Biomechanical properties of the keratoconic cornea: a review

Static magnetic field effects on the secondary structure and elasticity of collagen molecules; a possible biophysical approach to treat keratoconus

Type I collagen is among the major extracellular proteins that play a significant role in the maintenance of the cornea's structural integrity and is essential in cell adhesion, differentiation, growth, and integrity. Here, we investigated the effect of 300 mT Static Magnetic Field (300 mT SMF) on the structure and molecular properties of acid-solubilized collagens (ASC) isolated from the rat tail tendon. The SMF effects at molecular and atomic levels were investigated by various biophysical approaches like Circular Dichroism Spectropolarimetery (CD), Fourier Transform Infrared Spectroscopy (FTIR), Zetasizer light Scattering, and Rheological assay. Exposure of isolated type I collagen to 300 mT SMF retained its triple helix. The elasticity of collagen molecules and the keratoconus (KCN) cornea treated with SMF decreased significantly after 5 min and slightly after 10, 15, and 20 min of treatments. The exposure to 300 mT SMF shifted the Amid I bond random coil to antiparallel wave number from 1647 to 1631 cm-1. The pH of the 300 mT SMF treated collagen solution increased by about 25 %. The treatment of the KCN corneas with 300 mT SMF decreased their elasticity significantly. The promising results of the effects of 300 mT SMF on the collagen molecules and KCN cornea propose a novel biophysical approach capable of manipulating the collagen's elasticity, surface charges, electrostatic interactions, cross binding, network formation and fine structure. Therefore, SMF treatment may be considered as a novel non-invasive, direct, non-chemical and fast therapeutic and manipulative means to treat KCN cornea where the deviated physico-chemical status of collagen molecules cause deformation.

Current Scenario and Future Perspectives of Porcine Corneal Xenotransplantation

ABSTRACT

Corneal diseases represent a significant cause of blindness worldwide, with corneal transplantation being an effective treatment to prevent vision loss. Despite substantial advances in transplantation techniques, the demand for donor corneas exceeds the available supply, particularly in developing countries. Cornea xenotransplantation has emerged as a promising strategy to address the worldwide scarcity, notably using porcine corneas. In addition to the inherent immune privilege of the cornea, the low cost of porcine breeding and the anatomical and physiological similarities between humans and pigs have made porcine corneas a viable alternative. Nonetheless, ethical concerns, specifically the risk of xenozoonotic transmission and the necessity for stringent biosafety measures, remain significant obstacles. Moreover, the success of xenotransplantation is compromised by innate and adaptive immune responses, which requires meticulous consideration and further studies. Despite these challenges, recent breakthroughs have further contributed to reducing immunogenicity while preserving the corneal architecture. Advances in genetic engineering, such as the use of CRISPR-Cas9 to eliminate critical porcine antigens, have shown promise for mitigating immune reactions. Additionally, new immunosuppressive protocols, such as have techniques like decellularization and the use of porcine-derived acellular matrices, have greatly increased graft survival in preclinical models. Future research must focus on refining immunomodulatory strategies and improving graft preparation techniques to ensure the long-term survival and safety of porcine corneal xenotransplantation in clinical trials in humans.

Effects of Genipin Crosslinking of Porcine Perilimbal Sclera on Mechanical Properties and Intraocular Pressure

The mechanical properties of sclera play an important role in ocular functions, protection, and disease. Modulating the sclera's properties by exogenous crosslinking offers a way to expand the tissue's range of properties for study of the possible influences on the eye's behavior and diseases such as glaucoma and myopia. The focus of this work was to evaluate the effects of genipin crosslinking targeting the porcine perilimbal sclera (PLS) since the stiffness of this tissue was previously found in a number of studies to influence the eye's intraocular pressure (IOP). The work includes experiments on tensile test specimens and whole globes. The specimen tests showed decreased strain-rate dependence and increased relaxation stress due to the cross-linker. Whole globe perfusion experiments demonstrated that eyes treated with genipin in the perilimbal region had increased IOPs compared to the control globes. Migration of the cross-linker from the target tissue to other tissues is a confounding factor in whole globe, biomechanical measurements, with crosslinking. A novel quantitative genipin assay of the trabecular meshwork (TM) was developed to exclude globes where the TM was inadvertently crosslinked. The perfusion study, therefore, suggests that elevated stiffness of the PLS can significantly increase IOP apart from effects of the TM in the porcine eye. These results demonstrate the importance of PLS biomechanics in aqueous outflow regulation and support additional investigations into the distal outflow pathways as a key source of outflow resistance.

Early elastic and viscoelastic corneal biomechanical changes after photorefractive keratectomy and small incision lenticule extraction

PURPOSE

To compare early changes in the corneal biomechanical parameters after photorefractive keratectomy (PRK) and small incision lenticule extraction (SMILE) and their correlations with corneal shape parameters.

METHODS

One hundred twenty four eyes received myopic PRK and SMILE for similar amounts of myopia. Corneal tomography with Pentacam HR, biomechanical parameters using Corvis ST, and Ocular Response Analyzer (ORA) were evaluated before and 2 weeks after surgery. The change in each parameter was compared between groups, while the difference in central corneal thickness and cornea-compensated intraocular pressure measured before and after surgery were considered as covariates.

RESULTS

A significant reduction was seen in the corneal stiffness parameter at first applanation, and an increase in deformation amplitude ratio (DAR), and integrated inverse radius (IIR) in both groups after surgery (p < 0.001) Changes in DAR, and IIR were significantly greater in the SMILE than in the PRK group (p < 0.001) Corneal hysteresis (CH) and corneal resistance factor (CRF) decreased in both SMILE and PRK groups after surgery, (p < 0.001) with no statistically significant difference between groups (p > 0.05) Among new Corvis ST parameters, DAR showed a significant correlation with changes in Ambrosio relational thickness in both groups (p < 0.05).

CONCLUSIONS

Both techniques caused significant changes in corneal biomechanics in the early postoperative period, with greater elastic changes in the SMILE group compared to the PRK group, likely due to lower tension in the SMILE cap and thinner residual stromal bed in SMILE. There were no differences in viscoelastic changes between them, so the lower CH may reflect the volume of tissue removed.

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.

Improving early detection of keratoconus by Non Contact Tonometry. A computational study and new biomarkers proposal

Keratoconus is a progressive ocular disorder affecting the corneal tissue, leading to irregular astigmatism and decreased visual acuity. The architectural organization of corneal tissue is altered in keratoconus, however, data from ex vivo testing of biomechanical properties of keratoconic corneas are limited and it is unclear how their results relate to true mechanical properties in vivo. This study explores the mechanical properties of keratoconic corneas through numerical simulations of non-contact tonometry (NCT) reproducing the clinical test of the Corvis ST device. Three sensitivity analyses were conducted to assess the impact of corneal material properties, size, and location of the pathological area on NCT results. Additionally, novel asymmetry-based indices were proposed to better characterize corneal deformations and improve the diagnosis of keratoconus. Our results show that the weakening of corneal material properties leads to increased deformation amplitude and altered biomechanical response. Furthermore, asymmetry indices offer valuable information for locating the pathological tissue. These findings suggest that adjusting the Corvis ST operation, such as a camera rotation, could enhance keratoconus detection and provide insights into the relative position of the affected area. Future research could explore the application of these indices in detecting early-stage keratoconus and assessing the fellow eye's risk for developing the pathology.

Citation network analysis on keratoconus

OBJECTIVE

The high incidence of keratoconus has caused its management, etiology, and pathogenesis to be controversial topics in the ophthalmology field. This study aims to analyze the relationship between the different publications and authors through citation networks, as well as to identify the research areas and determine the most cited article.

METHODS

The search for publications was carried out through the Web of Science database, using the term "Keratoconus" between 1900 and December 2022. The Citation Network Explorer and CiteSpace software were used for the publication analysis.

RESULTS

9,655 publications were found, with 124,379 citations generated on the network. The year with the highest number of publications was 2021. The most cited publication was "Keratoconus" by Rabinowitz, published in 1998. Cluster function gave five groups of research areas about keratoconus: corneal signs and parameters, cross-linking efficiency and effects, clinical factors, keratoplasty, and treatment.

CONCLUSIONS

The citation network offers an objective and comprehensive analysis of the papers on keratoconus.

Corneal biomechanics and diagnostics: a review

PURPOSE

Corneal biomechanics is an emerging field and the interest into physical and biological interrelations in the anterior part of the eye has significantly increased during the past years. There are many factors that determine corneal biomechanics such as hormonal fluctuations, hydration and environmental factors. Other factors that can affect the corneas are the age, the intraocular pressure and the central corneal thickness. The purpose of this review is to evaluate the factors affecting corneal biomechanics and the recent advancements in non-destructive, in vivo measurement techniques for early detection and improved management of corneal diseases.

