Evaluation Of The Corneal Stromal Lenticule Surface Quality And Research On Its Water Loss Variation Model

PurposeIn this study,white light interferometry was used to measure the 3D coordinates of the corneal stromal lenticule surface from SMILE,and the surface roughness parameters were calculated after precise center positioning,noise removal,and filtering.The difference of roughness parameters in different areas,diopters and other conditions were compared.The related factors of roughness parameters to evaluate the surface micro-topography of the corneal stromal cutting surface after SMILE surgery were analyzed.Based on the 3D coordinates of the surface and the precise positioning of the center,the thickness of the corneal stromal lenticule was calculated,the water loss variation model was established,and the optimal corneal stroma exposure time was explored through data analysis to provide suggestion for improving the effect oflamellar corneal refractive surgery.Methods1.Explore surface roughness parameter data processing and water loss variation model characterization methods.A 3D optical profiler based on white light interferometry was used to measure the surface 3D coordinates of the corneal stromal lenticule.After the edge contour of the lenticule was extracted,the least squares method was used to fit it to obtain the coordinates of the center image of the stromal lenticule to achieve precise positioning of the lenticule center.After that,the noise and low frequency reference signal(waviness)in the signal were filtered through noise removal,filtering,and other steps,so as to realize the accurate calculation of the surface roughness parameters of the corneal stromal lenticule.After obtaining the 3D surface coordinates and accurately positioning the center based on the calculation of the corneal stromal lenticule central thickness,a variation model of water loss was established over time.2.Analysis of the surface roughness of the corneal stromal lenticule.The study included 102 eyes of 63 patients undergoing SMILE surgery.Based on the data processing method of the first part,the white light interferometer was used to measure the surface roughness of the corneal stromal lenticule after SMILE surgery.The paired t test was used to compare the surface roughness of the anterior and posterior side of corneal stromal lenticule and compare the surface roughness between left and right eyes.ANOVA was used to compare the surface roughness differences of different areas of the corneal stromal lenticules(central area,mid-peripheral area,and peripheral area)and the surface roughness differences among the astigmatism groups.The Student’s t test(two-tailed)was used to compare the difference of surface roughness between the low myopia group and the high myopia group.Partial Pearson’s correlation coefficient was used to analyze the correlation of surface roughness.Thus,the surface micro-topography information of the SMILE cutting surface could be obtained.3.The corneal stromal lenticule was used to simulate the water loss process during lamellar corneal refractive surgery,and a water loss variation model was established to guide the optimal corneal stroma exposure time.The study included 34 right eyes from34 patients undergoing SMILE surgery.The quadratic and linear thickness-time dehydration fitting curve of the central lenticule thickness over time were calculated and so was determination coefficient R~2.The Normal distribution of data was assessed using the Shapiro-Wilk test.ANOVA was used to compare the determination coefficient R~2 of quadratic equation and linear equation of different phases(linear phase1,2,3 and quadratic).The Student’s independent-samples t-test was used to investigate the difference of initial corneal thickness between the low and high myopic groups,the difference between the two myopia groups of the different allowable refractive error,the difference of the optimal corneal stroma exposure time under different optical zone conditions and the differences of the optimal corneal stroma exposure time between different genders.Mann-Whitney U test was used to compare the difference of dehydration duration between two myopic groups.Pearson correlation was used to calculate the correlation between the optimal corneal stroma exposure time and age.Results1.The white light interferometry is used to realize the rapid and accurate measurement of the surface coordinates of the corneal stromal lenticule,and the accurate calculation of the 3D information(roughness parameter)that characterizes the surface of the corneal stromal lenticule through data processing is used for the subsequent study of the surface roughness parameters of the corneal stroma lenticule.Continuously measure the surface of the center area of the lenticule and the surface of the glass slide,calculate the central thickness,and use a polynomial model to fit the central thickness data that changes with time.2.The surface roughness of the anterior part of the corneal stromal lenticule was smaller than that of the posterior part in different areas(P<0.01).The surface roughness of the central area was lower than that of the mid-peripheral and peripheral parts.The surface roughness of both anterior and posterior parts increased gradually from the center to the periphery(P<0.01).There was no significant difference of the central surface roughness on both the anterior and posterior sides