Corneal Biomechanical Metrics and Anterior Segment Parameters in Mild Keratoconus

The Utilization of Brillouin Microscopy in Corneal Diagnostics: A Systematic Review

Corneal biomechanical data has been used since 2005 to screen for keratoconus and corneal ectasia by corneal specialists. Older technology uses force applanation techniques over a 3 mm area in the central cornea, making it highly dependent on extraneous variables and unable to calculate the elasticity of the tissue. Brillouin microscopy is a newer method that uses a natural shift in the frequency of light as it passes through a material. This frequency shift can be used to estimate the viscoelasticity of the tissue. The advantage of Brillouin microscopy is that it can create a full three-dimensional (3D) map of the entire cornea without direct contact. A literature search was conducted using the databases PubMed, Google Scholar, and Ovid regarding the applications of Brillouin microscopy in corneal diagnostics. A final total of 16 articles was included describing the various ex vivo and in vivo studies conducted using Brillouin microscopy. Applications of this technology spanned from keratoconus diagnosis to post-corneal refractive surgery evaluation. All studies evaluated corneal biomechanics and other corneal properties through the quantification of Brillouin frequency shifts. Many of the studies found that this diagnostic device is capable of detecting subtle changes in corneal thickness and biomechanics in keratoconic corneas at a high level of specificity and sensitivity. However, limitations of Brillouin microscopy may include the duration of time required for use and fluctuations in accuracy depending on the corneal hydration state. Future technology seems to be geared toward a combination of optical coherence tomography (OCT) and Brillouin microscopy, using OCT as a three-dimensional pupil-tracking modality. Further research and understanding of the technology involved will lead to better care of patients in the field of ophthalmology.

Multi-modal imaging for the detection of early keratoconus: a narrative review

Keratoconus is a common progressive corneal disorder that can be associated with significant ocular morbidity. Various corneal imaging techniques have been used for the diagnosis of established cases. However, in the early stages of the disease, which include subclinical keratoconus and forme fruste keratoconus, detection of such cases can be challenging. The importance of detecting such cases is very important because early intervention can halt disease progression, improve visual outcomes and prevent postrefractive surgery ectasia associated with performing corneal refractive procedures in such patients. This narrative review aimed to examine several established and evolving imaging techniques for the detection of early cases of keratoconus. The utilization of combinations of these techniques may further increase their diagnostic ability.

New forays into measurement of ocular biomechanics

PURPOSE OF REVIEW

The field of corneal biomechanics has rapidly progressed in recent years, reflecting technological advances and an increased understanding of the clinical significance of measuring these properties. This review will evaluate in-vivo biomechanical properties obtained by current technologies and compare them regarding their relevance to established biomechanical properties obtained by gold-standard ex-vivo techniques normally conducted on elastic materials.

RECENT FINDINGS

Several new technologies have appeared in recent years, including vibrational optical coherence tomography (VOCT) and the corneal indentation device (CID). These techniques provide promising new opportunities for minimally invasive and accurate measurements of corneal viscoelastic properties.

SUMMARY

Alterations in corneal biomechanics are known to occur in several corneal degenerative diseases and after refractive surgical procedures. The measurement of corneal biomechanical properties has the capability to diagnose early disease and monitor corneal disease progression. Several new technologies have emerged in recent years, allowing for more accurate and less invasive measurements of corneal biomechanical properties, most notably the elastic modulus.

Corneal biomechanics in early diagnosis of keratoconus using artificial intelligence

Keratoconus is a blinding eye disease that affects activities of daily living; therefore, early diagnosis is crucial. Great efforts have been made toward an early diagnosis of keratoconus. Recent studies have shown that corneal biomechanics is associated with the occurrence and progression of keratoconus. Hence, detecting changes in corneal biomechanics may provide a novel strategy for early diagnosis. However, an early keratoconus diagnosis remains challenging due to the subtle and localized nature of its lesions. Artificial intelligence has been used to help address this problem. Herein, we reviewed the literature regarding three aspects of keratoconus (keratoconus, early keratoconus, and keratoconus grading) based on corneal biomechanical properties using artificial intelligence. Furthermore, we summarized the current research progress, limitations, and possible prospects.

Differences in anterior chamber depth in keratoconus patients with binocular very asymmetry ectasia

BACKGROUND

To evaluate the difference in anterior chamber depth (ACD) between two eyes among keratoconus patients with binocular very asymmetric ectasia (VAE) and to explore the influencing factors.

METHODS

The corneal curvature and ACD in both eyes of patients with VAE were measured by Sirius (version 3.2, CSO, Italy) at the following points: corneal vertex, maximum curvature (apex), and the 1.5 mm, 2.5 mm, and 3.5 mm superior-, inferior-, nasal-, temporal-paracentral from center. The mean pupil power (MPP) and corneal morphology parameters were also measured. Correlations between ACD and curvature and morphology parameters were analyzed by linear regression.

RESULTS

172 eyes of 86 patients (9 to 45 years) were classified into the VAE-N (n = 86) group and the VAE-E group (n = 86) based on the corneal morphology. The central (3.32 ± 0.27 mm versus 3.43 ± 0.29 mm, P < 0.001) and paracentral ACDs increased significantly in the VAE-E group, and the corneal morphology parameters were also significantly higher. The central ACD was significantly correlated with the MPP (r = 0.465), KVf/b (Keratoconus Vertex front/back) (r = 0.306, r = 0.327), and BCVf/b (Baiocchi Calossi Versaci front/back) (r = 0.356, r = 0.416). Linear regression showed good relationships between △ACD and △MPP (R2 = 0.429) and △KVf/b (R2 = 0.504, R2 = 0.536).

CONCLUSIONS

The ACD was larger in the VAE-E group. The difference in ACD between the VAE-E and VAE-N groups was significantly correlated with corneal curvature and the extent of corneal elevation, indicating the influences of both the corneal magnification effect and corneal ectasia on ACD.

Longitudinal Changes of Cornea Volume Measured by Means of Anterior Segment-Optical Coherence Tomography in Patients with Stable and Progressive Keratoconus

