Small incision lenticule extraction (SMILE) has garnered significant attention due to its advantages of small incisions, absence of corneal flaps, and preservation of cornea integrity. During SMILE, a laser is used to cut the corneal stromal layer, and the refractive lenticule is obtained and removed to correct myopia. The corneal incision alters the corneal structure, reducing its biomechanical strength and resistance to deformation. Latrogenic keratectasia occurs when the biomechanical strength is below the threshold required to maintain corneal shape. Thus, investigating the biomechanical changes in the cornea after SMILE surgery has become a key research area in the field of refractive surgery. Elastic modulus is a primary factor affecting the biomechanical changes of the cornea. Some studies have indicated changes in corneal elastic modulus after SMILE. However, there are notable variations among patients. The refractive lenticule and corneal cap thickness differ considerably in clinical practice. Consequently, the quantitative analysis of the corneal elastic modulus after SMILE remains controversial. This study aimed to quantitatively examine the influence of corrected refraction and corneal cap thickness on the corneal elastic modulus using the finite element method (FEM) after SMILE.

This study used Corvis ST technology and corneal topographic maps. A parametric modeling approach was applied to reconstruct a preoperative three-dimensional geometric model of rabbit eyes, including the lens, ciliary body, and aqueous humor. Additionally, the cutting surface and incision were fitted using point cloud data. Seven postoperative SMILE models with different corrected refractions were created while maintaining the same corneal cap thickness. Furthermore, four models with different corneal cap thicknesses were reconstructed with corrected refraction being maintained in the range of the -3 D and -6 D. The finite element software COMSOL Multiphysics 5.6 was used to simulate the corneal shear-wave optical coherence elastography (OCE) experiment. The intraocular pressure was set to 10 mmHg, which is consistent with the experimental intraocular pressure. A transverse propagating shear wave was generated in the cornea after transient excitation at the excitation source. The central point of the shear-wave vibration source was designated as the origin, and multiple detection points were set on its right side. The temporal displacement data were extracted at the detection points. The phase velocity algorithm was applied to obtain the dispersion curve of the phase velocity for the shear wave. The corneal elasticity was then quantified. Finally, the effects of corrected refraction and corneal cap thickness on the simulated corneal elastic modulus after SMILE were examined. The corneal elasticity in rabbit eyes after SMILE was measured non-invasively using ARF-OCE experiments. The obtained value of the corneal elastic modulus was used to validate the simulation results.

The results indicated that the simulated corneal Young’s modulus increased significantly by 192.26% when the corrected refraction changed from 0 D to -6 D. Furthermore, the result of the curve-fitting of shear-wave velocity at various corrected refractions was good (R2=0.9828). The curves indicated a nonlinear correlation between the shear-wave velocity and corrected refraction (Fig. 4). When the corrected refraction was -3 D and -6 D, with the corneal cap thickness increasing from 80 μm to 160 μm, the simulated corneal elastic modulus indicated a slight increase of 11.66% and 13.07%, respectively. The results revealed that as the corneal cap thickness increased, the simulated corneal elastic modulus increased; however, the effect was significantly less than the effect of the corrected refraction (Figs. 5 and 6). Subsequently, the ARF-OCE experiment was used to measure the elastic modulus of -3 D and -6 D cornea after SMILE in rabbit eyes. The measured values were 97.572 and 189.831 kPa, respectively, representing an increase of 94.55% (Fig. 8). These experimental results confirm the results of the FEM simulations. The elastic wave velocity errors of the corrected refraction (-3 D and -6 D) obtained by the FEM are 1.3% and 0.2%, respectively, compared with the elastic wave velocity measured by the ARF-OCE experiment. This demonstrates that FEM can effectively simulate OCE experiments and accurately quantify corneal elasticity.

A personalized three-dimensional FEM of rabbit eyes was reconstructed. The corneal OCE experiments in rabbit eyes after SMILE were simulated using the FEM and validated using ARF-OCE experiments. The findings indicate that increased corrected refraction and corneal cap thickness lead to increased simulated corneal elastic modulus. The former has a more significant influence, demonstrating a nonlinear increase. The latter has less significant effects than the former. This conclusion was also confirmed by the ARF-OCE experiments. This study theoretically examined the influence of single factors such as correction refraction on corneal elastic modulus and achieved an accurate quantification of corneal elasticity using FEM. This study provides theoretical insights into accurate experimental measurement of corneal elasticity and serves as a reference for the clinical characterization of elastic modulus.