between the low myopia group and the high myopia group(P=0.675,P=0.615).The surface roughness of the high myopia group was greater than that of the low myopia group(P=0.003,P=0.016)on the periphery part of both the anterior and posterior sides.In the central area of the anterior surface(F=0.301,P=0.741)and the central area of the posterior surface(F=1.167,P=0.316),there was no statistical difference among the different astigmatism groups.The roughness of the central area of the posterior surface is positively correlated with PTA(r=0.248,P=0.012);the roughness of the central area of the anterior and posterior surfaces were also positively correlated(r=0.466,P<0.01).There was no correlation between age and the roughness of the central area of both the anterior and posterior surfaces(r=0.091,P=0.369;r=0.114,P=0.260).There was no significant correlation of the central surface roughness between both eyes in anterior and posterior parts(r=0.273,P=0.093;r=0.003,P=0.985).The surface roughness of suction loss lenticule was greater than that of the matched lenticule.3.The water loss variation model was set up successfully.The linear and quadratic fitting curve were drawn respectively and the determination coefficient R~2 of which were very close to 1.The average value of quadratic R~2was 0.998±0.002,and the average value of R~2 for linear phases 1,2,and 3 was 0.995±0.007,0.996±0.003,0.984±0.035,respectively.The difference in coefficients a and b of the quadratic fitting curve between the two groups of myopia was-0.0001(P=0.317)and-0.1890(P=0.696),respectively,and the difference was not statistically significant.The absolute values of the two coefficients of the low myopia group were higher than those of the high myopia group.To simulate the water loss process after lifting the flap during LASIK procedure under various conventional conditions,and to obtain better postoperative refractive results,the allowable postoperative refractive error was set as0.25D,0.50D and 0.75D,respectively.The diameter of the zone was set as 6mm,6.5mm and 7mm respectively,and the optimal lifting time range of corneal flap under different parameter settings could be calculated.Taking the allowable error of 0.50D and the optical zone size of 6.5mm as an example,the allowable time for the low myopia group was 23.58±4.89s,and the allowable time for the high myopia group was23.95±4.48s.There was no statistically significant difference in the time of water loss between different gender(P=0.066).There was no statistical correlation between water loss time and age(P=0.089).Conclusion1.White light interferometry can be used to achieve rapid and accurate measurement of the surface coordinates of the corneal stromal lenticule,surface fitting based on a polynomial model,filtering out noise,and filtering out low-frequency reference information(waviness)based on Gaussian robust regression filtering,successfully high-frequency roughness Signal and perform roughness parameter calculation.The accurate calculation of the surface roughness parameters of the corneal stromal lenticule was realized.After continuous measurement of the surface of the center area of the stromal lenticule and the surface of the glass slide,the center thickness was calculated,and the center thickness data that changes with time was fitted with a polynomial model,to prepare for the selection of the subsequent polynomial model,the drawing and analysis of the corneal stroma lenticule central thickness-time water loss curve.2.The surface roughness of the corneal stromal lenticule is not uniform,and it gradually increases from the central area to the peripheral area,and the posterior surface roughness is greater than that of the anterior side.There is no statistical difference in the surface roughness of the central area between low myopia and high myopia,and the surface roughness is more affected by PTA.In a certain range(the astigmatism is less than 0.50D),the astigmatism will not affect the surface roughness.The surface roughness has no significant correlation with age,eye sides,etc.Intraoperative complications including OBL,and suction loss will affect the quality of surface cutting.This part of the research provides a basis for the setting of parameters in clinical surgery and do spadework for improving the postoperative visual quality of SMILE surgery.3.The water loss rate of the human corneal stroma is nonlinear,and the application of quadratic equation fitting is more in line with the actual water loss trend.As time goes by,the water loss rate gradually increases.The rate of corneal stromal water loss has nothing to do with thickness,and different gender or age will not affect the rate of water loss.The use of isolated corneal stromal tissue to establish a water loss model suggests that under different allowable diopter errors and different optical zone conditions,the optimal corneal stroma exposure time in lamellar corneal refractive surgery is different.The optimal corneal stroma exposure time increases as the optical zone and allowable diopter errors increase.Cutting within this time range will not cause cutting errors due to water loss.It is reminded that the refractive surgeon should pay attention to the excimer laser cutting within the recommended time range under different optical zone sizes and reposition the corneal flap in time and to obtain better postoperative results and avoid overcorrection or undercorrection.