Keratoconus is a corneal disease which results in progressive thinning and protrusion of the cornea leading to irregular astigmatism. The purpose of this study was to evaluate longitudinal changes in corneal volume (CV) occurring over time in keratoconus eyes. Consecutive patients affected by keratoconus were evaluated by means of anterior segment-optical coherence tomography (AS-OCT) at two different time points: baseline (T0) and after 1 year (T1). Anterior and posterior refractive value; corneal thickness at the thinnest point (TP) and corneal volume (CV) calculated within discs of 3, 5 and 8 mm of diameter; anterior chamber depth (ACD); and anterior chamber volume (ACV) were obtained. Enrolled patients were divided into 3 groups (groups 1, 2, 3) according to the increasing disease severity and into 2 groups (groups A, B) according to the progression or stability of the disease. Overall, 116 eyes of 116 patients (76 males and 40 females, mean age 34.76 ± 13.99 years) were included. For the entire group of keratoconus patients, in comparison with T0, mean TP decreased at T1 from 458.7 ± 52.2 µm to 454.6 ± 51.6 µm (p = 0.0004); in parallel, mean value of CV calculated at 5 mm and 8 mm decreased significantly (from 10.78 ± 0.8 at T0 to 10.75 ± 0.79 at T1 (p = 0.02), and from 32.03 ± 2.01 mm3 at T0 to 31.95 ± 1.98 at T1 (p = 0.02), respectively). Conversely, there were no statistically significant differences in CV at 3 mm from T0 to T1 (p = 0.08), as well as for ACD and ACV. Regarding the course of the disease, patients belonging to group A showed statistically significant differences from T0 to T1 for TP, and for CV at 3 mm, 5 mm and 8 mm (p < 0.0001, p < 0.0001, p < 0.001 and p = 0.0058 respectively). There were no statistically significant differences for ACD (p = 0.6916) and ACV calculated at 3, 5 and 8 mm (p = 0.7709, p = 0.3765, p = 0.2475, respectively) in group A. At the same time, no statistically significant differences for ACD (p = 0.2897) and ACV calculated at 3, 5 and 8 mm (p = 0.9849, p = 0.6420, p = 0.8338, respectively) were found in group B. There were statistically significant positive correlations between changes of TP and CV at 3 mm (r = 0.6324, p < 0.0001), 5 mm (r = 0.7622, p < 0.0001) and 8 mm (r = 0.5987 p < 0.0001). In conclusion, given the strong correlation with TP, CV might be considered an additional AS-OCT parameter to be used in association with conventional parameters when detecting longitudinal changes in keratoconic eyes.

Intereye Differences in the Clinical Assessment of Intraocular Pressure and Ocular Biomechanics

SIGNIFICANCE

Clinicians and researchers will have evidence whether intereye differences confound clinical measurements of intraocular pressure or of ocular biomechanical parameters.

PURPOSE

The purpose of this study was to determine whether intraocular pressure and biomechanical parameters, as measured by the Ocular Response Analyzer (ORA) and by Cornea Visualization with Scheimpflug Technology (CorVis ST), are different between the first and second eye measured.

METHODS

Intraocular pressure and biomechanical parameters were collected from both eyes of healthy participants (N = 139). The ORA measured corneal-compensated intraocular pressure, Goldmann-correlated intraocular pressure, and corneal hysteresis. The CorVis ST measured biomechanically corrected intraocular pressure, stiffness parameter at first applanation, and stiffness parameter at highest concavity. For each measurement, a paired t test compared the value of the first eye measured against that of the second eye measured.

RESULTS

For the ORA, Goldmann-correlated intraocular pressure was significantly higher ( P = .001) in the first eye (14.8 [3.45] mmHg) than in the second eye (14.3 [3.63] mmHg). For the CorVis ST, biomechanically corrected intraocular pressure was significantly higher ( P < .001) in the second eye (14.7 [2.14] mmHg) than in the first eye (14.3 [2.11] mmHg). Stiffness parameter at first applanation (intereye difference, 6.85 [9.54] mmHg/mm) was significantly ( P < .001) higher in the first eye than in the second eye. Stiffness parameter at highest concavity was significantly higher ( P = .01) in the second eye (14.3 [3.18] mmHg/mm) than in the first eye (14.0 [3.13] mmHg/mm).

CONCLUSIONS

Although there were statistically significant intereye differences in intraocular pressure and in biomechanical parameters for both devices, the variations were small and thus unlikely to affect clinical outcomes.

Motion-Tracking Brillouin Microscopy Evaluation of Normal, Keratoconic, and Post-Laser Vision Correction Corneas

PURPOSE

To characterize focal biomechanical differences between normal, keratoconic, and post-laser vision correction (LVC) corneas using motion-tracking Brillouin microscopy.

DESIGN

Prospective cross-sectional study.

METHODS

Thirty eyes from 30 patients (10 normal controls [Controls], 10 post-LVC, and 10 stage I or II keratoconus [KC]) had Scheimpflug and motion-tracking Brillouin microscopy imaging using a custom-built device. Mean, maximum (max) and minimum (min) Brillouin shift, spatial standard deviation, and max-min values were compared. Min values were correlated with local Brillouin values at multiple Scheimpflug imaging locations.

RESULTS

Mean (P < .0003), min (P < .00001), spatial standard deviation (P < .01), and max-min (P < .001) were significantly different between the groups. In post hoc pairwise comparisons, the best differentiators for group comparisons were mean (P = .0004) and min (P = .000002) for Controls vs KC, min (P = .0022) and max-min (P = .002) for Controls vs LVC, and mean (P = .0037) and min (P = .0043) for LVC vs KC. Min (area under the receiver operating characteristic = 1.0) and mean (area under the receiver operating characteristic =  0.96) performed well in differentiating Control and KC eyes. Min values correlated best with Brillouin shift values at the thinnest corneal point (r2 = 0.871, P = .001) and maximum keratometry value identified in the tangential curvature map (r2 = 0.840, P = .002).

CONCLUSIONS

Motion-tracking Brillouin microscopy effectively characterized focal corneal biomechanical alterations in LVC and KC and clearly differentiated these groups from Controls. Primary motion-tracking Brillouin metrics performed well in differentiating groups as compared with basic Scheimpflug metrics, in contrast to previous Brillouin studies, and identified focal changes after LVC where prior Brillouin studies did not.

Detection ability of corneal biomechanical parameters for early diagnosis of ectasia

PURPOSE

To assess the detection ability of corneal biomechanical parameters for early diagnosis of ectasia.

METHODS

This retrospective descriptive-analytical study included 134 normal eyes (control group) from 134 healthy subjects and 128 eyes with asymmetric contralateral corneal ectasia with normal topography (ACE-NT, study group) from 128 subjects with definite keratoconus in the opposite eye. Placido-disk-based corneal topography with TMS-4, Scheimpflug corneal tomography with Pentacam HR, and corneal biomechanical assessment with Corvis ST and ocular response analyzer (ORA) were performed. A general linear model was used to compare Corvis ST and ORA biomechanical parameters between groups, while central corneal thickness (CCT) and biomechanically corrected intraocular pressure (bIOP) were considered covariates. Receiving operator sensitivity curve (ROC) analysis was used to determine the cut-off point with the highest sensitivity and specificity along with the area under the curve (AUC) for each parameter.

RESULT

All parameters of Corvis ST and ORA showed a statistically significant difference between the two groups except for the first (P = 0.865) and second (P = 0.226) applanation lengths, and deformation amplitude (P = 0.936). The discriminative analysis of corneal biomechanical showed that the highest accuracy for the classic, new, and combined parameters of Corvis ST was related to HCR (AUC: 0.766), IR & DAR (0.846), and TBI (0.966), respectively. Using ORA, the corneal resistance factor (0.866) had a higher detection ability than corneal hysteresis (0.826).

CONCLUSIONS

TBI has the best accuracy and the highest effect size for differential diagnosis of normal from ACE-NT eyes with a cut-off point of 0.24.

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.

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.

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.

Evaluation of Corneal Biomechanical Indices in Distinguishing Between Normal, Very Asymmetric, and Bilateral Keratoconic Eyes

PURPOSE

To evaluate the ability of biomechanical indices provided by the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments) and dynamic Scheimpflug analyzer (Corvis ST; Oculus Optikgeräte GmbH) to distinguish between normal eyes and eyes with very asymmetric ectasia (VAE) and mild and moderate keratoconus.

