Research Of Corneal Biomechanics In Relation To Refractive Surgery

Refractive surgery is currently the most widely used method for the treatment of myopia in the world.Nowadays,people are paying more and more attention to the safety of the cornea after refractive surgery,the quality of vision and the stability of the refractive state.It is important to study the mechanical properties of cornea to reveal the mechanism of corneal recovery after surgery.At present,researches on the mechanical properties of the cornea,especially the values of its elastic modulus,are quite different,leading to differences on the safety and stability of the cornea after different refractive surgeries.Indentation can maintain the integrity of the cornea and is a common method to analyze its mechanical properties.The JKR model can obtain the elastic modulus of the cornea and the adhesion force in the indentation behavior by analyzing the indentation behavior.In this paper,the biomechanical parameters of intact cornea were analyzed by indentation method,and the reasons for the large difference in the elastic modulus of cornea were systematically analyzed.Furthermore,the safety limitations of refractive surgery were further discussed.The results of this study can provide reference for the analysis of mechanical properties of eyeballs and further discussion of the safety and stability of different refractive surgeries.The main work and conclusions of this study are as follows:(1)Intact corneal macroscopic indentation test at submillimeter scale was conducted to analyze the indentation behavior of the cornea under physiological pressure.Hertz and JKR models were used to calculate the elastic modulus of the intact cornea.The analysis shows that the elastic modulus obtained by JKR model considering adhesion energy is more than two times higher than that of Hertz model considering only external forces.Compared with the uniaxial tensile test of corneal strips in vitro,the elastic modulus of JKR model under physiological state was between the upper and lower limits of cornea,and it was consistent with the viscoelastic properties of cornea.This phenomenon suggests that the JKR model can be used to obtain a reasonable effective E value of cornea integrity under physiological pressure.(2)The calculation of three dimensional stress and strain on preoperative corneal refractive surgery,and the use of HGO model and Ogden model for normal cornea,LASIK postoperative corneal and primer and the stress distribution of finite element modeling analysis,the following conclusion: normal cornea volume strain value is almost the same in different thickness,the reason is that there exists antagonism between toroidal-radial,and refractive surgery melt some of the substrate,cornea is part of the radial stress exposure,in the use of the finite element model for the melting interface stress analysis,found that LASIK stress concentration is more serious than Smile.In addition,the recovery rate of the two kinds of refractive surgery was discussed according to the stress growth hypothesis.(3)After refractive surgery,there is adhesion behavior between the residual matrix bed and the damaged interface of corneal flap or corneal cap,which affects the safety and stability of cornea after refractive surgery.JKR model was used to analyze the adhesion of the cornea,and it was found that the compensatory tension of the cornea after surgery could not exceed the adhesion of the cornea itself.Otherwise,the flap or corneal cap will be separated from the remaining stromal bed,thus becoming a threat to the visual health of patients with corrected myopia.It is inferred that the adhesion energy is the limiting condition of the safety of refractive surgery.