Research Of Biomechanical Alteration And Corneal Ectasia After Refractive Surgery

During the past decades, the laser refractive surgical techniques have been rapidly developed in the treatment of visual corrections of refractive error such as myopia and astigmatism. The idea of laser refractive surgery is to ablate some corneal tissue in the central region in order to change the corneal curvature. Laser in situ keratomileusis (LASIK) is currently the most popular laser refractive surgery. Some cases were found cornea ectasia after LASIK surgery in a few years. By the background of treatment keratoconus with ultraviolet A/riboflavin (UVAR), much basic and clinical research has been done. The results indicate that ultraviolet A/riboflavin (UVAR) induced collagen crosslink has effect and steady outcomes. According to evidence based medical research approach, our experiment study beginning with animal model, and ending with numerical simulations to ask these questions,(1) discussion the effects of collagen cross-link on the interlamellar cohesive strength.(2) Corneal Hystersis and its related factors with intra ocular pressure (IOP), the viscoelasticity, mass and elasticity of the cornea based on experimental data and theoretical simulations.(3) Corneal biomechanics changes after different refractive surgeries.There are three parts in this paper:1. Collagen cross-linking’s effects on the interlamellar cohesivestrength of the porcine cornea. To determine the interlamellar cohesivestrength (ICS) of porcine cornea before and after collagen cross-linkingusing riboflavin and ultraviolet A (UVA) irradiation. The maximal ICSbetween corneal flap and the stroma was determined and histologicalexamination was performed. The mean stromal ICS was measured.Untreated eyes served as controls in all experiments. Cornea flaps werefixed in4%formaldehyde for a minimum of6days and embedded inparaffin. Thin paraffin sections (4mm) were stained withhematoxylin–eosin and observed under a light microscope.The results of this study using a porcine model suggest thatcross-linking as a result of treatment with riboflavin/UVA increased theICS of the cornea. Histological examination also suggested a connectionbetween the flap and underlying stroma as a result of cross-linking. Insummary, our study demonstrated an increase of ICS in the cornea ofporcine eyes after UVA/riboflavin treatment. These results suggest thatcorneal cross-linking may be used to prevent flap displacement afterLASIK because of the increase of cohesion between flap and stroma, andcorneal collagen cross-linking may be clinically useful to haltkeratoconus progression. 2. Corneal hysteresis and its related factors, experimental results andtheoretical simulations. In the experiment part, in-vitro porcine eyes wereused to investigate the relationship of CH and IOP. IOP was alteredwithin the range of10and60mmHg using a drip stand. CH and IOPwere measured with Ocular Response Analyzer (ORA) at different IOPvalues. Second-order polynomial regression method was employed toassess the nonlinear correlation of CH and IOP. To simulate the cornealmovement during the ORA measurement, a kinematic viscoelastic cornealmodel was developed involving the factors of corneal mass, dampingcoefficient, elasticity and IOP. Different parameter values were taken toinvestigate how factors would affected the cornea movement. The CHwas then explained based on the numerical results.RESULTS: CH kept approximately6mmHg when IOP ranged from10to25mmHg, and showed a decreasing tendency in higher IOP.Numerical results showed that during the ORA measurement there wouldbe vibration along with the cornea movement, and the dampingviscoelasticity of the cornea had the potential to reduce the vibrationamplitude. The overall vibration amplitude was smaller with higher IOP.3. Biomechanics comparisons of LASIK and LASEK before andafter surgeries. The purpose of this study is to investigate and comparethe biomechanical responses and refractive behaviors of the corneas afterrefractive surgeries of Laser in situ keratomileusis (LASIK) and lasersubepithelial keratomileusis (LASEK). Study the relationship withcorneal biomechanics changes and intra ocular pressure (IOP), cornealstroma ablation. The CH is low after two kinds of surgeries. There are nodifferences between two surgeries. So the selection of LASIK can beavoided after surgery uncomfortable and later recovery compare withLASEK.