Depth-Dependent Corneal Biomechanical Properties in Normal and Keratoconic Subjects by Optical Coherence Elastography

Anterior segment applications of optical coherence elastography in ophthalmic and vision science: a systematic review of intrinsic measurement techniques and clinical relevance

Optical coherence elastography (OCE) is a non-invasive imaging technique that measures the biomechanical properties of materials and tissues. This systematic review focuses on the applications of OCE in the anterior segment of the eye, including the cornea, iris, and crystalline lens, and its clinical relevance in diagnosing and managing ocular diseases. A systematic literature review was conducted using the PRISMA framework to identify studies published between 2014 and 2024. The review included studies that reported intrinsic biomechanical properties of anterior segment tissues measured using OCE. Databases searched included Scopus, Pub Med, and IEEE Xplore. Twenty-five studies met the inclusion criteria. The review found that OCE has been used to measure intrinsic biomechanical parameters such as Young's modulus and shear modulus in ocular tissues. OCE has been utilised to assess corneal stiffness in keratoconus, lens elasticity in presbyopia and cataract formation, and iris biomechanical changes under different lighting conditions. The studies demonstrated that OCE could detect subtle biomechanical changes associated with ocular diseases and measure treatment efficacy, such as collagen crosslinking for keratoconus management. The findings highlight the potential of OCE to enhance clinical diagnostics and patient care by providing detailed insights into the biomechanical properties of ocular tissues. However, variability in measurement techniques, the complexity of the method and reliance on animal models limit the current clinical translation of OCE. Standardised measurement protocols and further development andin vivovalidation are needed to overcome these barriers. OCE shows promise as a valuable non-invasive tool for high-resolution assessments of tissue biomechanics, which can subsequently support the diagnosis and management of ocular diseases. Future research should focus on standardising OCE methods and integrating them into clinical practice to fully realise their potential in improving patient outcomes.

Spatial relationship between histological staining intensity and corneal stiffness variations: Insights from AFM indentation in infant African green monkeys

This study investigated the spatial variations of mechanical properties and microstructure in the cornea using atomic force microscopy (AFM) indentation tests and histological analysis. Corneal samples were collected from three infant African green monkeys, approximately 6 months old. Hematoxylin and eosin (H&E) staining was performed on corneal cross-sections to examine microstructure and quantify staining intensity. AFM indentations were conducted to quantify stiffness variations through pathline scanning and 16 × 16 stiffness mappings. Results showed that the corneal microstructure transitions from thinner, denser lamellae in the anterior layer to thicker, looser lamellae in the posterior layer. Stiffness variations along pathlines in the central, paracentral, peripheral, and limbus regions correlate positively with the corresponding staining intensities. The average stiffness across all samples was highest at the central anterior cornea (392.6 ± 118.4 kPa) and anterior limbus (645.4 ± 158.1 kPa). Additionally, both the anterior and posterior layers showed higher stiffness than the middle layer, except in the central region. AFM stiffness maps further revealed the layered structure of the lamellae. The stiffness variations between layers may result from different orientations of collagen fibrils in each lamellae. These observations were expected to provide valuable insights into corneal microstructure and mechanical properties variations during the progression of corneal diseases, aiding in the design of optimal artificial corneas. While this study focuses on infant monkey eyes, further testing across different age and sex groups is needed to refine these observations.

Riboflavin-UV crosslinking of the cornea: Wound healing and biomechanics

The corneal wound healing response to Riboflavin-ultraviolet-crosslinking (RIB-UV-CXL) depends on the specific method used in treatment. The predominance of clinical evidence supports the classical "epithelium-off" RIB-UV-CXL method being more effective in halting ectasia progression than various "epithelium-on" methods, where the corneal epithelium is maintained intact. Corneal transparency results from the precise organization of collagen fibrils and extracellular matrix, along with transparent keratocytes. The mild and transient stromal opacity seen after standard RIB-UV-CXL is linked to changes in hydration, cellularity, and matrix composition. As hydration normalizes, opacity arises from the development of corneal fibroblasts and their secretion of disordered extracellular matrix materials including collagens. Over months, as the epithelial basement membrane regenerates, transitioning stromal cells either undergo apoptosis or revert to keratocan-positive keratocytes, restoring stromal transparency. In normal healing after standard RIB-UV-CXL, the stroma is eventually repopulated predominantly by keratocytes without significant persisting fibroblasts, immune cells, or myofibroblasts. Biomechanical studies have extensively explored how CXL strengthens corneal tissue, providing insight into its therapeutic mechanisms. The purpose of this review is to evaluate the wound healing response and biomechanical changes in the cornea following RIB-UV-CXL.

Regional Biomechanical Weakening in Keratoconus Corneas Detected by In Vivo High-Frequency Ultrasound Elastography

Purpose

In vivo biomechanical characterization of the cornea remains a challenge. We have developed a high-resolution ultrasound elastography technique, termed ocular pulse elastography (OPE), to measure corneal deformation in response to the intraocular pressure (IOP) pulsation at each heartbeat. In this study, we aimed to compare corneal axial strains (CASs) between patients with keratoconus and normal subjects and evaluate the spatial mapping of CAS in high grade keratoconus.

Methods

Forty patients with keratoconus (63 eyes) and 40 normal controls (80 eyes) were enrolled in this study. Each eye underwent 4 ultrasound measurements using the Vevo2100 high-frequency ultrasound system. Each measurement acquired 1000 continuous B-mode scans in 8 seconds. Corneal axial displacements and strains were quantified using an ultrasound speckle tracking algorithm.

Results

CAS magnitude was significantly higher in keratoconus than normal corneas (-0.13% ± 0.09% vs. -0.06% ± 0.04%, P < 0.001) with an increasing trend in higher grades (P < 0.001). CAS in keratoconus corneas had a greater spatial variance as higher strains were observed in the cone center than its surrounding regions in grade 3 and 4 keratoconus corneas.

Conclusions

Our results showed that high-frequency ultrasound elastography was able to detect and quantify the larger deformation of keratoconus corneas than normal corneas in response to the natural fluctuations of IOP at each heartbeat, and it also detected the spatial variance showing greater deformation in the cone region.

Translational Relevance

High-resolution ultrasound may provide a sensitive tool for quick, spatially resolved characterization of corneal biomechanics to aid keratoconus detection and diagnosis.

Thickness Speed Progression Index: Machine Learning Approach for Keratoconus Detection

PURPOSE

To develop and validate a pachymetry-based machine learning (ML) index for differentiating keratoconus, keratoconus suspect, and normal corneas.

DESIGN

Development and validation of an ML diagnostic algorithm.

METHODS

This retrospective study included 349 eyes of 349 patients with normal, frank keratoconus (KC), and KC suspect (KCS) corneas. KCS corneas included topographically/tomographically normal (TNF) and borderline fellow eyes (TBF) of patients with asymmetric KC. Six parameters were derived from the corneal thickness progression map on the Galilei Dual Scheimpflug-Placido system and fed into a machine-learning algorithm to create the Thickness Speed Progression Index. The model was trained with 5-fold cross-validation using a random search over 7 different ML algorithms, and the best model and hyperparameters were selected.

