Stress-Strain Index Map: A New Way to Represent Corneal Material Stiffness

Implantation of Intracorneal Ring Segments in Keratectasia: Effects on Corneal Biomechanics in 112 Eyes

PURPOSE

The aim of this study was to analyze changes in corneal biomechanical properties after implantation of intracorneal ring segments (ICRSs) in keratectasia.

METHODS

This retrospective single-center study included 112 patient eyes that underwent femtosecond laser-assisted ICRS implantation (Intacs SK; Addition Technology Inc, Des Plaines, IL) for keratectasia. Biomechanical analysis was performed using the Ocular Response Analyzer (ORA; Reichert Inc, Depew, NY), with determination of corneal resistance factor, corneal hysteresis, and Keratoconus Match Index, as well as by Corvis ST (OCULUS, Wetzlar, Germany), with determination of stiffness parameter A1, Ambrosio relational thickness to the horizontal profile (Arth), integrated radius, deformation amplitude ratio, and stress-strain index as well as Corvis Biomechanical Index and Tomographic Biomechanical Index. Data collection was performed preoperatively and 6 months postoperatively for ORA and Corvis ST and additionally after 1 and 2 years for ORA.

RESULTS

The corneal resistance factor decreased significantly postoperatively (5.8 ± 1.7 mm Hg) compared with preoperatively (6.75 ± 3.7 mm Hg; P = 0.021) and increased again during follow-up (6.2 ± 1.9 mm Hg; P = 0.024), without regaining preoperative values. Corneal hysteresis and Keratoconus Match Index did not change significantly. Stiffness parameter A1 ( P = 0.045) increased significantly after ICRS implantation and Arth decreased significantly from 181 ± 85 to 150 ± 92 ( P = 0.016). However, there was no significant postoperative change for others Corvis parameters.

CONCLUSIONS

Corneal biomechanical properties showed inconsistent changes after ICRS implantation. Classical corneal biomechanical parameters (using single central air-puff tonometers) do not seem to be suitable for follow-up after ICRS implantation.

[Anisotropy and viscoelasticity of different corneal regions in rabbit corneal ectasia model]

The mechanical properties of the cornea in corneal ectasia disease undergo a significant reduction, yet the alterations in mechanical properties within distinct corneal regions remain unclear. In this study, we established a rabbit corneal ectasia model by employing collagenase II to degrade the corneal matrix within a central diameter of 6 mm. Optical coherence tomography was employed for the in vivo assessment of corneal morphology (corneal thickness and corneal curvature) one month after operation. Anisotropy and viscoelastic characteristics of corneal tissue were evaluated through biaxial and uniaxial testing, respectively. The results demonstrated a marked decrease in central corneal thickness, with no significant changes observed in corneal curvature. Under different strains, the elastic modulus of the cornea exhibited no significant differences in the up-down and naso-temporal directions between the control and model groups. However, the cornea in the model group displayed a significantly lower elastic modulus compared to the control group. Specifically, the elastic modulus of the central region cornea in the model group was significantly lower than that of the entire cornea within the same group. Moreover, in comparison to the control group, the cornea in the model group exhibited a significant increase in both creep rate and overall deformation rate. The instantaneous modulus and equilibrium modulus were significantly reduced in the model cornea. No significant differences were observed between the entire cornea and the central cornea concerning these parameters. The results indicate that corneal anisotropy remains unchanged in collagenase-induced ectatic cornea. However, a significant reduction in viscoelastic properties is noticed. This study provides valuable insights for investigating changes in corneal mechanical properties within different regions of ectatic corneal disease.

Altered Corneal Biomechanics According to the Biomechanical E-Staging in Pellucid Marginal Degeneration

PURPOSE

The purpose of this study was to investigate corneal biomechanics in pellucid marginal degeneration (PMD) compared with healthy controls using Corvis ST (Oculus, Germany) by using the new biomechanical E-staging (based on the Corvis Biomechanical Factor, the linearized Corvis Biomechanical Index) together with tomographic parameters.

METHODS

Corneal biomechanical and topographic data of 75 eyes of 75 patients with PMD and 75 eyes of 75 age-matched and sex-matched healthy controls were investigated. Topographic parameters (K1, K2, Kmax, central corneal thickness (CCT), and Belin/Ambrósio Deviation Index (BAD-D) were evaluated in dependence of and correlated with the biomechanically defined E-stages. Biomechanical parameters were also recorded for the 2 groups.

