Establishing Standardization Guidelines For Finite-Element Optomechanical Simulations of Refractive Laser Surgeries: An Application to Photorefractive Keratectomy.

Purpose: Computational models can help clinicians plan surgeries by accounting for factors such as mechanical imbalances or testing different surgical techniques beforehand. Different levels of modeling complexity are found in the literature, and it is still not clear what aspects should be included to obtain accurate results in finite-element (FE) corneal models. This work presents a methodology to narrow down minimal requirements of modeling features to report clinical data for a refractive intervention such as PRK. Methods: A pipeline to create FE models of a refractive surgery is presented: It tests different geometries, boundary conditions, loading, and mesh size on the optomechanical simulation output. The mechanical model for the corneal tissue accounts for the collagen fiber distribution in human corneas. Both mechanical and optical outcome are analyzed for the different models. Finally, the methodology is applied to five patient-specific models to ensure accuracy. Results: To simulate the postsurgical corneal optomechanics, our results suggest that the most precise outcome is obtained with patient-specific models with a 100 µm mesh size, sliding boundary condition at the limbus, and intraocular pressure enforced as a distributed load. Conclusions: A methodology for laser surgery simulation has been developed that is able to reproduce the optical target of the laser intervention while also analyzing the mechanical outcome. Translational Relevance: The lack of standardization in modeling refractive interventions leads to different simulation strategies, making difficult to compare them against other publications. This work establishes the standardization guidelines to be followed when performing optomechanical simulations of refractive interventions.

Optical coherence elastography under homolateral parallel acoustic radiation force excitation for ocular elasticity quantification.

Alteration in the elastic properties of biological tissues may indicate changes in the structure and components. Acoustic radiation force optical coherence elastography (ARF-OCE) can assess the elastic properties of the ocular tissues non-invasively. However, coupling the ultrasound beam and the optical beam remains challenging. In this Letter, we proposed an OCE method incorporating homolateral parallel ARF excitation for measuring the elasticity of the ocular tissues. An acoustic-optic coupling unit was established to reflect the ultrasound beam while transmitting the light beam. The ARF excited the ocular tissue in the direction parallel to the light beam from the same side of the light beam. We demonstrated the method on the agar phantoms, the porcine cornea, and the porcine retina. The results show that the ARF-OCE method can measure the elasticity of the cornea and the retina, resulting in higher detection sensitivity and a more extensive scanning range.

Investigation of the optimal cutting depth in small incision lenticule extraction based on a collagen fibril crimping constitutive model of the cornea.

To investigate the optimal cutting depth (Cap) in small incision lenticule extraction from the perspective of corneal biomechanics, a three-dimensional finite element model of the cornea was established using a stromal sub-regional material model to simulate small incision lenticule extraction. The displacement difference PΔ at the central point of the posterior corneal surface before and after lenticule extraction, as well as the von Mises stress at four points of different thicknesses in the center of the cornea, were analyzed using the finite element model considering the hyperelastic property and the difference in stiffness between the anterior and posterior of the cornea. The numerical curves of PΔ-Cap and von Mises Stress-Cap relations at different diopters show that the displacement difference PΔ has a smallest value at the same diopter. In this case, the von Mises stress at four points with different thicknesses in the center of the cornea was also minimal. Which means that the optimal cutting depth exsisting in the cornea. Moreover, PΔ-Cap curves for different depth of stromal stiffness boundaries show that the optimal cap thickness would change with the depth of the stromal stiffness boundary. These results are of guiding significance for accurately formulating small incision lenticule extraction surgery plans and contribute to the advancement of research on the biomechanical properties of the cornea.

Ex vivo, in vivo and in silico studies of corneal biomechanics: a systematic review.