RESULTS

A total of 133 normal eyes, 141 KC eyes, and 75 KCS eyes, subdivided into 34 TNF and 41 TBF eyes, were included. In experiment 1 (normal and KC), the best model (Random Forest) achieved an accuracy of 100% and area under the receiver operating characteristic (AUROC) of 1.00 for both normal and KC groups. In experiment 2 (normal, KCS, and KC), the model achieved an overall accuracy of 91%, and AUROC curves of 0.93, 0.83, and 0.99 in detecting normal, KCS, and KC corneas respectively. In experiment 3 (normal, TNF, TBF, and KC), the model achieved an accuracy of 87% with AUROC curves of 0.91, 0.60, 0.77, and 0.94 for normal, TNF, TBF, and KC corneas, respectively.

CONCLUSIONS

Using data solely based on pachymetry, ML algorithms such as the Thickness Speed Progression Index are able to discriminate normal corneas from KC and KCSs corneas with reasonable accuracy.

The Contribution of the Limbus and Collagen Fibrils to Corneal Biomechanical Properties: Estimation of the Low-Strain In Vivo Elastic Modulus and Tissue Strain

We have compared the biomechanical properties of human and porcine corneas using vibrational optical coherence tomography (VOCT). The elastic modulus of the cornea has been previously reported in the literature to vary from about several kPa to more than several GPa based on the results of different techniques. In addition, the formation of corneal cones near the central cornea in keratoconus has been observed in the clinic. Measurements of the resonant frequency and morphology of human and porcine corneas were used to evaluate the role of the limbus in corneal stabilization, the effect of Bowman's layer, and the effect of collagen content on the low-strain corneal biomechanics. The results of these studies indicate that limbus stability plays an important anatomic role in preventing folding, corneal slippage, and cone formation. Machine learning studies of both human and porcine corneas indicate that Bowman's membrane, like that of the collagen fibrils found in the anterior corneal stroma, contributes to the 110-120 Hz resonant frequency peak. Finite element and SOLIDWORKS models of normal and keratoconus corneas suggest that the deformation of the cornea is the highest at the central zone and is higher in keratoconus corneas compared to normal controls. VOCT results suggest that although collagen fibril slippage occurs first at the limbus, cone formation in keratoconus occurs centrally/paracentrally, where stress concentration and deformation due to intraocular forces are the highest. Cone formation occurs at the points of maximum curvature. Results of these studies indicate the elastic modulus of cornea fibrillar collagen dictates the corneal elastic modulus at low strains. These results suggest that tension in the cornea at the limbus results in deformation into the low modulus region of the J-shaped stress-strain curve, resulting in an in vivo strain of less than about 10%. We propose that tension in the cornea provides a baseline force that regulates corneal epithelial regeneration as well as corneal lamellae composition and matrix turnover.

Mechanical properties of stromal striae, and their impact on corneal tissue behavior

Cornea is an essential element of our eye. The refractive power of the cornea is closely related to its shape, which depends on the balance between its mechanical properties and the intraocular pressure. However, in keratoconus, the shape of the cornea is altered, and the mechanical properties (i.e., elastic modulus and viscosity) are reduced. These alterations have been associated with the development of striae within the cornea. Recently, such striae have been observed in healthy corneas as well, but with slightly different shapes. Our study investigated the mechanical role of these striae. To this end, we performed an inflation test under Optical Coherence Tomography: tomographic volumes were acquired in the central zone of eleven human corneas during an inflation test. Striae planes were extracted from the segmented images, and principal deformation maps were obtained by Digital Volume Correlation (DVC). We observe that the pattern of the striae does not change with pressure, even far above physiological pressure. Maximum principal strains are co-localized with the striae and are oriented perpendicular to the striae. We also observe that principal deformations on the striae increase with depth in the cornea. Our results show that striae lead to greater deformability in the direction perpendicular to the striae, especially in the posterior part of the cornea where they are the most visible. This supports the idea that the striae are undulations in the cornea collagenous microstructure, which are progressively unfolded under loading. They decrease the global stiffness of the cornea, in particular in the posterior part, and thus may help in accommodating deformations.

Dynamic evaluation of corneal cross-linking and osmotic diffusion effects using optical coherence elastography

Dynamic deformation events induced by osmosis or photochemical stiffening substantially influence geometrical and mechanical assessments in post-mortem corneas, therefore need to be carefully monitored in experimental settings. In this study, we employed optical coherence elastography (OCE) to quantify dynamic deformation processes at high resolution in freshly enucleated porcine corneas. Osmotic effects were studied by immerging n = 9 eyes in preservation media of three different tonicities. Dynamic processes underlying corneal cross-linking (CXL) were studied by subjecting n = 6 eyes to standard Dresden treatment, while three control groups were used. The entire procedures were performed under an OCE setup during up to 80 min, acquiring a volumetric scan every 20 s. Changes in OCE-derived axial deformations were incrementally calculated between consecutive scans. Preservation conditions had a strong influence on the observed strain patterns, which were consistent with the tonicity of the medium (swelling in hypotonic, deswelling in hypertonic environment). In the CXL group, we observed deswelling of the anterior stroma 10 min after starting the UV irradiation, which was not observed in any control group (p = 0.007). The presented results proved OCE to be a valuable technique to quantify subtle dynamic biomechanical alterations in the cornea resulting from CXL and preservation solutions.

Keratectasia severity staging and progression assessment based on the biomechanical E-staging

Until recently, corneal topography has been the gold standard in detecting keratectasia and monitoring its progression. The recently introduced ABCD tomographic keratoconus staging system focuses on anterior ("A") and posterior ("B") radius of curvature, thinnest corneal thickness ("C"), best-corrected visual acuity with spectacles ("D") and is supplemented with the introduction of the biomechanical E-staging (BEST, "E"). The need for biomechanical staging arose from the fact of altered biomechanical characteristics of keratectasia in comparison to healthy corneas. Ectatic corneas usually exhibit a biomechanical weakening and greater deformation than healthy corneas when exposed to a biomechanical stressor such as a standardized air puff indentation as provided by the Corvis ST® (CST, Oculus, Wetzlar, Germany). The BEST is based on the linear term of the Corvis Biomechanical Index (CBI) and provides a biomechanical keratoconus severity staging and progression assessment within the CST software. This review traces the development of the BEST as an addition to the tomographic ABCD staging system and highlights its strengths and limitations when applied in daily practice for the detection, monitoring and progression assessment in keratectasia.

Dual-channel air-pulse optical coherence elastography for frequency-response analysis

Microliter air-pulse optical coherence elastography (OCE) has recently been proposed for the characterization of soft-tissue biomechanics using transient, sub-nanometer to micrometer-scale natural frequency oscillations. However, previous studies have not been able to provide real-time air-pulse monitoring during OCE natural frequency measurement, which could lead to inaccurate measurement results due to the unknown excitation spectrum. To address this issue, we introduce a dual-channel air-pulse OCE method, with one channel stimulating the sample and the other being simultaneously measured with a pressure sensor. This allows for more accurate natural frequency characterization using the frequency response function, as proven by a comprehensive comparison under different conditions with a diverse range of excitation spectra (from broad to narrow, clean to noisy) as well as a diverse set of sample response spectra. We also demonstrate the capability of the frequency-response analysis in distinguishing samples with different stiffness levels: the dominant natural frequencies increased with agar concentrations (181-359 Hz, concentrations: 1-2%, and maximum displacements: 0.12-0.47 µm) and intraocular pressures (IOPs) for the silicone cornea (333-412 Hz, IOP: 5-40 mmHg, and maximum displacements: 0.41-0.52 µm) under a 200 Pa stimulation pressure. These frequencies remained consistent across different air-pulse durations (3 ms to 35 ms). The dual-channel OCE approach that uses transient, low-pressure stimulation and high-resolution imaging holds the potential to advance our understanding of sample frequency responses, especially when investigating delicate tissues such as the human cornea in vivo.