RESULTS

Patients with PMD showed higher K2, Kmax, BAD-D, and Corvis Biomechanical Factor values and a lower CCT compared with healthy controls (P < 0.001). The E-stage was positively correlated with K1, K2, Kmax, BAD-D, and intraocular pressure difference and negatively correlated with CCT. Stage-dependent analysis revealed a significant increase in K1, K2, Kmax (P < 0.001), and BAD-D (P = 0.041) in stage E3 compared with E0 and a significant decrease in stage E2 in CCT (P = 0.009) compared with E0.

CONCLUSIONS

This study showed that patients with PMD may have a reduced corneal stiffness compared with healthy controls which worsens with increasing E-stage. Significant changes in topographic parameters were observed at stage E2 for CCT and at stage E3 for K1, K2, Kmax, and BAD-D when compared with stage E0.

Corneal stress-strain index in myopic Indian population

AIM

The purpose is to study the corneal stress-strain index (SSI) in myopic refractive error among Indian subjects.

METHODS

A retrospective study where young myopic subjects aged between 11 and 35 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Subjects with central corneal thickness (CCT) <500 μ, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease or any previous ocular surgery, high astigmatism, corneal disease such as keratoconus were excluded. Subjects with missing data or having poor quality scan were excluded. Corneal biomechanical properties and corneal SSI were assessed using Corvis ST. For statistical purposes, eyes were divided into four different groups and were analyzed using one-way ANOVA.

RESULTS

Nine hundred and sixty-six myopic eyes with mean ± standard deviation age, IOP, and CCT of 26.89 ± 4.92 years, 16.94 ± 2.00 mmHg, and 540.18 ± 25.23 microns, respectively, were included. There were 311, 388, 172, and 95 eyes that were low, moderate, severe, and extreme myopic. Deformation amplitude ratio at 1 mm and 2 mm were similar across different myopic groups. A significant increase in max inverse radius, ambrosia relational thickness, biomechanically corrected IOP, integrated radius was noted with an increase in myopic refractive error. Corvis biomechanical index, corneal SSI was found to be decreased significantly with an increase in myopic refractive error. We noted a significant positive association between myopic refractive error and SSI (P < 0.001).

CONCLUSION

Corneal SSI was found to be reduced in extreme myopic eyes.

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.

Corneal Biomechanics Assessment with Ultra High Speed Scheimpflug Camera in Primary Open Angle Glaucoma Compared with Healthy Subjects: A meta-analysis of the Literature

PURPOSE

The aim of this meta-analysis of the literature is to provide a comprehensive analysis of the differences in Corvis ST dynamic corneal response (DCR) parameters between primary open-angle glaucoma (POAG) patients and healthy controls.

METHODS

A quantitative meta-analysis was conducted on articles published before September 10, 2021 identified by searching PubMed, EMBASE, and Web of Science. Prospective studies comparing DCR Corvis ST parameter in high tension POAG and healthy controls were included. The random-effects model was conducted. Assessment of heterogeneity was based on the calculation of I². Funnel plots evaluation and meta-regression were performed in case of detection of high heterogeneity.

RESULTS

The selection process resulted in the inclusion of six articles. Pooled analysis revealed that POAG corneas respond to mechanical stimulus with a smaller concavity, showing lower deformation amplitude (DA) (CI95% -0.991 to -0.578; p < .001; I² = 0%), higher highest concavity radius (HCR; confidence interval [CI]95% -0.01 to 0.34; p = .058; I² = 6.7%), and lower peak distance (PD; CI95% -1.06 to -0.024; p = .040; I² = 86.5%). They also show a slower loading phase, with lower highest concavity time (HCT; CI95% -0.39 to -0.02; p = .029; I² = 3.3%) and lower applanation velocity-1 (CI95% -0.641 to -0.127; p = .003; I² = 34.6%), and a faster restoration to the original form, shown by lower applanation time-2 (CI95% -1.123 to -0.544; p = .001; I² = 44.8%) compared to healthy subjects.

CONCLUSIONS

High tension POAG patients are characterized by stiffer corneas compared to healthy controls. These differences are valid also after removing the effect of age, corneal thickness, and intraocular pressure (IOP).

