| Myopic refractive error is the most common ophthalmic disease,and it has shown a trend of younger age in recent years.Refractive surgery has gradually become a routine method for the treatment of myopic refractive errors for more and more patients,among which femtosecond laser small incision lenticule extraction(SMILE)is a new type of treatment.Refractive surgery uses a femtosecond laser to precisely cut and make a lens in the corneal stroma,and remove the lens through a tiny incision to achieve the purpose of refractive correction.Because of its unique advantages of "no flap" and "minimally invasive",SMILE has received widespread clinical attention and has been gradually promoted.Experts recommend that the residual stromal thickness(RST)during refractive surgery should be no less than 250 μm(about 50% of the central corneal thickness).There are certain limitations in patients with thinner CCTs and high myopia.Theoretically,the SMILE surgical approach preserves denser anterior stroma,with better corneal structural integrity and mechanical load-bearing properties.Therefore,it is not clear whether SMILE surgery can break through the clinical minimum limit of RST for the correction of myopic refractive errors because of the above advantages.In this paper,the SMILE animal model with corneal cap thickness 1/3 of CCT was prepared,and RST was set to 30% of CCT(group A,corrected degree ～-9D)and 50% of CCT(group B,corrected degree ～-3D).The changes of corneal morphology and biomechanical parameters in vivo were compared between the two groups at 1 week,1 month and 3 months after surgery.We explored whether SMILE can correct high myopia under the condition of preparing thicker corneal cap and retaining less residual stromal bed,in order to provide experimental basis for parameter optimization of SMILE.The main work and conclusions are as follows:(1)The basic parameters and comprehensive parameters of corneal morphology in vivo were measured by Pentacam three-dimensional anterior segment analyzer.Spectral domain optical coherence tomography(SD-OCT)was used to measure corneal thickness,corneal epithelium thickness,corneal cap thickness and RST at different orientations.The results showed that the changes of the curvature of the corneal posterior surface,the height of the corneal posterior surface,the corneal volume and the comprehensive parameters with time were basically same in the two groups.There was basically no significant difference in the above parameters between the two groups at 3 months after the surgery,and there was no tendency to keratoconus,indicating that RST has little effect on corneal morphology when it is 30% and50% of CCT.The corneal thickness,corneal epithelium thickness,corneal cap thickness and RST in the two groups were all greater than the theoretical value corrected after surgery at 3months after surgery,and were significantly increased compared with that at 1 month after surgery.It suggested that the corneal epithelium and stroma had a certain degree of repair at 3months after operation.(2)Corneal visualization scheimpflug technology(Corvis ST)was used to measure the in vivo corneal biomechanical basic parameters and comprehensive parameters.The results showed that in the comparison between different groups at the same time point,the SP-A1 of group A was significantly higher than that of group B at 1 week after surgery.One month after surgery,the A1 V,A2V,Def Amp Max and IR values of group A were significantly lower than those of group B.It is suggested that within 1 month after surgery,the corneal tissue stiffness increased as the residual stromal bed decreased.At 3 months after surgery,ARTh in group A was significantly lower than that in group B,and there were no significant differences in other parameters between groups,suggesting that group A may have slightly lower resistance to deformation than group B at 3 months after surgery.The changes of most biomechanical parameters at different time points before and after surgery in the two groups were basically same,and they basically returned to the preoperative level at 3 months after surgery.The results of correlation analysis showed that postoperative CCT and RST were the main influencing factors of corneal biomechanical parameters after SMILE.To sum up,when the thickness of the corneal cap is 1/3 of the CCT,even if the RST is lower to 30% of the CCT to correct high myopia(～-9D),in the short term(3 months after the operation),there was no keratoconus predisposition was seen from corneal morphology and biomechanics.This may be related to the small incision,no flap,and the preservation of denser anterior matrix with strong mechanical properties during SMILE. |
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