Longitudinal Analysis of Corneal Biomechanics of Suspect Keratoconus: A Prospective Case-Control Study

BACKGROUND

To evaluate the corneal biomechanics of stable keratoconus suspects (Stable-KCS) at 1-year follow-up and compare them with those of subclinical keratoconus (SKC).

METHODS

This prospective case-control study included the eyes of 144 patients. Biomechanical and tomographic parameters were recorded (Corvis ST and Pentacam). Patients without clinical signs of keratoconus in both eyes but suspicious tomography findings were included in the Stable-KCS group (n = 72). Longitudinal follow-up was used to evaluate Stable-KCS changes. Unilateral keratoconus contralateral eyes with suspicious tomography were included in the SKC group (n = 72). T-tests and non-parametric tests were used for comparison. Multivariate general linear models were used to adjust for confounding factors for further analysis. Receiver operating characteristic (ROC) curves were used to analyze the distinguishability.

RESULTS

The biomechanical and tomographic parameters of Stable-KCS showed no progression during the follow-up time (13.19 ± 2.41 months, p > 0.05). Fifteen biomechanical parameters and the Stress-Strain Index (SSI) differed between the two groups (p < 0.016). The A1 dArc length showed the strongest distinguishing ability (area under the ROC = 0.888) between Stable-KCS and SKC, with 90.28% sensitivity and 77.78% specificity at the cut-off value of -0.0175.

CONCLUSIONS

The A1 dArc length could distinguish between Stable-KCS and SKC, indicating the need to focus on changes in the A1 dArc length for keratoconus suspects during the follow-up period. Although both have abnormalities on tomography, the corneal biomechanics and SSI of Stable-KCS were stronger than those of SKC, which may explain the lack of progression of Stable-KCS.

Quantitative assessment of corneal elasticity distribution after FS-LASIK using optical coherence elastography

Quantifying corneal elasticity after femtosecond laser-assisted in situ keratomileusis (FS-LASIK) procedure plays an important role in improving surgical safety and quality, since some latent complications may occur ascribing to changes in postoperative corneal biomechanics. Nevertheless, it is suggested that current research has been severely constrained due to the lack of an accurate quantification method to obtain postoperative corneal elasticity distribution. In this paper, an acoustic radiation force optical coherence elastography system combined with the improved phase velocity algorithm was utilized to realize elasticity distribution images of the in vivo rabbit cornea after FS-LASIK under various intraocular pressure levels. As a result, elasticity variations within and between the regions of interest could be identified precisely. This is the first time that elasticity imaging of in vivo cornea after FS-LASIK surgery was demonstrated, and the results suggested that this technology may hold promise in further exploring corneal biomechanical properties after refractive surgery.

Corneal Biomechanical Properties to Predict Prognosis of Abnormal Tomographic Corneas: A Prospective Cohort Study

PURPOSE

To analyze the corneal biomechanical properties in patients with abnormal corneal tomography (ACT) and predict their stability using the biomechanical stability index (BSI).

DESIGN

Prospective cohort study.

METHODS

Setting: Multicenter study.

STUDY POPULATION

This study included 385 eyes of 278 patients with stable ACT (n = 70), subclinical keratoconus (SKC, n = 65), keratoconus (n = 65), normal controls (NL, n = 142). Forty-three eyes with first-visit ACT were included in a separate cohort (follow-up ACT group).

OBSERVATION PROCEDURE

Tomographical and biomechanical parameters (Pentacam and Corvis ST) were recorded.

MAIN OUTCOME MEASURES

Nonparametric tests were used for comparison. Logistic regression was employed to introduce BSI to separate stable ACT and SKC accurately. An independent dataset of 43 first-visit ACT eyes was followed up for 1 year to validate BSI's accuracy and positive and negative predictive values (PPV, NPV).

RESULTS

The tomographical and biomechanical parameters in patients with Stable ACT remained stable over the follow-up period (12.73 ± 2.57 months, P > .05). Stable ACT had 12/14 biomechanical parameters different (P < .05) from SKC but not different from NL (P > .05). With a cut-off value of 0.585, BSI demonstrated the strongest ability to distinguish between stable ACT and SKC (area under the receiver operating characteristic curve = 0.991), with 93.85% sensitivity and 97.14% specificity. During the 1-year follow-up of 43 eyes (follow-up ACT group), 30 remained stable. The accuracy, PPV, and NPV of the BSI were 95.35%, 100%, and 93.75%, respectively.

CONCLUSIONS

Biomechanical properties of patients with stable abnormal tomography corneas were stronger than SKC and close to normal corneas, which may explain the reason for tomographic stability. The BSI may be useful for predicting disease progression in patients with ACT and the possible management of corneal cross-linking at the first visit.

Subclinical Keratoconus Detection and Characterization Using Motion-Tracking Brillouin Microscopy

PURPOSE

To characterize focal biomechanical alterations in subclinical keratoconus (SKC) using motion-tracking (MT) Brillouin microscopy and evaluate the ability of MT Brillouin metrics to differentiate eyes with SKC from normal control eyes.

DESIGN

Prospective cross-sectional study.

PARTICIPANTS

Thirty eyes from 30 patients were evaluated, including 15 eyes from 15 bilaterally normal patients and 15 eyes with SKC from 15 patients.

METHODS

All patients underwent Scheimpflug tomography and MT Brillouin microscopy using a custom-built device. Mean and minimum MT Brillouin values within the anterior plateau region and anterior 150 μm were generated. Scheimpflug metrics evaluated included inferior-superior (IS) value, maximum keratometry (Kmax), thinnest corneal thickness, asymmetry indices, Belin/Ambrosio display total deviation, and Ambrosio relational thickness. Receiver operating characteristic (ROC) curves were generated for all Scheimpflug and MT Brillouin metrics evaluated to determine the area under the ROC curve (AUC), sensitivity, and specificity for each variable.

MAIN OUTCOME MEASURES

Discriminative performance based on AUC, sensitivity, and specificity.

RESULTS

No significant differences were found between groups for age, sex, manifest refraction spherical equivalent, corrected distance visual acuity, Kmax, or KISA% index. Among Scheimpflug metrics, significant differences were found between groups for thinnest corneal thickness (556 μm vs. 522 μm; P < 0.001), IS value (0.29 diopter [D] vs. 1.05 D; P < 0.001), index of vertical asymmetry (IVA; 0.10 vs. 0.19; P < 0.001), and keratoconus index (1.01 vs. 1.05; P < 0.001), and no significant differences were found for any other Scheimpflug metric. Among MT Brillouin metrics, clear differences were found between control eyes and eyes with SKC for mean plateau (5.71 GHz vs. 5.68 GHz; P < 0.0001), minimum plateau (5.69 GHz vs. 5.65 GHz; P < 0.0001), mean anterior 150 μm (5.72 GHz vs. 5.68 GHz; P < 0.0001), and minimum anterior 150 μm (5.70 GHz vs. 5.66 GHz; P < 0.001). All MT Brillouin plateau and anterior 150 μm mean and minimum metrics fully differentiated groups (AUC, 1.0 for each), whereas the best performing Scheimpflug metrics were keratoconus index (AUC, 0.91), IS value (AUC, 0.89), and IVA (AUC, 0.88).