In Vivo Corneal Stiffness Mapping by the Stress-Strain Index Maps and Brillouin Microscopy

The study of corneal stiffness in vivo has numerous clinical applications such as the measurement of intraocular pressure, the preoperative screening for iatrogenic ectasia after laser vision correction surgery and the diagnosis and treatment of corneal ectatic diseases such as keratoconus. The localised aspect of the microstructure deterioration in keratoconus leading to local biomechanical softening, corneal bulging, irregular astigmatism and ultimately loss of vision boosted the need to map the corneal stiffness to identify the regional biomechanical failure. Currently, two methods to map the corneal stiffness in vivo are integrated into devices that are either already commercially available or about to be commercialised: the stress-strain index (SSI) maps and the Brillouin Microscopy (BM). The former method produces 2D map of stiffness across the corneal surface, developed through numerical simulations using the corneal shape, its microstructure content, and the deformation behaviour under air-puff excitation. It estimates the whole stress-strain behaviour, making it possible to obtain the material tangent modulus under different intraocular pressure levels. On the other hand, BM produces a 3D map of the corneal longitudinal modulus across the corneal surface and thickness. It uses a low-power near-infrared laser beam and through a spectral analysis of the returned signal, it assesses the mechanical compressibility of the tissue as measured by the longitudinal modulus. In this paper, these two techniques are reviewed, and their advantages and limitations discussed.

Biomechanics in Keratoconus Diagnosis

Purpose: To prospectively review the importance of biomechanical assessment in the screening, diagnosis, prognosis, individualized planning, and clinical follow-up for ectatic corneal diseases.Methods: We demonstrate two commercially available devices to assess the corneal biomechanics in vivo, the Ocular Response Analyzer (ORA, Reichester, NY, USA) and the Corvis ST (Oculus, Wetzlar, Germany). Novel devices have been demonstrated to provide in vivo biomechanical measurements, including Brillouin optical microscopy and OCT elastography. Conclusion: The integration of biomechanical data and other data from multimodal refractive imaging using artificial intelligence demonstrated the ability to enhance accuracy in diagnosing ectatic corneal diseases.

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.

In Vivo Biomechanical Measurements of the Cornea

In early corneal examinations, the relationships between the morphological and biomechanical features of the cornea were unclear. Although consistent links have been demonstrated between the two in certain cases, these are not valid in many diseased states. An accurate assessment of the corneal biomechanical properties is essential for understanding the condition of the cornea. Studies on corneal biomechanics in vivo suggest that clinical problems such as refractive surgery and ectatic corneal disease are closely related to changes in biomechanical parameters. Current techniques are available to assess the mechanical characteristics of the cornea in vivo. Accordingly, various attempts have been expended to obtain the relevant mechanical parameters from different perspectives, using the air-puff method, ultrasound, optical techniques, and finite element analyses. However, a measurement technique that can comprehensively reflect the full mechanical characteristics of the cornea (gold standard) has not yet been developed. We review herein the in vivo measurement techniques used to assess corneal biomechanics, and discuss their advantages and limitations to provide a comprehensive introduction to the current state of technical development to support more accurate clinical decisions.

Corneal Biomechanical Characteristics in Osteogenesis Imperfecta With Collagen Defect

Purpose

To identify the characteristic corneal biomechanical properties of osteogenesis imperfecta (OI), and to compare the corneal biomechanical properties between OI and keratoconus.

Methods

We included 46 eyes of 23 patients with OI, 188 eyes of 99 keratoconus patients, and 174 eyes of 92 normal controls to compare corneal biomechanical parameters between OI corneas, keratoconus, and normal controls by using Corneal Visualization Scheimpflug Technology (Corvis ST).

Results

Patients with OI had significantly higher Corvis biomechanical index (CBI) (P < 0.001), higher tomographic and biomechanical index (TBI) (P = 0.040), lower Corvis Biomechanical Factor (CBiF) (P = 0.034), and lower stiffness parameter at first applanation (SP-A1) (P < 0.001) compared with normal controls. In contrast, OI group showed lower CBI (P < 0.001), lower TBI (P < 0.001), higher CBiF (P < 0.001), and higher SP-A1 (P = 0.020) than keratoconus group. Notably, the stress-strain index (SSI) was not significantly different between the OI and normal controls (P = 1.000), whereas keratoconus showed the lowest SSI compared with OI group (P = 0.025) and normal controls (P < 0.001).

