Long-term Evaluation of Corneal Biomechanical Properties After Corneal Cross-linking for Keratoconus: A 4-Year Longitudinal Study

Long-term outcomes of corneal crosslinking

PURPOSE OF REVIEW

This manuscript summarizes contemporary research from 2018 to 2023 evaluating long-term (≥2 years) outcomes of corneal crosslinking (CXL) for progressive keratoconus (KCN).

RECENT FINDINGS

The standard Dresden protocol (SDP) has been utilized clinically since the early 2000 s to treat ectatic disorders, primarily progressive KCN and postrefractive ectasia. Various modifications have since been introduced including accelerated and transepithelial protocols, which are aimed at improving outcomes or reducing complications. This review summarizes data demonstrating that the SDP halts disease progression and improves various visual and topographic indices (UDVA, CDVA, Kmax, K1, K2) up to 13 years postoperatively. Accelerated and transepithelial protocols have been found to be well tolerated alternatives to SDP with similar efficacy profiles. Studies focusing on pediatric populations identified overall higher progression rates after CXL. All protocols reviewed had excellent safety outcomes in adults and children.

SUMMARY

Recent studies revealed that SDP successfully stabilizes KCN long term, and a variety of newer protocols are also effective. Pediatric patients may exhibit higher progression rates after CXL. Further research is required to enhance the efficacy and ease of these protocols.

Effect of corneal cross-linking on biomechanical changes following transepithelial photorefractive keratectomy and femtosecond laser-assisted LASIK

Purpose: To evaluate the change in corneal biomechanics in patients with postoperative ectasia risk when combining two common laser vision correction procedures (tPRK and FS-LASIK) with cross-linking (in tPRK Xtra and FS-LASIK Xtra). Methods: The study included 143 eyes of 143 myopic, astigmatic patients that were divided into non-cross-linked refractive surgery groups (non-Xtra groups, tPRK and FS-LASIK) and cross-linked groups (Xtra groups, tPRK Xtra and FS-LASIK Xtra) according to an ectasia risk scoring system. The eyes were subjected to measurements including the stress-strain index (SSI), the stiffness parameter at first applanation (SP-A1), the integrated inverse radius (IIR), the deformation amplitude at apex (DA), and the ratio of deformation amplitude between apex and 2 mm from apex (DARatio2mm). The measurements were taken preoperatively and at 1, 3, and 6 months postoperatively (pos1m, pos3m, and pos6m). Posterior demarcation line depth from the endothelium (PDLD) and from the ablation surface (DLA) were recorded at pos1m. Results: SP-A1 significantly decreased, while IIR, deformation amplitude, and DARatio2mm increased significantly postoperatively in all four groups (p < 0.01)-all denoting stiffness decreases. In the FS-LASIK group, the changes in IIR, DA, and DARatio2mm were 32.7 ± 15.1%, 12.9 ± 7.1%, and 27.2 ± 12.0% respectively, which were significantly higher (p < 0.05) compared to 20.1 ± 12.8%, 6.4 ± 8.2%, and 19.7 ± 10.4% in the FS-LASIK Xtra group. In the tPRK group, the change in IIR was 27.3 ± 15.5%, significantly larger than 16.9 ± 13.4% in the tPRK Xtra group. The changes of SSI were minimal in the tPRK (-1.5 ± 21.7%, p = 1.000), tPRK Xtra (8.4 ± 17.9%, p = 0.053), and FS-LASIK Xtra (5.6 ± 12.7%, p = 0.634) groups, but was significant in the FS-LASIK group (-12.1 ± 7.9%, p < 0.01). After correcting for baseline biomechanical metrics, preoperative bIOP and the change in central corneal thickness (△CCT) from pre to pos6m, the changes in the IIR in both FS-LASIK and tPRK groups, as well as DA, DARatio2mm and SSI in the FS-LASIK group remained statistically greater than their corresponding Xtra groups (all p < 0.05). Most importantly, after correcting for these covariates, the changes in DARatio2mm in the FS-LASIK Xtra became statistically smaller than in the tPRK Xtra (p = 0.017). Conclusion: The statistical analysis results indicate that tPRK Xtra and FS-LASIK Xtra effectively reduced the biomechanical losses caused by refractive surgery (tPRK and FS-LASIK). The decrease in corneal overall stiffness was greater in FS-LASIK than in tPRK, and the biomechanical enhancement of CXL was also higher following LASIK than after tPRK.

Changes in corneal biomechanical parameters in keratoconus eyes with various severities after corneal cross-linking (CXL): A comparative study

OBJECTIVES

To compare changes in corneal biomechanical parameters one year after corneal cross-linking (CXL) in keratoconus (KCN) eyes of different severities.

METHODS

Seventy-five eyes with mild, moderate, and severe grades of KCN (n = 24, 31, and 20 eyes, respectively) that were treated with CXL, based upon the standard Dresden protocol, were included. The corneal biomechanical assessment was performed using Corvis ST and Ocular Response Analyzer (ORA). Changes in Corvis's dynamic corneal response (DCR) parameters and ORA's derived parameters (corneal hysteresis (CH), and corneal resistance factor (CRF)) were assessed whilst the corneal thickness and intraocular pressure were considered as covariates.

RESULTS

There was no statistically significant difference in the corneal biomechanical parameters obtained using both devices after surgery separately in different KCN grades, except for the deformation amplitude (DA) in the severe KCN group (P = 0.017). Changes in the classic parameters of the highest concavity phase of Corvis ST (peak distance, radius, and DA) were more positive and in the newer parameters (integrated inverse radius (IIR), deformation amplitude ratio (DAR)) more negative in the severe group compared to the other groups. Also, the mean change in CH (P = 0.710), and CRF (P = 0.565), showed a negative shift in higher grades of KCN; however, there was no significant difference in the mean changes of all parameters between different groups. (P > 0.05).