CONCLUSIONS

Motion-tracking Brillouin microscopy metrics effectively characterize focal corneal biomechanical alterations in eyes with SKC and clearly differentiated these eyes from control eyes, including eyes that were not differentiated accurately using Scheimpflug metrics.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

In Vivo Evaluation of Corneal Biomechanics Following Cross-Linking Surgeries Using Optical Coherence Elastography in a Rabbit Model of Keratoconus

Purpose

Validation of the feasibility of novel acoustic radiation force optical coherence elastography (ARF-OCE) for the evaluation of biomechanical enhancement of the in vivo model of keratoconus by clinical cross-linking (CXL) surgery.

Methods

Twelve in vivo rabbit corneas were randomly divided into two groups. Both groups were treated with collagenase type II, and a keratoconus model was obtained. Then, the two groups were treated with CXL procedures with different irradiation energy of 15 J and 30 J (CXL-15 J and CXL-30 J, respectively). An ARF-OCE probe with an ultrasmall ultrasound transducer was used to detect the biomechanical properties of cornea. An antisymmetric Lamb wave model was combined with the frequency dispersion relationship to achieve depth-resolved elastography.

Results

Compared with the phase velocity of the Lamb wave in healthy corneas (approximately 3.96 ± 0.27 m/s), the phase velocity of the Lamb wave was lower in the keratoconus region (P < 0.05), with an average value of 3.12 ± 0.12 m/s. Moreover, the corneal stiffness increased after CXL treatment (P < 0.05), and the average phase velocity of the Lamb wave was 4.3 ± 0.19 m/s and 4.54 ± 0.13 m/s after CXL-15 J and CXL-30 J treatment.

Conclusions

The Young's moduli of the keratoconus regions were significantly lower than the healthy corneas. Moreover, the Young's modulus of the keratoconus regions was significantly higher after CXL-30 J treatment than after CXL-15 J treatment. We demonstrated that the ARF-OCE technique has great potential in screening keratoconus and guiding clinical CXL treatment.

Translational Relevance

This work accelerates the clinical translation of OCE systems using ultrasmall ultrasound transducers and is used to guide CXL procedures.

Optomechanical assessment of photorefractive corneal cross-linking via optical coherence elastography

Purpose: Corneal cross-linking (CXL) has recently been used with promising results to positively affect corneal refractive power in the treatment of hyperopia and mild myopia. However, understanding and predicting the optomechanical changes induced by this procedure are challenging. Methods: We applied ambient pressure modulation based optical coherence elastography (OCE) to quantify the refractive and mechanical effects of patterned CXL and their relationship to energy delivered during the treatment on porcine corneas. Three different patterned treatments were performed, designed according to Zernike polynomial functions (circle, astigmatism, coma). In addition, three different irradiation protocols were analyzed: standard Dresden CXL (fluence of 5.4 J/cm2), accelerated CXL (fluence of 5.4 J/cm2), and high-fluence CXL (fluence of 16.2 J/cm2). The axial strain distribution in the stroma induced by ocular inflation (Δp = 30 mmHg) was quantified, maps of the anterior sagittal curvature were constructed and cylindrical refraction was assessed. Results: Thirty minutes after CXL, there was a statistically significant increase in axial strain amplitude (p < 0.050) and a reduction in sagittal curvature (p < 0.050) in the regions treated with all irradiation patterns compared to the non-irradiated ones. Thirty-6 hours later, the non-irradiated regions showed compressive strains, while the axial strain in the CXL-treated regions was close to zero, and the reduction in sagittal curvature observed 30 minutes after the treatment was maintained. The Dresden CXL and accelerated CXL produced comparable amounts of stiffening and refractive changes (p = 0.856), while high-fluence CXL produced the strongest response in terms of axial strain (6.9‰ ± 1.9‰) and refractive correction (3.4 ± 0.9 D). Tripling the energy administered during CXL resulted in a 2.4-fold increase in the resulting refractive correction. Conclusion: OCE showed that refractive changes and alterations in corneal biomechanics are directly related. A patient-specific selection of both, the administered UV fluence and the irradiation pattern during CXL is promising to allow customized photorefractive corrections in the future.

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.

Screening of sensitive in vivo characteristics for early keratoconus diagnosis: a multicenter study

Purpose: To analyze and compare sensitive in vivo characteristics for screening early keratoconus. Methods: This multicenter, case-control study included 712 eyes, after matching for age and biomechanically corrected intraocular pressure, from three clinics in different cities. The keratoconus (n = 288), early keratoconus (n = 91), and normal cornea (n = 333) groups included eyes diagnosed with bilateral keratoconus, fellow eyes with relatively normal topography with unilateral keratoconus, and normal eyes before refractive surgery, respectively. After adjusting for central corneal thickness, differences in vivo characteristics were analyzed among the three groups. The in vivo characteristics were measured by Pentacam and Corvis ST. Fifty-four indices were evaluated to screen for a sensitive index for the detection of early keratoconus. Results: Significant differences were observed in 26 of the 36 corneal biomechanical indeces between the early keratoconus and normal corneas. The area under the receiver operating characteristic curve of tomographic and biomechanical index, Belin/Ambrósio deviation, and Da in differentiating keratoconus from normal cornea was 1.000. Among the top five indeces of the area under the receiver operating characteristic curve for detecting early keratoconus, the corneal biomechanical-related index accounted for 80% (4/5), including A1 dArc length, highest concavity radius, A2 time, and tomographic and biomechanical index, of which the area under the receiver operating characteristic curve of A1 dArc length was 0.901. Conclusion: A1 dArc length and several corneal biomechanical indices are highly sensitive for the detection of early keratoconus, even in the absence of topographic abnormalities. Ophthalmologists should focus on the clinical application of corneal biomechanics and combine corneal tomography for the timely and accurate detection of early keratoconus.

Clinical applications of anterior segment swept-source optical coherence tomography: A systematic review

Optical coherence tomography (OCT) is a non-invasive method that provides the opportunity to examine tissues by taking cross-sectional images. OCT is increasingly being used to evaluate anterior segment (AS) pathologies. Swept-source (SS) OCT allows greater penetration and achieves better visualization of the internal configuration of AS tissues due to the longer wavelength employed and high scan speeds. We reviewed the utilization of AS SS-OCT in various conditions including glaucoma, ocular surface pathologies, iris tumors, refractive surgery, cataract surgery, and scleral diseases. A systematic literature search was carried out on PubMed, Scopus, and Web of Science databases between January 1, 2008, and September 1, 2022 using the following keywords: AS SS-OCT; dry eye and SS-OCT; ocular surface and SS-OCT; cornea and SS-OCT; dystrophy and SS-OCT; glaucoma and SS-OCT; ocular surface tumors and SS-OCT; conjunctival tumors and SS-OCT; refractive surgery and SS-OCT; cataract and SS-OCT; biometry and SS-OCT; sclera and SS-OCT; iris and SS-OCT; ciliary body and SS-OCT; artificial intelligence and SS-OCT. A total of 221 studies were included in this review. Review of the existing literature shows that SS-OCT offers several advantages in the diagnosis of AS diseases. Exclusive features of SS-OCT including rapid scanning, deeper tissue penetration, and better image quality help improve our understanding of various AS pathologies.

