the changes of surgically-induced astigmatism at different times after small incision lenticule extraction（SMILE）,and to evaluate the possible predictors.Methods:This retrospective,longitudinal study enrolled105 eyes from 84 patients with simple myopia treated with SMILE.Refractive astigmatism and corneal astigmatism were measured at preoperative,1-day,and 1-,3-,6-month postoperatively.Changes in corneal astigmatism before and 6 months after surgery were analyzed using Fourier analysis.Generalized estimating equations were used to evaluate the factors associated with surgically-induced astigmatism in preoperative parameters.Results:The mean cylinder were-0.36±0.34 D,-0.30±0.25 D and-0.30±0.26 D at 1 day,1 month,and 6 months after surgery respectively which was introduced by 66%eyes at 6 months postoperatively,and 92%of those eyes introducing astigmatism within-0.50D.CJ₀（t（208）=-6.13,P<0.01,d=-0.44 at 1 months,t（208）=-3.5,P=0.02,d=-0.3 at6 months）show an increase compared to pre-operation,and CJ₄₅(t（208）=-3.5,P=0.02,d=-0.3 increased at 6 months too.Compared with OBL,ATR got larger cylinder（β=0.646,P<0.001）andΔCJ₀（β=0.136,P=0.003）,WTR got larger cylinder（β=0.287,P=0.007）,ΔCJ₄₅（β=0.09,P=0.017）and TJ₀（β=0.085,P=0.016）.Compared with low myopia,moderate and high myopia had lessΔCJ₀（β=-0.201,P=0.032）,larger TJ₀（β=0.144,P=0.037）and TJ₄₅（β=0.102,P=0.011）.The cap thickness of 130μm had smaller cylinder compared with 120μm（β=-0.092,P=0.017）.The smaller of the optical zone,the lager of cylinder（β=-0.339,P=0.029）,TJ₀（β=-0.259,P<0.001）and TJ₄₅（β=-0.268,P<0.001）.With preoperative corneal astigmatism increasing,cylinder（β=0.34,P=0.003）andΔCJ₄₅（β=0.249,P=0.04）increased andΔCJ₀（β=-0.323,P<0.001）and TJ₀（β=-0.194,P=0.013）decreased.Conclusion:There was a small amount of introduction of surgically-induced astigmatism after SMILE which related to the type and magnitude of preoperative corneal astigmatism,degree of myopia,optical zone and cap thickness.In clinical practice,patients with large corneal astigmatism,high myopia should be explained the risks of surgically-induced astigmatism,and the surgery should be carried out cautiously.