Healthy cornea guarantees the refractive power of the eye and the protection of the inner components, but injury, trauma or pathology may impair the tissue shape and/or structural organization and therefore its material properties, compromising its functionality in the ocular visual process. It turns out that biomechanical research assumes an essential role in analysing the morphology and biomechanical response of the cornea, preventing pathology occurrence, and improving/optimising treatments. In this review, ex vivo, in vivo and in silico methods for the corneal mechanical characterization are reported. Experimental techniques are distinct in testing mode (e.g., tensile, inflation tests), samples' species (human or animal), shape and condition (e.g., healthy, treated), preservation methods, setup and test protocol (e.g., preconditioning, strain rate). The meaningful results reported in the pertinent literature are discussed, analysing differences, key features and weaknesses of the methodologies adopted. In addition, numerical techniques based on the finite element method are reported, incorporating the essential steps for the development of corneal models, such as geometry, material characterization and boundary conditions, and their application in the research field to extend the experimental results by including further relevant aspects and in the clinical field for diagnostic procedure, treatment and planning surgery. This review aims to analyse the state-of-art of the bioengineering techniques developed over the years to study the corneal biomechanics, highlighting their potentiality to improve diagnosis, treatment and healing process of the corneal tissue, and, at the same, pointing out the current limits in the experimental equipment and numerical tools that are not able to fully characterize in vivo corneal tissues non-invasively and discourage the use of finite element models in daily clinical practice for surgical planning.

Spatial mapping of corneal biomechanical properties using wave-based optical coherence elastography.

Quantifying the mechanical properties of the cornea can provide valuable insights into the occurrence and progression of keratoconus, as well as the effectiveness of corneal crosslinking surgery. This study presents a non-contact and non-invasive wave-based optical coherence elastography system that utilizes air-pulse stimulation to create a two-dimensional map of corneal elasticity. Homogeneous and dual concentration phantoms were measured with the sampling of 25 × 25 points over a 6.6 × 6.6 mm2 area, to verify the measurement capability for elastic mapping and the spatial resolution (0.91 mm). The velocity of elastic waves distribution of porcine corneas before and after corneal crosslinking surgery were further mapped, showing a significant change in biomechanics in crosslinked region. This system features non-invasiveness and high resolution, holding great potential for application in ophthalmic clinics.

Smoking, Corneal Biomechanics, and Glaucoma: Results From Two Large Population-Based Cohorts.

Purpose: Smoking may influence measured IOP through an effect on corneal biomechanics, but it is unclear whether this factor translates into an increased risk for glaucoma. This study aimed to examine the association of cigarette smoking with corneal biomechanical properties and glaucoma-related traits, and to probe potential causal effects using Mendelian randomization (MR). Methods: Cross-sectional analyses within the UK Biobank (UKB) and Canadian Longitudinal Study on Aging (CLSA) cohorts. Multivariable linear and logistic regression models were used to assess associations of smoking (status, intensity, and duration) with corneal hysteresis (CH), corneal resistance factor, IOP, inner retinal thicknesses, and glaucoma. Two-sample MR analyses were performed. Results: Overall, 68,738 UKB (mean age, 56.7 years; 54.7% women) and 22 845 CLSA (mean age, 62.7 years; 49.1% women) participants were included. Compared with nonsmokers, smokers had a higher CH (UKB, +0.48 mm Hg; CLSA, +0.57 mm Hg; P < 0.001) and corneal resistance factor (UKB, +0.47 mm Hg; CLSA, +0.60 mm Hg; P < 0.001) with evidence of a dose-response effect in both studies. Differential associations with Goldmann-correlated IOP (UKB, +0.25 mm Hg; CLSA, +0.36 mm Hg; P < 0.001) and corneal-compensated IOP (UKB, -0.28 mm Hg; CLSA, -0.32 mm Hg; P ≤ 0.001) were observed. Smoking was not associated with inner retinal thicknesses or glaucoma status in either study. MR provided evidence for a causal effect of smoking on corneal biomechanics, especially higher CH. Conclusions: Cigarette smoking seems to increase corneal biomechanical resistance to deformation, but there was little evidence to support a relationship with glaucoma. This outcome may result in an artefactual association with measured IOP and could account for discordant results with glaucoma in previous epidemiological studies.

A new R,R-RvD6 isomer with protective actions following corneal nerve injury.

