Three-dimensional mapping of the orientation of collagen corneal lamellae in healthy and keratoconic human corneas using SHG microscopy

Imaging with a twist: Three-dimensional insights of the chiral nematic phase of cellulose nanocrystals via SHG microscopy

Cellulose nanocrystals (CNCs) are bio-based nanoparticles that, under the right conditions, self-align into chiral nematic liquid crystals with a helical pitch. In this work, we exploit the inherent confocal effect of second-harmonic generation (SHG) microscopy to acquire highly resolved three-dimensional (3D) images of the chiral nematic phase of CNCs in a label-free manner. An in-depth analysis revealed a direct link between the observed variations in SHG intensity and the pitch. The highly contrasted 3D images provided unprecedented detail into liquid crystal's native structure. Local alignment, morphology, as well as the presence of defects are readily revealed, and a provisional framework relating the SHG response to the orientational distribution of CNC nanorods within the liquid crystal structure is presented. This paper illustrates the numerous benefits of using SHG microscopy for visualizing CNC chiral nematic systems directly in the suspension-liquid phase and paves the road for using SHG microscopy to characterize other types of aligned CNC structures, in wet and dry states.

Spectroscopic analysis of the sum-frequency response of the carbon-hydrogen stretching modes in collagen type I

We studied the origin of the vibrational signatures in the sum-frequency generation (SFG) spectrum of fibrillar collagen type I in the carbon-hydrogen stretching regime. For this purpose, we developed an all-reflective, laser-scanning SFG microscope with minimum chromatic aberrations and excellent retention of the polarization state of the incident beams. We performed detailed SFG measurements of aligned collagen fibers obtained from rat tail tendon, enabling the characterization of the magnitude and polarization-orientation dependence of individual tensor elements Xijk2 of collagen's nonlinear susceptibility. Using the three-dimensional atomic positions derived from published crystallographic data of collagen type I, we simulated its Xijk2 elements for the methylene stretching vibration and compared the predicted response with the experimental results. Our analysis revealed that the carbon-hydrogen stretching range of the SFG spectrum is dominated by symmetric stretching modes of methylene bridge groups on the pyrrolidine rings of the proline and hydroxyproline residues, giving rise to a dominant peak near 2942 cm-1 and a shoulder at 2917 cm-1. Weak asymmetric stretches of the methylene bridge group of glycine are observed in the region near 2870 cm-1, whereas asymmetric CH2-stretching modes on the pyrrolidine rings are found in the 2980 to 3030 cm-1 range. These findings help predict the protein's nonlinear optical properties from its crystal structure, thus establishing a connection between the protein structure and SFG spectroscopic measurements.

Comparing the clinical applicability of wavefront phase imaging in keratoconus versus normal eyes

The aim of this work is to quantitatively assess the wavefront phase of keratoconic eyes measured by the ocular aberrometer t·eyede (based on WaveFront Phase Imaging Sensor), characterized by a lateral resolution of 8.6 µm without requiring any optical element to sample the wavefront information. We evaluated the parameters: root mean square error, Peak-to-Valley, and amplitude of the predominant frequency (Fourier Transform analysis) of a section of the High-Pass filter map in keratoconic and healthy cohorts. Furthermore, we have analyzed keratoconic eyes that presented dark-light bands in this map to assess their period and orientation with the Fourier Transform. There are significant statistical differences (p value < 0.001) between healthy and keratoconic eyes in the three parameters, demonstrating a tendency to increase with the severity of the disease. Otherwise, the quantification of the bands reveals that the width is independent of eye laterality and keratoconic stage as orientation, which tends to be oblique. In conclusion, the quantitative results obtained with t·eyede could help to diagnose and monitor the progression of keratoconus.

Fourier transform-based method for quantifying the three-dimensional orientation distribution of fibrous units

Several materials and tissues are characterized by a microstructure composed of fibrous units embedded in a ground matrix. In this paper, a novel three-dimensional (3D) Fourier transform-based method for quantifying the distribution of fiber orientations is presented. The method allows for an accurate identification of individual fiber families, their in-plane and out-of-plane dispersion, and showed fast computation times. We validated the method using artificially generated 3D images, in terms of fiber dispersion by considering the error between the standard deviation of the reconstructed and the prescribed distributions of the artificial fibers. In addition, we considered the measured mean orientation angles of the fibers and validated the robustness using a measure of fiber density. Finally, the method is employed to reconstruct a full 3D view of the distribution of collagen fiber orientations based on in vitro second harmonic generation microscopy of collagen fibers in human and mouse skin. The dispersion parameters of the reconstructed fiber network can be used to inform mechanical models of soft fiber-reinforced materials and biological tissues that account for non-symmetrical fiber dispersion.

