The Distribution And Influencing Factors Of Posterior-to-anterior Corneal Curvature Radii Ratio And Corneal Volume In Chinese Myopic Patients

Part 1 The distribution and influencing factors of posterior-toanterior corneal curvature radii ratio in Chinese myopic patientsObjectiveWe aimed to investigate the normal range and frequency distribution pattern of posterior-to-anterior corneal curvature radii ratio(P/A ratio)in Chinese myopic patients.At the same time,we further explored the correlation between the P/A ratio and other ocular biological parameters and analyzed the influence of P/A ratio on the difference between the actual corneal refractive power and simulated corneal refractive power(ΔK).MethodsThis study included a total of 7893 myopic patients who underwent corneal refractive surgery from 2017 to 2020.Pentacam was used to measure the P/A ratio and other ocular biological parameters before surgery.The normal range of P/A ratio and its frequency distribution were statistically analyzed.Spearman’s correlation test was used to analyze the relationship between P/A ratio and other ocular biological parameters as well as ΔK.The linear regression model was applied to predict P/A ratio and ΔK.ResultsThe distribution of P/A ratio in Chinese myopic patients was slightly negatively skewed(skewness:-0.140,KS P<0.001).The mean P/A ratio was 0.82±0.01 with a 95%confidence interval(CI)of 0.8148～0.8154.The 95%normal range of P/A ratio in this population was 0.79～0.84.P/A ratio was positively correlated with mean posterior corneal radius(r=0.482),posterior corneal asphericity(r=0.373)and corneal diameter(r=0.369)and negatively correlated with posterior corneal astigmatism(r=-0.215),posterior corneal eccentricity(r=-0.363),central corneal thickness(r=-0.402),corneal volume-3mm(r=-0.438),corneal volume-5mm(r=-0.512),corneal volume-7mm(r=-0.594)(all P<0.05).Multiple linear regression analysis showed that central corneal thickness and posterior corneal astigmatism were significantly correlated with P/A ratio(regression coefficients:-1.59×10-4,-0.027,respectively;standardized regression coefficients:-0.336,-0.271,respectively,all P<0.05).There was a significant correlation between P/A ratio and ΔK in all eyes(r=0.976,P<0.05).The regression equation was:ΔK=7.548×P/A ratio-6.191(R2=0.998,P<0.05).Thus,a change of 0.1 in P/A ratio would lead to a change of 0.75 D in ΔK.Conclusions1.The distribution of P/A ratio in Chinese myopic patients was slightly negatively skewed.It has a range of fluctuations,rather than being a constant.2.Central corneal thickness and posterior corneal astigmatism were the main independent influencing factors of P/A ratio.3.P/A ratio was significantly positively correlated with ΔK.An increase of 0.1 in P/A ratio caused an average increase of 0.75 D in ΔK.It is necessary for ophthalmologists to consider the actual P/A ratio when calculating the intraocular lens power.Part 2 The distribution and influencing factors of corneal volume in Chinese myopic patientsObjectivewe aimed to investigate the normal range and frequency distribution pattern of corneal volume(CV)in Chinese myopic patients.At the same time,we further explored the correlation between CV and other ocular biological parameters,and predicted CV in different corneal measurement diameter regions.MethodsThis study included a total of 7893 myopic patients who underwent corneal refractive surgery from 2017 to 2020.Pentacam was used to measure CV at 3.0 mm,5.0 mm,and 7.0 mm regions centered on the corneal apex(CV-3mm,CV-5mm,CV7mm,respectively)and other ocular biological parameters before surgery.The normal range and frequency distribution of CV-3mm,CV-5mm,CV-7mm were calculated.Spearman’s correlation test and restricted cubic splines were used to determine multiple ocular indicators associated with CV,which were integrated into a multiple linear regression model to predict CV.ResultsThe distribution of CV-3mm,CV-5mm and CV-7mm were slightly positively skewed(skewness:0.209,0.218,0.229,respectively,KS all P<0.001),The mean values of CV-3mm,CV-5mm and CV-7mm were 3.93±0.21 mm3,11.53±0.60 mm3,and 24.84±1.26 mm3,respectively and 2.5th to 97.5th percentiles were 3.6～4.4 mm3,10.4～12.8 mm3,22.5～27.5 mm3,respectively.CV-3mm,CV-5mm and CV-7mm were significantly positive correlated with central corneal thickness(CCT)(rcv-3mm=0.987,rcv-5mm=0.987,rcv-7mm=0.956,all P<0.05),while weakly negative correlated with mean posterior corneal radius,posterior corneal asphericity