METHODS

This prospective, observational, and monocentric study included normal eyes (defined as keratoconus percentage index < 60, Belin/Ambrósio total deviation value [BAD-D] < 1.6, inferior-superior keratometry [I-S value] < 1.45 and maximum keratometry [Kmax] < 47) and eyes with clinical bilateral keratoconus (mild and moderate) and VAE (defined as unilateral keratoconus, where one eye showed a clinical keratoconus and the fellow eye was topographically normal [VAE-NT] or topographically and tomographically normal [VAE-NTT]). All eyes were measured by the Pentacam (Oculus Optikgeräte GmbH), ORA, and Corvis ST. Receiver operating characteristic curve analysis was performed to test the diagnostic ability.

RESULTS

Fifty-eight normal eyes and 238 ectatic eyes were included. The highest area under the curve (AUC) was provided by the Corvis Biomechanical Index (CBI) with an AUC of 0.979, followed by ORA corneal resistance factor (0.865), and corneal hysteresis (0.824) separating normal eyes from all ectatic subgroups. The AUC of the CBI was statistically significantly higher than all other parameters (DeLong test, P < .001). A sensitivity of 100% and 70.9%, respectively, and a specificity of 93.1% was found to distinguish normal eyes from VAE-NT and VAE-NTT using a cut-off value of 0.2.

CONCLUSIONS

The assessment of biomechanical properties is an additional important method to evaluate corneal ectasia independent of its stage. The CBI provides further information for ectasia screening in cases where corneal topography and tomography are clinically not suspicious by using a cutoff of 0.2. [J Refract Surg. 2022;38(6):364-372.].

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.

The link between Keratoconus and posterior segment parameters: An updated, comprehensive review

Keratoconus (KCN) has been typically known as a disorder with effects limited to the cornea. Because of this viewpoint, less attention has been devoted to its effects on the posterior segment structures. We aimed to provide a comprehensive review of the literature to understand the potential link between KCN and posterior segment structures and their functions. It is clear from the extensive evidence in the literature that KCN can be associated with morphological and functional changes in different parts of the posterior segment. It is worth noting that anatomical changes have been not only noted in several layers of the retina but also in the optic nerve head and the choroid. Several mechanisms have been proposed to explain this observation, including incidents induced by oxidative stress in keratoconic corneas and retinal adaptions to the distorted image that lands on the retina. Consequently, when KCN has been diagnosed, it seems practical to consider assessing the retinal and choroidal profile using optical coherence tomography and potentially functional abnormalities through electrophysiology procedures.

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.

Association between diabetes and keratoconus-a systematic review and meta-analysis

PURPOSE

To assess the association between diabetes mellitus and keratoconus.

METHODS

PubMed, Google Scholar, Web of Science, and Scopus databases were searched for literature on the association between diabetes and keratoconus. The last literature search was conducted on April 4, 2021. A secondary form of the literature search was conducted by manually scanning the reference list of retrieved eligible articles. Included studies were cohort, case-control, or cross-sectional study design that used odds ratio or risk ratio to evaluate the relationship between keratoconus and diabetes. Egger's test was used to assess the presence of publication bias. The quality of eligible studies was assessed using the Newcastle-Ottawa Scale.

RESULTS

Nine studies (six case-control and three cohort studies) published between 2000 and 2021 were included. The total number of keratoconus patients and controls were 27,311 and 53,732. respectively. Meta-analysis revealed no significant association between diabetes mellitus and keratoconus; the pooled odds ratio was 0.87 (95% confidence interval: 0.66-1.14; p = 0.314). There was significant heterogeneity (Q (df = 7) = 33.36, p < 0.001;I² = 79.01, p < 0.001). Age of participants (p < 0.0001), study design (p < 0.001), and sample size (p = 0.024) were significant sources of heterogeneity. There was no evidence of publication bias.

CONCLUSION

The current meta-analysis revealed no significant association between diabetes mellitus and keratoconus. Well-designed longitudinal prospective studies are, however, needed to investigate any association between diabetes mellitus and keratoconus.

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.

The effect of corneal crosslinking on the rigidity of the cornea estimated using a modified algorithm for the Schiøtz tonometer

Purpose:

The aim of this study was to test a method for estimating corneal rigidity before and after cross-linking (CXL) using a Schiøtz tonometer.

Methods:

The study was performed in the Kyiv City Clinical Ophthalmological Hospital “Eye Microsurgical Center”, Ukraine. This was a prospective, consecutive, randomized, masked, case-by-case, clinical study. Corneal rigidity, indicated by the gradient (G) between lg applied weight and corresponding lg scale reading during Schiøtz tonometry, were obtained by increasing (A-mode) then reducing (D-mode) weights by two operators [A] in keratoconus, post-CXL and control subjects for estimation of (i) interoperator and (ii) intersessional errors, (iii) intergroup differences; [B] before and after CXL. Central corneal thickness CCT was measured by scanning slit pachymetry. ANOVA, t tests, linear regression were the statistical tools used.

Results:

Average interoperator difference (ΔG) was –0.120 (SD = ±0.294, 95%CI = –0.175 to –0.066). A significant correlation between ΔG and the mean of each pair of G values was found (r = –0.196, n = 112, P = 0.038). Intersessional differences in mean G values were insignificant (P > 0.05). There was a significant correlation between G at first session (X₁) and difference between sessions (ΔG) [Operator 1, ΔG = 0.598x₁–0.461, r = 0.601, n = 27, P = 0.009]. Significant intergroup differences in G were found (Operator 1, one-way ANOVA, F = 4.489, P = 0.014). The difference (Δ) between the pre-(X₂) and post-CXL treatment G values was significantly associated with the pre-CXL treatment value (Operator 1, Δ = 1.970x₂-1.622, r = 0.642, n = 18, P = <.001). G values were correlated with CCT in keratoconus and post-CXL.

Conclusion:

Corneal rigidity (G) estimated using the Schiøtz tonometer can be useful for detecting changes after CXL. However, G values are linked to CCT, can vary from time-to-time and the procedure is operator dependent.

Excimer laser tissue interactions in the cornea

Excimer lasers induces significant changes to corneal structure and corneal biomechanics. The aim of this paper is to describe all laser-tissue interactions which are relevant for clinical practice, particularly, we will focus on laser ablations profiles, causes of regression and haze and prevention of those. At last the manuscript will describe the impact on corneal biomechanics of different Laser Vision Corrections techniques.

Relationship between corneal biomechanical parameters and corneal sublayer thickness measured by Corvis ST and UHR-OCT in keratoconus and normal eyes

BACKGROUND

To explore the relationship between corneal biomechanical parameters and corneal sublayer thickness using corneal visualization Scheimpflug technology (Corvis ST) and ultrahigh-resolution optical coherence tomography (UHR-OCT) in clinical and suspected keratoconus and normal eyes.

METHODS

Cross-sectional prospective study. A total of 94 eyes of 70 participants were recruited. Twenty five eyes of 19 keratoconus patients, 52 eyes of 34 patients showing high risk of developing keratoconus according to the Belin/Ambrosio Enhanced Ectasia Display, and each eye of 17 normal subjects were enrolled. All participants underwent Corvis ST, Pentacam, and UHR-OCT examinations at the same time. Stiffness parameter A1 (SP-A1), deformation amplitude ratio (DA ratio), and other biomechanical parameters were recorded using Corvis ST. The vertical and horizontal thickness profiles of central 3 mm corneal epithelium, Bowman's layer, and stroma as measured by the perpendicular distance between the neighboring interfaces were generated using UHR-OCT. The flat keratometry and steep keratometry were obtained using Pentacam. Analysis of correlation was applied to explore the association between variables.

