Measurement And Comparison Study Of The Effective Optical Zone After Corneal Refractive Surgery For Myopia

PurposeUsing two different objective methods to measure the effective optical zone(EOZ)after small incision lenticule extraction(SMILE),and compare the change of EOZ after SMILE and femtosecond laser-assisted excimer laser in situ keratomileusis(FS-LAISK).The related factors affecting the EOZ were analyzed.Furthermore,the changes in epithelial thickness profile within the optical zone and its related factors following transepithelial photorefractive keratectomy(TransPRK)for myopia was also discussed.Methods1.Forty-three myopic patients(43 eyes)who underwent SMILE pre-and6-month postoperative were enrolled in this retrospective study.The EOZ was measured in two ways.First,a topographic(Pentacam)effective optical zone was measured on the subtraction tangential curvature map before and after 6 months.Second,functional optical zone(FOZ)was determined from the radial MTF,established at the retinal plane as a function of pupil size.The repeatability of data was analyzed by intraclass correlation coefficient(ICC)and Cronbach'S Alpha coefficients,and the data were compared using a paired t test and Bland-Ahman plot.Correlations between EOZ and the amount of correction,centeral corneal thickness(CCT),ablation depth,Q-value changes(?Q)and K value changes were analyzed statistically.2.In this prospective non-randomized controlled study,76 right eyes of 76 myopic patients were included under the informed consent.The eyes were divided into SMILE group for 45 eyes and FS-LASIK group for 31 eyes.Effective optical zone was measured using topographic tangential curvature method preoperatively and at 1,3,and 6 months postoperatively.The results were compared between the two groups.Repeat-measure ANOVA was used to analysis the changes of EOZ pre-and postoperatively,and comparison between different meridian EOZ of the cornea.The correlations between EOZ with operative parameters were analyzed.3.In this retrospective non-randomized controlled study,forty-three patients(43eyes)who underwent TransPRK with the spherical equivenlent refraction-1.25 to6.25 D were included under the informed consent.Epithelial thickness was measured using spectral-domain optical coherence tomography at different corneal zones(central,2mm;paracentral,2-5mm;and mid-peripheral,5-6mm)preoperatively and at 1 week and 1,3,and 6 months postoperatively.Correlations between epithelial thickness changes and the amount of correction,effective optical zone,and ?Q were analyzed 6 months postoperatively.Results1.The ICC and Cronbach's Alpha coefficients of EOZ measured by Pentacam were more than 0.9,which showed high intraobserver repeatability.The Bland-Ahman plots displayed relatively good agreement between the two methods.The 95% limits of agreement was-0.47 to 0.91?m.The EOZ measured by Pentacam and FOZ measured by MTF were(5.33±0.22)mm and(5.11±0.25)mm,respectively.A statistically significant differences were found between the two methods(t=-4.14,P=0.000).The negative correlations were found between the EOZ and ?Q 6 months after SMILE(r=-0.451,P=0.005).While no correlations were found between FOZ and ?Q(r=0.144,P=0.396).2.There was no significant difference among different time points after SMILE and FS-LASIK(Ftime =0.991,P=0.382).The mean EOZ diameters after SMILE were(5.39±0.27),(5.33±0.30)and(5.34±0.23)mm at 1 week,1 month or 3 months after surgery,respectively.The mean EOZ diameters after FS-LASIK were(5.06±0.36),(4.98±0.39)and(5.01±0.31)mm at different time points.Statistically significant differences were found in the SMILE group compared with FS-LASIK group during the follow-up time(Fgroup =14827.325,P=0.000)?The positive correlations were found between the ?EOZ and ?Q after SMILE and FS-LASIK(r=0.487,0.411,P<0.05).A significant positive relationship was observed between ?EOZ and epithelial thickening 3 months after SMILE(r=0.479,P=0.001).3.The mean epithelial thickness in the central zone were(52.37±3.42),(53.97±4.33),(51.03±4.12),(55.14±5.52)and(56.68±5.06)?m at 5 days,1 month,3months and 6 months after surgery,respectively.The epithelium were thicker at 3 and6 months after surgery compared to preoperative measurements(P<0.05).Comparedto preoperative values,the epithelial thickness was(3.69 ± 4.23),(5.19 ± 3.88)and(6.23 ±3.91)?m thicker at the center,paracenter,and mid-periphery,respectively,at 6months postoperatively(P<0.01).A significant positive relationship was observed between epithelial thickening and programmed SE correction and between epithelial thickening and ?Q.The EOZ diameter was(1.26±0.22),(1.26±0.26)and(1.34±0.28)?m thinner at 1 month,3 months and 6 months after surgery compared to the programmed optical zone diameter(t1m=14.659,t3m=14.497,t6m=20.969,all P<0.01).No significant correlation was found between epithelial thickening and ?EOZ(t0-2mm=-0.163;t2-5mm=-0.119;t5-6mm=-0.025,all P>0.05).Conclusion1.Topographic tangential curvature map measurement of effective optical zone after SMILE is of high repeatability and is consistent with MTF method.2.Significant effective optical zone decreased was observed 6 months after SMILE.The optical zone was approximately 5.33 mm and 5.13 mm measured by two different methods with a programmed optical zone diameter of 6.5mm.Different results may be attributed to different measurement principles.3.SMILE created larger corneal topographic EOZs than FS-LASIK.The reduction of the EOZ seemed to be independent of the change of the asphericity of the cornea.4.Significant epithelial thickening was observed after T-PRK and showed a lenticular change with more thickening mid-peripherally,resulting in increased oblateness postoperatively.Epithelial remodeling have no significant effect on the reduction of the EOZ.