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

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

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.

Motion-tracking Brillouin microscopy for in-vivo corneal biomechanics mapping

Corneal biomechanics play a critical role in maintaining corneal shape and thereby directly influence visual acuity. However, direct corneal biomechanical measurement in-vivo with sufficient accuracy and a high spatial resolution remains an open need. Here, we developed a three-dimensional (3D) motion-tracking Brillouin microscope for in-vivo corneal biomechanics mapping. The axial tracking utilized optical coherence tomography, which provided a tracking accuracy better than 3 µm. Meanwhile, 10 µm lateral tracking was achieved by tracking pupils with digital image processing. The 3D tracking enabled reconstruction of depth-dependent Brillouin distribution with a high spatial resolution. This superior technical performance enabled the capture of high-quality mechanical mapping in vivo even while the subject was breathing normally. Importantly, we improved Brillouin spectral measurements to achieve relative accuracy better than 0.07% verified by rubidium absorption frequencies, with 0.12% stability over 2000 seconds. These specifications finally yield the Brillouin measurement sensitivity that is required to detect ophthalmology-relevant corneal biomechanical properties.

Biomechanical properties analysis of forme fruste keratoconus and subclinical keratoconus

PURPOSE

To analyze the biomechanical properties of the eye in patients with unilateral keratoconus with normal (forme fruste keratoconus [FFKC]) or abnormal topography (subclinical keratoconus [SKC]).

METHODS

This study included 153 eyes of 153 participants, including 95 eyes of patients with unilateral keratoconus, and 58 eyes of 58 healthy controls. Contralateral eyes with unilateral keratoconus were divided into two groups according to clinical manifestations and global consensus: FFKC (n = 30) and SKC (n = 65). The biomechanical characteristics were analyzed using non-parametric tests; further analysis thereof was performed after adjusting for confounding factors (i.e., intraocular pressure, age, and corneal thickness). Receiver operating characteristic curve (ROC) was used to analyze the ability of the biomechanical parameters to distinguish FFKC from SKC.

RESULTS

Statistically significant differences between the FFKC and SKC groups were found in 9 of the 18 corneal biomechanical parameters analyzed using non-parametric tests. After adjusting for confounding factors, the multivariate analysis still revealed significant statistical differences in A1-time (P = 0.017), integrated radius (IR) (P = 0.024), and tomographic and biomechanical index (TBI, P < 0.001) between the FFKC and SKC groups. Stiffness parameter at first applanation (SP-A1) (Area under ROC [AUROC] = 0.765) demonstrated the strongest distinguishing ability, except for TBI (AUROC = 0.858) and Corvis Biomechanical Index (AUROC = 0.849), however, there was no statistically significant difference in SP-A1 (P = 0.366) between FFKC and SKC.

CONCLUSIONS

Biomechanical parameters A1-time and IR have a high diversity between FFKC and SKC, besides TBI, and may reflect more subtle changes in corneal biomechanical properties (BPs) preceding SP-A1. The BPs of SKC are weaker than FFKC, which might be a basic and clue for the classification and diagnosis of the severity of early keratoconus in terms of biomechanics.

Acoustic Micro-Tapping Optical Coherence Elastography to Quantify Corneal Collagen Cross-Linking: An Ex Vivo Human Study

Purpose

To evaluate changes in the anisotropic elastic properties of ex vivo human cornea treated with ultraviolet cross-linking (CXL) using noncontact acoustic micro-tapping optical coherence elastography (AμT-OCE).

Design

Acoustic micro-tapping OCE was performed on normal and CXL human donor cornea in an ex vivo laboratory study.

Subjects

Normal human donor cornea (n = 22) divided into 4 subgroups. All samples were stored in optisol.

Methods

Elastic properties (in-plane Young's, E, and out-of-plane, G, shear modulus) of normal and ultraviolet CXL-treated human corneas were quantified using noncontact AμT-OCE. A nearly incompressible transverse isotropic model was used to reconstruct moduli from AμT-OCE data. Independently, cornea elastic moduli were also measured with destructive mechanical tests (tensile extensometry and shear rheometry).

Main Outcome Measures

Corneal elastic moduli (in-plane Young's modulus, E, in-plane, μ, and out-of-plane, G, shear moduli) can be evaluated in both normal and CXL treated tissues, as well as monitored during the CXL procedure using noncontact AμT-OCE.

Results

Cross-linking induced a significant increase in both in-plane and out-of-plane elastic moduli in human cornea. The statistical mean in the paired study (presurgery and postsurgery, n = 7) of the in-plane Young's modulus, E = 3 μ , increased from 19 MPa to 43 MPa, while the out-of-plane shear modulus, G, increased from 188 kPa to 673 kPa. Mechanical tests in a separate subgroup support CXL-induced cornea moduli changes and generally agree with noncontact AμT-OCE measurements.

Conclusions

The human cornea is a highly anisotropic material where in-plane mechanical properties are very different from those out-of-plane. Noncontact AμT-OCE can measure changes in the anisotropic elastic properties in human cornea as a result of ultraviolet CXL.

High-frequency ultrasound detects biomechanical weakening in keratoconus with lower stiffness at higher grade

In vivo biomechanical characterization of the cornea remains a challenge. We have developed a high-frequency ultrasound elastography method, the ocular pulse elastography (OPE), to measure corneal axial displacement (CAD) induced by the ocular pulse. Here we compared CAD and a stiffness index derived from CAD between keratoconus patients and normal controls. We also explored the trend of these parameters with keratoconus grade. Twenty normal subjects and twenty keratoconus patients were recruited in this study. Corneal topography, tomography, intraocular pressure (IOP) and ocular pulse amplitude (OPA) were obtained in each measured eye. The cornea’s heartbeat-induced cyclic axial displacements were measured by high-frequency (50 MHz) ultrasound. A corneal stiffness index (CSI) was derived from CAD normalized against OPA. CAD and CSI were compared between normal and keratoconus groups, and across keratoconus grades. Keratoconus corneas had significantly greater CAD and lower CSI than normal controls (p’s<0.01). Both parameters correlated strongly with grade, in which CAD increased significantly (p = 0.002) and CSI decreased significantly (p = 0.011) with grade. These results suggested a biomechanical weakening in keratoconus which worsens at higher disease severity. This study also demonstrated the ability of high-frequency ultrasound elastography to provide a safe, quick, and accurate evaluation of the cornea’s biomechanical condition in vivo. The OPE-measured biomechanical metrics, when integrated with existing diagnostic criteria, may aid the decision-making in the early and definitive diagnosis and staging of keratoconus.

In Vivo Determination of the Human Corneal Elastic Modulus Using Vibrational Optical Coherence Tomography

Purpose

To determine the in vivo elastic modulus of the human cornea using vibrational optical coherence tomography (VOCT).

Methods

Vibrational analysis coupled with optical coherence tomography (OCT) was used to obtain the resonant frequency (RF) and elastic modulus of corneal structural components. VOCT corneal thickness values were measured using OCT images and correlated with corneal thickness determined with Pentacam (Oculus, Wetzlar, Germany). Moduli were obtained at two locations: central cornea (CC) and inferior cornea (IC). Measurements were obtained with and without anesthetic eye drops to assess their effect on the modulus measurements.