RESULTS

Most of the biomechanical parameters and corneal sublayer thickness profiles showed statistical differences among three groups. A statistically significant linear relationship was noted between SP-A1 and DA ratio in all three groups. SP-A1 was found to be positively correlated with epithelial and Bowman's layer thickness in the keratoconus (KC) group, and with stromal thickness in all three groups. In the normal and suspected keratoconus (SKC) groups, only stromal thickness was included in the stepwise linear regression to predict SP-A1, whereas in the KC group, steep keratometry and Bowman's layer thickness were included.

CONCLUSIONS

Significant and different correlations were noted between corneal stiffness and corneal sublayer thickness in different groups, indicating that corneal sublayers may play different roles in maintaining corneal biomechanical stability between keratoconus and normal eyes.

A Combined Biomechanical and Tomographic Model for Identifying Cases of Subclinical Keratoconus

PURPOSE

To develop a combined biomechanical and tomographic model for identifying eyes with subclinical keratoconus (SKC) that are categorized as normal or borderline in the Pentacam Belin/Ambrósio Enhanced Ectasia Display.

METHODS

This case-control study comprised 62 eyes with SKC and randomly selected eyes of 186 age-matched healthy controls. SKC was defined as the presence of the following: 1) normal topography, topometric indices, and slit lamp; 2) normal or borderline Belin/Ambrósio Enhanced Ectasia Display D index, back and front elevation difference; and 3) keratoconus in the fellow eye. Stepwise logistic regression analysis was performed to identify the best variable combination for detecting SKC cases from Ocular Response Analyzer and Pentacam parameters. Receiver operating characteristic curve analysis was used to determine the predictive accuracy [area under the curve (AUC)] of the model. Based on the predictors in the final logistic regression model, a linear equation was derived using the discriminant function analysis.

RESULTS

The final model (AUC: 0.948, sensitivity: 87.1%, and specificity: 91.4%) chose corneal hysteresis (CH) and D index from a total of 63 candidate variables. The final model had a higher AUC compared with D (0.933, P = 0.053) and CH (0.80, P < 0.001) alone. According to the discriminant function analysis, a higher CH was required with increasing D index to classify an eye as normal.

CONCLUSIONS

The proposed combined model provided varying cutoffs for CH and D as a function of the other. The probability plot as a function of CH and D index may be used for identifying eyes with SKC.

Comprehensive evaluation of corneas from normal, forme fruste keratoconus and clinical keratoconus patients using morphological and biomechanical properties

OBJECTIVE

To more comprehensively evaluate the ability of the parameters reflecting the morphological and biomechanical properties of the cornea to distinguish clinical keratoconus (CKC) and forme fruste keratoconus (FFKC) from normal.

METHODS

Normal eyes (n = 50), CKC (n = 45) and FFKC (n = 15) were analyzed using Pentacam, Corvis ST and ORA. Stepwise logistic regression of all parameters was performed to obtain the optimal combination model capable of distinguishing CKC, FFKC from normal, named SLR1 and SLR2, respectively. Receiver operating characteristic (ROC) curves were applied to determine the predictive accuracy of the parameters and the two combination models, as described by the area under the curve (AUC). AUCs were compared using the DeLong method.

RESULTS

The SLR1 model included only the TBI output by Pentacam, while the SLR2 model included the morphological parameter F.Ele.Th and two parameters from the Corvis ST, HC DfA and SP-A1. The majority of the parameters had sufficient strength to differentiate the CKC from normal corneas, even the seven separate parameters and the SLR1 model had a discrimination efficiency of 100%. The predictive accuracy of the parameters was moderate for FFKC, and the SLR2 model (0.965) presented an excellent AUC, followed by TBI, F.Ele.Th and BAD-D.

CONCLUSION

The F.Ele.Th from Pentacam was the most sensitive morphological parameter for FFKC, and the combination of F.Ele.Th, HC DfA and SP-A1 made the diagnosis of FFKC more efficient. The CRF and CH output by ORA did not improve the combined diagnosis, despite the corneal combination of morphological and biomechanical properties that optimized the diagnosis of FFKC.

Study on change in corneal biomechanics and effect of percent tissue altered in myopic laser-assisted in situ keratomileusis

Purpose

To evaluate corneal biomechanical changes and their correlation with the percentage of tissue altered (PTA) in myopic femtosecond (FS)-flap LASIK.

Methods

Prospective longitudinal observational study of 80 eyes of FS LASIK. Demographic details, LASIK parameters, preoperative and postoperative (day 1, month 1, 3, and 6), UCVA, BCVA, refraction, corneal topography, corneal hysteresis (CH), and a corneal resistance factor (CRF) were noted. Change in CH and CRF and its correlation with PTA were analyzed. Data were analyzed in three subgroups [subgroup 1: PTA 23 to <27%; subgroup 2: 27 to <33%; subgroup 3: 33 to <40%].

Results

FS LASIK for MRSE -3.5D ± 1.6D with mean PTA of 31.6 ± 4.4% (range 23.8-39.8%), showed statistically significant decrease in CH and CRF. Mean CH decreased from a preoperative value of 10.4 ± 1.9 to 8.1 ± 1.1; mean CRF from 10.5 ± 1.6 to 7.5 ± 1.3 at 6-months postoperative period, respectively. Mean preoperative CH decreased by 25%, 24%, 23%, and 21% and mean preoperative CRF decreased by 34%, 28%, 28%, and 28% at postoperative day 1, month 1, 3, and 6 follow-ups. Mean CH and CRF showed a significant negative correlation with PTA (CH: r = - 0.33 [P = <0.0001], CRF: r = -0.34 [P = <0.001]. Subgroup analysis noted greater decrease in CRF and CH in eyes with higher PTA (subgroup 3).

Conclusion

Myopic FS LASIK causes a decrease in corneal biomechanics with a significant negative correlation with PTA indicating a greater decrease in corneal biomechanics with higher PTA.

Age-related variation in corneal biomechanical parameters in healthy Indians

Purpose:

To report age-related variations in corneal biomechanical parameters in healthy Indians.

Methods:

A retrospective study where healthy Indian individuals aged between 5 and 70 years having undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2018 and having best corrected visual acuity of 20/20 were enrolled. Subjects with central corneal thickness <500 microns, intra-ocular pressure (IOP) ≥ 22 mmHg, refractive error ≥ 6.00D, history of any systemic and ocular disease, previous ocular surgery, poor scans quality, and subjects with any missing data were also excluded. Corneal biomechanical parameters were noted and compared across different age groups.

Results:

Total of 3125 eyes had undergone the Corvis ST analysis. After applying exclusion criteria, 718 right eyes of 718 patients were included for the analysis and were further divided into different age groups as per each decade (sample size), such as 5-10 (37), 11-20 (113), 21-30 (396), 31-40 (116), 41-50 (39), 50 and above (17). All the subjects were matched for IOP and central corneal thickness (p > 0.05). A total of 19 out of 26 corneal biomechanical parameters were significantly different across age groups (p < 0.05). Vinciguerra screening parameters, such as deformation amplitude ratio max, biomechanically corrected IOP, and stiffness parameter A1 were significantly different across different age groups (p < 0.05).

