The Effect Of Anterior Segment Changes On IOP Detection After SMILE Surgery

Objiective:To discuss the changes of the anterior segment parameters changes after small incision lenticule extraction（SMILE）,and analyze the effect of the changes on the postoperative intraocular pressure（IOP）measurements,and to obtain the correction equation for the postoperative IOP measurements.Methods:In this prospective case series study,we collected 141 patients（258 eyes）with myopia astigmatism who underwent SMILE surgery at the Affiliated Hospital of Qingdao University from June to November 2020,and they were divided into three groups according to the residual stromal thickness（RST）:280μm-320μm in group A,320μm-360μm in group B,>360μm in group C。Tomography and biomechanical parameters of all eyes were obtained with the Pentacam and Corvis ST respectively.To observe the changes of parameters at preoperatively and at 1、3 and 6 months postoperatively.Repeated measures analysis of variance was used for preoperative and postoperative comparison of the parameters of each group,pairwise comparisons using the LSD-t-test.Using the Pearson correlation coefficient to evaluate the relationship between preoperative and postoperative IOPs and influencing factors.The postoperative IOP expression model was established by phased multiple linear regression analysis.Results:1.Except for the anterior chamber angle（ACA）,postoperative CCT,central anterior chamber depth（ACD）,anterior chamber volume（ACV）and corneal volume（CV）in each group were generally lower than those before surgery,with statistically significant differences（all P<0.05）.2.At 6 months after surgery,the first flattening time（A1T）,corneal hardness parameters（SP-A1）,horizontal direction Ambrosio related thickness（ARTh）,peak distance（PD）decreased,and the second flattening time（A2T）deformation amplitude（DA）increased;Pairwise comparison between the groups,the range of changes in△A1T,△A1V,△DA,and△PD in group A was greater than that of the other two groups,with statistically significant differences（at all P<0.05）.3.△IOP and spherical equivalent（SE）,preoperative IOP（IOPpre）,△CCT,changes in corneal volume（△CV）,changes in central anterior chamber depth（△ACD）and the change of anterior chamber volume（△ACV）are positively correlated（all P<0.05）,no correlation with the change of anterior chamber angle（△ACA）（all P>0.05）.In the correlation between△IOP and△ACD,group A showed a moderate correlation（r=0.500,P<0.001）,no correlation in group B（r=0.245,P<0.001）,and a low correlation in group C（r=0.324,P<0.001）.4.The correlation between the changes in the anterior chamber parameters in group A was shown as:△ACD and△ACV are positively medium and low correlated with△A1T（r=0.328,P=0.002;r=0.303,P=0.005）,△SP-A1（r=0.689,P<0.001;r=0.410,P<0.001）,and△ARTh（r=0.426,P<0.001;r=0.318,P=0.048）,and negatively low correlated with△DA（r=-0.321,P=0.003;r=-0.315,P=0.004）.5.A multiple linear regression equation of△IOP was established for each group,respectively,the correction equation for the postoperative IOP was obtained:Group A IOP_{post corrected}=IOP_{post measured}+0.572×IOP_pre+15.759×△ACD-2.401（after adjustment R2=0.553,P<0.001）;Group B IOP_{post corrected}=IOP_{post measured}+0.471×IOP_pre+0.54×SE-1.099×△CV-3.551（after adjustment R²=0.504,P<0.001）;Group C IOP_{post corrected}=IOP_{post measured}+0.447×IOP_pre+0.304×SE-4.013（after adjustment R²=0.337,P<0.001）.Conclusion:1.The anterior segment parameters changed after SMILE compared with preoperative surgery,which showed that the anterior chamber depth became shallower,the volume of anterior chamber became smaller,and the biomechanical stability of cornea decreased.2.The positive correlation of changes in anterior chamber parameters after SMILE and△IOP,especially when RST is below the 320μm range,the effect of changes in anterior chamber parameters needs to be considered in the measurement of IOP.