CONCLUSIONS

Similar changes in the Corvis ST and ORA parameters in mild, moderate, and severe KCN indicate biomechanical stability and the effective role of CXL in stopping the progressive nature of keratoconus in eyes of varying severities one year after CXL.

Evaluation of Biomechanical Changes After Accelerated Cross-Linking in Progressive Keratoconus: A Prospective Follow-Up Study

PURPOSE

The aim of this study was to analyze the biomechanical effect of accelerated corneal cross-linking (9*10) in progressive keratoconus (KC) in comparison to untreated fellow eyes using Scheimpflug-based tonometry (Corvis ST, CVS).

METHODS

Forty-three eyes of 43 patients with KC showed progressive KC and were treated using accelerated corneal cross-linking. Twenty-five untreated fellow eyes were used as the control group. All eyes were examined biomechanically (CVS) and tomographically (Pentacam) at baseline, after 1-month, 6-month, and 12-month follow-up. Statistical analysis was performed using a linear mixed model. A logistic regression was performed to attribute the effects of changes in each parameter to treatment status (treated or untreated).

RESULTS

Maximum keratometry values decreased statistically significantly at 12 months by -1.1 D (95 confidence interval: -2.0 to -0.1, P = 0.025) compared with baseline. Thinnest corneal thickness decreased significantly after 1 month ( P < 0.001) and recovered to baseline after 12 months ( P = 0.752). In the corneal cross-linking (CXL) group, biomechanical changes were observed by an increased bIOP, a shorter A2 time, and a lower integrated radius after 1 month (all P < 0.05). No biomechanical and tomographical changes were observed in the control group (all P > 0.05). Logistic regression pointed out that treated eyes can be separated from untreated eyes by differences in bIOP, corneal thickness, A1 velocity, integrated radius, and Kc mean at 1, 6, and 12 months.

CONCLUSIONS

The alterations in biomechanical parameters indicated a corneal stiffening effect after CXL treatment, which was mostly detectable 1 month after treatment, although corneal thickness was reduced. The logistic regression model showed an adequate separation between CXL-treated and untreated eyes.

[Corneal biomechanics before and after cross-linking in patients with keratoconus]

OBJECTIVE

The aim of this study was to analyze the effect of corneal cross-linking (CXL) on corneal biomechanics and visual acuity.

PATIENTS AND METHODS

The examination results before and after CXL in 56 eyes of 56 patients between 2017 and 2021 were evaluated retrospectively. The last preoperative examination was compared to the postoperative follow-up values after 6 and 12 months. The main outcome measures included various biomechanical parameters from the Corvis ST (CST), Pentacam and the visual acuity (logMAR, "logarithm of the Minimal Angle of Resolution"). For longitudinal evaluation, a general linear model for repeated measurements was used. A p-value of less than 0.05 was considered to show a statistically significant result. Bonferroni correction was applied for multiple comparisons.

RESULTS

The maximum corneal refractive power Kmax decreased slightly without statistical significance from 57.1 ± 6.1 diopters (dpt) to 56.6 ± 6.3 dpt after 6 months (p = 0.076) and 56.8 ± 6.6 dpt after 12 months (p = 0.443). The Pentacam parameter Belin/Ambrósio Enhanced Ectasia Total Deviation Display (BAD D) showed a statistically significant increase from the preoperative value of 8.4 ± 3.7 to the postoperative value of 9.1 ± 3.6 after 6 months (p < 0.001) and to 8.9 ± 3.5 after 12 months (p = 0.051). The CST parameter Ambrósio's relational thickness to horizontal profile (ARTh) decreased statistically significantly from 229.9 ± 109.6 to 204.8 ± 84.9 at 6 months (p = 0.017) and 205.3 ± 93.7 at 12 months (p = 0.022). The CST parameter stiffness parameter A1 (SP A1) increased slightly from the preoperative value 69.9 ± 17.2 to 70.4 ± 17.2 after 6 months (p = 1) and 71 ± 18.2 after 1 year (p = 1). Mean best-corrected visual acuity (logMAR) showed an improvement from 0.39 ± 0.3 to 0.34 ± 0.3 at 6 months (p = 0.286) and to 0.31 ± 0.3 at 12 months (p = 0.077). Regarding the ABCD classification, the parameters were determined preoperatively with an average of A2B3C1D2. They showed the same value of A2B3C1D2 after 6 and 12 months.

CONCLUSION

In progressive keratoconus, corneal cross-linking has the potential to positively influence the biomechanics of the cornea and visual acuity as a low complication treatment option.

Short- and mid-term changes in CORVIS ST parameters in successful, adult orthokeratology patients

CLINICAL RELEVANCE

The changes in various biomechanical and tomographic characteristics of the cornea associated with orthokeratology may allow us to identify potential mid- and long-term structural alterations, resulting in a better understanding of the governing mechanisms of this procedure and in its optimisation.

BACKGROUND

The study aimed at describing short and mid-term changes in CORVIS ST® parameters and indices in orthokeratology (ortho-k), and their diurnal variations.

METHODS

A prospective observational study was designed in which several CORVIS ST® parameters of 75 new adult participants successfully fitted with overnight ortho-k Seefree® (Conóptica - Hecht Contactlinsen) contact lenses were explored. Measurements were conducted in baseline (BL) conditions and in the morning and evening at the one-night (1 NM/1NT), one-week (1WM/1 WT) and 3-month (3 MM/3MT) follow-up visits.