METHODS

Until recently, corneal biomechanics could not be directly assessed in humans and were instead inferred from geometrical cornea analysis and ex vivo biomechanical testing. The current research has made strides in studying and creating non-destructive and contactless techniques to measure the biomechanical properties of the cornea in vivo.

RESULTS

Research has indicated that altered corneal biomechanics contribute to diseases such as keratoconus and glaucoma. The identification of pathological corneas through the new measurement techniques is imperative for preventing postoperative complications.

CONCLUSIONS

Identification of pathological corneas is crucial for the prevention of postoperative complications. Therefore, a better understanding of corneal biomechanics will lead to earlier diagnosis of ectatic disorders, improve current refractive surgeries and allow for a better postoperative treatment.

Comparison of Ectasia Detection in Early Keratoconus Using Scheimpflug-Based Corneal Tomography and Biomechanical Assessments

PURPOSE

The aim of this study was to determine the detection of keratoconus using corneal biomechanical parameters only, a corneal tomographic parameter only, and a parameter that combines corneal biomechanical and tomographic indices.

METHODS

The discriminatory power of the Pentacam Random Forest Index (PRFI), Belin/Ambrósio Enhanced Ectasia Display (BAD-D) index, Corvis Biomechanical Index (CBI), and Tomographic and Biomechanical Index (TBI) to differentiate between normal eyes (n = 84), eyes with very asymmetric corneal ectasia (VAE-E, n = 21), and the fellow eyes without apparent ectasia based on normal tomography (VAE-NT, n = 21) was assessed. Statistical analyses were completed with R software using t -tests, Wilcoxon rank sum tests, and receiver operating characteristic (ROC) curves. The DeLong test was used to compare the area under the ROC curve (AUROC).

RESULTS

The TBI and PRFI had the highest AUROC when distinguishing between normal and VAE-E corneas (AUROC = 1.00, 95% CI = 1.00-1.00); however, they were not statistically superior to the CBI (AUROC = 0.97, P = 0.27) or BAD-D (AUROC = 1.00, P = 0.34). The TBI (AUROC = 0.92, 95% CI = 0.86-0.98) was superior to CBI (AUROC = 0.78, P = 0.02) and BAD-D (AUROC = 0.81, P = 0.02) when distinguishing between healthy and VAE-NT corneas. At a threshold of 0.72, the TBI had 99% sensitivity, 67% specificity, and 92% accuracy in distinguishing normal and VAE-NT corneas.

CONCLUSIONS

The TBI is a useful parameter for the screening of subclinical and frank keratoconus in tomographically normal eyes.

Corneal Biomechanics in Non-infectious Uveitis Measured by Corvis ST: A Pilot Study

PURPOSE

To assess differences between corneal biomechanical properties in patients with non-infectious uveitis and healthy subjects using CorVis.

METHODS

77 patients with non-infectious uveitis and 47 control subjects were recruited. Biomechanical parameters were measured: deformation amplitude (DA), A-1 length and A-2 length (L1, L2), A-1 velocity and A-2 velocity (V1, V2), peak distance (PD) and HC radius (highest concavity radius). AUC ROC and correlation between clinical variables and biomechanical properties were determined.

RESULTS

Lower HC Radius and IOPb and higher DA and V1 was found in uveitis group. Statistical differences between cases using systemic medications and those with topical treatment were found in L1. Differences were showed between those cases with active and inactive uveitis in PD, DA, V2 and L2. The biomechanical parameter with the best discriminatory capacity of uveitis disease was HC Radius.

CONCLUSION

Differences in corneal biomechanical properties between non-infectious uveitis and healthy eyes were found.

A review of the epithelial and stromal effects of corneal collagen crosslinking

Induced corneal collagen crosslinking and mechanical stiffening via ultraviolet-A photoactivation of riboflavin (UVA CXL) is now a common treatment for corneal ectasia and Keratoconus. Some effects of the procedure such as induced mechanical stiffening, corneal flattening, and cellular toxicity are well-known, but others remain more controversial. Authors report a variety of contradictory effects, and provide evidence based on individual results and observations. A full understanding of the effects of and mechanisms behind this procedure are essential to predicting its outcome. A growing interest in modifications to the standard UVA CXL protocol, such as transepithelial or accelerated UVA CXL, makes analyzing the literature as a whole more urgent. This review presents an analysis of both the agreed-upon and contradictory results reported and the various methods used to obtain them.

Corneal Biomechanical Measures for Glaucoma: A Clinical Approach

Over the last two decades, there has been growing interest in assessing corneal biomechanics in different diseases, such as keratoconus, glaucoma, and corneal disorders. Given the interaction and structural continuity between the cornea and sclera, evaluating corneal biomechanics may give us further insights into the pathogenesis, diagnosis, progression, and management of glaucoma. Therefore, some authorities have recommended baseline evaluations of corneal biomechanics in all glaucoma and glaucoma suspects patients. Currently, two devices (Ocular Response Analyzer and Corneal Visualization Schiempflug Technology) are commercially available for evaluating corneal biomechanics; however, each device reports different parameters, and there is a weak to moderate agreement between the reported parameters. Studies are further limited by the inclusion of glaucoma subjects taking topical prostaglandin analogues, which may alter corneal biomechanics and contribute to contradicting results, lack of proper stratification of patients, and misinterpretation of the results based on factors that are confounded by intraocular pressure changes. This review aims to summarize the recent evidence on corneal biomechanics in glaucoma patients and insights for future studies to address the current limitations of the literature studying corneal biomechanics.

Comparison of bilateral differential characteristics of corneal biomechanics between keratoconus and normal eyes

Purpose: To compare bilateral differences in corneal biomechanics between keratoconus and normal eyes. Methods: In this case-control study, 346 eyes of 173 patients (aged 22.1 ± 6.1 years) with keratoconus (KC group) and 378 eyes of 189 patients (aged 26.7 ± 5.6 years) with ametropia (control group) were enrolled. Corneal tomography and biomechanical properties were examined using Pentacam HR and Corvis ST, respectively. The corneal biomechanical parameters were compared between eyes with forme fruste keratoconus (FFKC) and normal eyes. Bilateral differences in corneal biomechanical parameters were compared between the KC and control groups. Receiver operating characteristic (ROC) analysis was used to assess discriminative efficacies. Results: The areas under the ROC curves (AUROCs) of stiffness parameter at the first applanation (SP-A1) and Tomographic and Biomechanical Index (TBI) for identifying FFKC were 0.641 and 0.694, respectively. The bilateral differential values of major corneal biomechanical parameters were significantly increased in the KC group (all p < 0.05), except for the Corvis Biomechanical Index (CBI). The AUROCs of the bilateral differential values of the deformation amplitude ratio at 2 mm (ΔDAR2), Integrated Radius (ΔIR), SP-A1 (ΔSP-A1), and the maximum inverse concave radius (ΔMax ICR) for discriminating keratoconus were 0.889, 0.884, 0.826, and 0.805, respectively. The Logistic Regression Model-1 (comprising of ΔDAR2, ΔIR, and age) and the Logistic Regression Model-2 (comprising of ΔIR, ΔARTh, ΔBAD-D, and age) had AUROCs of 0.922 and 0.998, respectively, for discriminating keratoconus. Conclusion: The bilateral asymmetry of corneal biomechanics was significantly increased in keratoconus compared with normal eyes, which may be helpful for the early detection of keratoconus.

The Corneal Ectasia Model of Rabbit: A Validity and Stability Study

Keratoconus is a bilateral progressive degenerative corneal disease characterized by localized corneal thinning and dilatation. The pathogenesis of keratoconus is not fully elucidated. To gain a better understanding of the pathophysiology of this disease and to explore potential treatments, animal models are essential for basic research. Several attempts have been made to establish animal models of corneal ectasia by using collagenase. However, continuous changes of the cornea have not been well-tracked for the model. In this study, corneal morphology and biomechanical behavior in vivo were determined before and after collagenase Ⅱ treatment at 2, 4, and 8 weeks. The elastic modulus and histology of cornea tissues ex vivo were measured at 8 weeks postoperatively. The results showed that the posterior corneal curvature (Km B) increased and central corneal thickness (CCT) decreased after collagenase treatment. The mechanical properties of ectatic corneas weakened significantly and the collagen fiber interval in the stromal layer was increased and disorganized. This study provides insights into the changes of corneal morphology and biomechanical properties in a rabbit model of corneal ectasia. Changes observed at 8 weeks indicated that the cornea was still undergoing remodeling.

The Evolution of Diagnostics for Keratoconus: From Ophthalmometry to Biomechanics

PURPOSE

To enumerate the various diagnostic modalities used for keratoconus and their evolution over the past century.

METHODS

A comprehensive literature search including articles on diagnosis on keratoconus were searched on PUBMED and summarized in this review.