The transparent cornea is the most densely innervated tissue in the body, primarily by sensory nerves originating from the trigeminal ganglia (TG). Damage to corneal nerves reduces sensitivity and tear secretion and results in dry eye. Consequently, ocular pain, for which no satisfactory therapies exist, arises in many cases. Treatment of injured corneas with pigment epithelium-derived factor (PEDF) combined with docosahexaenoic acid (DHA) stimulates nerve regeneration in models of refractive surgery, which damages nerves. The mechanism involves the synthesis of a stereoisomer of resolvin D6 (R,R-RvD6) formed after incorporating DHA into membrane lipids. Activation of a PEDF receptor (PEDF-R) with phospholipase activity releases DHA to synthesize the new resolvin isomer, which is secreted via tears. Topical treatment of mice corneas with R,R-RvD6 shows higher bioactivity in regenerating nerves and increasing sensitivity compared to PEDF+DHA. It also stimulates a transcriptome in the TG that modulates genes involved in ocular pain. Our studies suggest an important therapeutic role for R,R-RvD6 in regenerating corneal nerves and decreasing pain resulting from dry eye.

The inclusion of the epithelium in numerical models of the human cornea.

We present a patient-specific finite element model of the human cornea that accounts for the presence of the epithelium. The thin anterior layer that protects the cornea from the external actions has a scant relevance from the mechanical point of view, and it has been neglected in most numerical models of the cornea, which assign to the entire cornea the mechanical properties of the stroma. Yet, modern corneal topographers capture the geometry of the epithelium, which can be naturally included into a patient-specific solid model of the cornea, treated as a multi-layer solid. For numerical applications, the presence of a thin layer on the anterior cornea requires a finer discretization and the definition of two constitutive models (including the corresponding properties) for stroma and epithelium. In this study, we want to assess the relevance of the inclusion of the epithelium in the model of the cornea, by analyzing the effects in terms of uncertainties of the mechanical properties, stress distribution across the thickness, and numerical discretization. We conclude that if the epithelium is modeled as stroma, the material properties should be reduced by 10%. While this choice represents a sufficiently good approximation for the simulation of in vivo mechanical tests, it might result into an under-estimation of the postoperative stress in the simulation of refractive surgery.

Development of a Corneal and Eye Protection Strategy in Domestic Swine.

Large animal models are essential to research in facial paralysis, face transplant, craniofacial surgery, and ophthalmology. Pigs are a well-studied species with high similarity to human anatomy and physiology for these research areas. However, in contrast to cats and dogs protecting the cornea and eye is difficult in swine due to the inability to use an Elizabethan collar (E-collar) and the complexity of placing and maintaining a temporary tarsorrhaphy for corneal protection due to the strength of the pig levator muscle. This study presents an effective method to provide corneal and eye protection in the domestic swine for at least 50 d. Furthermore, protection of the eye and face is achieved through the innovative use of a modified ophthalmologic face shield. The findings from this study will advance large animal research in these fields, enabling innovation in surgery and tissue engineering in areas of both craniofacial and ophthalmologic research.

Corneal biomechanics after small incision lenticule extraction and femtosecond laser in situ keratomileusis: A systematic review and meta-analysis.