RESULTS

Statistically significant differences were found in DARatio_2 mm, IntRad, ARTh, CBI and TBI following overnight ortho-k, when compared with BL values, with most values reaching stability at 1WM or reverting to BL values at 3 MM. The ARTh and CBI parameters showed some of the most significant temporal variations (both p < 0.001), probably reflecting the encountered differences in central corneal thickness between BL and 1WM (p = 0.010) and between BL and 3 MM (p = 0.016). In general, corneal rigidity was higher in the morning at all follow-up visits, and decreased during the day. No statistically significant changes in adjusted intraocular pressure values were found.

CONCLUSION

Ortho-k in adults may be considered a safe procedure in terms of short and mid-term changes in CORVIS ST® parameters. The observed alterations in most of the parameters provided by the Corvis ST® probably responded to the well-described changes in corneal pachymetry and tomography, rather than to actual alterations in corneal rigidity.

Repeatability of corneal deformation response parameters by dynamic ultra-high-speed Scheimpflug imaging before and after corneal crosslinking

PURPOSE

To evaluate the repeatability of deformation corneal response (DCR) parameters before and after corneal crosslinking (CXL) compared with their untreated fellow eyes (uFEs).

SETTING

University Hospital Carl Gustav Carus, Dresden, Germany; IRCCS Humanitas Research Hospital, Rozzano, Milan, Italy.

DESIGN

Multicenter, interventional reliability analysis.

METHODS

53 eyes of 53 patients with keratoconus who received CXL treatment after the disease progression (CXL group) were included. Patients were measured 3 times using a dynamic Scheimpflug analyzer to determine repeatability before and 1 month after CXL treatment. The uFEs were measured in the same way (uFE group). Reliability of DCR parameters was assessed by a coefficient of repeatability, coefficient of variation, and intraclass correlation coefficient (ICC).

RESULTS

The repeatability of DCR parameters did not change after CXL compared with the preoperative values for all investigated DCR parameters ( P > .05). In the uFE group, no statistically significant shift was observed regarding the repeatability ( P > .05). An ICC greater than 0.75 was achieved in both groups for almost all parameters. Concerning the biomechanical stiffening induced by CXL, integrated inverse radius and stress-strain index were found to be statistically significantly decreased and increased ( P < .001), respectively, both indicating stiffening. No changes were observed for the uFE group.

CONCLUSIONS

The study demonstrated highly repeatable measurements of the dynamic Scheimpflug analyzer before and after CXL. The improvement of certain DCR parameters after CXL confirmed the capability of the device to detect the stiffening effect.

Ocular Biomechanics and Glaucoma

Biomechanics is a branch of biophysics that deals with mechanics applied to biology. Corneal biomechanics have an important role in managing patients with glaucoma. While evidence suggests that patients with thin and stiffer corneas have a higher risk of developing glaucoma, it also influences the accurate measurement of intraocular pressure. We reviewed the pertinent literature to help increase our understanding of the biomechanics of the cornea and other ocular structures and how they can help optimize clinical and surgical treatments, taking into consideration individual variabilities, improve the diagnosis of suspected patients, and help monitor the response to treatment.

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.

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.

Clinical Ocular Biomechanics: Where Are We after 20 Years of Progress?

Purpose: Ocular biomechanics is an assessment of the response of the structures of the eye to forces that may lead to disease development and progression, or influence the response to surgical intervention. The goals of this review are (1) to introduce basic biomechanical principles and terminology, (2) to provide perspective on the progress made in the clinical study and assessment of ocular biomechanics, and (3) to highlight critical studies conducted in keratoconus, laser refractive surgery, and glaucoma in order to aid interpretation of biomechanical parameters in the laboratory and in the clinic.Methods: A literature review was first conducted of basic biomechanical studies related to ocular tissue. The subsequent review of ocular biomechanical studies was limited to those focusing on keratoconus, laser refractive surgery, or glaucoma using the only two commercially available devices that allow rapid assessment of biomechanical response in the clinic.Results: Foundational studies on ocular biomechanics used a combination of computer modeling and destructive forces on ex-vivo tissues. The knowledge gained from these studies could not be directly translated to clinical research and practice until the introduction of non-contact tonometers that quantified the deformation response of the cornea to an air puff, which represents a non-destructive, clinically appropriate load. The corneal response includes a contribution from the sclera which may limit corneal deformation. Two commercial devices are available, the Ocular Response Analyzer which produces viscoelastic parameters with a customized load for each eye, and the Corvis ST which produces elastic parameters with a consistent load for every eye. Neither device produces the classic biomechanical properties reported in basic studies, but rather biomechanical deformation response parameters which require careful interpretation.Conclusions: Research using clinical tools has enriched our understanding of how ocular disease alters ocular biomechanics, as well as how ocular biomechanics may influence the pathophysiology of ocular disease and response to surgical intervention.

Evaluation of Dynamic Corneal Response Parameters and the Biomechanical E-Staging After Accelerated Corneal Cross-Linking in Keratoconus

PURPOSE

This study evaluated the biomechanical E-staging in progressive keratoconus (KC) corneas before and after epithelium-off accelerated corneal cross-linking (CXL, 9 mW/cm2, 10 min, 5.4 J/cm2).

DESIGN

German university-based retrospective longitudinal cohort study.

METHODS

The biomechanical E-staging for ectatic corneal diseases was applied retrospectively on 49 progressive KC corneas of 41 patients who underwent CXL. Main outcome parameters included the Corvis Biomechanical Factor (CBiF, the linearized Corvis Biomechanical Index), the biomechanical E-staging (E1 to E4 result of dividing the CBiF value range into 5 groups), maximal anterior keratometry (Kmax), anterior radius of curvature (ARC), and thinnest corneal thickness (TCT). They were evaluated at 2.1±2.0 months preoperatively (n=49 corneas, 41 patients) and postoperatively after 5.4±1.4, 11.3±1.8, and 23.4±1.6 months.