RESULTS

Initially diagnosed in later stages of the disease process through clinical signs and retinoscopy, the initial introduction of corneal topography devices like Placido disc, photokeratoscopy, keratometry and computer-assisted videokeratography helped in the earlier detection of keratoconus. The evolution of corneal tomography, initially with slit scanning devices and later with Scheimpflug imaging, has vastly improved the accuracy and detection of clinical and sub-clinical disease. Analyzing the alteration in corneal biomechanics further contributed to the earlier detection of keratoconus even before the tomographic changes became evident. Anterior segment optical coherence tomography has proven to be a helpful adjuvant in diagnosing keratoconus, especially with epithelial thickness mapping. Confocal microscopy has helped us understand the alterations at a cellular level in keratoconic corneas.

CONCLUSION

Thus, the collective contribution of the various investigative modalities have greatly enhanced earlier and accurate detection of keratoconus, thus reducing the disease morbidity.

Ultra-High Resolution Optical Aberrometry in Patients with Keratoconus: A Cross-Sectional Study

INTRODUCTION

This study performs optical aberration assessment in patients using a novel ultra-high-resolution device. The objective of this study is to analyze optical aberrations, especially the very high order wavefront (more than 10th order of Zernike coefficients), and compare between keratoconus and healthy patients.

METHODS

In this cross-sectional study, we analyzed 43 eyes from 25 healthy patients and 43 eyes from 27 patients with keratoconus using corneal tomography and a very high-resolution (8.55 µm) aberrometer prototype (T-eyede) outfitted with a sensor originally developed for use in the field of astrophysics. Corneal aberration values were assessed using an optical model built with Zemax optical software, while ocular aberrations were assessed using T-eyede. In addition, image-processing analysis was performed of the wavefront phase, creating a high-pass filter map.

RESULTS

We found lower values for ocular aberrations than corneal aberrations in both groups (p < 0.001). Specifically, we found a reduction in primary astigmatism (0.145 µm) and primary coma (0.017 µm). Also, the keratoconus group showed significantly higher wavefront aberration values compared with controls (p < 0.001). An analysis of the high-pass filter map revealed 2 contrasting results: one smooth or clear, while the other presented a banding pattern. Almost all in the control group (95%) showed the first pattern, while 77% of the keratoconus group showed a banding pattern on the filtered map (chi-squared test, p < 0.001).

CONCLUSION

This device provides reliable, precise measurements of ocular aberrations that correlate well with corneal aberrations. Furthermore, the extraordinary high-resolution measurements revealed unprecedented micro changes in the wavefront phase of patients with keratoconus that varied with disease stage. These findings could lead to new screening or follow-up methods.

Comparison of corneal biomechanical properties in healthy thin corneas with matched keratoconus eyes

PURPOSE

To compare corneal biomechanical parameters of normal thin corneas with matched keratoconus eyes.

SETTING

Eye Research Center, Mashhad University of Medical Sciences, Mashhad, Iran.

DESIGN

Cross-sectional comparative study.

METHODS

Dynamic corneal response parameters of Corvis ST were compared in 61 eyes with keratoconus with 61 matched healthy thin corneas (corneal thinnest point <500 μm), while corneal thickness, biomechanically corrected intraocular pressure, and age were considered covariates. The receiving operator sensitivity curve analysis was used to determine the cutoff point with the highest sensitivity and specificity, and the area under the curve (AUC) for each parameter.

RESULTS

All biomechanical parameters were statistically significant between the 2 groups except for the first ( P = .947) and second ( P = .582) applanation length, first ( P = .783) and second ( P = .301) applanation velocity, and deformation amplitude in the highest concavity phase ( P = .106). The highest mean difference between groups (12.89 ± 2.03 mm Hg/mm) was related to the stiffness parameter at the first applanation (SPA1). Although the Corvis biomechanical index and tomographic biomechanical index had the highest detection ability based on their AUC (0.912 and 0.959, respectively), among the standard and combined biomechanical parameters except for keratoconus screening parameters, the highest discriminative ability was related to SPA1 with AUC, sensitivity, and specificity of 0.793, 60.66%, and 90.16%, respectively.

CONCLUSIONS

Keratoconus corneas were significantly softer compared with healthy thin corneas of matched thickness. Optimal cutoff points close to the maximum value defined for screening parameters limit their clinical use for differentiation purposes in these particular types of cases.

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.

Two-photon collagen crosslinking in ex vivo human corneal lenticules induced by near-infrared femtosecond laser

Myopia and keratoconus have become common corneal diseases that threaten the quality of human vision, and keratoconus is one of the most common indications for corneal transplantation worldwide. Collagen crosslinking (CXL) using riboflavin and ultraviolet A (UVA) light is an effective approach for treating ophthalmic disorders and has been shown clinically not only to arrest further progression of keratoconus but also to improve refractive power for cornea. However, CXL surgery irradiated by UVA has various potential risks such as surface damage and endothelial cell damage. Here, near-infrared femtosecond laser-based two-photon CXL was first applied to ex vivo human corneal stroma, operating at low photon energy with high precision and stability. After two-photon CXL, the corneal stiffness can be enhanced by 300% without significantly reducing corneal transparency. These findings illustrate the optimized direction that depositing high pulses energy in corneal focal volume (not exceeding damage threshold), and pave the way to 3D CXL of in vivo human cornea with higher safety, precision, and efficacy.

In-vivo high-speed biomechanical imaging of the cornea using Corvis ST and digital image correlation

In-vivo corneal biomechanical characterization has gained significant clinical relevance in ophthalmology, especially in the early diagnosis of eye disorders and diseases (e.g. keratoconus). In clinical medicine, the air-puff-based tonometers such as Ocular Response Analyzer (ORA) and Corvis ST have been used in the in-vivo biomechanical testing. In the test, the high-speed dynamic deformation of the cornea under air-puff excitation is analyzed to identify the abnormities in the morphological and biomechanical properties of the cornea. While most existing measurements reflect the overall corneal biomechanical properties, in-vivo high-speed strain and strain rate fields at the tissue level have not been assessed. In this study, 20 subjects were classified into two different groups: the normal (NORM, N = 10) group and the keratoconus (KC, N = 10) group. Image sequences of the horizontal cross-section of the human cornea under air puff were captured by the Corvis ST tonometer. The macroscale mechanical response of the cornea was determined through image analysis. The high-speed evolution of full-field corneal displacement, strain, velocity, and strain rate was reconstructed using the incremental digital image correlation (DIC) approach. Differences in the parameters between the NORM and KC groups were statistically analyzed and compared. Statistical results indicated that compared with the NORM group, the KC corneas absorbed more energy (KC: 8.98 ± 2.76 mN mm; NORM: 4.79 ± 0.62 mN mm; p-value <0.001) with smaller tangent stiffness (KC: 22.49 ± 2.62 mN/mm; NORM: 24.52 ± 3.20 mN/mm; p-value = 0.15) and larger maximum deflection (KC: 0.99 ± 0.07 mN/mm; NORM: 0.92 ± 0.06 mN/mm; p-value <0.05) on the macro scale. Further, we also observed that The maximum displacement (KC: 1.17 ± 0.06 mm; NORM: 1.06 ± 0.07 mm; p-value <0.005), velocity (KC: 236 ± 29 mm/s; NORM: 203 ± 17 mm/s; p-value <0.01), shear strain (KC: 24.43 ± 2.59%; NORM: 20.26 ± 1.54%; p-value <0.001), and shear strain rate (KC: 69.74 ± 11.99 s-1; NORM: 54.84 ± 3.03 s-1; p-value <0.005) in the KC group significantly increased at the tissue level. This is the first time that the incremental DIC method was applied to the in-vivo high-speed corneal deformation measurement in combination with the Corvis ST tonometer. Through the image registration using incremental DIC analysis, spatiotemporal dynamic strain/strain rate maps of the cornea can be estimated at the tissue level. This is constructive for the clinical recognition and diagnosis of keratoconus at a more underlying level.

Keratoconus: A Biomechanical Perspective

PURPOSE

The relevance of corneal biomechanics and the importance of including it in the clinical assessment of corneal ectasias are being increasingly recognized. The connection between corneal ultrastructure, biomechanical properties, and optical function is exemplified by a condition like keratoconus. Biomechanical instability is seen as the underlying basis for the secondary morphological changes in the cornea. Asymmetric biomechanical weakening is believed to drive progressive corneal steepening and thinning. Biomechanical strengthening is the principle of collagen crosslinking that has been shown to effectively arrest progression of the keratoconus. Corneal biomechanics has therefore ignited the interest of researchers and clinicians alike and has given us new insights into the cause and course of the disease. This article is an overview of the extensive work published, predominantly in the last two decades, on the biomechanical aspect of keratoconus.