BACKGROUND: Small incision lenticule extraction (SMILE) and femtosecond laser in situ keratomileusis (FS-LASIK) have been extensively studied as the main surgical methods for corneal refractive surgery. However, there is no consensus on whether SMILE is superior to FS-LASIK in corneal biomechanics. Therefore, this systematic review and meta-analysis used the results of ocular response analyzer and corvis ST to explore whether SMILE is superior to FS-LASIK in corneal biomechanics. METHODS: The literature was searched in PubMed, EMBASE, and Controlled Trials Register databases. The Cochrane Collaboration's "risk of bias" tool was used to evaluate the quality of the included randomized clinical trials, and the Newcastle-Ottawa Scale was used to evaluate the included non-randomized controlled trials. The results were analyzed using Revman 5.3. RESULTS: Sixteen studies (3 randomized clinical trials and 13 non-randomized controlled trials) were included in this meta-analysis. There was no statistical difference in corneal biomechanics between SMILE and FS-LASIK in corneal hysteresis [mean difference (MD), 0.20; 95% confidence interval (CI): -0.09, 0.49; P = .18] and corneal resistant factor (MD, 0.31; 95% CI: -0.09, 0.71; P = .13), A1 time (MD, -0.02; 95% CI: -0.11, 0.07; P = .66), A1 length (MD, 0.01; 95% CI: -0.01, 0.03; P = .42), A1 velocity (MD, 0.00; 95% CI: -0.01, 0.01; P = .85), A2 velocity (MD, -0.01; 95% CI: -0.11, 0.09; P = .86), HC time (MD, 0.12; 95% CI: -0.13, 0.38; P = .35), The stiffness parameter at first applanation (MD, -7.91; 95% CI: -17.96, 2.14; P = .12), The ratio between the deformation amplitude 2 mm away from apex and the apical deformation (MD, 0.01; 95% CI: -0.26, 0.27; P = .96). CONCLUSION: A comprehensive assessment of the parameters of ocular response analyzer and corvis ST showed that SMILE is not superior to LASIK in corneal biomechanics 3 months post-surgery.

Corneal regeneration strategies: From stem cell therapy to tissue engineered stem cell scaffolds.

Corneal epithelial defects and excessive wound healing might lead to severe complications. As stem cells can self-renew infinitely, they are a promising solution for regenerating the corneal epithelium and treating severe corneal epithelial injury. The chemical and biophysical properties of biological scaffolds, such as the amniotic membrane, fibrin, and hydrogels, can provide the necessary signals for stem cell proliferation and differentiation. Multiple researchers have conducted investigations on these scaffolds and evaluated them as potential therapeutic interventions for corneal disorders. These studies have identified various inherent benefits and drawbacks associated with these scaffolds. In this study, we provided a comprehensive overview of the history and use of various stem cells in corneal repair. We mainly discussed biological scaffolds that are used in stem cell transplantation and innovative materials that are under investigation.

The Changes in Ocular Biomechanical Response Parameters and Intraocular Pressure After Surgical Treatment for Thyroid Eye Disease.

Purpose: To investigate changes in ocular biomechanical response parameters and intraocular pressure (IOP) in patients with thyroid eye disease (TED) undergoing orbital decompression or anterior blepharotomy. Methods: Eighty-three eyes from 46 patients receiving orbital decompression (the orbital decompression group) and 45 eyes from 28 patients receiving anterior blepharotomy (the anterior blepharotomy group) were retrospectively enrolled from a tertiary center. Corvis ST tonometry was used to assess ocular biomechanical response and biomechanically corrected IOP (bIOP) pre- and postoperatively. Non-contact tonometry (IOP-NCT) was also performed. Results: In the anterior blepharotomy group, the margin reflex distance decreased (P < 0001). The highest concavity radius (P = 0.026) and whole eye movement (P = 0.003) increased. Neither IOP-NCT nor bIOP had a significant change. In the orbital decompression group, the extent of exophthalmos decreased (P < 0.001). The A2 length (P = 0.009) decreased. The bIOP did not show a significant change (16.4 ± 2.7 vs. 16.7 ± 4.5; P = 0.415), but the IOP-NCT decreased significantly (17.5 ± 3.3 vs. 16.0 ± 3.3; P < 0.001). Higher baseline IOP-NCT (ß = -0.40, P < 0.001) and greater reduction in stiffness parameter A1 (SP-A1; ß = 0.05, P = 0.002) were associated with more significant IOP-NCT reduction after the orbital decompression. Conclusions: Ocular biomechanical response parameters may change after TED surgery, potentially affecting IOP measurements, particularly in patients receiving orbital decompression.

Interobserver agreement between central topography and Scheimpflug tomography for premium intraocular lens selection.

