Comparison Of Effective Optical Zone Of 110μm And 120μm Cap In SMILE

PurposeTo investigate the changes of the effective optical zone（EOZ）and its influencing factors after small incision lenticle extraction（SMILE）with a corneal cap thickness of 110μm and 120μm.MethodFifty-three patients（106 eyes）with myopia or myopic astigmatism corrected by SMILE in the Second Hospital of Jilin University from December 2020 to December 2021 were included.The right eyes of the patients were randomly grouped by coin toss.The front face（national emblem）represented the right eye that was included in the 110μm cap thickness group（53 eyes）,and the left eye was then automatically included in the 120μm cap thickness group（53 eyes）.The preoperative optical zone was 6.5 mm.The uncorrected visual acuity（UCVA）,corrected distance visual acuity（CDVA）,objective refraction,intraocular pressure,and subjective refraction were recorded in all patients preoperatively,1 week,1 month,3 months,and 6 months postoperatively.The EOZ was calculated using the tangential curvature difference map in Pentacam anterior segment analysis system.The posterior surface elevation map was used to evaluate the change in the posterior cornea apex elevation.The Zernike polynomial（sixth order,scanning diameter of 6mm）was used to describe the corneal aberration.The pre-operative optical zone and post-operative EOZ were compared using single sample t-test,and the parameters such as EOZ were compared between the two groups using paired sample t-test.The change of EOZ over time was examined by multi-factor repeated measures variance test.Pearson linear correlation analysis was performed to determine the parameters that were related to EOZ,and one-dimensional linear correlation analysis was performed.Result1.There was no statistical difference in preoperative CDVA,spherical equivalent（SE）,corneal asphericity（Q）,corneal volume,lenticule thickness,corneal spherical aberration,corneal vertical coma,corneal horizontal coma,corneal total higher-order aberrations,and total corneal refractive power between the 110μm group and the 120μm group（P > 0.05）.While the preoperative remaining stromal thickness（RST）of the 110μm group was larger.（P < 0.05）.2.There was no statistical significance in UCVA,CDVA,SE,corneal volume,corneal spherical aberration,corneal vertical coma,corneal horizontal coma,corneal total higher-order aberrations,Q,and total corneal refractive power between the two groups six months after surgery（P < 0.05）.3.The posterior corneal apex elevation in both groups was lower than that before surgery from one week to six months after surgery（P <0.05）.The posterior corneal apex elevation shifted further in the 110μm group than in the 120μm group at 6 months after surgery（P=0.003）,but there was no difference between the two groups for the rest of the followup time.4.From 1 week to 6 months after operation,the EOZ of 110μm group was larger than that of 120μm group（P < 0.05）.In addition,the postoperative EOZ of both groups was smaller than preoperative optical zone（6.50mm）（P=0.00）,among which the EOZ of 110μm and 120μm groups decreased by 1.12 mm and 1.27 mm respectively after operation.There was no difference in EOZ between different follow-up times（F=1.58,P=0.19）,and the change of EOZ with time was not affected by the thickness of the two groups（F=0.07,P=0.98）.5.Six months after surgery,in the 110μm group,the change of EOZ was negatively correlated with the change of SE and corneal horizontal coma,and positively correlated with the change of corneal spherical aberration,the change of corneal total higher-order aberrations,and the change of Q value.In the 120μm group,the variation of EOZ is negatively correlated with SE,and positively correlated with the variation of corneal spherical aberration,the variation of corneal total higher-order aberration,and the variation of Q value.6.At 6 months postoperatively,there was no statistical difference in the EOZ between the 110μm and 120μm corneal cap thicknesses in the SE≤-3D or SE≥-6D groups（P > 0.05）,and the change of EOZ in the110μm group was less in-3D < SE <-6D（P=0.02）.Conclusion1.SMILE with the thickness of 110μm and 120μm corneal cap is effective,safe and predictable for the correction of myopia and myopic astigmatism.2.The posterior corneal apex elevation in 110μm and 120μm groups has no ectasia trend.3.The postoperative EOZ of the 110μm group and the 120μm group was lower than that before operation and did not change with time.4.The EOZ is larger for the 110μm group than for the 120μm group.In addition,preoperative SE,Q and corneal aberration may be factors affecting EOZ.5.In patients with high myopia,on the basis of ensuring equal EOZ after operation,it seems that 110μm corneal cap can reserve a safer RST for patients than that of 120μm.