METHODS

Published articles on corneal biomechanics in the specific context of keratoconus were reviewed, based on an electronic search using PubMed, Elsevier, and Science Direct. The search terms used included "Corneal Biomechanics," "Mechanical properties of the cornea," "Corneal ultrastructure," "Corneal Collagen," and "Keratoconus". Articles pertaining to refractive surgery, keratoplasty, collagen crosslinking, or intrastromal rings were excluded.

RESULTS

The electronic search revealed more than 500 articles, from which 80 were chosen for this article.

CONCLUSIONS

The structural and organizational pattern of the corneal stroma determines its mechanical properties and are responsible for the maintenance of the normal shape and function of the cornea. Changes in the ultrastructure are responsible for the biomechanical instability that leads to corneal ectasia. As non-invasive methods for evaluating corneal biomechanics in vivo evolve, our ability to diagnose subclinical keratoconus will improve, allowing identification of patients at risk to develop ectasia and to allow early treatment to arrest progression of the disease.

Comprehensive Assessment of Corvis ST Biomechanical Indices in Normal and Keratoconus Corneas with Reference to Corneal Enantiomorphism

The aim of this study was to assess Corvis ST biomechanical indices in reference to corneal enantiomorphism. In a prospective observational cohort study, 117 eyes from 63 patients with normal or keratoconus corneas were assessed by three independent observers. In the control group (n = 62), no significant differences were observed between the three observers for all indices. The best reproducibility was obtained with pachymetry and the weakest with CBI. All indices but CBI and arc length featured COV < 10%. All indices except the PD and SSI correlated with pachymetry; all but Rad correlated with IOP. The comparison of the thinnest with the thickest corneas showed no significant differences for any index except pachymetry. In the keratoconus group (n = 55), loss of corneal enantiomorphism was confirmed for all indices except the arc length, velocity, and PD. Significant differences between both groups were found for all indices, even after adjustment for pachymetry and intraocular pressure. The CBI featured the best accuracy (92%), sensitivity (91%), and graphical relevance for keratoconus diagnosis. However, its reproducibility was weak in normal corneas and was strongly dependent on corneal thickness. The SSI was independent of corneal thickness, highly reproducible, and provided the expected enantiomorphism characteristics in both groups, making it a relevant biomarker of biomechanical corneal behavior.

Unraveling the mechanobiology of cornea: From bench side to the clinic

The cornea is a transparent, dome-shaped structure on the front part of the eye that serves as a major optic element and a protector from the external environment. Recent evidence shows aberrant alterations of the corneal mechano-environment in development and progression of various corneal diseases. It is, thus, critical to understand how corneal cells sense and respond to mechanical signals in physiological and pathological conditions. In this review, we summarize the corneal mechano-environment and discuss the impact of these mechanical cues on cellular functions from the bench side (in a laboratory research setting). From a clinical perspective, we comprehensively review the mechanical changes of corneal tissue in several cornea-related diseases, including keratoconus, myopia, and keratectasia, following refractive surgery. The findings from the bench side and clinic underscore the involvement of mechanical cues in corneal disorders, which may open a new avenue for development of novel therapeutic strategies by targeting corneal mechanics.

Quantitative Assessment of Biomechanical Properties of the Human Keratoconus Cornea Using Acoustic Radiation Force Optical Coherence Elastography

Purpose

Quantification of biomechanical properties of keratoconus (KC) corneas has great significance for early diagnosis and treatment of KC, but the corresponding clinical measurement remains challenging. Here, we developed an acoustic radiation force (ARF) optical coherence elastography technique and explored its potential for evaluating biomechanical properties of KC corneas.

Methods

An ARF system was used to induce the tissue deformation, which was detected by an optical coherence tomography system, and thus the localized point-by-point Young's modulus measurements were achieved. Then, two healthy rabbit eyes were imaged to test the system, after which the human keratoconus cornea was evaluated by using the same method. Three regions were selected for biomechanics analysis: the conical region, the transitional region, and the peripheral region.

Results

Young's moduli of transitional region ranged from 53.3 to 58.5 kPa. The corresponding values for the peripheral region were determined to be 58.6 kPa and 63.2 kPa, respectively. Young's moduli of the conical region were gradually increased by 18.3% from the center to the periphery, resulting in the minimum and maximum values of 44.9 kPa and 53.1 kPa, respectively. Furthermore, Young's moduli of the anterior and posterior of the center were determined to be 44.9 kPa and 50.7 kPa, respectively.

Conclusions

Differences in biomechanical properties between the three regions and slight variations within the conical region were clearly distinguished. Biomechanical weakening of the keratoconus cornea was mainly localized in the conical region, especially in the vertex position.

Translational Relevance

The system may provide a promising clinical tool for the noninvasive evaluation of local corneal biomechanics and thus may have potential applications in early keratoconus detection with further optimization.

Accelerated corneal crosslinking causes pseudoprogression in keratoconus within the first 6 weeks without affecting posterior corneal curvature

PURPOSE

To evaluate the effectiveness of epithelium-off (epi-off) accelerated corneal crosslinking (A-CXL, 9 mW/cm², 10 min) in adult keratoconus (KC) patients.

METHODS

The study included 151 KC corneas (124 patients) after A-CXL. The parameters best corrected visual acuity (BCVA) and the tomographic readings (Pentacam HR, Oculus, Germany) were analysed at 24, 12 and 6 months preoperatively, prior to surgery; and 6 weeks, 6 months, 1, 2 and >2 years postoperatively. The demarcation line was assessed by anterior segment optical coherence tomography (Tomey SS-1000, CASIA 2 (Tomey, Nagoya, Japan)).

RESULTS

Comparing pre- to postoperative findings 6 weeks after A-CXL with paired t-test, the anterior steep (46.8 ± 4.0|47.1 ± 4.1), flat (50.2 ± 4.3|50.6 ± 4.6) and maximal keratometry (57.6 ± 6.8|58.3 ± 6.8) increased (p < 0.05), while the thinnest pachymetry decreased significantly (459 ± 39|444 ± 42, p < 0.05). Lateron, however, there was a decreasing anterior flat (1, 2 and >2 years; p < 0.0001), mean (1 year; p = 0.01 and 2 years; p = 0.03) and maximal keratometry (1, 2 and >2 years; p < 0.0001). The posterior corneal keratometry readings did not change significantly until >2 years after A-CXL (MANOVA; steep, p = 0.008; flat, p = 0.027; mean, p = 0.007). The mean depth of the demarcation line was 242 ± 62 µm (53.6%). The preoperative logMAR BCVA (0.35 ± 0.02) decreased 6 weeks after A-CXL (0.39 ± 0.03) followed by a continuous improvement until the latest follow-up (0.18 ± 0.04).

CONCLUSION

A-CXL constitutes a successful method for KC stabilization. Signs of KC progression occur within the first 6 weeks postoperatively ("pseudoprogression"), but this is not indicative of the long-term effect.

Determine Corneal Biomechanical Parameters by Finite Element Simulation and Parametric Analysis Based on ORA Measurements

Purpose: The Ocular Response Analyzer (ORA) is one of the most commonly used devices to measure corneal biomechanics in vivo. Until now, the relationship between the output parameters and corneal typical biomechanical parameters was not clear. Hence, we defined the output parameters of ORA as ORA output parameters. This study aims to propose a method to determine corneal biomechanical parameters based on ORA measurements by finite element simulation and parametric analysis.

Methods: Finite element analysis was used to simulate the mechanics process of ORA measurements with different intraocular pressure (IOP), corneal geometrical parameters and corneal biomechanical parameters. A simplified geometrical optics model was built to simulate the optical process of the measurements to extract ORA output parameters. After that, 70% of the simulated data was used to establish the quantitative relationship between corneal biomechanical parameters and ORA output parameters by parametric analysis and 30% of the simulated data was used to validate the established model. Besides, ten normal subjects were included to evaluate the normal range of corneal biomechanical parameters calculated from ORA.

Results: The quantitative relationship between corneal biomechanical parameters and ORA output parameters is established by combining parametric analysis with finite element simulation. The elastic modulus (E) and relaxation limit (G_∞) of the ten normal subjects were 0.65 ± 0.07 MPa and 0.26 ± 0.15, respectively.

Conclusions: A method was proposed to determine corneal biomechanical parameters based on the results of ORA measurements. The magnitude of the corneal biomechanical parameters calculated according to our method was reasonable.

Long-Term Clinical Outcomes of Small-Incision Femtosecond Laser-Assisted Intracorneal Concave Lenticule Implantation in Patients with Keratoconus

Purpose

The purpose of this study was to evaluate the long-term prognosis of small-incision femtosecond laser-assisted intracorneal concave lenticule implantation (SFII) in correction of human keratoconus.