Conclusion:

Corneal biomechanical parameters are affected by age as cornea becomes progressively stiffer. The information reported here would serve as a reference for future corneal biomechanical researches and would help in differentiating the abnormal eyes from normal healthy eyes.

Relationship between Corneal Morphogeometrical Properties and Biomechanical Parameters Derived from Dynamic Bidirectional Air Applanation Measurement Procedure in Keratoconus

The morphogeometric analysis of the corneal structure has become a clinically relevant diagnostic procedure in keratoconus (KC) as well as the in vivo evaluation of the corneal biomechanical properties. However, the relationship between these two types of metrics is still not well understood. The current study investigated the relationship of corneal morphogeometry and volume with two biomechanical parameters: corneal hysteresis (CH) and corneal resistance factor (CRF), both provided by an Ocular Response Analyzer (Reichert). It included 109 eyes from 109 patients (aged between 18 and 69 years) with a diagnosis of keratoconus (KC) who underwent a complete eye examination including a comprehensive corneal topographic analysis with the Sirius system (CSO). With the topographic information obtained, a morphogeometric and volumetric analysis was performed, defining different variables of clinical use. CH and CRF were found to be correlated with these variables, but this correlation was highly influenced by corneal thickness. This suggests that the mechanical properties of KC cornea contribute only in a partial and limited manner to these biomechanical parameters, being mostly influenced by morphogeometry under normal intraocular pressure levels. This would explain the limitation of CH and CRF as diagnostic tools for the detection of incipient cases of KC.

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.

Obstructive Sleep Apnea and Keratoconus: A Systematic Review and Meta-analysis

BACKGROUND

Despite the increasing number of studies focused on the association between obstructive sleep apnea (OSA) and keratoconus (KC), to date, no comprehensive meta-analysis or systematic review was published.

OBJECTIVE

The objective of this study was to evaluate the association between OSA and KC.

DATA SOURCES

Sources of data were PubMed, Scopus, and Web of Science databases.

STUDY ELIGIBILITY CRITERIA

The criteria for study eligibility were case-control studies and cohort studies reporting data on the association of OSA with KC with risk ratio, odds ratio, or hazard ratio with 95% confidence intervals or sufficient raw data for calculation.

STUDY APPRAISAL AND SYNTHESIS METHODS

Meta-analysis was conducted with a random-effects model using odds ratio with 95% confidence interval as the effect size. Heterogeneity was evaluated using the Q and I tests. Sensitivity analysis and assessment of publication bias were performed.

RESULTS

Five studies (four case-control studies and one cohort study) published between 2012 and 2016 and involving 33,844 subjects (16,922 patients with KC, 16,922 controls) were included in this meta-analysis. A significant association between OSA and KC has been shown (pooled odds ratio, 1.841; 95% confidence interval, 1.163 to 2.914; P = .009). A significant heterogeneity was observed (Q = 15.8, I = 74.6%). There was no evidence of significant publication bias (P = .07). The sensitivity analyses indicated the stability of results.

LIMITATIONS

Heterogeneity across the studies was observed. Data from four hospital-based case-control studies and one large population-based cohort study were combined. Most of the included studies ascertained OSA by the Berlin Questionnaire, which is a screening tool.

CONCLUSIONS

This meta-analysis provides significant evidence that OSA is associated with KC. Therefore, a proper screening for OSA is warned for KC patients for the prevention of various cardiovascular comorbidities. Further prospective studies are warranted to explore more in-depth the casual relationship between the two conditions.

Corneal Topographic, Anatomic, and Biomechanical Properties in Severe Obstructive Sleep Apnea-Hypopnea Syndrome

PURPOSE

To determine corneal topographic, anatomic, and biomechanical properties in patients newly diagnosed with severe obstructive sleep apnea-hypopnea syndrome (OSAHS).

METHODS

This is a cross-sectional study including 25 patients recently diagnosed with severe OSAHS (apnea-hypopnea index above 30) and a paired control group of 25 healthy subjects. All patients underwent a complete eye examination with an elevation topography Pentacam Scheimpflug study and a study with Reichert Ocular Response Analyzer, collecting several topographic, anatomic, and biomechanical variables.

RESULTS

Fifty eyes of 25 patients (23 of them were men) diagnosed with OSAHS by somnography and the same number of healthy subjects (23 of them were men) were included, with an average age of 64 ± 11 years (range 45-78 years) for cases and an average age of 64 ± 11 years (range 45-81 years) for the controls. No differences were found in keratometry, cylinder, refractive indexes, Bad-D, or pachymetry. The mean corneal volume for cases was 58.64 ± 3.05 mm and for the controls 60.48 ± 3.33 mm (P = 0.005). The mean minimum radius for cases was 7.49 ± 0.31 and for the controls 7.36 ± 0.30 (P = 0.035). The mean elevation in apex for cases was 8.46 ± 5.18 and for the controls 2.38 ± 2.36 (P ≤ 0.001). Two eyes with a topographic diagnosis of keratoconus (KC) and another 6 with subclinical KC were detected using the Pentacam in the OSAHS group.

CONCLUSIONS

Many of the corneal topographic and biomechanical variables in patients with severe OSAHS present different values from the general population with a trend toward KC values, such as keratoconus index or paired keratoconus index. Compared with the control group, significant differences were found in corneal volume, corneal elevation, and minimum radius.

Anterior Segment Analysis and Evaluation of Corneal Biomechanical Properties in Children with Joint Hypermobility

Objectives:

To compare anterior segment parameters and biomechanical analysis of the cornea in children with joint hypermobility (JH) and healthy children.

Materials and Methods:

Cross-sectional case-control study. Fifty eyes of 25 children with JH were compared with 74 eyes of 37 healthy age- and sex-matched controls in terms of refractive, anterior segment topographic, and corneal biomechanical measurements. Axial length (AL) was measured with a Nidek AL-Scan biometry device; corneal-compensated intraocular pressure (IOPcc), Goldmann-correlated IOP (IOPg), corneal hysteresis (CH), and corneal resistance factor (CRF) were measured with a Reichert ocular response analyzer (ORA). Central corneal thickness (CCT), anterior chamber depth (ACD), K1/K2 values, iris diameter, and anterior chamber volume (ACV) were measured with a Sirius topography device.

Results:

Mean age in the JH group was 10.56±4.03 years, while that of the control group was 11.27±2.59 years (p=0.23). Spherical equivalent was -0.22±1.02 diopter (D) in the JH group and -0.12±1.12 D in the control group (p=0.60); CCT was 23.01±0.82 µm in the JH group and 23.17±0.82 µm in the control group (p=0.33). There were no significant differences between the two groups in terms of age, sex, IOP, IOPcc, IOPg, CH, CRF, AL, K1, K2, iris diameter, ACD, and ACV.

Conclusion:

JH, which causes increased flexibility of the joints, was concluded not to cause a significant change in the corneal biomechanical markers of CRF and CH or in anterior segment topographic parameters.

Association between corneal biomechanical parameters and myopic refractive errors in young Indian individuals

PURPOSE

To report corneal biomechanical parameters in young myopic Indian individuals.