Results

VOCT thickness values correlated positively (R² = 0.97) and linearly (y = 1.039x–16.89) with those of Pentacam. Five RF peaks (1–5) were present, although their presence was variable across eyes. The RF for peaks 1 to 5 in the CC and IC ranged from 73.5 ± 4.9 to 239 ± 3 Hz and 72.1 ± 6.3 to 238 ± 4 Hz, respectively. CC and IC moduli for peaks 1 to 5 ranged from 1.023 ± 0.104 to 6.87 ± 0.33 MPa and 0.98 ± 0.15 to 6.52 ± 0.79 MPa, respectively. Topical anesthesia did not significantly alter the modulus (P > 0.05 for all), except for peak 2 in the CC (P < 0.05).

Conclusions

This pilot study demonstrates the utility of VOCT as an in vivo, noninvasive technology to measure the elastic modulus in human corneas. The structural origin of these moduli is hypothesized based on previous reports, and further analyses are necessary for confirmation.

Translational Relevance

This work presents VOCT as a novel approach to assess the in vivo elastic modulus of the cornea, an indicator of corneal structural integrity and health.

Properties of the acellular porcine cornea crosslinked with UVA/riboflavin as scaffolds for Boston Keratoprosthesis

The Boston Keratoprosthesis type I (B-KPro) is widely used in the world, but the lack of donor corneas limits its application. This study aims to prepare the acellular porcine cornea (APC) crosslinked with ultraviolet A (UVA)/riboflavin instead of donor corneas as the scaffold for B-KPro. Decellularization of freeze-thaw combined with biological enzymes resulted in approximately 5 ng/mg DNA residue, the a-Gal removal rate of 99%, and glycosaminoglycans retention at a high level of 46.66 ± 2.59 mg/mg. UVA/ riboflavin cross-linking was adopted to induce the formation of new chemical bonds between adjacent collagen chains in the corneal stroma to improve the mechanical properties and resistance to enzymatic hydrolysis. Through comprehensive analysis of the biomechanics, enzyme degradation, immunogenicity and histological structure of the APC crosslinked at different times, CL3 (irradiation conditions, 365 nm, 3 mW/cm, 80 min, both sides) was selected and transplanted into the rabbit cornea model through interlamellar keratoplasty and penetrating keratoplasty as the scaffold of the B-KPro. Compared with the native porcine cornea (NPC) and APC, the experiment of interlamellar pocket indicated that the structure of CL3 was homogeneous without degradation and vascularization in vivo at 12 weeks after surgery. Simultaneously, the results of transplantation of B-KPro showed complete epithelialization of CL3 within 1 week, and neovascularization of the cornea indicated rejection but could be controlled with immunosuppressants. At 3 months postoperatively, the lens of B-KPro remained transparent, and the structure of CL3 was compact and uniform, accompanied by the migration and proliferation of a large number of stromal cells without degradation, suggesting the CL3 could be a promising corneal substitute.

Differences of Corneal Biomechanics Among Thin Normal Cornea, Forme-Fruste Keratoconus, and Cornea After SMILE

Background: To compare the corneal biomechanics of thin normal cornea (TNC) with thinnest corneal thickness (TCT) (≤500 µm), forme-fruste keratoconus (FFKC) and cornea after small incision lenticule extraction (Post-SMILE) had their central corneal thickness (CCT) matched by Corneal Visualization Scheimpflug Technology (Corvis ST).

Methods: CCT were matched in 23 eyes with FFKC, 23 eyes by SMILE in 3 months post-operatively, and 23 TNC eyes. The differences in corneal biomechanics by Corvis ST among the three groups were compared.

Results: There was no significant difference in CCT among the three groups, and the biomechanically corrected intraocular pressure (bIOP) did not differ significantly among the three groups (all p &gt; 0.05). There were significant differences in most DCR parameters between pre- and post-operatively (all p &lt; 0.05). Compared with TNC, the values of corneal deflection amplitude during the first applanation (A1DA), length at the first applanation (A1L), corneal deflection amplitude during the second applanation (A2DA), and maximum deformation amplitude (DA) decreased in 3 months after SMILE (all p &lt; 0.05), these values increased in the FFKC (all p &lt; 0.05).

Conclusion: The majority of the DCR parameters were different among the three groups. The parameters A1DA, A1L, A2DA, and DA may be different between TNC and Post-SMILE, TNC and FFKC, and Post-SMILE and FFKC.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Multimodal Heartbeat and Compression Optical Coherence Elastography for Mapping Corneal Biomechanics

The biomechanical properties of the cornea have a profound influence on the health, structural integrity, and function of the eye. Understanding these properties may be critical for diagnosis and identifying disease pathogenesis. This work demonstrates how two different elastography techniques can be combined for a multimodal approach to measuring corneal biomechanical properties. Heartbeat optical coherence elastography (Hb-OCE) and compression OCE were performed simultaneously to measure the stiffness of the cornea in an in vivo rabbit model. Measurements were further performed after collagen crosslinking to demonstrate how the combined technique can be used to measure changes in corneal stiffness and map mechanical contrast. The results of this work further suggest that measurements from Hb-OCE and compression OCE are comparable, meaning that Hb-OCE and compression OCE may be used interchangeably despite distinct differences in both techniques.

Correlation of the Corvis Biomechanical Factor with tomographic parameters in keratoconus

PURPOSE

To investigate the relationship between corneal biomechanics and keratoconus (KC) severity as described by tomographic parameters.

SETTING

University-based German ophthalmology department.

DESIGN

Retrospective cross-sectional study.

METHODS

A total of 448 KC corneas of the Homburg Keratoconus Center and 112 healthy corneas (448+112 patients) were examined by Pentacam high-resolution and Corneal Visualization Scheimpflug Technology (Pentacam HR and Corvis ST). The KC population included a wide spectrum of disease severity based on Belin's ABCD classification. Linear regression analysis was performed between the linear term of the Corvis Biomechanical Index (CBI) (CBI beta) and the tomographic values anterior radius of curvature (ARC), posterior radius of curvature (PRC), and thinnest corneal thickness (TCT). A linear transformation of the CBI beta was performed to provide an intuitive scaling, which was referred to as the Corvis Biomechanical Factor (CBiF = -0.24294226 × CBI beta + 6.02). This scaling adjusted the CBI beta to the same scale as posterior corneal curvature (PRC).

RESULTS

There was a high correlation of the CBI beta and its modification, the CBiF, with TCT (Pearson, r = -0.775), ARC (r = -0.835), and PRC (r = -0.839) in the KC population (P < .001). In the control corneas, the correlation between the CBI beta and ARC was weak (r = -0.216, P = .022), not significant (PRC, r = -0.146, P = .125), or moderate (TCT, r = -0.628, P < .001).

CONCLUSIONS

The linear term of the CBI was highly associated with KC severity as defined by corneal tomography. The CBiF represents a new scale based on biomechanical characteristics in KC, which could serve as a basis for a biomechanical KC classification in the future.