RESULTS

The CBiF decreased (5.1±0.5 | 5.0±0.5, P=0.0338) and the E-staging increased significantly (2.4±0.9 | 2.6±0.8, P=0.0035) from preoperatively to the first postoperative follow-up. The difference was not significant after 11 months and there were same values after 23 months. Kmax, ARC, and TCT slightly decreased (Kmax: 56.9±6.3, 54.3±5.1, 56.2±6.6, 54.0±5.2; ARC: 49.8±3.5, 48.9±3.2, 50.8±5.6, 49.0±3.7; TCT: 470±34, 454±36, 459±35, 466±39; preoperatively and 5, 11, and 23 months postoperatively). A postoperatively decreased TCT was associated with an increased E-stage, whereas an equal or increased TCT measurement after CXL was associated with equal or lower E-staging results.

CONCLUSIONS

The biomechanical E-staging in KC corneas is influenced by TCT measurements and increases within the first postoperative months after CXL. On the long term, it indicates a postoperative KC stabilization, with comparable E-values to preoperatively at 11 and 23 months after CXL.

Reliability analysis of successive Corvis ST® measurements in keratoconus 2 years after accelerated corneal crosslinking compared to untreated keratoconus corneas

Purpose

To assess the reliability of successive Corvis ST® measurements (CST, Oculus, Wetzlar, Germany) in keratoconus (KC) ≥ 2 years after accelerated corneal crosslinking (9 mW/cm2, 10 min, 5.4 J/cm2) compared to untreated KC corneas.

Methods

Three successive CST measurements per eye were performed in ≥ 2 years after CXL (CXLG, n = 20 corneas of 16 patients) and a control group consisting of non-operated, ABC-stage-matched KC corneas according to Belin’s ABCD KC grading (controls, n = 20 corneas, 20 patients). Main outcome measures included maximal keratometry (Kmax), the Belin/Ambrósio-Enhanced-Ectasia-Deviation-Index BAD-D; the biomechanical parameters A1 velocity, deformation amplitude (DA) ratio 2 mm, Ambrósio relational thickness to the horizontal profile (ARTh), integrated radius, stiffness parameter A1 (SP-A1), and the Corvis Biomechanical Factor (CBiF, the linearized term of the Corvis Biomechanical Index). Mean values, standard deviations, and Cronbach’s alpha (CA) were calculated.

Results

Both groups were tomographically comparable (BAD: 11.5 ± 4.7|11.2 ± 3.6, p = 0.682, Kmax: 60.5 ± 7.2|60.7 ± 7.7, p = 0.868 for controls|CXLG, paired t-test). A1 velocity (mean ± SD: 0.176 ± 0.02|0.183 ± 0.02, p = 0.090, CA: 0.960|0.960), DA ratio 2 mm (6.04 ± 1.13|6.14 ± 1.03, p = 0.490, CA: 0.967|0.967), integrated radius (12.08 ± 2.5|12.42 ± 1.9, p = 0.450, CA: 0.976|0.976), and CBiF (4.62 ± 0.6|4.62 ± 0.4, p = 0.830, CA: 0.965|0.965) were also comparable (controls|CXLG). ARTh was significantly higher in controls (177.1 ± 59, CA: 0.993) than after CXL (155.21 ± 65, p = 0.0062, CA: 0.993) and SP-A1 was significantly higher after CXL (59.2 ± 13, CA: 0.912) than in controls (52.2 ± 16, p = 0.0018, CA: 0.912).

Conclusion

ARTh and SP-A1 differed significantly between controls and CXLG. Biomechanical measurements were generally of excellent reliability in both groups. CXL seems to affect biomechanical measurements of human corneas over more than 2 years.

A Critical Assessment of Friedenwald’s Technique for Estimating the Coefficient of Rigidity of the Cornea

Purpose

To determine if Friedenwald's technique for estimating the coefficient of corneal rigidity (Ko, units mmHg/μL), could differentiate between the cornea in keratoconus, normal eyes, and after crosslinking (CXL).

Methods

Two operators (1 and 2) independently measured Ko in three groups (keratoconus, normal, and post-CXL corneas), and repeated the procedure in some where their care remained unchanged and others after routine CXL (>28 days postop, epi-off treatment, 3.0 mW/cm², 30 min). The data were subsequently used to quantify interoperator error, test-retest/intersessional reliability for estimation of Ko, the significance of intergroup differences, and the effect of CXL on Ko.

Results

The major findings were: (i) Ko values were not normally distributed; (ii) mean (±sd, 95% CI) interoperator error was -0.002 (±0.019, −0.006 to 0.003, n = 95) and the limit of agreement between the operators was ±0.039; (iii) RMS differences in the intersessional estimation of Ko values were 0.011 (operator 1) and 0.012 (operator 2); (iv) intergroup differences in Ko were not significant (p > 0.05); (v) intersessional change in Ko (y) was linearly related to Ko estimated (x) at 1^st session (for operator 2 y = 1.187x−0.021, r = 0.755, n = 16, p < 0.01); and (vi) change in Ko (y₁) after CXL was linearly related to Ko (x₁) at preop (for operator 2 y₁ = 0.880x₁−0.016, r = 0.935, n = 20, p < 0.01).

Conclusion

Friedenwald's technique for estimating the Ko is prone to substantial interoperator error and intersessional differences. According to the technique, the change in Ko following CXL is on par with the expected intersessional change observed in controls.

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.

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.

Systemic supplemental oxygen therapy during accelerated corneal crosslinking for progressive keratoconus: randomized clinical trial

PURPOSE

To investigate the potential additive effect of systemic supplemental oxygen administered during accelerated corneal crosslinking (CXL) for progressive keratoconus (KC).

SETTING

Academic center.