Methods

This was a prospective study for 11 patients who received SFII after being diagnosed as progressive keratoconus based on the Amsler–Krumeich classification system. Clinical assessment was performed for all the patients prior to and postsurgically at different time points for 5 years. These included uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), biomechanically corrected intraocular pressure (bIOP), corneal topography, anterior segment optical coherence tomography (AS-OCT), confocal microscopy, and biomechanical assessment with Corvis ST.

Results

Comparison of preoperative and 60-month postoperative UDVA and CDVA (P_60months=0.081 and 0.001, respectively), all eyes showed an improvement in CDVA. Corneal topography showed no significant changes in corneal anterior K1, K2, posterior K1, K2, posterior elevation, or corneal densitometry compared with preoperative levels (P > 0.05). Corvis ST showed that central corneal thickness (CCT) and stiffness at applanation 1 (SP-A1) were significantly greater 1 week postsurgically when compared to the baseline (P < 0.05) and remained stable thereafter. The lenticule under the AS-OCT remained transparent throughout the entire postsurgical period. Under confocal microscopy, corneal edema and an increase in cell activation and reflectivity were observed at the lenticule-stromal interface within 1 week postoperatively. These reactions gradually subsided with time within 6 months.

Conclusion

SFII is an effective procedure to prevent the progression of keratoconus due to its minimal invasiveness and capability of maintaining a steady biometry of the cornea.

Risk Factors for Corneal Striae in Eyes After Glaucoma Surgery

PRCIS

Eyes with corneal striae had steeper cornea, induced astigmatism, and higher corneal hysteresis (CH), which implies a relationship between striae, corneal shape, and the cornea's resistance to deformation at low intraocular pressures (IOPs).

BACKGROUND

Anterior corneal striae (ACS) are associated with low IOP. However, the clinical significance of ACS is unclear. Here, we aim to evaluate differences in eyes with striae compared with eyes without striae.

METHODS

Adults with ACS (cases) and without ACS (controls) ≥8 weeks after glaucoma surgery with an IOP ≤10 mm Hg were enrolled. Optical coherence tomography and optical biometry were performed. CH, defined as the difference in pressure between corneal indentation and reformation in response to an air jet, was obtained by the ocular response analyzer. Hypotony maculopathy (HM) was defined as optic disc swelling, vascular tortuosity attributed to hypotony, or clinical presence of chorioretinal folds confirmed on OCT.

RESULTS

One hundred sixteen eyes (76 cases, 40 controls) were included. Cases had a lower IOP compared with controls (6.5±2.3 vs. 8.5±1, P<0.0001). A 1 mm Hg increase in CH increased ACS odds [odds ratio (OR)=1.51, P=0.01]. A 1 D increase in the flattest presurgical and postsurgical corneal power increased ACS odds by 1.83 (P=0.01) and 1.41 (P=0.02), respectively. Astigmatism increased in eyes with ACS by 1.11 D (P<0.001). ACS odds were increased with every 1 minute increase in mitomycin-C duration (OR=1.58, P=0.047) and decreased with the use of topical glaucoma medication (OR=0.62, P=0.03). Visual acuity decreased from logarithm of the minimal angle of resolution 0.22 (20/33 Snellen) presurgery to 0.28 (20/38) postsurgery (P=0.008), independent of ACS. HM occurred in 19% of cases (P=0.05). A higher postsurgical CH increased HM odds (OR=1.8, P=0.003). HM predicted a 0.41 mm decrease in axial length (P<0.0001), independent of IOP.

CONCLUSION

ACS were associated with a steeper cornea, induced astigmatism, and higher CH, suggesting a relationship between striae, corneal shape, and the cornea's ability to resist deformations at lower IOP. CH, HM, and axial length shortening were associated independently of IOP.

A Potential Screening Index of Corneal Biomechanics in Healthy Subjects, Forme Fruste Keratoconus Patients and Clinical Keratoconus Patients

Purpose: This study aims to evaluate the validity of corneal elastic modulus (E) calculated from corneal visualization Scheimpflug technology (Corvis ST) in diagnosing keratoconus (KC) and forme fruste keratoconus (FFKC).

Methods: Fifty KC patients (50 eyes), 36 FFKC patients (36 eyes, the eyes were without morphological abnormality, while the contralateral eye was diagnosed as clinical keratoconus), and 50 healthy patients (50 eyes) were enrolled and underwent Corvis measurements. We calculated E according to the relation between airpuff force and corneal apical displacement. One-way analysis of variance (ANOVA) and receiver operating characteristic (ROC) curve analysis were used to identify the predictive accuracy of the E and other dynamic corneal response (DCR) parameters. Besides, we used backpropagation (BP) neural network to establish the keratoconus diagnosis model.

Results: 1) There was significant difference between KC and healthy subjects in the following DCR parameters: the first/second applanation time (A1T/A2T), velocity at first/second applanation (A1V/A2V), the highest concavity time (HCT), peak distance (PD), deformation amplitude (DA), Ambrosio relational thickness to the horizontal profile (ARTh). 2) A1T and E were smaller in FFKC and KC compared with healthy subjects. 3) ROC analysis showed that E (AUC = 0.746) was more accurate than other DCR parameters in detecting FFKC (AUC of these DCR parameters was not more than 0.719). 4) Keratoconus diagnosis model by BP neural network showed a more accurate diagnostic efficiency of 92.5%. The ROC analysis showed that the predicted value (AUC = 0.877) of BP neural network model was more sensitive in the detection FFKC than the Corvis built-in parameters CBI (AUC = 0.610, p = 0.041) and TBI (AUC = 0.659, p = 0.034).

Conclusion: Corneal elastic modulus was found to have improved predictability in detecting FFKC patients from healthy subjects and may be used as an additional parameter for the diagnosis of keratoconus.

Keratoconus: An updated review

Keratoconus is a bilateral and asymmetric disease which results in progressive thinning and steeping of the cornea leading to irregular astigmatism and decreased visual acuity. Traditionally, the condition has been described as a noninflammatory disease; however, more recently it has been associated with ocular inflammation. Keratoconus normally develops in the second and third decades of life and progresses until the fourth decade. The condition affects all ethnicities and both sexes. The prevalence and incidence rates of keratoconus have been estimated to be between 0.2 and 4,790 per 100,000 persons and 1.5 and 25 cases per 100,000 persons/year, respectively, with highest rates typically occurring in 20- to 30-year-olds and Middle Eastern and Asian ethnicities. Progressive stromal thinning, rupture of the anterior limiting membrane, and subsequent ectasia of the central/paracentral cornea are the most commonly observed histopathological findings. A family history of keratoconus, eye rubbing, eczema, asthma, and allergy are risk factors for developing keratoconus. Detecting keratoconus in its earliest stages remains a challenge. Corneal topography is the primary diagnostic tool for keratoconus detection. In incipient cases, however, the use of a single parameter to diagnose keratoconus is insufficient, and in addition to corneal topography, corneal pachymetry and higher order aberration data are now commonly used. Keratoconus severity and progression may be classified based on morphological features and disease evolution, ocular signs, and index-based systems. Keratoconus treatment varies depending on disease severity and progression. Mild cases are typically treated with spectacles, moderate cases with contact lenses, while severe cases that cannot be managed with scleral contact lenses may require corneal surgery. Mild to moderate cases of progressive keratoconus may also be treated surgically, most commonly with corneal cross-linking. This article provides an updated review on the definition, epidemiology, histopathology, aetiology and pathogenesis, clinical features, detection, classification, and management and treatment strategies for keratoconus.

Optical coherence elastography for assessing the influence of intraocular pressure on elastic wave dispersion in the cornea

The cornea is a highly specialized organ that relies on its mechanical stiffness to maintain its aspheric geometry and refractive power, and corneal diseases such as keratoconus have been linked to abnormal tissue stiffness and biomechanics. Dynamic optical coherence elastography (OCE) is a clinically promising non-contact and non-destructive imaging technique that can provide measurements of corneal tissue stiffness directly in vivo. The method relies on the concepts of elastography where shear waves are generated and imaged within a tissue to obtain mechanical properties such as tissue stiffness. The accuracy of OCE-based measurements is ultimately dependent on the mathematical theories used to model wave behavior in the tissue of interest. In the cornea, elastic waves propagate as guided wave modes which are highly dispersive and can be mathematically complex to model. While recent groups have developed detailed theories for estimating corneal tissue properties from guided wave behavior, the effects of intraocular pressure (IOP)-induced prestress have not yet been considered. It is known that prestress alone can strongly influence wave behavior, in addition to the associated non-linear changes in tissue properties. This present study shows that failure to account for the effects of prestress may result in overestimations of the corneal shear moduli, particularly at high IOPs. We first examined the potential effects of IOP and IOP-induced prestress using a combination of approximate mathematical theories describing wave behavior in thin plates with observations made from data published in the OCE literature. Through wave dispersion analysis, we deduce that IOP introduces a tensile hoop stress and may also influence an elastic foundational effect that were observable in the low-frequency components of the dispersion curves. These effects were incorporated into recently developed models of wave behavior in nearly incompressible, transversely isotropic (NITI) materials. Fitting of the modified NITI model with ex vivo porcine corneal data demonstrated that incorporation of the effects of IOP resulted in reduced estimates of corneal shear moduli. We believe this demonstrates that overestimation of corneal stiffness occurs if IOP is not taken into consideration. Our work may be helpful in separating inherent corneal stiffness properties that are independent of IOP; changes in these properties and in IOP are distinct, clinically relevant issues that affect the cornea health.