METHODS

It is a retrospective study where young myopic individuals aged between 19 and 36 years who have undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2017 were enrolled. Individuals with central corneal thickness (CCT) <500 microns, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease, any previous ocular surgery, high astigmatism, corneal disease such as keratoconus, poor scans quality, and individuals with any missing data were also excluded. Corneal biomechanical parameters were noted in mild to moderate and high myopia.

RESULTS

We analyzed the 266 eyes of 266 myopic individuals, of which 167 and 99 eyes had mild to moderate and high myopia, respectively. All the individuals were matched for age, IOP, and CCT (P > 0.05). Twenty-three of 32 parameters were similar in different degrees of myopia whereas 9 parameters were significantly different in high myopes as compared to low to moderate myopes. First applanation (A1) parameters and Vinciguerra screening parameters were similar in both the groups (P > 0.05). Second applanation (A2) parameters were similar in both the groups (P > 0.05) except A2 time, A2 deformation, amplitude (DA) (P < 0.05). Highest concavity (HC) parameters were significantly different in both the groups (P < 0.05) except HCDA, HC deflection length, and HC delta arc length (P > 0.05).

CONCLUSIONS

High myopic eyes showed a significantly higher maximum deflection amplitude, lesser A2 time and HC time, less A2DA, smaller HC radius than mild to moderate myopia indicating softer, more deformable corneas. However, better predictor of corneal biomechanics such as Stiffness parameters at A1 (SPA1), DA ratio max, integrated radius, and Corvis Biomechanical Index were similar among both the groups of myopia.

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.

Comparison of Anterior Chamber Depth between Normal and Keratoconic Eyes: A Systematic Review and Meta-Analysis

Purpose:

To review the published data about changes in the anterior chamber depth (ACD) in keratoconus patients.

Methods:

In this systematic review and meta-analysis of observational studies, we reviewed the available and relevant literature on anterior segment changes in keratoconic eyes, with a special focus on the ACD, an effective factor in many surgical methods. Articles published up to December 2017 were identified in the following data sources: PubMed, Scopus, Ovid, ISI, ScienceDirect, and Google Scholar. Databases were comprehensively searched using the key words “Anterior Chamber Depth AND Anterior segment AND Keratoconus”.

Results:

A total of 496 studies including these key words were detected. Four hundred fifty-three studies were excluded, and overall 16 studies which precisely described the change in ACD were included in the literature review. The results show that with respect to the applied device, there was a statistically significant difference in ACD between keratoconic eyes and normal eyes except for Galilei analyzer.

Conclusion:

Summarizing the results of studies, this review revealed that ACD is significantly deeper in keratoconic eyes as compared with normal eyes, which could be explained by the steeper corneal curvature.

Evaluation of Central and Peripheral Corneal Thicknesses in Patients with Systemic Lupus Erythematosus

Purpose

To evaluate the corneal pachymetric and topographic parameters of systemic Lupus Erythematosus (SLE) patients using Dual Scheimpflug Imaging.

Methods

This observational cross-sectional controlled study included the right eye of 30 SLE patients and 30 age-matched controls. Corneal measurements were acquired by dual Scheimpflug imaging including anterior and posterior corneal curvatures, central, mid-peripheral corneal thickness (measured at the 5 mm zone) and peripheral pachymetry (measured at the 7 mm zone). SLE disease activity index (SLEDAI) was calculated and correlated with corneal pachymetry.

Results

SLE patients had significantly thicker corneal periphery than controls. Mean central corneal pachymetry was 530.4± 27.3 microns (SD) in SLE and 547.5±31.5 microns (SD) in control group, p = 0.032. The corneal periphery – except superiorly – was significantly thicker in SLE patients than controls (p ˂0.001). Nasal peripheral corneal thickness positively correlated with disease activity index SLEDAI (p=0.03).

Conclusion

SLE patients present with thicker corneal periphery than controls characteristically sparing the superior quadrant. Possible corneal photosensitivity leading to peripheral immune complex deposition as well as flatter posterior corneal surface at the periphery are proposed explanations for these findings.

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.

Custom air puff-derived biomechanical variables in a refractive surgery screening setting: Study from 2 centers

PURPOSE

To assess the ability of air puff-derived biomechanical variables to predict surgeon-perceived candidacy for laser in situ keratomileusis (LASIK).

SETTING

Cole Eye Institute, Cleveland Clinic, Cleveland, Ohio, and Emory Eye Institute, Emory University, Atlanta, Georgia, USA.

DESIGN

Retrospective case series.

METHODS

Data were collected from refractive surgery screening examinations by 2 surgeons at 2 centers. Disqualified cases (19 eyes and 28 eyes from each surgeon) were judged not to be candidates based on available data including standard variables from the Ocular Response Analyzer. Controls consisted of LASIK candidates (n = 26 and 23). Three custom biomechanical variables not available during screening were calculated and compared by group and surgeon.

RESULTS

The hysteresis loop area was significantly different between disqualified cases and controls for both surgeons (Surgeon 1: controls, 121.50 ± 25.38 [SD], disqualified, 107.62 ± 18.50, P = .04; Surgeon 2: controls, 135.89 ± 22.47, disqualified, 106.11 ± 16.40, P < .001). The area under the curves of the receiver operating characteristics and the cutoff values were statistically significant for the concavity minimum and hysteresis loop area for Surgeon 1 and for all variables except concavity minimum for Surgeon 2. The hysteresis loop area had the highest odds ratio (Surgeon 1, 4.48, Surgeon 2, 20.00). Adjusted R² in best-subsets regressions were 40.2% for Surgeon 1 and 62.9% for Surgeon 2.

CONCLUSIONS

The hysteresis loop area was predictive of which patients were disqualified for LASIK at different sites. Certain measures of the corneal dynamic response to an air puff might serve as correlates to clinically perceived ectasia risk.

Corneal Ectasia Risk And Percentage Tissue Altered In Myopic Patients Presenting For Refractive Surgery

Purpose

A percentage tissue altered (PTA) score of ≥40% has been advocated as an independent indicator of post-operative ectasia risk following laser in-situ keratomileusis (LASIK). This study was performed to test the hypothesis that refractive procedures, such as laser-assisted sub-epithelial keratectomy (LASEK) or small incision lenticule extraction (SMILE), may alter the range of PTA, within which refractive corneal surgery can be safely performed.

Setting

Refractive department, tertiary ophthalmic hospital.

Design

Retrospective observational study.

Methods

Review of case notes was performed for patients who presented for refractive surgeries, other than LASIK. To determine the risk of corneal ectasia for each patient prior to refractive surgery, we estimated what each patient’s PTA would have been if they had undergone LASIK. The Randleman Ectasia Risk Score System (ERSS) was also calculated.

Results

114 eyes (66 patients) were included. 94 eyes underwent SMILE. 20 eyes underwent LASEK. A significant proportion of eyes had PTA ≥40% – SMILE eyes: up to 31.9%, LASEK eyes: up to 60.0% (at presumed LASIK flap of 120 μm). The maximum calculated PTA was up to 47.9% in the SMILE group and up to 51.5% in the LASEK group. Using ERSS, 12.8–16% of SMILE eyes and 15.0–80.0% of LASEK eyes would have been considered to have moderate-to-high ectasia risk. No post-surgical ectasia was observed at 3 years.