DESIGN

Randomized clinical trial.

METHODS

Eyes with progressive KC randomized to 3 different CXL protocols were included. The first group (OA-CXL) included 19 eyes that underwent an accelerated CXL protocol (9 mW/cm2 for 10 minutes) while receiving systemic oxygen at a rate of 5 L/min for 10 minutes. The second group consisted of 14 eyes undergoing the same accelerated CXL protocol without supplemental oxygen therapy (A-CXL). The third group (C-CXL) comprised 14 eyes undergoing conventional CXL according to the Dresden protocol. All subjects were followed up for at least 6 months. Visual acuity, keratometry and corneal biomechanical parameters including corneal hysteresis and corneal resistance factor (CRF) were measured preoperatively and 6 months postoperatively.

RESULTS

Reduction in maximum keratometry (Kmax) was significantly greater in the OA-CXL group (P = .01). At baseline, the mean Kmax was 54.31 ± 3.64 diopters (D) in the OA-CXL group, 54.66 ± 4.99 D in the A-CXL group, and 56.03 ± 5.28 D in the C-CXL group (P = .58), which reached 53.58 ± 3.24 D, 54.59 ± 4.65 D, and 55.87 ± 4.73 D at 6 months in the 3 study groups, respectively (P = .115). The mean CRF increased significantly only in the OA-CXL group from a baseline value of 6.32 ± 2.12 mm Hg to 7.38 ± 1.88 mm Hg at 6 months (P = .009).

CONCLUSIONS

This study suggests superior efficacy of an accelerated CXL protocol coupled with systemic oxygen supplementation when compared with the accelerated CXL protocol and the conventional protocol in eyes with progressive KC. In addition to greater reduction in Kmax as the primary outcome, improvement in corneal biomechanics was also observed at 6 months.

Comparison of corneal biological parameters between transepithelial and epithelium-off corneal cross-linking in keratoconus

AIM

To evaluate the differences in corneal biological parameters between transepithelial and epithelium-off corneal cross-linking in keratoconus.

METHODS

In our prospective clinical trial, 40 patients (60 eyes) with progressive keratoconus were randomized to undergo corneal cross-linking with transepithelial (TE group, n=30) or epithelium-off (EO group, n=30) keratoconus. Examinations comprised topography, corneal biomechanical analysis and specular microscopy at 6mo postoperatively.

RESULTS

The keratometer values were not significantly different between the TE and EO corneal cross-linked groups in different periods (each P>0.05). The corneal thickness of the EO group was greater than that of the TE group at 1wk after the operation (each P<0.05). Regarding corneal biomechanical responses, the EO group showed a longer second applanation length than TE group (P=0.003). Regarding the corneal endothelial function, standard deviation of the endothelial cell size, and coefficient of variation in the cell area, the values of EO group were larger than those of TE group at 1wk (P=0.011, 0.026), and the percentage of hexagonal cells in EO group was lower than that in TE group at 1 and 6mo (P=0.018, 0.019).

CONCLUSION

Epithelium-off corneal cross-linking may strengthen corneal biomechanics better than TE procedure can. However, the TE procedure with a lower ultraviolet-A irradiation intensity would be safer for corneal endothelial function.

One-Year Follow-Up of Corneal Biomechanical Changes After Accelerated Transepithelial Corneal Cross-Linking in Pediatric Patients With Progressive Keratoconus

Aims: This study aimed to investigate the corneal biomechanical changes and topographic outcomes of accelerated transepithelial corneal cross-linking (ATE-CXL) in pediatric progressive keratoconus.

Methods: In this prospective longitudinal study, 31 eyes of 28 pediatric patients with keratoconus (21 boys and 7 girls; mean age, 14.35 ± 2.68 years) undergoing ATE-CXL (epithelium-on procedure with 45 mW/cm² for 320 s) were included. Corvis ST was used to measure dynamic corneal response parameters at baseline and at 12 month after ATE-CXL. Corneal keratometry and corneal thickness were measured using Pentacam pre-operatively and 1, 6, and 12 month post-operatively.

Results: No serious complications occurred during or after ATE-CXL. The maximum keratometry values were 60.10 ± 7.51 D pre-operatively and 61.42 ± 8.92, 61.17 ± 7.96, and 60.02 ± 7.58 D at 1, 6, and 12 month after ATE-CXL (P > 0.05), respectively. Corneal thickness remained stable during the 12-month follow-up (P > 0.05). At post-operative 12 month, first applanation time (P < 0.001), first applanation length (P = 0.004), second applanation velocity (P = 0.014), highest concavity time (P = 0.022), and radius of curvature at highest concavity (P = 0.031) increased significantly. The value of stiffness parameter at first applanation was significantly increased from 57.70 ± 27.57 pre-operatively to 63.36 ± 27.09 at 12 months after ATE-CXL (P = 0.018).

Conclusions: ATE-CXL is safe and effective in stabilizing the progression of pediatric keratoconus. Changes in corneal biomechanical response consistent with stiffening following ATE-CXL were observed in pediatric patients with keratoconus.

Comparison of Corneal Biomechanical Properties and Corneal Tomography Between Customized and Accelerated Corneal Crosslinking in Eyes with Keratoconus

PURPOSE

To compare the changes in corneal biomechanical properties and corneal tomography between transepithelial customized corneal crosslinking (C-CXL) and epithelium-off accelerated corneal crosslinking (A-CXL) in eyes with keratoconus.