Prospective one year study of corneal biomechanical changes following high intensity, accelerated cornea cross-linking in patients with keratoconus using a non-contact tonometer

PURPOSE

To characterize corneal biomechanical properties utilizing a dynamic ultra-high-speed Scheimpflug camera equipped with a non-contact tonometer (CorVis ST, CST) in keratoconic corneas following continuous high intensity, high irradiance corneal cross-linking.

DESIGN

Prospective longitudinal single-centre study at a tertiary referral center.

METHODS

Corneal biomechanical properties were measured in patients with progressive keratoconus undergoing high intensity (30 mW/cm2), high irradiance (5.4 J/cm2), accelerated corneal cross-linking with continuous exposure to ultraviolet-A for 4 min. CST was used to assess corneal biomechanical properties pre-operatively and at 1, 3, 6 and 12 months post-operatively. CST output videos were further analyzed using several previously reported algorithms.

RESULTS

A total of 25 eyes of 25 participants were examined. The mean age of participants was 20.9 ± 5.3 years; 56% were male and 80% were of Māori or Pacific Island origin. Energy absorbed area (mN mm), was the only significantly changed parameter compared to baseline at all time points measuring 3.61 ± 1.19 preoperatively, 2.81 ± 1.15 at 1 month (p = 0.037), 2.79 ± 0.81 (p = 0.033) at 3 months, 2.76 ± 0.95 (p = 0.028) at 6 months and 2.71 ± 1.18 (p = 0.016) at 12 months.

CONCLUSIONS

The significant difference between the pre and post-operative energy absorbed area appears to reflect changes in corneal viscous properties that occur following corneal cross-linking.

Exploring the Biomechanical Properties of the Human Cornea In Vivo Based on Corvis ST

Purpose: The aim of this study was to provide a method to determine corneal nonlinear viscoelastic properties based on the output data of corneal visualization Scheimpflug technology (Corvis ST).

Methods: The Corvis ST data from 18 eyes of 12 healthy humans were collected. Based on the air-puff pressure and the corneal displacement from the Corvis ST test of normal human eyes, the work done by the air-puff attaining the whole corneal displacement was obtained. By applying a visco-hyperelastic strain energy density function of the cornea, in which the first-order Prony relaxation function and the first-order Ogden strain energy were employed, the corneal strain energy during the Corvis ST test was calculated. Then the work done by the air-puff attaining the whole corneal displacement was completely regarded as the strain energy of the cornea. The identification of the nonlinear viscoelastic parameters was carried out by optimizing the sum of difference squares of the work and the strain energy using the genetic algorithm.

Results: The visco-hyperelastic model gave a good fit to the data of corneal strain energy with time during the Corvis ST test (R² > 0.95). The determined Ogden model parameter μ ranged from 0.42 to 0.74 MPa, and α ranged from 32.76 to 55.63. The parameters A and τ in the first-order Prony function were 0.09–0.36 and 1.21–1.95 ms, respectively.

Conclusion: It is feasible to determine the corneal nonlinear viscoelastic properties based on the corneal contour information and air-puff pressure of the Corvis ST test.

Femtosecond Laser-assisted Allogenic Additive Stromal Keratoplasty With or Without Excimer Laser Donor Keratomileusis for Management of Keratoconus

We describe a modified allogenic intrastromal lenticule implantation technique for management of keratoconus (KCN). Patients with advanced KCN already scheduled for corneal transplantation were enrolled. An allogenic corneal lenticule was implanted inside a stromal pocket created by femtosecond laser. In three cases, the estimated refractive error of the recipient eyes was corrected on the donor lenticules using an Excimer laser. All operated eyes underwent corneal crosslinking at the time of surgery. This method was named “Femtosecond Laser-assisted Allogenic Stromal Keratoplasty Without and With Excimer Laser-assisted Donor Keratomileusis”; briefly called FASK and FASK Plus EDK, respectively. Two out of five patients were satisfied with the results. There was a decrease in the average simulated keratometric values as well as myopia when FASK Plus EDK was performed. Increased corneal thickness was achieved in all cases. Graft edema gradually decreased over weeks but interface wrinkling and lenticule folds in the visual axis remained as a problem during follow-up period. No other complications were encountered.

Comparisons of corneal biomechanical and tomographic parameters among thin normal cornea, forme fruste keratoconus, and mild keratoconus

Background

To compare the dynamic corneal response (DCR) and tomographic parameters of thin normal cornea (TNC) with thinnest corneal thickness (TCT) (≤ 500 µm), forme fruste keratoconus (FFKC) and mild keratoconus (MKC) had their central corneal thickness (CCT) matched by Scheimpflug imaging (Pentacam) and corneal visualization Scheimpflug technology (Corvis ST).

Methods

CCT were matched in 50 eyes with FFKC, 50 eyes with MKC, and 53 TNC eyes with TCT ≤ 500 µm. The differences in DCR and tomographic parameters among the three groups were compared. The receiver operating characteristic (ROC) curve was used to analyze the diagnostic significance of these parameters. Back propagation (BP) neural network was used to establish the keratoconus diagnosis model.

Results

Fifty CCT-matched FFKC eyes, 50 MKC eyes and 50 TNC eyes were included. The age and biomechanically corrected intraocular pressure (bIOP) did not differ significantly among the three groups (all P > 0.05). The index of height asymmetry (IHA) and height decentration (IHD) differed significantly among the three groups (all P < 0.05). IHD also had sufficient strength (area under the ROC curves (AUC) > 0.80) to differentiate FFKC and MKC from TNC eyes. Partial DCR parameters showed significant differences between the MKC and TNC groups, and the deflection amplitude of the first applanation (A1DA) showed a good potential to differentiate (AUC > 0.70) FFKC and MKC from TNC eyes. Diagnosis model by BP neural network showed an accurate diagnostic efficiency of about 91%.

Conclusions

The majority of the tomographic and DCR parameters differed among the three groups. The IHD and partial DCR parameters assessed by Corvis ST distinguished FFKC and MKC from TNC when controlled for CCT.

Corneal biomechanical properties following corneal cross-linking: Does age have an effect?

PURPOSE

To explore the effect of age on corneal biomechanical properties following corneal cross-linking (CXL).

METHODS

A total of 12 pairs of human eye-banked corneas (24 corneas, from 14 females and 10 males) were used in the study. The mean donor age was 48.5 years (ranging from 26 to 71 years). Corneas were divided into three age groups: A (26-41 years), B (42-57 years) and C (58-71 years), with four pairs in each group. For each pair, the right corneas were cross-linked using accelerated CXL with UVA (10 mW/cm²) and riboflavin, while the left corneas served as controls and were not exposed to either UVA irradiation or riboflavin. The corneal elastic modulus of the anterior, mid and posterior corneal stroma was measured using nanoindentation.

RESULTS

The difference in the corneal elastic modulus following CXL was significant in the anterior (p = 0.00002) and mid stroma (p = 0.001); however, the difference was not significant in the posterior stroma (p = 0.27) when compared to control corneas. The corneal elastic modulus of the anterior stroma increased by 178.44% in Group A, 119.7% in Group B and 50.73% in Group C compared to control corneas. For the mid stroma, the elastic modulus increased by 47.35% in Group A, 25% in Group B and 24.56% in Group C. No differences were observed in the posterior stroma between age groups.

CONCLUSIONS

Corneal elasticity showed a greater response to CXL in the younger group compared to older groups. CXL treatment showed effectiveness in enhancing stromal strength, and the effect was concentrated in the anterior and mid stroma with minimal impact on the posterior stroma in all age groups.