Conclusion

SMILE and LASEK alter the range of PTA, within which corneal refractive surgery may be performed with a lower risk of developing post-operative corneal ectasia; a safe PTA threshold needs to be determined for these procedures before recommendations for clinical practice can be made.

Floppy eyelid syndrome: A comprehensive review

Floppy eyelid syndrome (FES) is defined as eyelid hyperlaxity with reactive palpebral conjunctivitis. It is a common condition that can be associated with significant ocular irritation. FES presents with easily everted eyelids and chronic papillary conjunctivitis in the upper eyelids. It is frequently associated with ocular and systemic diseases, notably keratoconus and obstructive sleep apnea, respectively. This comprehensive review describes the epidemiology, pathological changes, proposed pathogenesis, clinical manifestations, and a variety of treatment options for this condition. Conservative treatment of FES includes aggressive lubrication, nighttime eye shield, and avoiding sleeping on the affected eye. Patients with FES and obstructive sleep apnea may have an improvement in their ocular signs and symptoms after long-term therapy with continuous positive airway pressure. In refractory cases, a corrective surgery that addresses the eyelid laxity can result in significant improvement. All patients with ocular irritation should be evaluated for the presence of FES.

Genetic Variants Associated With Corneal Biomechanical Properties and Potentially Conferring Susceptibility to Keratoconus in a Genome-Wide Association Study

Importance

Keratoconus is an important cause of visual loss in young adults, but little is known about its genetic causes. Understanding the genetic determinants of corneal biomechanical factors may in turn teach us about keratoconus etiology.

Objectives

To identify genetic associations with corneal biomechanical properties and to examine whether these genetic variants are associated with keratoconus.

Design, Setting, and Participants

A stage 1 discovery and replication genome-wide association study (GWAS) of corneal biomechanical properties was performed in 2 cross-sectional populations (6645 participants from the European Prospective Investigation into Cancer and Nutrition [EPIC]-Norfolk Eye Study and 2384 participants from the TwinsUK study). In stage 2, the association of genetic determinants identified in stage 1 with keratoconus was examined in a case-control study. A total of 752 patients with keratoconus were compared with 974 TwinsUK participants (undergoing direct sequencing) or 13 828 EPIC-Norfolk participants (undergoing genotyping and imputation) who were not part of the stage 1 analysis. Data were collected from March 1, 1993, through March 13, 2017, and analyzed from November 1, 2015, through February 1, 2018.

Exposures

In stage 1, allele dosage at genome-wide single-nucleotide polymorphisms (SNPs); in stage 2, allele dosage at SNPs with genome-wide significance (P < 5 × 10-8) in stage 1 and not previously reported as associated with corneal disease.

Main Outcomes and Measures

In stage 1, corneal hysteresis (CH) and corneal resistance factor (CRF), measured with the Ocular Response Analyzer (ORA); in stage 2, association with keratoconus compared with controls.

Results

Among 6645 participants in the discovery cohort (3635 women (54.7%); mean age, 69 years [range, 48-92 years]), 7 genome-wide significant loci associated with CH or CRF were identified that were independently replicated. Two further suggestive loci were identified after meta-analysis. To date, 5 of the identified loci, at ANAPC1, ADAMTS8, ADAMTS17, ABCA6, and COL6A1, have not previously been reported as associated with corneal disease. The ABCA6 locus (rs77542162) was associated with keratoconus using the TwinsUK (odds ratio [OR], 0.50; 95% CI, 0.27-0.92; P = .03) and EPIC-Norfolk controls (OR, 0.39; 95% CI, 0.22-0.70; P = .002). The other loci were associated with keratoconus using TwinsUK (OR per effect allele for ADAMTS8, 0.51 [95% CI, 0.37-0.71; P = 7.9 × 10-5]; for COL6A1, 1.65 [95% CI, 1.05-2.59; P = .03]) or EPIC-Norfolk (OR per effect allele for ANAPC1, 0.78 [95% CI, 0.68-0.89; P = 3.7 × 10-4]; for ADAMTS17, 0.82 [95% CI, 0.68-0.99; P = .04]) controls.

Conclusions and Relevance

Five loci that are associated with corneal biomechanical properties and that have suggestive associations with keratoconus were reported. These findings suggest the role of type VI collagen, extracellular matrix, and connective-tissue development for corneal biomechanics and keratoconus and the role of CH and CRF as biomarkers for keratoconus.

Comparative analysis of biomechanically corrected intraocular pressure with corneal visualization Scheimpflug technology versus conventional noncontact intraocular pressure

PURPOSE

To compare the difference between biomechanically corrected intraocular pressure (bIOP) and noncontact IOP measurement (IOP_NCT) and to investigate the effect of corneal biomechanical properties on IOP.

METHODS

IOP was evaluated in 1046 myopic eyes (544 subjects) using a conventional noncontact tonometer and a novel corneal visualization Scheimpflug technology (Corvis ST). Corneal biomechanical parameters were measured using the Corvis ST.

RESULTS

The mean IOP_NCT and bIOP values were significantly different (15.59 ± 2.56 mmHg and 15.89 ± 1.75 mmHg, respectively; P < 0.001). The bIOP showed a less correlation with central corneal thickness (CCT), compared with IOP_NCT (P < 0.01). The IOP_NCT was lower than the bIOP when the thickness of cornea was ≤ 550 μm but higher than bIOP when it was ≥ 550 μm (P < 0.01). A strong association was found between IOP_NCT and deflection amplitude and deflection area at the highest concavity (HC DefA and HC DefArea), stiff parameter, maximum deformation amplitude (DAmax), and maximum deflection amplitude (DefAmax), as well as for bIOP (r > 0.500, P < 0.001). The bIOP could be calculated based on IOP_NCT according to different values of CCT (P < 0.01).

CONCLUSIONS

The bIOP was less affected by CCT as compared to IOP_NCT. IOP_NCT may be underestimated when the cornea is thinner and overestimated when the cornea is thicker because of the difference in corneal biomechanics.

Corneal biomechanics and intraocular pressure assessment after penetrating keratoplasty for non keratoconic patients, long term results

Background

To evaluate corneal biomechanical properties by the Ocular Response Analyzer (ORA) in non keratoconic patients underwent penetrating keratoplasty (PK).

Methods

Corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann- correlated intraocular pressure (IOPg), cornea-compensated IOP (IOPcc) using the ORA, and central graft thickness (CGT) were measured in 30 eyes at least two years after penetrating keratoplasty for non keratoconic indications. IOP using the Goldmann applanation tonometer (GAT) was also obtained after compensation for graft thickness and astigmatism.

Results

The mean age of patients was 33.1 ± 10.13 years; indications for PK were herpetic corneal scar (53.3%), corneal stromal dystrophy (23.3%), traumatic corneal opacity (10%), chemical corneal opacity (6.7%), and Fuchs endothelial dystrophy (6.7%). Mean CH and CRF were 8.52 ± 1.81 mmHg, and 8.56 ± 1.59 mmHg, respectively. Mean CGT was 532.43 ± 30 μm. Mean IOP GAT, IOPg, and IOPcc were 11.88 ± 3.66, 14.64 ± 4.08, and 17.27 ± 4.60 mmHg, respectively (P < 0.001). No significant association was found between CGT and IOP readings obtained using either the ORA or GAT. There were significant negative association between CH with both IOP GAT and IOPcc, while CRF had significant positive association with IOPg.