METHODS

Twenty eyes in 20 consecutive patients who underwent C-CXL (C-CXL group) and 20 eyes in 20 patients who underwent A-CXL (A-CXL group) were included in this retrospective comparative study. The corneal biomechanical properties were analyzed using a Scheimpflug-based tonometer, and all corneas were examined by anterior segment optical coherence tomography (AS-OCT) before and 3 months after surgery. The corneal biomechanical parameters analyzed were the maximum inverse radius, deformation amplitude (DA) ratio max (2 mm), stiffness parameter at applanation 1, and integrated radius. The AS-OCT parameters analyzed included average keratometry, corneal astigmatism, maximum keratometry reading (Kmax), higher-order irregularity, and asymmetry.

RESULTS

In the C-CXL group, there were significant improvements in biomechanical parameters, including the maximum inverse radius, the DA ratio max (2 mm), and the integrated radius after surgery (P = 0.037, P = 0.002, and P = 0.003, respectively). In the C-CXL group, there was a significant decrease in the Kmax, higher-order irregularity, and asymmetry components (P = 0.014, P = 0.008, and P = 0.016, respectively). The biomechanical properties and AS-OCT parameters did not change significantly in the A-CXL group after surgery. According to multiple regression analyses, C-CXL had a greater effect than A-CXL in improving the maximum inverse radius, DA ratio max (2 mm), integrated radius, Kmax, asymmetry component, and higher-order irregularity component.

CONCLUSIONS

C-CXL might improve the biomechanical properties and irregular shape of the cornea from the early postoperative period to a greater extent than A-CXL.

Biomechanics of Ophthalmic Crosslinking

Crosslinking involves the formation of bonds between polymer chains, such as proteins. In biological tissues, these bonds tend to stiffen the tissue, making it more resistant to mechanical degradation and deformation. In ophthalmology, the crosslinking phenomenon is being increasingly harnessed and explored as a treatment strategy for treating corneal ectasias, keratitis, degenerative myopia, and glaucoma. This review surveys the multitude of exogenous crosslinking strategies reported in the literature, both "light" (involving light energy) and "dark" (involving non-photic chemical processes), and explores their mechanisms, cytotoxicity, and stage of translational development. The spectrum of ophthalmic applications described in the literature is then discussed, with particular attention to proposed therapeutic mechanisms in the cornea and sclera. The mechanical effects of crosslinking are then discussed in the context of their proposed site and scale of action. Biomechanical characterization of the crosslinking effect is needed to more thoroughly address knowledge gaps in this area, and a review of reported methods for biomechanical characterization is presented with an attempt to assess the sensitivity of each method to crosslinking-mediated changes using data from the experimental and clinical literature. Biomechanical measurement methods differ in spatial resolution, mechanical sensitivity, suitability for detecting crosslinking subtypes, and translational readiness and are central to the effort to understand the mechanistic link between crosslinking methods and clinical outcomes of candidate therapies. Data on differences in the biomechanical effect of different crosslinking protocols and their correspondence to clinical outcomes are reviewed, and strategies for leveraging measurement advances predicting clinical outcomes of crosslinking procedures are discussed. Advancing the understanding of ophthalmic crosslinking, its biomechanical underpinnings, and its applications supports the development of next-generation crosslinking procedures that optimize therapeutic effect while reducing complications.

The Role of Corneal Biomechanics in the Assessment of Ectasia Susceptibility Before Laser Vision Correction

Purpose

To describe the tomographic and corneal biomechanical status of a sample of eyes excluded from LVC and to present the differences in biomechanical behavior in relation to cutoffs of clinical- and tomography-based screening methods used in clinical practice.

Patients and Methods

Observational cross-sectional study including 61 eyes from 32 consecutive patients who were excluded from LVC in our department. Clinical and demographic data were collected from the patients’ clinical records. Tomographic data was assessed with a Scheimpflug camera (Pentacam, OCULUS^®). Ablation depth (µm) and residual stromal bed (µm) were calculated by the WaveLight^® EX500 laser system software (Alcon, EUA). The corneal biomechanical assessment was made through ultra-high speed Scheimpflug imaging during noncontact tonometry (Corvis ST, OCULUS^®). Several ectasia risk scores were analyzed.

Results

Mean age was 31.0±6 years old and mean manifest spherical equivalent was −2.01 ± 2.3D. Belin–Ambrósio deviation index was the tomographic parameter with higher proportion of eyes within the ectasia high risk interval. In the biomechanical assessment, more than 95% of eyes met the criteria for ectasia susceptibility in four of the first generation and in two of the second generation parameters. In a cutoff based comparative analysis, eyes with Kmax ≥45.5 D, eyes with VCOMA <0 and eyes with ARTmax ≤350 presented significantly softer corneal biomechanical behavior.

Conclusion

The majority of eyes excluded from LVC in the present study met the criteria for ectasia susceptibility in several biomechanical parameters, validating the clinical and tomographic based screening prior to LVC in our center. Differences found in the biomechanical assessment regarding cutoffs used in clinical practice highlight its differential role in characterizing risk profile of these patients. Tomography should not be overlooked and the integration of all data, including treatment-related parameters, can be the future of risk ectasia screening prior LVC.

Association between Corneal Stiffness Parameter at the First Applanation and Keratoconus Severity

Objective

The study aimed to evaluate the character of corneal stiffness parameter at the first applanation (SP-A1) in normal and keratoconus eyes and explore the association between SP-A1 and keratoconus severity indicators.

Methods

A total of 351 normal and 351 keratoconus eyes were included in the current study. Keratoconus was diagnosed according to the corneal topography map and slit-lamp examination. The severity of keratoconus was classified to mild (steep keratometry (Ks) < 48D), moderate (48 ≤ Ks < 55D), and severe (Ks ≥ 55D). The SP-A1 was measured using the Corvis ST software. The correlation analyses and receiver operating characteristic (ROC) curve were performed in the current analysis.