and anterior chamber volume(|r|<0.35,P<0.05).The correlation coefficient between CV and CCT showed an inconsistent trend with the increase of age.The correlation coefficient between C V and CCT did not change significantly with the increase of diopter in low to moderate myopia,but fluctuated significantly in high myopia(SE<-6.0 D).CV-3mm,CV-5mm,CV-7mm had a negative nonlinear relationship with mean posterior corneal radius and a positive nonlinear relationship with posterior corneal eccentricity(P and Pn<0.05).Multiple linear regression analysis showed that CCT was significantly correlated with CV-3mm,CV,5mm and CV-7mm(regression coefficients:0.007,0.020 and 0.039,respectively;standardized regression coefficients:0.970,0.936 and 0.871,respectively,all P<0.05).Conclusions1.The distribution of CV-3mm,CV-5mm and CV-7mm in Chinese myopic patients were slightly positively skewed and the 95%normal ranges were 3.6～4.4 mm3,10.4～12.8 mm3,22.5～27.5 mm3,respectively.Ophthalmic surgeons should be cautious in screening candidates for corneal refractive surgery whose CV is below the lower limit of the normal reference range.2.CV-3mm,CV-5mm,and CV-7mm were significantly correlated with central corneal thickness,and this correlation changed with age and refractive status.3.Central corneal thickness was the main influencing factor of CV.An increase of 1μm in CCT caused an average increase of 0.007 mm3,0.020 mm3 and 0.039 mm3 in CV-3mm,CV-5mm and CV-7mm,respectively.Part 3 The effects of corneal refractive surgery on posterior-toanterior corneal curvature radii ratio and corneal volumeObjectivewe aimed to investigate the variation characteristics of posterior-to-anterior corneal curvature radii ratio(P/A ratio)and corneal volume(CV)following small incision lenticule extraction(SMILE)surgery and femtosecond laser-assisted in situ keratomileusis(FS-LASIK)surgery,and further explored the correlations between surgical-related factors and P/A ratio changes(ΔP/A ratio),CV changes(ΔCV-3mm,ΔCV-5mm,ΔCV-7mm).MethodsThis study included a total of 321 myopic patients who underwent corneal refractive surgery from 2017 to 2020,containing 93 patients(93 eyes)underwent SMILE surgery and 228 patients(228 eyes)received FS-LASIK surgery.All subjects underwent Pentacam examination preoperatively,at 1 week and 3 months postoperatively.The P/A ratio and CV-3mm,CV-5mm,CV-7mm were measured and the changes of each index(Δ)after surgery were calculated.The variation characteristics of P/A ratio and CV-3mm,CV-Smm,CV-7mm after SMILE and FSLASIK surgery were analyzed.Spearman’s correlation tests were used to determine the correlations between surgical-related factors and ΔP/A ratio,ΔCV-3mm,ΔCV-5mm,ΔCV-7mm and multiple linear regression model was used to predict ΔP/A ratio and ACV.ResultsP/A ratio and CV-3mm5 CV-5mm,CV-7mm decreased after SMILE and FSLASIK surgery compared to preoperative levels(P<0.05).In the SMILE group,the mean values of each parameter were 0.74±0.02,3.44±0.17 mm3,10.39±0.49 mm3,23.31±1.03 mm3 at 1 week postoperatively and were 0.74±0.02,3.47±0.17 mm3,10.48±0.48 mm3,23.46±1.04 mm3 at 3 months postoperatively.In the FS-LASIK group,the mean values of each parameter were 0.73±0.03,3.32±0.25 mm3,10.13±0.65 mm3,22.93±1.27 mm3 at 1 week postoperatively and were 0.74±0.03,3.36±0.24 mm3,10.21±0.65 mm3,23.01±1.26 mm3 at 3 months postoperatively.ΔP/A ratio and CV-3mm,CV-5mm,CV-7mm were negative correlated with the change in spherical equivalent(ΔSE)and positive correlated with the change in central corneal thickness(ΔCCT)and the change in mean corneal curvature(ΔKm)at 3 months postoperatively(all P<0.05).Multiple linear regression analysis showed that the regression coefficient of ΔKm for ΔP/A ratio was 0.019(P<0.05)and the regression coefficients of ACCT for ΔCV-3mm,ΔCV-5mm and ΔCV-7mm were 0,002,0.007,and 0.014,respectively(all P<0.05).Conclusions1.P/A ratio and CV significantly decreased after SMILE and FS-LASIK surgery.CV increased slightly at 3 months after surgery compared to 1 week postoperatively.2.ΔP/A ratio and ΔCV were significantly correlated with surgical-related factors(ΔSE,ΔCCT,and ΔKm)at 3 months after SMILE and FS-LASIK surgery.3.ΔKm was an independent influencing factor of ΔP/A ratio.An increase of 1 D in ΔKm caused an average increase of 0.019 in ΔP/A ratio.ΔCCT was the main influencing factor of ΔCV.An increase of 1 μm in ΔCCT caused an average increase of 0.002 mm3,0.007 mm3 and 0.014 mm3 in ΔCV-3mm,ΔCV-5mm and ΔCV-7mm,respectively.