Aberration Compensation Between Anterior And Posterior Corneal Surface In Myopia Subjects And After SMILE And FS-LASIK

ObjectiveTo study the distribution and compensation mechanism of higher order aberrations between the anterior and the posterior corneal surface,and to investigate the correlations between the corneal aberrations and K values,intraocular pressure and central corneal thickness.What’s more,to investigate the aberration compensation mechanism between the anterior and the posterior corneal surface after small incision lenticule extraction（SMILE）and femtosecond laser-assisted Laser in-situ keratomileusis（FS-LASIK）.MethodsOne hundred and sixty-one subjects（303 eyes）with myopia and myopic astigmatism were recruited randomly.Corneal aberrations（the anterior corneal surface,the posterior corneal surface and the whole cornea）in three different optical zones（2mm,4mm,6mm）,which was centered on the corneal vertex,were assessed with corneal topographer（Sirius）.We also calculated a compensation factor（CF）,CF=1–（aberration of the total cornea/aberration of the anterior surface）,as a measure of the relative efficiency of the aberration compensation mechanism.Mann-Whitney U test were utilized for the comparison between the CFs of different part of aberrations.Spearman correlation was applied to analyze the correlation between corneal aberrations and K values,intraocular pressure and central corneal thickness.In addition,fifty five subjects（55 eyes）undergoing SMILE and 51 subjects（51 eyes）undergoing FS-LASIK were recruited randomly from January 2011 to November 2013 in Department of Refractive Surgery,Tianjin Eye Hospital.Wave-front aberrations（vertical coma,horizontal coma,spherical aberration,and the total higher order aberration）of the anterior and the posterior corneal surface and the whole cornea at 6mm diameter were measured using a Scheimpflug Camera（Pentacam）preoperatively and 1,3 and 6 months postoperatively.The compensation factor（CF）and the change of CF were calculated.Independent-Samples T Test was used for the comparison of the baseline characteristic and the aberrations between SMILE and FS-LASIK group preoperatively.Repeated ANOVA was used for the comparison of aberrations preoperatively and at 1,3 and 6 months postoperatively in each group.LSD test was used for the comparision between postoperative time points.And any significant difference of CF in each group was compared by Friedman test.Mann-Whitney U test was utilized to compareΔCF between the groups at 1,3 and 6months postoperatively.Results1.The larger the analyzing zone,the larger the wave-front aberrations were.And as the order of the aberrations elevated from third-order to seventh-order,the Zernike coefficient decreased significantly.The larger the optical zone,the greater the values of aberrations and the lower the percentage of the compensation mechanism among all the Zernike terms.At the same time,as the order of the aberrations increased,the predominance of the compensation mechanism was more obvious.2.Slight compensation of spherical aberration（C₄⁰）between the anterior and the posterior corneal surface was observed in peripheral（6mm）,with augmentation of spherical aberration（C₄⁰）at 2mm in diameter.While in the center of the analyzing zones（2mm）,compensation mechanism represented in coma(C₃^±1),and it disappeared in peripheral（4mm and 6mm）.Augmentation was found in trefoil(C₃^±3)at each diameter.Nevertheless,a tendency towards compensation was discovered with the expending of the optical zones.In addition,compensation of the secondary spherical aberration（C₆⁰）between the anterior and the posterior corneal surface was observed at different analyzing zones（2mm,4mm and 6mm）.3.Similar compensation was discovered between the right eye and left eye in different analyzing optical zones.4.There were significant correlations between K values and higher order aberrations in the anterior and the posterior corneal surface and the whole cornea.When the optical zone expanded to 6mm,C₃^±1 and C₄⁰ significantly decreased with intraocular pressure(r_coma=-0.188,P<0.05,r_{spherical aberration}=-0.147,P<0.05).No correlation was found between various aberrations and central corneal thickness（CCT）（P>0.05）.5.Wave-front aberrations increased significantly after both SMILE and FS-LASIK surgeries.6.In an analyzing area of 6 mm in diameter,the spherical aberration of posterior surface and the total cornea remained stable after SMILE surgery（P>0.05）.However,the spherical aberration of posterior surface increased significantly at 6 months in the FS-LASIK group（P<0.05）.7.The total higher order aberration（t HOA）of the anterior surface and the whole cornea at 6 months were lower than the values at 1 month and 3 months（P=0.001,0.001）in the FS-LASIK group.In the SMILE group,there was no significant difference of t HOA between postoperative time points（P>0.05）.8.There were significant decreases of the compensation factor（CF）of total higher order aberration（t HOA）in both groups(P<0.05.Differences of the CF of vertical coma were found at 3 and 6 months postoperatively in the FS-LASIK group compared with the preoperative values（P=0.021,0.008,respectively）.The CF of spherical aberration reduced significantly in both SMILE and FS-LASIK group at 1month,3 months and 6 months postoperatively（P<0.05）.9.The changes of CF（ΔCF）of spherical aberration were smaller in the SMILE group than in the FS-LASIK group at 1 and 3 months postoperatively（P=0.003,0.000,respectively）.In subjects with high myopia,ΔCF of spherical aberration were smaller in the SMILE group than in the FS-LASIK group at 1 and 3 months postoperatively（P=0.009,0.003,respectively）.ΔCF of spherical aberration were significantly lower in moderate myopia subjects than in high myopia subjects at 1month in the SMILE group（P=0.041）,and 1,3 and 6 months in the FS-LASIK group（P=0.014,0.020,and 0.004,respectively）.Conclusions1.Compensation dominated in center cornea,while the percentage decreased gradually as the optical zone extended.2.Obvious aberration compensation between the anterior and the posterior corneal surface was observed in most of the higher order aberrations in the centeral cornea,including coma.No compensation for spherical aberration was found in center,while there was slight compensation in peripheral.It might be contribute to increase the visual quality in scotopic condition.Complete compensation for secondary spherical aberration was found at 2mm and 4mm in diameter,with partial compensation at 6mm in diameter.Augmentation for trefoil between the anterior and the posterior corneal surface was observed at different diameters.We assume that it might be the survival of the fittest determined genetically through evolution.3.Siginificant correlations were found between corneal aberrations and K values.C₃^±1 and C₄⁰ decreased significantly with intraocular pressure in peripheral.4.The posterior corneal surface played an important role in compensating the spherical aberration of anterior corneal surface.5.The compensation mechanism of spherical aberration and higher order aberration between the anterior and the posterior corneal surface was disrupted by SMILE and FS-LASIK process.6.The changes of the CF of spherical aberration were smaller in the SMILE group compared with that in the FS-LASIK group,especially in subjects with high myopia.