PURPOSE: To assess the inter-observer agreement and reliability of central topography (CT) of IOL Master 700 with that of Pentacam HR, as a screening tool for determining the suitability for premium (toric, monofocal and presbyopia correcting) intraocular lens (IOL) implants. SETTING: Nethradhama Superspeciality Eye Hospital, Bangalore, India. METHOD: 200 eyes of eligible patients undergoing unilateral or bilateral phacoemulsification with premium IOL's were included in the study. Preoperatively, IOL Master 700 CT and Pentacam scans were performed for corneal topography evaluation. 2 experienced clinicians were randomised to interpret the axial map of either of the scans at a time and their responses were then enclosed in an envelope, and forwarded to a third observer for analysis. If there was a conflict, final decision was based on the interpretation of the Pentacam scan. RESULTS: Of the total scans interpreted by the 2 observers, 77.5% (n = 155) showed agreement whereas 22.5% (n = 45) showed disagreement. The sensitivity, specificity, positive predictive value and negative predictive value (NPV) of IOL Master 700 CT in diagnosing irregular corneas when compared with Pentacam HR, was 72.73%, 78.09%, 29.09% and 95.86% respectively. The Cohen's κ inter-observer agreement value was 0.44, signifying a moderate degree of agreement, which was statistically significant (P < .00). CONCLUSIONS: High NPV of IOL Master CT suggested that the device efficiently ruled out irregular corneas, while any suspicious scan should be confirmed with Pentacam HR or a similar device for appropriate decision-making regarding premium IOL selection. TRIAL REGISTRATION NUMBER: CTRI/2021/11/038053.

Human cornea thermo-viscoelastic behavior modelling using standard linear solid model.

BACKGROUND: Corneal biomechanics is of great interest to researchers recently. Clinical findings relate them to corneal diseases and to outcomes of refractive surgery. To have a solid understanding of corneal diseases' progression, it is important to understand corneal biomechanics. Also, they are essential for better explaining outcomes of refractive surgeries and their undesired consequences. There is a difficulty for studying corneal biomechanics in-vivo and multiple limitations arise for ex-vivo studies. Hence mathematical modelling is considered as a proper solution to overcome such obstacles. Mathematical modelling of cornea in-vivo allows studying corneal viscoelasticity with taking into consideration all boundary conditions existing in real in-vivo situation. METHODS: Three mathematical models are used to simulate corneal viscoelasticity and thermal behavior in two different loading situations: constant and transient loading. Two models of the three are used for viscoelasticity simulation which are Kelvin-Voigt and standard linear solid models. Also, temperature rise due to the ultrasound pressure push is calculated using bioheat transfer model for both the axial direction and as a 2D spatial map using the third model (standard linear solid model). RESULTS: Viscoelasticity simulation results show that standard linear solid model is efficient for describing the viscoelastic behavior of human cornea in both loading conditions. Results show also that the deformation amplitude obtained from standard linear solid model is more reasonable for corneal soft-tissue deformation with respect to corresponding clinical findings than that obtained from Kelvin-Voigt model. Thermal behavior results estimated corneal temperature rise to be roughly 0.2 °C, which conforms with FDA regulations for soft tissue safety. CONCLUSION: Standard Linear Solid (SLS) model is better describing the human corneal behavior in response to constant and transient load more efficiently. Temperature rise (TR) for the corneal tissue of about 0.2 °C is conforming with FDA regulations and even less than the FDA regulations for soft tissue safety.

Biaxial hyperelastic and anisotropic behaviors of the corneal anterior central stroma along the preferential fibril orientations. Part II: Quantitative computational analysis of mechanical response of stromal components.

To study the hyperelastic and anisotropic behaviors of the central anterior stroma for patients with myopia, 40 corneal stromal specimens extracted after small incision lenticule extraction (SMILE) surgery were used in the biaxial extension test along two preferential fibril orientations. An improved collagen fibril crimping constitutive model with a specific physical meaning was proposed to analyze the hyperelasticity and anisotropy of the stroma. The effective elastic modulus of the two families of preferentially oriented collagen fibrils and the stiffness of the non-collagenous matrix along all three directions were compared according to the specific physical meaning of the parameters. Anisotropic behavior was found in the hyperelastic properties of the corneal anterior central stroma in the preferential fibril orientations. The stiffness of non-collagenous matrix is significantly larger in the optical axis direction than in the nasal-temporal (NT) and superior-inferior (SI) directions. Moreover, individual differences between males and females slightly impact on hyperelastic and anisotropic behaviors. The differences of these behaviors were significant in the comparison of the left and right eyes. These results have a guiding significance for the accurate design of surgical plans for refractive surgery according to a patient's condition and have a driving value for the further exploration of the biomechanical properties of the whole cornea.