Anterior pituitary, sex hormones, and keratoconus: Beyond traditional targets

"The Diseases of the Horny-coat of The Eye", known today as keratoconus, is a progressive, multifactorial, non-inflammatory ectatic corneal disorder that is characterized by steepening (bulging) and thinning of the cornea, irregular astigmatism, myopia, and scarring that can cause devastating vision loss. The significant socioeconomic impact of the disease is immeasurable, as patients with keratoconus can have difficulties securing certain jobs or even joining the military. Despite the introduction of corneal crosslinking and improvements in scleral contact lens designs, corneal transplants remain the main surgical intervention for treating keratoconus refractory to medical therapy and visual rehabilitation. To-date, the etiology and pathogenesis of keratoconus remains unclear. Research studies have increased exponentially over the years, highlighting the clinical significance and international interest in this disease. Hormonal imbalances have been linked to keratoconus, both clinically and experimentally, with both sexes affected. However, it is unclear how (molecular/cellular signaling) or when (age/disease stage(s)) those hormones affect the keratoconic cornea. Previous studies have categorized the human cornea as an extragonadal tissue, showing modulation of the gonadotropins, specifically luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Studies herein provide new data (both in vitro and in vivo) to further delineate the role of hormones/gonadotropins in the keratoconus pathobiology, and propose the existence of a new axis named the Hypothalamic-Pituitary-Adrenal-Corneal (HPAC) axis.

Mechanical Properties of Bioengineered Corneal Stroma

For the majority of patients with severe corneal injury or disease, corneal transplantation is the only suitable treatment option. Unfortunately, the demand for donor corneas greatly exceeds the availability. To overcome shortage issues, a myriad of bioengineered constructs have been developed as mimetics of the corneal stroma over the last few decades. Despite the sheer number of bioengineered stromas developed , these implants fail clinical trials exhibiting poor tissue integration and adverse effects in vivo. Such shortcomings can partially be ascribed to poor biomechanical performance. In this review, existing approaches for bioengineering corneal stromal constructs and their mechanical properties are described. The information collected in this review can be used to critically analyze the biomechanical properties of future stromal constructs, which are often overlooked, but can determine the failure or success of corresponding implants.

Application of a scheimpflug-based biomechanical analyser and tomography in the early detection of subclinical keratoconus in chinese patients

Background

In vivo corneal biomechanics evaluation has been used to help screen early keratoconus in recent years. This study is to evaluate the value of a Scheimpflug-based biomechanical analyser combined with tomography in detecting subclinical keratoconus by distinguishing normal eyes from frank keratoconus (KC) and forme frusta keratoconus (FFKC) eyes in Chinese patients.

Methods

Study design: diagnostic test. This study included 31 bilateral frank keratoconus patients, 27 unilateral clinically manifesting keratoconus patients with very asymmetric eyes, and 79 control subjects with normal corneas. Corneal morphological and biomechanical parameters were measured using a Pentacam HR and a Corvis ST (OCULUS, Wetzlar, Germany). The diagnostic ability of computed parameters reflecting corneal biomechanical and morphological traits [including the Belin-Ambrósio deviation index (BAD_D), the Corvis biomechanical index (CBI) and the tomographic and biomechanical index (TBI)] was determined using receiver operating characteristic (ROC) curve analysis and compared by the DeLong test. Additionally, the area under the curve (AUC), the best cut-off values, and the Youden index for each parameter were reported. A novel corneal stiffness parameter, the stress-strain index (SSI), was also compared between KC, FFKC and normal eyes.

Results

Every morphological and biomechanical index analysed in this study was significantly different among KC, FFKC and normal eyes (P = 0.000). The TBI was most valuable in detecting subclinical keratoconus (FFKC eyes), with an AUC of 0.928 (P = 0.000), and both forms of corneal ectasia (FFKC and frank KC eyes), with an AUC of 0.966 (P = 0.000). The sensitivity and specificity of the TBI was 97.5 and 77.8 % in detecting FFKC and 97.5 and 89.7 % in detecting any KC, respectively, with a cut-off value of 0.375. The morphological index BAD_D and the biomechanical index CBI were also very useful in distinguishing eyes with any KC from normal eyes, with AUCs of 0.965 and 0.934, respectively. The SSI was significantly different between KC, FFKC and normal eyes (P = 0.000), indicating an independent decrease in corneal stiffness in KC eyes.

Conclusions

The combination of a Scheimpflug-based biomechanical analyser and tomography could increase the accuracy in detecting subclinical keratoconus in Chinese patients. The TBI was the most valuable index for detecting subclinical keratoconus, with a high sensitivity and specificity. Evaluation of corneal biomechanical properties in refractive surgery candidates could be helpful for recognizing potential keratoconic eyes and increasing surgical safety.

Regional variation of corneal stromal deformation measured by high-frequency ultrasound elastography

The cornea's mechanical response to intraocular pressure elevations may alter in ectatic diseases such as keratoconus. Regional variations of mechanical deformation in normal and keratoconus eyes during intraocular pressure elevation have not been well-characterized. We applied a high-frequency ultrasound elastography technique to characterize the regional deformation of normal and keratoconus human corneas through the full thickness of corneal stroma. A cross-section centered at the corneal apex in 11 normal and 2 keratoconus human donor eyes was imaged with high-frequency ultrasound during whole globe inflation from 5 to 30 mmHg. An ultrasound speckle tracking algorithm was used to compute local tissue displacements. Radial, tangential, and shear strains were mapped across the imaged cross-section. Strains in the central (1 mm surrounding apex) and paracentral (1 to 4 mm from apex) regions were analyzed in both normal and keratoconus eyes. Additional regional analysis was performed in the eye with severe keratoconus presenting significant thinning and scarring. Our results showed that in normal corneas, the central region had significantly smaller tangential stretch than the paracentral region, and that within the central region, the magnitudes of radial and shear strains were significantly larger than that of tangential strain. The eye with mild keratoconus had similar shear strain but substantially larger radial strains than normal corneas, while the eye with severe keratoconus had similar overall strains as in normal eyes but marked regional heterogeneity and large strains in the cone region. These findings suggested regional variation of mechanical responses to intraocular pressure elevation in both normal and keratoconus corneas, and keratoconus appeared to be associated with mechanical weakening in the cone region, especially in resisting radial compression. Comprehensive characterization of radial, tangential, and shear strains through corneal stroma may provide new insights to understand the biomechanical alterations in keratoconus.

Quantitative structural imaging of keratoconic corneas using polarization-resolved SHG microscopy

The human cornea is mainly composed of collagen fibrils aligned together within stacked lamellae. This lamellar structure can be affected in pathologies such as keratoconus, which is characterized by progressive corneal thinning and local steepening. In this study, we use polarization-resolved second harmonic generation (P-SHG) microscopy to characterize 8 control and 6 keratoconic human corneas. Automated processing of P-SHG images of transverse sections provides the collagen orientation in every pixel with sub-micrometer resolution. Series of P-SHG images recorded in the most anterior region of the stroma evidence sutural lamellae inclined at 22° ± 5° to the corneal surface, but show no significant difference between control and keratoconic corneas. In contrast, series of P-SHG images acquired along the full thickness of the stroma show a loss of order in the lamellar structure of keratoconic corneas, in agreement with their defective mechanical properties. This structural difference is analyzed quantitatively by computing the entropy and the orientation index of the collagen orientation distribution and significant differences are obtained along the full thickness of the stroma. This study shows that P-SHG is an effective tool for automatic quantitative analysis of structural defects of human corneas and should be applied to other collagen-rich tissues.

Diagnosis of Subclinical Keratoconus with a Combined Model of Biomechanical and Topographic Parameters

This study sought to develop a diagnostic model with aberrometry and biomechanical variables for subclinical keratoconus. The design was a cross-sectional study. The topographic data were obtained with a rotating Scheimpflug camera (Pentacam HR), and biomechanical data were obtained with Corvis ST. The study included 81 eyes distributed in 61 healthy corneas and 20 subclinical keratoconus (SCKC), defined as eyes with suspicious topographic findings, normal slit-lamp examination, and a manifestation of keratoconus. Analyses of the topographic and biomechanical data were performed, and a classifying model of SCKC was elaborated. The model for the diagnosis of SCKC includes posterior coma to 90°, Ambrósio's Relational Thickness in the horizontal profile (ARTh), and velocity when the air pulse is off (A2 velocity). The sensitivity was 89.5%, specificity 96.7%, accuracy 94.9%, and precision 89.5%. The area under the curve (AUC) of the receiver operating characteristic (ROC) curve for the model was 0.951. Diagnosis of subclinical keratoconus depends on the aberrometry variable posterior coma to 90° and the biomechanical variables A2 velocity and ARTh.