Conclusions

Although the corneal structures of OI patients are less stable and easier to deform as compared to those of the control group, there is no significant difference in material stiffness observed between the OI and normal controls. In contrast, the corneas of keratoconus showed not only lower structural stability and higher deformability but also lower material stiffness compared with those of OI cornea and normal controls.

Translational Relevance

The biomechanical alterations are different between OI corneas and keratoconus.

Change in the corneal material mechanical property for small incision lenticule extraction surgery

Purpose: To assess the distribution characteristics and related factors of stress-strain index (SSI) values and discuss changes in biomechanical parameters, including SSI, after small incision lenticule extraction (SMILE) surgery. Methods: This study included 253 patients who underwent SMILE (253 eyes). SSI and other biomechanical parameters were measured using corneal visualization Scheimpflug technology before and 3 months after surgery. The data collected included SSI, central corneal thickness (CCT), and eight other dynamic corneal response parameters. The Kolmogorov-Smirnov test, Pearson and partial correlation analyses, and paired-sample t-tests were used for statistical analyses. Results: Both pre-op SSI and ΔSSI follow a normal distribution, while post-op SSI does not follow a normal distribution. The decline in SSI after SMILE surgery was not statistically significant, and the data dispersion of SSI after SMILE surgery was close to that before surgery (p > 0.05). No statistical correlation was noted between SSI values and age and pre-op CCT (all p > 0.05). However, both pre- and post-op SSI values decreased with increasing degree of myopia (all p < 0.05), and weakly correlated with preoperative intraocular pressure and biomechanically corrected intraocular pressure (all p < 0.05). Other biomechanical parameters changed significantly after surgery (all p < 0.001). After SMILE, the magnitude of the deformation at the highest concave, deformation ratio, and integral radius increased significantly (all p < 0.001), while the Ambrosio relational thickness horizontal, stiffness parameter A1, and Corvis biomechanical index decreased significantly (p < 0.001). Conclusion: SSI, which reflects essential corneal material attributes, differs from other corneal biomechanical parameters and remains stable before and after SMILE surgery, and can be used as an indicator to evaluate changes in corneal material properties after SMILE surgery.

Biomechanical analysis of ocular diseases and its in vitro study methods

Ocular diseases are closely related to the physiological changes in the eye sphere and its contents. Using biomechanical methods to explore the relationship between the structure and function of ocular tissue is beneficial to reveal the pathological processes. Studying the pathogenesis of various ocular diseases will be helpful for the diagnosis and treatment of ocular diseases. We provide a critical review of recent biomechanical analysis of ocular diseases including glaucoma, high myopia, and diabetes. And try to summarize the research about the biomechanical changes in ocular tissues (e.g., optic nerve head, sclera, cornea, etc.) associated with those diseases. The methods of ocular biomechanics research in vitro in recent years are also reviewed, including the measurement of biomechanics by ophthalmic equipment, finite element modeling, and biomechanical analysis methods. And the preparation and application of microfluidic eye chips that emerged in recent years were summarized. It provides new inspiration and opportunity for the pathogenesis of eye diseases and personalized and precise treatment.

Effect of myopia and astigmatism deepening on the corneal biomechanical parameter stress-strain index in individuals of Chinese ethnicity

Purpose: To investigate the differences in corneal biomechanical parameter stress–strain index (SSI) among different degrees of myopic eyes in Chinese individuals and to analyze the relevant factors of the SSI.

Methods: This study analyzed the right eyes of 240 participants (240 eyes) aged 18–34 years. The participants were divided into low-, moderate-, high-, and ultra-high myopia groups according to their spherical equivalent (SE), with 60 eyes included in each group. Spherical, cylinder, and SE were measured via automatically integrated optometry. Intraocular pressure (IOP) was measured using a non-contact tonometer. AL was measured using an IOLMaster device. Corneal curvature and central corneal thickness (CCT) were measured using a Pentacam. SSI and biomechanical corrected IOP (bIOP) were measured via corneal visualization Scheimpflug technology (Corvis ST). The statistical analyses included one-sample Kolmogorov–Smirnov tests and normal distribution histogram methods, Levene variance homogeneity tests, Pearson’s correlation analyses, multiple linear stepwise regression analyses, one-way ANOVA, and LSD t-tests.