Role of tear vasoactive intestinal peptide on dry eyes after laser keratorefractive surgery.

BACKGROUND: To explore the changes in vasoactive intestinal peptide (VIP) concentration in tears post laser-assisted sub-epithelial keratomileusis (LASEK) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) surgeries and related factors, possible association between postoperative dry eye symptoms and VIP concentration in tears, and factors influencing dry eye symptoms after different periods post LASEK and FS-LASIK surgeries. METHODS: In this prospective, non-randomized, longitudinal cohort study, 23 and 22 subjects were recruited and underwent LASEK and FS-LASIK, respectively. After conducting an intact ophthalmic examination and collecting relevant surgical data, all subjects were examined for VIP concentration in their tears using ELISAs, tear-film breakup time, ocular staining and ocular surface disease index questionnaire before surgery and 1 day, 1 week, and 1 month post-surgery. RESULTS: Tear VIP concentration increased significantly after both LASEK and FS-LASIK, with the highest concentration observed 1 week post-surgery (P ≤ 0.05). Tear VIP concentration correlated negatively with corneal ablation depth (AD). The extent of dry eyes was related to the operation method employed and postoperative recovery period. In FS-LASIK and LASEK subjects, dry eyes were mainly affected by the basic ocular surface status before surgery, and VIP concentration. Furthermore, in LASEK subjects, dry eyes were negatively correlated with AD. CONCLUSION: VIP was stimulated and mobilized as an emergency protection post-refractive surgery and a trauma model affected by AD. It can indirectly indicate the inevitable relationship between postoperative dry eye and nerve injury. Elevated post-surgery tear VIP relieves dry eye symptoms, showing its neuroimmune role in regulating adverse injury stimulation. The present study provides a solution to the pathogenesis of postoperative dry eyes. TRIAL REGISTRATION: The trial registration number: 2021JS22. Date of registration: 10 May 2021.

[Corneal epithelial biomechanics: Resistance to stress and role in healing and remodeling]. / Biomécanique de l'épithélium cornéen : résistance au stress et implications dans la cicatrisation et le remodelage.

The corneal epithelium is one of the first tissue barriers of the eye against the environment. In recent years, many studies provided better knowledge of its healing, its behavior and its essential role in the optical system of the eye. At the crossroads of basic science and clinical medicine, the study of the mechanical stresses applied to the cornea makes it possible to learn the behavior of epithelial cells and better understand ocular surface disease. We describe herein the current knowledge about the adhesion systems of the corneal epithelium and their resistance to mechanical stress. We will also describe the involvement of these mechanisms in corneal healing and their role in epithelial dynamics. Adhesion molecules of the epithelial cells, especially hemidesmosomes, allow the tissue cohesion required to maintain the integrity of the corneal epithelium against the shearing forces of the eyelids as well as external forces. Their regeneration after a corneal injury is mandatory for the restoration of a healthy epithelium. Mechanotransduction plays a significant role in regulating epithelial cell behavior, and the study of the epithelium's response to mechanical forces helps to better understand the evolution of epithelial profiles after refractive surgery. A better understanding of corneal epithelial biomechanics could also help improve future therapies, particularly in the field of tissue engineering.

Smoking, alcohol consumption and corneal biomechanical parameters among Chinese university students.