Corneal biomechanical parameters in keratoconus eyes with abnormal elevation on the back corneal surface only versus both back and front surfaces

Corneal biomechanical parameters were compared in 100 keratoconus eyes with abnormal elevation on the back corneal surface only (group 1), versus both the back and front surfaces (group 2). Scheimpflug tomography with Pentacam HR, corneal biomechanical assessments using Corvis ST and Ocular Response Analyzer (ORA) and corneal epithelium thickness maps using anterior segment optical coherence tomography were assessed. There were no significant differences in the IOP measured using Corvis ST and ORA, age or sex between the two groups. Statistically significant differences were found in all corneal shape parameters and all new parameters of Corvis ST: corneal stiffness parameter at first applanation (SP-A1), integrated inverse radius (IR) and deformation amplitude ratio (DAR)) between groups (p < 0.001). The classic parameters of ORA including corneal hysteresis (CH) and corneal resistance factor (CRF) were about 1.00 mmHg higher in group 1 (p < 0.001). In conclusion, keratoconus eyes with abnormal elevation limited to the back corneal surface have lower grade, stiffer corneal biomechanical parameters and less asymmetric shape. This is consistent with progressive biomechanical weakening from the first detectable back surface elevation to manifestation on the front surface as the severity overwhelms the ability of the epithelium to compensate.

Keratoconus Epidemiology Presentations at Najran Province, Saudi Arabia

AIM

Our study aims to produce an estimation of the keratoconus disease epidemiology and important demographic factors in disease presentation in Najran city in the south of Saudi Arabia.

METHODS

Keratoconus patients between 9 and 29 years of age at the eye clinic of King Khaled General Hospital in Najran Province with a confirmed diagnosis of keratoconus were recruited over one year period. Visual acuity, keratometry, corneal thickness and demographic data were collected and analysed.

RESULTS

The keratoconus prevalence in Najran Province was found to be 87.3 cases per 100,000 people with an incidence of 28.47 per 100,000 cases. The disease presented more frequently in male patients (67.9%) with a statistically significant difference in the mean age between genders (p=0.014). Most cases presented at a moderate stage, and almost half of the cases reported a family history of the disease. There were statistically significant differences in best visual acuity and corneal thickness between genders (p<0.05). The majority of the cases were managed with contact lenses (56.08%).

CONCLUSION

Genetic and environmental factors could have a substantial role in the increased rate of keratoconus presentation in Najran Province. Screening programmes should dedicate more attention to late presentation to improve prevention and early detection. More studies on keratoconus epidemiology in Saudi Arabia are needed.

Measurement of In Vivo Biomechanical Changes Attributable to Epithelial Removal in Keratoconus Using a Noncontact Tonometer

PURPOSE

To compare the biomechanical properties of the cornea after epithelial removal in eyes with keratoconus undergoing corneal cross-linking.

METHODS

Prospective interventional case series at a university hospital tertiary referral center. Corneal biomechanical properties were measured in patients with keratoconus undergoing corneal cross-linking, immediately before and after epithelial debridement by using a dynamic ultrahigh-speed Scheimpflug camera equipped with a noncontact tonometer.

RESULTS

The study comprised 45 eyes of 45 patients with a mean age of 19.6 ± 4.9 years (range 14-34). The cornea was found to be 23.7 ± 15.7 μm thinner after epithelial removal (P < 0.01). Corneal stiffness was reduced after epithelial removal as demonstrated by a significant decrease of parameters such as stiffness parameter A1 (12.31, P < 0.01), stiffness parameter-highest concavity (2.25, P < 0.01), A1 length (0.13 mm, P = 0.04), highest concavity radius of curvature (0.26 mm, P = 0.01), highest concavity time (0.22 ms, P = 0.04) and an increase in A1 velocity (-0.01 m/s, P = 0.01), A1 deformation amplitude (-0.03 mm, P ≤ 0.01), A1 deflection length (-0.32 mm, P < 0.01), A2 deformation amplitude (-0.03 mm, P = 0.01), and A2 deflection length (-1.00 mm, P < 0.01). There were no significant differences in biomechanical intraocular pressure (0.15 mm Hg, P = 0.78), deformation amplitude (0.03, P = 0.54), maximum inverse radius (-0.01 mm, P = 0.57), and whole eye movement length (-0.02 mm, P = 0.12).

CONCLUSIONS

Dynamic ultrahigh-speed Scheimpflug camera equipped with a noncontact tonometer offers an alternative method for in vivo measurements of the epithelial layer's contribution to corneal biomechanical properties. Our results suggest that corneal epithelium may play a more significant role in corneal biomechanical properties in patients with keratoconus than previously described.

Evaluation of Intraocular Pressure and Other Biomechanical Parameters to Distinguish between Subclinical Keratoconus and Healthy Corneas

(1) Purpose: To assess the main corneal response differences between normal and subclinical keratoconus (SCKC) with a Corvis® ST device. (2) Material and Methods: We selected 183 eyes of normal patients, of a mean age of 33 ± 9 years and 16 eyes of patients with SCKC of a similar mean age. We measured best corrected visual acuity (BCVA) and corneal topography with a Pentacam HD device to select the SCKC group. Biomechanical measurements were performed using the Corvis® ST device. We carried out a non-parametric analysis of the data with SPSS software (Wilcoxon signed rank-test). (3) Results: We found statistically significant differences between the control and SCKC groups in some corneal biomechanical parameters: first and second applanation time (p = 0.05 and p = 0.02), maximum deformation amplitude (p = 0.016), highest concavity radius (p = 0.007), and second applanation length and corneal velocity ((p = 0.039 and p = 0.016). (4) Conclusions: Our results show that the use of normalised biomechanical parameters provided by noncontact tonometry, combined with a discriminant function theory, is a useful tool for detecting subclinical keratoconus.

Changes in Intraocular Pressure after Transepithelial Photorefractive Keratectomy and Femtosecond Laser In Situ Keratomileusis

PURPOSE

To investigate the changes in intraocular pressure (IOP) and biomechanically corrected IOP (bIOP) in patients undergoing transepithelial photorefractive keratectomy (TPRK) and femtosecond laser in situ keratomileusis (FS-LASIK) and to determine the effects of preoperative biomechanical factors on IOP and bIOP changes after FS-LASIK and TPRK.

DESIGN

A retrospective comparative study.

METHODS

We retrospectively investigated the IOP and corneal biomechanical changes in 93 eyes undergoing FS-LASIK and 104 eyes undergoing TPRK in a clinical setting. Preoperative and postoperative data on ophthalmic and Corvis ST examinations, in vivo Young's modulus, and noncontact tonometry were analyzed. Marginal linear regression models with generalized estimating equations were used for intragroup and intergroup comparisons of IOP and bIOP changes.

RESULTS

In the univariate model, IOP reduction after FS-LASIK was 2.49 mmHg higher than that after TPRK. In addition, bIOP reduction after FS-LASIK was 1.85 mmHg higher than that after TPRK. In the multiple regression model, we revealed that IOP reduction after FS-LASIK was 1.75 mmHg higher than that after TPRK. Additionally, bIOP reduction after FS-LASIK was 1.64 mmHg higher than that after TPRK. Postoperative changes in bIOP were less than those in IOP. In addition, Young's modulus and CBI had no significant effect on postoperative IOP and bIOP changes. We establish a biomechanically predictive model using the available data to predict postoperative IOP and bIOP changes after TPRK and FS-LASIK.

CONCLUSIONS

Reductions in IOP and bIOP after FS-LASIK were 1.75 mmHg and 1.64 mmHg, respectively, more than those after TPRK, after adjustment for confounders. We revealed that the type of refractive surgery and peak distance (PD) were significant predictors of postoperative IOP and bIOP changes. By contrast, depth of ablation showed a significant effect on only IOP changes.

Keratoconus Diagnosis: Validation of a Novel Parameter Set Derived from IOP-Matched Scenario

Purpose

Considering that intraocular pressure (IOP) is an important confounding factor in corneal biomechanical evaluation, the notion of matching IOP should be introduced to eliminate any potential bias. This study aimed to assess the capability of a novel parameter set (NPS) derived from IOP-matched scenario to diagnose keratoconus.

Methods

Seventy samples (training set; 35 keratoconus and 35 normal corneas; pairwise matching for IOP) were used to determine NPS by forward logistic regression. A large validation dataset comprising 62 matching samples (31 keratoconus and 31 normal corneas) and 203 unmatching samples (112 keratoconus and 91 normal corneas) was used to evaluate its clinical significance. To further assess its diagnosis capability, NPS was compared with the other two prior biomechanical indexes.

Results

NPS was comprised of three biomechanical parameters, namely, DA Ratio Max 1 mm (DRM1), the first applanation time (AT1), and an energy loading parameter (Eload). NPS was successfully applied to the validation dataset, with a higher accuracy of 96.8% and 95.6% in the IOP-matched and -unmatched scenarios, respectively. More surprisingly, accuracy of NPS was 95.5% in the combined validation, an improvement compared to the two prior biomechanical indexes.

Conclusions

This is the first study taking IOP bias into consideration to determine a biomechanical parameter set. Our study shows that NPS indeed offers comparable performance in keratoconus diagnosis. Translational Relevance. Determining a parameter set after eliminating the influence from IOP is useful in revealing the essential differences between keratoconus and normal corneas and possibly facilitating further progress in keratoconus diagnosis.