Results

The SP-A1 values of keratoconus were lower than that of normal eyes (72.11 (57.02, 83.08) mmHg/mm vs 110.89 (100.45, 122.47) mmHg/mm, P < 0.001). With the severity of keratoconus increasing, the SP-A1 decreased and the value of SP-A1 was 79.54 (70.30, 90.93) mmHg/mm, 65.11 (53.14, 77.46) mmHg/mm, and 47.59 (37.50, 62.14) mmHg/mm in mild, moderate, and severe keratoconus eyes, respectively (P < 0.001). The negative association between SP-A1 and Ks was found in mild, moderate, and severe keratoconus eyes (r _mild = -0.171, r _moderate = -0.317, r _severe = -0.288, all P < 0.05). A positive association between SP-A1 and the thinnest corneal thickness (TCT) was found in all eyes (r_normal = 0.687, r _mild = 0.519, r _moderate = 0.488, r _severe = 0.382, all P < 0.05). SP-A1 was found to be statistically positively associated with intraocular pressure (IOP), biomechanical corrected IOP (bIOP), time from the initiation of air puff until the first applanation (A1T), corneal velocity at the second applanation (A2V), and negatively associated with deformation amplitude (DA), peak distance (PD), corneal velocity at the first applanation (A1V), time from the initiation of air puff until the second applanation (A2T), and DA Ratio Max [2 mm] both in normal and keratoconus eyes (all P < 0.05). The ROC analysis indicated that the AUC (95% CI) of SP-A1 was 0.952 (0.934-0.967) and 0.930 (0.904-0.951) in detecting keratoconus eyes and mild keratoconus eyes from normal eyes, respectively.

Conclusions

The SP-A1 value decreased while the keratoconus severity increased. It was lower in keratoconus than that in normal eyes and could be helpful in identifying keratoconus eyes from normal eyes. Further researches would be warranted to expand the clinical utility of SP-A1.

Ectatic diseases

Ectatic corneal disease (ECD) comprises a group of disorders characterized by progressive thinning and subsequent bulging of the corneal structure. Different phenotypes have been recognized, including keratoglobus, pellucid marginal degeneration (PMD), and keratoconus (KC). Keratoconus has been widely investigated throughout the years, but the advent of laser refractive surgery boosted an immediate need for more knowledge and research about ectatic diseases. This article discusses nomenclature of ectatic disease, etiology and pathogenesis, along with treatment options, with special focus ok KC and forme fruste keratoconus.

Correlation Between Corneal Biomechanical Indices and the Severity of Keratoconus

PURPOSE

To investigate the correlations between the biomechanical indices determined in Scheimpflug-based corneal biomechanical assessments and the severity of keratoconus (KC) based on corneal tomographic assessments in patients with different stages of KC.

METHODS

Fifty-three patients who presented with clinical KC in 1 eye and KC suspect in the fellow eye were included. Corneal tomographic and biomechanical assessments were performed using the Pentacam HR and Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany). Correlations between the tomographic indices and biomechanical indices were assessed, including the anterior radius of curvature (ARC) and posterior radius of curvature (PRC) at a 3.0-mm optical zone and the thinnest pachymetry (Tmin), deformation amplitude ratio max 2 mm (DAR2mm), integrated radius, stiffness parameter at the first applanation, and linear Corvis Biomechanical Index (beta).

RESULTS

DAR2mm correlated negatively with ARC (R = -0.722), PRC (R = -0.677), and Tmin (R= -0.650) (P < 0.001 for all). Integrated radius correlated negatively with ARC (R = -0.700), PRC (R = -0.668), and Tmin (R= -0.648) (P < 0.001 for all). Stiffness parameter at the first applanation correlated positively with ARC (R = 0.622), PRC (R = 0.601), and Tmin (R = 0.703) (P < 0.001 for all). The Corvis Biomechanical Index beta correlated negatively with ARC (R = -0.754), PRC (R = -0.755), and Tmin (R= -0.765) (P < 0.001 for all).

CONCLUSIONS

Corneal biomechanical indices correlated with corneal tomographic parameters in patients with KC. These findings support the possibility of developing biomechanical-based staging classification for KC in combination with topographic or tomographic indices.

Corneal biomechanical outcome of collagen cross-linking in keratoconic patients evaluated by Corvis ST

PURPOSE

A 6-month evaluation of the topographic and biomechanical changes induced by corneal collagen cross-linking (CXL) in keratoconic eyes using Pentacam and Corvis ST.

DESIGN

Longitudinal prospective case series.

METHODS

In this study, 67 eyes of 67 patients with progressive keratoconus (KCN) treated with "Epithelium-off" CXL were evaluated. Patients with stages 1 or 2 of KCN and a corneal thickness of at least 400 μm at the thinnest point were included. Standard ophthalmologic examinations were carried out for all patients. The topographic and biomechanical measurements of the cornea were obtained by Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany) and Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) preoperatively and 6-month postoperatively.

RESULTS

The mean age of the participants was 21.68 ± 4.23 years. There was significant difference in mean spherical equivalent (SE) before and 6 months after CXL. Uncorrected and best corrected visual acuity improved postoperatively, although not statistically significant. The mean and maximum keratometry showed a significant decrease 6 months after CXL (0.93 ± 0.38 D and 1.43 ± 0.62 D, respectively p < 0.001). Among Corvis ST parameters, first applanation length and velocity (AL1 and AV1) showed statistically significant changes. The radius at highest concavity changed significantly (0.13 ± 0.37 mm mean increase after CXL; p < 0.001). A significant increase was observed in stiffness parameter A1 (SP-A1; p < 0.001) and significant decreases were noted in integrated radius (IR) and deformation amplitude ratio (DAR; p < 0.001).

CONCLUSION

Analyzing biomechanical changes after corneal cross-linking can provide basis for efficient KCN treatment. Corvis ST parameters demonstrated changes in corneal biomechanical characteristics indicative of stiffing after CXL.