Conclusion

After penetrating keratoplasty for non keratoconic patients, graft biomechanics does not return to average values even 2 years after the operation; moreover, intraocular pressure measurement with ORA gives higher values than thickness compensated GAT.

Longitudinal study on ocular manifestations in a cohort of patients with Fabry disease

Purpose

This study aims to assess the evolution of ocular manifestations in a cohort of Fabry patients.

Methods

This is a prospective observational study conducted from 2013 to 2017 (5 consecutive exams). All subjects underwent a comprehensive ocular examination including oriented case history, refraction, corneal topography, biomechanical corneal properties and pachometry assessments, aberrometry, anterior segment evaluation, double-frequency visual field (FDT), intra-ocular pressure, and ocular fundus. At baseline, 41 subjects enrolled but 9 dropped-out and 4 files were not kept for analysis (missing data). Remaining 28 subjects were classified into: Group 1 -hemizygotes (HMZ), all on enzyme replacement therapy (ERT) (N = 10); Group 2 -heterozygotes (HTZ) actively ERT-treated (N = 8), and Group 3 -HTZ not treated (N = 10).

Results

There is a high intra and inter-subjects variability. At baseline, prevalence of the ocular manifestations found is similar to published data: cornea verticillata (89.2%), conjunctival vessels tortuosity (85.7%), corneal haze (67.8%), retinal vessels tortuosity (64.2%), anterior cataract (39.2%) and posterior cataract (28.5%). Prevalence for new elements are found: upper lid vessels toricity (96.4%) and micro-aneurysms (42.8%). At the end, micro-aneurysms (+82%), posterior cataract (+75%) corneal haze (+21%) anterior cataract (+17%) and retinal vessels tortuosities (+4%) evolved in prevalence and severity despite the fact that 68% of the patients were on ERT. Treated heterozygotes evolved more than other groups (p>0.05).

Conclusion

ERT does not seem to halt the clinical evolution of several ocular manifestations. Longer observational time and objective grading systems may be required to fully confirm these findings.

Relationship Among Corneal Stiffness, Thickness, and Biomechanical Parameters Measured by Corvis ST, Pentacam and ORA in Keratoconus

Purpose

To assess the relationship among corneal stiffness, thickness, and biomechanical parameters in keratoconus.

Setting

The EENT Hospital of Fudan University, Shanghai, China.

Design

Comparative study.

Methods

In this cross-sectional prospective study, 75 keratoconic eyes of 44 patients were recruited. Eyes were divided three groups according to the steepest K-readings (Kmax): mild (31 eyes; 42.1-54.5D); moderate (27 eyes, 55.0-61.6D); and severe (17 eyes, 65.2-94.5D). Thirty-one healthy subjects were recruited as the control group. All patients underwent Corvis ST, Pentacam and ORA examinations at the same time. Stiffness parameter A1 (SP-A1) and other dynamic parameters were assessed using the Corvis ST. Kmax and thinnest corneal thickness (TCT) was obtained using the Pentacam. Corneal resistance factor (CRF) and corneal hysteresis (CH) were measured using the ORA. Analysis of correlation was applied to investigate the association between variables.

Results

There was a decrease in SP-A1 in different stages of keratoconus compared with controls (P ≤ 0.001): with increasing severity, the value of SP-A1 became smaller (P < 0.05). A statistically significant linear relationship was noted between SP-A1 and TCT in each subgroup of keratoconus (P ≤ 0.001). In all three groups, SP-A1 was found to be positively correlated with first applanation time (P < 0.01), while negatively correlated with deformation amplitude (P < 0.05). Analysis of SP-A1 with regard to CRF and CH indicated statistically positive correlation in keratoconus (P < 0.05).

Conclusion

Significant decreases in corneal stiffness were noted in kerotoconic eyes compared with normal eyes. The stiffness parameter could be a valuable clinical tool enables biomechanically track progression with keratoconus.

Synopsis

Our study found that corneal thinning and biomechanical decreasing synchronize with one another throughout the progression of the keratoconus, and SP-A1 could be a potential biomarker evaluating disease progression.

Comparative analysis of biomechanical parameters of the corneas following Descemet membrane endothelial keratoplasty and contralateral healthy corneas

PURPOSE

To compare the biomechanical properties of the unilateral operated corneas in patients who had undergone Descemet membrane endothelial keratoplasty (DMEK) for pseudophakic bullous keratopathy (PBK) with those of the contralateral normal corneas.

METHODS

This was a retrospective cohort study conducted at university hospitals (Department of Ophthalmology, Rabin Medical Center, Petach Tikva, Israel, and S. Fyodorov Eye Microsurgery State Institution, Moscow, Russia). Forty eyes of 20 patients who underwent DMEK for unilateral PBK 3.5 to 36 months ago and with normal fellow eyes were included in the study. An ocular response analyzer was used to measure the corneal biomechanical properties in the operated and normal fellow eyes. The main outcome measures were corneal hysteresis (CH) and corneal resistance factor (CRF).

RESULTS

The mean CH (8.4 ± 1.5 mmHg vs. 8.2 ± 1.5 mmHg, P = 0.707) and the mean CRF (8.7 ± 1.6 mmHg vs. 8.3 ± 1.6 mmHg, P = 0.419) values did not show any statistically significant difference between the operated and the normal fellow eyes.

CONCLUSIONS

In our study, the corneas that underwent DMEK for PBK showed normal values for biomechanical parameters. These findings support the previous studies that have reported near complete visual, functional, and ultra-structural rehabilitation of the corneas following DMEK.

The Role of Ocular Response Analyzer in Differentiation of Forme Fruste Keratoconus From Corneal Astigmatism

PURPOSE

To determine the diagnostic accuracy of corneal biomechanical factors in differentiating patients with forme fruste keratoconus (FFKC) from astigmatic and normal cases.

METHODS

A total of 50 eyes with FFKC, 50 with astigmatism and 50 normal eyes, were included in this study. All patients had a detailed ophthalmologic examination including slit-lamp evaluation, Goldmann tonometry, indirect fundoscopy, topography by Scheimpflug imaging biomicroscopic anterior and posterior segment examination, and corneal biomechanical and intraocular pressure evaluation with ocular response analyzer (ORA).

RESULTS

All topographic findings were statistically significant among the three groups (P>0.05). Although there was no statistically significant difference in the corneal-compensated intraocular pressure (IOPcc) among the three groups, the Goldmann-correlated intraocular pressure (IOPg), corneal hysteresis (CH), and corneal resistance factor (CRF) were statistically significantly lower in the FFKC group, compared with the other groups (P<0.001). There were no statistically significant difference in the IOPg, CH, and CRF between astigmatism and control groups (P=0.99, 0.79, and 0.86, respectively). The area under the receiver operating characteristic (AUROC) curve was greater than 0.85 for IOPg (0.80), CH (0.85), and CRF (0.90) for discriminating between FFKC and controls; whereas the AUROC was greater than 0.85 for IOPg (0.80), CH (0.79), and CRF (0.85) for discriminating between FFKC and astigmatism groups.

CONCLUSION

Based on our study results, in differentiation of patients with FFKC from normal control cases or astigmatic patients, corneal biomechanical parameters play a role particularly in patients with suspicious results. We suggest using ORA in combination with corneal topography for better and more accurate diagnosis of FFKC.