Impact Of Ocular Residual Astigmatism On Visual Outcomes Of Myopic Astigmatism Correction After Small Incision Lenticule Extraction

Objective:To calculate ocular residual astigmatism(ORA)by vector analysis and to evaluate the impact of ocular residual astigmatism on refractive and visual outcomes after correction of myopic astigmatism using small incision lenticule extraction(SMILE).Methods:In this prospective case-series study,115 eyes with myopic astigmatism from Jan 2019 to Aug 2019 at Department of Ophthalmology of China Medical University affiliated Shengjing Hospital were treated with SMILE.These cases were calculated ORA by vector analysis and divided into two groups according to the magnitude of ORA: High ORA group(ORA?1.00 D)with 42 eyes(average ORA: 1.25±0.41 D)and Low ORA group(ORA<1.00 D)with 73 eyes(average ORA: 0.54±0.20 D).The follow-up period was 3 months after operation.Measured best corrected visual acuity(BCVA)before operation and uncorrected visual acuity(UCVA).And measured refractive outcomes,total higher order aberrations(t HOAs),vertical trefoil(Z9),vertical coma(Z7),horizontal coma(Z8),oblique trefoil(Z10),oblique quadrafoil(Z15),oblique secondary astigmatism(Z13),spherical aberration(Z11),vertical secondary astigmatism(Z12),vertical quadrafoil(Z14).Measured contrast sensitivity of 85 cd/m2 and 3 cd/m2 respectively from 1.5 c/d to 18 c/d.Then calculate axis shift,error vector,error of magnitude,error of angle,index of success and index of correction using vector analysis.The data were analyzed by SPSS 20.0 using independent-samples t test and Mann-Whitney U test to compare the difference between two groups.Result:3 months postoperatively,refractive outcomes shown postoperative residual astigmatism of high ORA group(-0.41±0.21 D)was significantly higher than low ORA group(-0.26±0.24 D)(P<0.05).Vector analysis shown error vector(high ORA group: 0.41±0.17 D,low ORA group: 0.27±0.23 D),absolute error of angle(high ORA group: 11.7±15.4°,low ORA group: 7.5±12.4°)and index of success(high ORA group:0.53±0.41,low ORA group: 0.33±0.34)were higher in high ORA group(P<0.05).The axis shift of high ORA group less than15°,15° to 30° and more than 30° was 45%,29% and 26% respectively.And for low ORA group,axis shift was 53%,33% and 10% corresponding to high ORA group.After SMILE,all cases has achieved desirable visual acuity(high ORA group:-0.15 ± 0.05,low ORA group:-0.15 ± 0.05)without statistic difference between two groups.An obvious difference was found in contrast sensitivity which shown a higher amount at 18 c/d spatial frequency under 85 cd/m2 in low ORA group(1.39±0.14)than high ORA group(1.30±0.18)(P<0.05),but this tendency wasn't shown in 3 cd/m2.There was no statistic difference in higher order aberrations and Pythagorean length between two groups(P>0.05).Conclusion:Ocular residual astigmatism impacts the precision of SMILE in correcting myopic astigmatism which leads to a tendency of more residual astigmatism in refractive outcomes,and reduce the contrast sensitivity at high spatial frequency,but it doesn't affect effectiveness of obtaining good visual qualities both visual acuity and higher order aberrations.