BACKGROUND/OBJECTIVES: Smoking and alcohol consumption are important risk factors for several ocular disorders, but their effects on corneal biomechanics remain unclear. Our study aims to explore the association between smoking and alcohol consumption with corneal biomechanical parameters measured by Corvis-ST (CST) among university students. SUBJECTS/METHODS: A total of 1645 healthy university students from a university-based study were included, and all participants underwent corneal biomechanical parameters measurement by CST. We selected 10 reliable parameters that can reflect the corneal deformation response. All participants had a standardised interview to determine their smoking and alcohol consumption status. RESULTS: The mean age of the participants was 19.0 ± 0.9 years, and 1132 (68.8%) were women. Smoking was significantly associated with stiffer corneas. Smokers showed significantly slower second applanation velocity (A2v) (ß = 0.007 m/s, 95% confidence interval 0.001 to 0.014, P = 0.032) and lower integrated radius (IR) (ß = -0.214 mm-1, 95% confidence interval -0.420 to -0.007, P = 0.043) than non-smokers after adjusting for age, gender, eye-rubbing, myopia, and body mass index (BMI). Smokers with BMI no less than 24.0 showed slower A2v and lower IR. Alcohol consumption and passive smoking were found no significant association with corneal biomechanics. CONCLUSION: Smokers had less deformable corneas, especially those with BMI no less than 24.0. Our findings provide new evidence for the association between smoking and ocular disorders associated with corneal biomechanics like glaucoma.

Corneal Biomechanics Losses Caused by Refractive Surgery.

Recent advances, specifically in the understanding of the biomechanical properties of the cornea and its response to diseases and surgical interventions, have significantly improved the safety and surgical outcomes of corneal refractive surgery, whose popularity and demand continue to grow worldwide. However, iatrogenic keratectasia resulting from the deterioration in corneal biomechanics caused by surgical interventions, although rare, remains a global concern. On one hand, in vivo biomechanical evaluation, enabled by clinical imaging systems such as the ORA and the Corvis ST, has significantly improved the risk profiling of patients for iatrogenic keratectasia. That is despite the fact the biomechanical metrics provided by these systems are considered indicators of the cornea's overall stiffness rather than its intrinsic material properties. On the other hand, new surgical modalities including SMILE were introduced to offer superior biomechanical performance to LASIK, but this superiority could not be proven clinically, creating more myths than answers. The literature also includes sound evidence that tPRK provided the highest preservation of corneal biomechanics when compared to both LASIK and SMILE. The aim of this review is twofold; to discuss the importance of corneal biomechanical evaluation prior to refractive surgery, and to assess the current understanding of cornea's biomechanical deterioration caused by mainstream corneal refractive surgeries. The review has led to an observation that new imaging techniques, parameters and evaluation systems may be needed to reflect the true advantages of specific refractive techniques and when these advantages are significant enough to offer better protection against post-surgery complications.

[Effects of SMILE with different residual stromal thicknesses on corneal biomechanical properties of rabbits in vivo].

Femtosecond laser small incision lenticule extraction (SMILE) with different residual stromal thicknesses (RST) is set to investigate its effect on corneal biomechanical properties of rabbits in vivo. In this study, 24 healthy adult Japanese rabbits were randomly divided into group A and B. The RST of group A was set 30% of the corneal central thickness (CCT), and the RST of group B was 50% of the CCT. The thickness of the corneal cap in both groups was set one third of CCT. Corneal visualization Scheimpflug technology (Corvis ST) and Pentacam three-dimensional anterior segment analyzer were used to determine corneal biomechanical and morphological parameters before surgery, and 1 week, 1 month and 3 months after surgery. Pearson correlation analysis was used to analyze factors affecting corneal biomechanical parameters after SMILE. The results showed that the corneal stiffness of group A was significantly higher than that of group B at 1 week and 1 month after surgery, and most biomechanical parameters returned to preoperative levels at 3 months postoperatively. The results of correlation analysis showed that postoperative CCT and RST were the main factors affecting corneal biomechanical parameters after SMILE. There was no significant difference in corneal posterior surface height (PE) between 3 months after surgery and before surgery in both two groups. It indicates that although the ability to resist deformation of cornea decreases in SMILE with thicker corneal cap and less RST, there is no tendency to keratoconus, which may be related to the preservation of more anterior stromal layer.