Optical Coherence Elastography as a Tool for Studying Deformations in Biomaterials: Spatially-Resolved Osmotic Strain Dynamics in Cartilaginous Samples

This paper presents a recently developed variant of phase-resolved Optical Coherence Elastography (OCE) enabling non-contact visualization of transient local strains of various origins in biological tissues and other materials. In this work, we demonstrate the possibilities of this new technique for studying dynamics of osmotically-induced strains in cartilaginous tissue impregnated with optical clearing agents (OCA). For poroelastic water-containing biological tissues, application of non-isotonic OCAs, various contrast additives, as well as drug solutions administration, may excite transient spatially-inhomogeneous strain fields of high magnitude in the tissue bulk, initiating mechanical and structural alterations. The range of the strain reliably observed by OCE varied from ±10-3 to ±0.4 for diluted and pure glycerol, correspondingly. The OCE-technique used made it possible to reveal previously inaccessible details of the complex spatio-temporal evolution of alternating-sign osmotic strains at the initial stages of agent diffusion. Qualitatively different effects produced by particular hydrophilic OCAs, such as glycerol and iohexol, are discussed, as well as concentration-dependent differences. Overall, the work demonstrates the unique abilities of the new OCE-modality in providing a deeper insight in real-time kinetics of osmotically-induced strains relevant to a broad range of biomedical applications.

Keratoconus: An updated review

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

Optical coherence tomography

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Quantitative compressive optical coherence elastography using structural OCT imaging and optical palpation to measure soft contact lens mechanical properties

In this study, the principle of 'optical palpation' was applied to a compression optical coherence elastography (OCE) method using spectral domain optical coherence tomography (OCT). Optical palpation utilizes a compliant transparent material of known mechanical properties, which acts as a stress sensor, in order to derive the mechanical properties of a sample material under examination. This technique was applied to determine the mechanical properties of soft contact lenses, with one lens being used as the compliant stress sensor and the other as the sample under investigation to extract the mechanical properties. This compliant stress sensor allowed for the stress of the compression to be measured without the use of a force sensor. The strain of the materials was measured through an automatic boundary segmentation that tracks the material thickness (of the sensor and the sample) during compression through sequential structural OCT images. A total of five contact lens combinations were tested, using three separate commercially available contact lenses with unique mechanical properties. Various combinations of contact lens materials were used to further validate the technique. The Young's modulus derived from this method was compared to nominal manufacturer's values. Both accuracy and repeatability were assessed, with highly accurate measurements obtained, with a percentage difference between the nominal and experimentally derived Young's modulus being less than 6% for all the tested combinations as well as providing a Young's modulus that was not statistically significant different (p > 0.01) to the nominal value. The results demonstrate the potential of optical palpation in OCE to accurately measure the mechanical properties of a material without the use of sophisticated electronics to capture the stress of the sample. These findings have potential to be translated into a method for tissue mechanical testing with ex vivo and in vivo clinical applications.

Real-Time Strain and Elasticity Imaging in Phase-Sensitive Optical Coherence Elastography Using a Computationally Efficient Realization of the Vector Method

We present a real-time realization of OCT-based elastographic mapping local strains and distribution of the Young’s modulus in biological tissues, which is in high demand for biomedical usage. The described variant exploits the principle of Compression Optical Coherence Elastography (C-OCE) and uses processing of phase-sensitive OCT signals. The strain is estimated by finding local axial gradients of interframe phase variations. Instead of the popular least-squares method for finding these gradients, we use the vector approach, one of its advantages being increased computational efficiency. Here, we present a modified, especially fast variant of this approach. In contrast to conventional correlation-based methods and previously used phase-resolved methods, the described method does not use any search operations or local calculations over a sliding window. Rather, it obtains local strain maps (and then elasticity maps) using several transformations represented as matrix operations applied to entire complex-valued OCT scans. We first elucidate the difference of the proposed method from the previously used correlational and phase-resolved methods and then describe the proposed method realization in a medical OCT device, in which for real-time processing, a “typical” central processor (e.g., Intel Core i7-8850H) is sufficient. Representative examples of on-flight obtained elastographic images are given. These results open prospects for broad use of affordable OCT devices for high-resolution elastographic vitalization in numerous biomedical applications, including the use in clinic.

Enhanced medical diagnosis for dOCTors: a perspective of optical coherence tomography

SIGNIFICANCE

After three decades, more than 75,000 publications, tens of companies being involved in its commercialization, and a global market perspective of about USD 1.5 billion in 2023, optical coherence tomography (OCT) has become one of the fastest successfully translated imaging techniques with substantial clinical and economic impacts and acceptance.

AIM

Our perspective focuses on disruptive forward-looking innovations and key technologies to further boost OCT performance and therefore enable significantly enhanced medical diagnosis.

APPROACH

A comprehensive review of state-of-the-art accomplishments in OCT has been performed.

RESULTS

The most disruptive future OCT innovations include imaging resolution and speed (single-beam raster scanning versus parallelization) improvement, new implementations for dual modality or even multimodality systems, and using endogenous or exogenous contrast in these hybrid OCT systems targeting molecular and metabolic imaging. Aside from OCT angiography, no other functional or contrast enhancing OCT extension has accomplished comparable clinical and commercial impacts. Some more recently developed extensions, e.g., optical coherence elastography, dynamic contrast OCT, optoretinography, and artificial intelligence enhanced OCT are also considered with high potential for the future. In addition, OCT miniaturization for portable, compact, handheld, and/or cost-effective capsule-based OCT applications, home-OCT, and self-OCT systems based on micro-optic assemblies or photonic integrated circuits will revolutionize new applications and availability in the near future. Finally, clinical translation of OCT including medical device regulatory challenges will continue to be absolutely essential.

CONCLUSIONS

With its exquisite non-invasive, micrometer resolution depth sectioning capability, OCT has especially revolutionized ophthalmic diagnosis and hence is the fastest adopted imaging technology in the history of ophthalmology. Nonetheless, OCT has not been completely exploited and has substantial growth potential-in academics as well as in industry. This applies not only to the ophthalmic application field, but also especially to the original motivation of OCT to enable optical biopsy, i.e., the in situ imaging of tissue microstructure with a resolution approaching that of histology but without the need for tissue excision.

Depth-resolved Corneal Biomechanical Changes Measured Via Optical Coherence Elastography Following Corneal Crosslinking

Purpose

To evaluate depth-resolved changes of corneal biomechanical properties in eyes with corneal ectasia after corneal crosslinking (CXL) using optical coherence elastography.

Methods

In a prospective pilot series of eyes with corneal ectasia, a custom high-speed swept source optical coherence tomography system was used to image the cornea before and 3 months after CXL during a low-speed applanating deformation while monitoring applanation force. Cross-correlation was applied to track frame-by-frame two-dimensional optical coherence tomography speckle displacements, and the slope of force versus local axial displacement behavior during the deformation was used to produce a two-dimensional array of axial stiffness (k). These values were averaged for anterior (k_a) and posterior (k_p) stromal regions and expressed as a ratio (k_a/k_p) to assess depth-dependent differences in stiffness. CXL was performed according to the Dresden protocol with a system approved by the U.S. Food and Drug Administration.

Results

Four eyes from four patients with keratoconus (n = 3) or post-LASIK ectasia (n = 1) underwent optical coherence elastography before and 3 months after CXL. The mean k_a/k_p was 1.03 ± 0.07 before CXL compared with 1.34 ± 0.17 after the CXL procedure. All four eyes demonstrated at least a 20% increase in the k_a/k_p.

Conclusions

Preferential stiffening of the anterior stroma with the standard CXL protocol was demonstrated with optical coherence elastography in live human subjects.

Translational Relevance

Although ex vivo studies have demonstrated anterior stiffening effects after CXL using various destructive and nondestructive methods, this report presents the first evidence of such changes in serial live human measurements.