Results: The mean (±SD) age of the 240 participants was (24.97 ± 4.16) years. The SSI was positively correlated with spherical, cylinder, SE, CCT, IOP, and bIOP and negatively correlated with K1 and AL (r = 0.475, 0.371, 0.497, 0.169, 0.291, 0.144, −0.154, and −0.464, respectively; all p < 0.05), but were not correlated with age, K2, or Km (all p > 0.05). Multiple linear regression analysis performed with SSI as the dependent variable, and spherical, cylinder, K1, CCT, and IOP as independent variables produced the following regression equation: SSI = 0.989 + 0.017 spherical + 0.042 cylinder +0.018 IOP (R² = 0.402, F = 31.518, p < 0.001). The SSI values in the low-, moderate-, high-, and ultra-high myopia groups were 0.945 ± 0.135, 0.940 ± 0.128, 0.874 ± 0.110, and 0.771 ± 0.104, respectively. The values decreased sequentially, and the differences between pairs were statistically significant (all p < 0.05), except for that between the low- and moderate-myopia groups (p > 0.05).

Conclusion: SSI decreased with increasing myopia and astigmatism in the Chinese participants. The SSI was significantly lower in high and ultra-high myopia, especially ultra-high myopia. These findings indicate that increased corneal elasticity may be related to the pathogenesis of high and ultra-high myopia.

Changes in Corneal Biomechanical Properties After Small-Incision Lenticule Extraction and Photorefractive Keratectomy, Using a Noncontact Tonometer

PURPOSE

The aim of this study was to evaluate and compare early corneal biomechanical changes after small-incision lenticule extraction (SMILE) and photorefractive keratectomy (PRK).

METHODS

The study comprised 74 patients eligible for refractive surgery, equally allocated to PRK (37 patients) and SMILE (37 patients). Corneal biomechanical properties were recorded and compared between the 2 groups at preoperatively and 3 months after surgery using a dynamic ultra-high-speed Scheimpflug camera equipped with a noncontact tonometer.

RESULTS

Both procedures significantly affected corneal biomechanical properties at 3 months after surgery. Patients in the PRK group showed significantly better results for deformation amplitude ratio (DA ratio) ( P = 0.03), maximum inverse radius (InvRadMax) ( P = 0.02), and A2 time ( P = 0.03). The mean changes in DA ratio, HC radius, InvRadMax, and Ambrosio relational thickness were significantly higher in the SMILE group in comparison with those of the PRK group (all, P < 0.05). In both groups, change in CCT was significantly correlated with changes in DA ratio and InvRadMax ( P < 0.05).

CONCLUSIONSS

Both SMILE and PRK refractive surgeries significantly altered corneal biomechanical properties but the changes were more prominent after SMILE.

In vivo Assessment of Localised Corneal Biomechanical Deterioration With Keratoconus Progression

Purpose: To evaluate the regional corneal biomechanical deterioration with keratoconus (KC) progression as measured by the Stress-Strain Index (SSI) maps.

Methods: The preoperative examinations of 29 progressive KC cases that were submitted to corneal cross-linking (CXL) were evaluated. The examinations included the tomography and the SSI measured by the Pentacam HR and the Corvis ST (Oculus, Wetzlar, Germany), respectively. The results were recorded twice, the latter of which was at the last visit before the CXL procedure. The patient-specific SSI maps were built, using data at each examination, based on finite element modelling and employing inverse analysis to represent the regional variation of biomechanical stiffness across the cornea.

Results: All cases presented significant shape progression (above the 95% CI of repeatability) in anterior and posterior curvatures and minimum thickness. The overall corneal stiffness as measured by the SSI within the central 8 mm-diameter area underwent slight but significant reductions from the first to the last examination (−0.02 ± 0.02, range: −0.09 to 0, p < 0.001). In all 29 cases, the reduction in stiffness was localised and concentred in the area inside the keratoconus cone. The SSI values inside the cone were significantly lower in the last examination (by 0.15 ± 0.09, range: −0.42 to −0.01, p < 0.001), while the SSI outside the cone presented minimal, non-significant variations (0 ± 0.01, range: −0.04 to 0.01, p = 0.999).

Conclusion: It has been observed through the SSI maps that the regional deterioration in stiffness was concerted inside the area of pathology, while only mild non-significant alterations were observed outside the area of pathology.

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.