Comparison of waveform-derived corneal stiffness and stress-strain extensometry-derived corneal stiffness using different cross-linking irradiances: an experimental study with air-puff applanation of ex vivo porcine eyes

Purpose

To assess corneal stiffening of standard (S-CXL) and accelerated (A-CXL) cross-linking protocols by dynamic corneal response parameters and corneal bending stiffness (Kc[mean/linear]) derived from Corvis (CVS) Scheimpflug-based tonometry. These investigations were validated by corneal tensile stiffness (K[ts]), derived from stress-strain extensometry in ex vivo porcine eyes.

Methods

Seventy-two fresh-enucleated and de-epithelized porcine eyes were soaked in 0.1% riboflavin solution including 10% dextran for 10 min. The eyes were separated into four groups: controls (n = 18), S-CXL (intensity in mW/cm²*time in min; 3*30) (n = 18), A-CXL (9*10) (n = 18), and A-CXL (18*5) (n = 18), respectively. CXL was performed using CCL Vario. CVS measurements were performed on all eyes. Subsequently, corneal strips were extracted by a double-bladed scalpel and used for stress-strain measurements. K[ts] was calculated from a force-displacement curve. Mean corneal stiffness (Kc[mean]) and constant corneal stiffness (Kc[linear]) were calculated from raw CVS data.

Results

In CVS, biomechanical effects of cross-linking were shown to have a significantly decreased deflection amplitude as well as integrated radius, an increased IOP, and SP A1 (P < 0.05). Kc[mean]/Kc[linear] were significantly increased after CXL (P < 0.05). In the range from 2 to 6% strain, K[ts] was significantly higher in S-CXL (3*30) compared to A-CXL (9*10), A-CXL (18*5), and controls (P < 0.05). At 8% to 10% strain, all protocols induced a higher stiffness than controls (P < 0.05).

Conclusion

Several CVS parameters and Kc[mean] as well as Kc[linear] verify corneal stiffening effect after CXL on porcine eyes. S-CXL seems to have a higher tendency of stiffening than A-CXL protocols have, which was demonstrated by Scheimpflug-based tonometry and stress-strain extensometry.

The Role of Corneal Biomechanics for the Evaluation of Ectasia Patients

Purpose: To review the role of corneal biomechanics for the clinical evaluation of patients with ectatic corneal diseases. Methods: A total of 1295 eyes were included for analysis in this study. The normal healthy group (group N) included one eye randomly selected from 736 patients with healthy corneas, the keratoconus group (group KC) included one eye randomly selected from 321 patients with keratoconus. The 113 nonoperated ectatic eyes from 125 patients with very asymmetric ectasia (group VAE-E), whose fellow eyes presented relatively normal topography (group VAE-NT), were also included. The parameters from corneal tomography and biomechanics were obtained using the Pentacam HR and Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany). The accuracies of the tested variables for distinguishing all cases (KC, VAE-E, and VAE-NT), for detecting clinical ectasia (KC + VAE-E) and for identifying abnormalities among the VAE-NT, were investigated. A comparison was performed considering the areas under the receiver operating characteristic curve (AUC; DeLong’s method). Results: Considering all cases (KC, VAE-E, and VAE-NT), the AUC of the tomographic-biomechanical parameter (TBI) was 0.992, which was statistically higher than all individual parameters (DeLong’s; p < 0.05): PRFI- Pentacam Random Forest Index (0.982), BAD-D- Belin -Ambrosio D value (0.959), CBI -corneal biomechanical index (0.91), and IS Abs- Inferior-superior value (0.91). The AUC of the TBI for detecting clinical ectasia (KC + VAE-E) was 0.999, and this was again statistically higher than all parameters (DeLong’s; p < 0.05): PRFI (0.996), BAD-D (0.995), CBI (0.949), and IS Abs (0.977). Considering the VAE-NT group, the AUC of the TBI was 0.966, which was also statistically higher than all parameters (DeLong’s; p < 0.05): PRFI (0.934), BAD- D (0.834), CBI (0.774), and IS Abs (0.677). Conclusions: Corneal biomechanical data enhances the evaluation of patients with corneal ectasia and meaningfully adds to the multimodal diagnostic armamentarium. The integration of biomechanical data and corneal tomography with artificial intelligence data augments the sensitivity and specificity for screening and enhancing early diagnosis. Besides, corneal biomechanics may be relevant for determining the prognosis and staging the disease.

Biomechanical diagnostics of the cornea

Corneal biomechanics has been a hot topic for research in contemporary ophthalmology due to its prospective applications in diagnosis, management, and treatment of several clinical conditions, including glaucoma, elective keratorefractive surgery, and different corneal diseases. The clinical biomechanical investigation has become of great importance in the setting of refractive surgery to identify patients at higher risk of developing iatrogenic ectasia after laser vision correction. This review discusses the latest developments in the detection of corneal ectatic diseases. These developments should be considered in conjunction with multimodal corneal and refractive imaging, including Placido-disk based corneal topography, Scheimpflug corneal tomography, anterior segment tomography, spectral-domain optical coherence tomography (SD-OCT), very-high-frequency ultrasound (VHF-US), ocular biometry, and ocular wavefront measurements. The ocular response analyzer (ORA) and the Corvis ST are non-contact tonometry systems that provide a clinical corneal biomechanical assessment. More recently, Brillouin optical microscopy has been demonstrated to provide in vivo biomechanical measurements. The integration of tomographic and biomechanical data into artificial intelligence techniques has demonstrated the ability to increase the accuracy to detect ectatic disease and characterize the inherent susceptibility for biomechanical failure and ectasia progression, which is a severe complication after laser vision correction.