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

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

Ultrasound Shear Wave Elastography and Transient Optical Coherence Elastography: Side-by-Side Comparison of Repeatability and Accuracy

Objective: We compare the repeatability and accuracy of ultrasound shear wave elastography (USE) and transient optical coherence elastography (OCE). Methods: Elastic wave speed in gelatin phantoms and chicken breast was measured with USE and OCE and compared with uniaxial mechanical compression testing. Intra- and Inter-repeatability were analyzed using Bland-Altman plots and intraclass correlation coefficients (ICC). Results: OCE and USE differed from uniaxial testing by a mean absolute percent error of 8.92% and 16.9%, respectively, across eight phantoms of varying stiffness. Upper and lower limits of agreement for intrasample repeatability for USE and OCE were ±0.075 m/s and −0.14 m/s and 0.13 m/s, respectively. OCE and USE both had ICCs of 0.9991. In chicken breast, ICC for USE was 0.9385 and for OCE was 0.9924. Conclusion: OCE and USE can detect small speed changes and give comparable measurements. These measurements correspond well with uniaxial testing.

Compressional Optical Coherence Elastography of the Cornea

Assessing the biomechanical properties of the cornea is crucial for detecting the onset and progression of eye diseases. In this work, we demonstrate the application of compression-based optical coherence elastography (OCE) to measure the biomechanical properties of the cornea under various conditions, including validation in an in situ rabbit model and a demonstration of feasibility for in vivo measurements. Our results show a stark increase in the stiffness of the corneas as IOP was increased. Moreover, UV-A/riboflavin corneal collagen crosslinking (CXL) also dramatically increased the stiffness of the corneas. The results were consistent across 4 different scenarios (whole CXL in situ, partial CXL in situ, whole CXL in vivo, and partial CXL in vivo), emphasizing the reliability of compression OCE to measure corneal biomechanical properties and its potential for clinical applications.

Corneal biomechanics: Measurement and structural correlations

The characterization of corneal biomechanical properties has important implications for the management of ocular disease and prediction of surgical responses. Corneal refractive surgery outcomes, progression or stabilization of ectatic disease, and intraocular pressure determination are just examples of the many key clinical problems that depend highly upon corneal biomechanical characteristics. However, to date there is no gold standard measurement technique. Since the advent of a 1-dimensional (1D) air-puff based technique for measuring the corneal surface response in 2005, advances in clinical imaging technology have yielded increasingly sophisticated approaches to characterizing the biomechanical properties of the cornea. Novel analyses of 1D responses are expanding the clinical utility of commercially-available air-puff-based instruments, and other imaging modalities-including optical coherence elastography (OCE), Brillouin microscopy and phase-decorrelation ocular coherence tomography (PhD-OCT)-offer new opportunities for probing local biomechanical behavior in 3-dimensional space and drawing new inferences about the relationships between corneal structure, mechanical behavior, and corneal refractive function. These advances are likely to drive greater clinical adoption of in vivo biomechanical analysis and to support more personalized medical and surgical decision-making.

Heartbeat optical coherence elastography: corneal biomechanics in vivo

SIGNIFICANCE

Mechanical assessment of the cornea can provide important structural and functional information regarding its health. Current clinically available tools are limited in their efficacy at measuring corneal mechanical properties. Elastography allows for the direct estimation of mechanical properties of tissues in vivo but is generally performed using external excitation force.

AIM

To show that heartbeat optical coherence elastography (Hb-OCE) can be used to assess the mechanical properties of the cornea in vivo.

APPROACH

Hb-OCE was utilized to detect Hb-induced deformations in the rabbit cornea in vivo without the need for external excitation. Furthermore, we demonstrate how this technique can distinguish corneal stiffness between untreated (UT) and crosslinked (CXL) tissue.

RESULTS

Our results demonstrate that stiffness changes in the cornea can be detected using only the Hb-induced deformations in the cornea. Additionally, we demonstrate a statistically significant difference in strain between the UT and CXL corneas.

CONCLUSIONS

Hb-OCE may be an effective tool for assessing the mechanical properties of the cornea in vivo without the need for external excitation. This tool may be effective for clinical assessment of corneal mechanical properties because it only requires optical coherence tomography imaging and data processing.

Heartbeat-Induced Corneal Axial Displacement and Strain Measured by High Frequency Ultrasound Elastography in Human Volunteers

Purpose

The purpose of this study was to establish in vivo data acquisition and processing protocols for repeatable measurements of heartbeat-induced corneal displacements and strains in human eyes, using a high-frequency ultrasound elastography method, termed ocular pulse elastography (OPE).

Methods

Twenty-four volunteers with no known ocular diseases were recruited for this study. Intraocular pressure (IOP) and ocular pulse amplitude (OPA) were measured using a PASCAL Dynamic Contour Tonometer (DCT). An in vivo OPE protocol was developed to measure heartbeat-induced corneal displacements. Videos of the central 5.7 mm of the cornea were acquired using a 50-MHz ultrasound probe at 128 frames per second. The radiofrequency data of 1000 frames were analyzed using an ultrasound speckle tracking algorithm to calculate corneal displacements and quantify spectral and temporal characteristics. The intrasession and intersession repeatability of OPE- and DCT-measured parameters were also analyzed.

Results

The in vivo OPE protocol and setup were successful in tracking heartbeat-induced corneal motion using high-frequency ultrasound. Corneal axial displacements showed a strong cardiac rhythm, with good intrasession and intersession repeatability, and high interocular symmetry. Corneal strain was calculated in two eyes of two subjects, showing substantially different responses.

Conclusions

We demonstrated the feasibility of high-frequency ultrasound elastography for noninvasive in vivo measurement of the cornea's biomechanical responses to the intrinsic ocular pulse. The high intrasession and intersession repeatability suggested a robust implementation of this technique to the in vivo setting.

Translational Relevance

OPE may offer a useful tool for clinical biomechanical evaluation of the cornea by quantifying its response to the intrinsic pulsation.

Advances in the diagnosis and treatment of keratoconus

Keratoconus had traditionally been considered a rare disease at a time when the imaging technology was inept in detecting subtle manifestations, resulting in more severe disease at presentation. The increased demand for refractive surgery in recent years also made it essential to more effectively detect keratoconus before attempting any ablative procedure. Consequently, the armamentarium of tools that can be used to diagnose and treat keratoconus has significantly expanded. The advances in imaging technology have allowed clinicians and researchers alike to visualize the cornea layer by layer looking for any early changes that might be indicative of keratoconus. In addition to the conventional geometrical evaluation, efforts are also underway to enable spatially resolved corneal biomechanical evaluation. Artificial intelligence has been exploited in a multitude of ways to enhance diagnostic efficiency and to guide treatment. As for treatment, corneal cross-linking treatment remains the mainstay preventive approach, yet the current main focus of research is on increasing oxygen availability and developing new strategies to improve riboflavin permeability during the procedure. Some new combined protocols are being proposed to simultaneously halt keratoconus progression and correct refractive error. Bowman layer transplantation and additive keratoplasty are newly emerging alternatives to conventional keratoplasty techniques that are used in keratoconus surgery. Advances in tissue engineering and regenerative therapy might bring new perspectives for treatment at the cellular level and hence obviate the need for invasive surgeries. In this review, we describe the advances in the diagnosis and treatment of keratoconus primarily focusing on newly emerging approaches and strategies.