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

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

Unveiling the lamellar structure of the human cornea over its full thickness using polarization-resolved SHG microscopy

A key property of the human cornea is to maintain its curvature and consequently its refraction capability despite daily changes in intraocular pressure. This is closely related to the multiscale structure of the corneal stroma, which consists of 1-3 µm-thick stacked lamellae made of thin collagen fibrils. Nevertheless, the distribution, size, and orientation of these lamellae along the depth of the cornea are poorly characterized up to now. In this study, we use second harmonic generation (SHG) microscopy to visualize the collagen distribution over the full depth of 10 intact and unstained human corneas (500-600 µm thick). We take advantage of the small coherence length in epi-detection to axially resolve the lamellae while maintaining the corneal physiological curvature. Moreover, as raw epi-detected SHG images are spatially homogenous because of the sub-wavelength size of stromal collagen fibrils, we use a polarimetric approach to measure the collagen orientation in every voxel. After a careful validation of this approach, we show that the collagen lamellae (i) are mostly oriented along the inferior-superior axis in the anterior stroma and along the nasal-temporal axis in the posterior stroma, with a gradual shift in between and (ii) exhibit more disorder in the anterior stroma. These results represent the first quantitative characterization of the lamellar structure of the human cornea continuously along its entire thickness with micrometric resolution. It also shows the unique potential of P-SHG microscopy for imaging of collagen distribution in thick dense tissues.

Long-Term Zonal Average Analysis of Corneal Tomography in Keratoconus Eyes

PURPOSE

The scope of this study was to investigate keratoconus progression using zonal average analysis of corneal tomography.

METHODS

The corneal tomographies of patients participating in initial baseline and all scheduled follow-up visits up to 4 years were analyzed. Data were exported in custom software, which delineated 4 zones of analysis and calculated the average values of the anterior and posterior curvature and the average thickness for each zone at each visit. In particular, a 3.1 mm 2 area containing the K max , termed "keratoconus cone zone," was defined for assessing disease progression during the follow-up.

RESULTS

A total of 201 patients were enrolled in this prospective study. At 4 years, 31% of the eyes (n = 62) had an average increase of ≥1.0 D in the keratoconus cone zone in baseline visit, whereas only 11% of the eyes (n = 22) had ≥1.0 D K max increase in the same period. The zonal anterior average curvature (+1.1 D; P < 0.001) and thickness (-14 μm; P < 0.001) values of the keratoconus cone zone progressed significantly during the follow-up. A high correlation was found between the 4-year changes of K max and central corneal thickness values and the change of the average anterior curvature and thickness values in the keratoconus cone zone. The posterior cornea did not show significant average changes (<-0.2 D; P > 0.05) during the follow-up.

CONCLUSIONS

Single-point tomography indexes for keratoconus progression did not capture the overall structure and shape changes of the cornea. It would be recommended to update criteria for keratoconus management including the zonal average analysis of curvature and thickness values for tracking disease progression over observation periods longer than 1 year.

Two-Photon Imaging for Non-Invasive Corneal Examination

Two-photon imaging (TPI) microscopy, namely, two-photon excited fluorescence (TPEF), fluorescence lifetime imaging (FLIM), and second-harmonic generation (SHG) modalities, has emerged in the past years as a powerful tool for the examination of biological tissues. These modalities rely on different contrast mechanisms and are often used simultaneously to provide complementary information on morphology, metabolism, and structural properties of the imaged tissue. The cornea, being a transparent tissue, rich in collagen and with several cellular layers, is well-suited to be imaged by TPI microscopy. In this review, we discuss the physical principles behind TPI as well as its instrumentation. We also provide an overview of the current advances in TPI instrumentation and image analysis. We describe how TPI can be leveraged to retrieve unique information on the cornea and to complement the information provided by current clinical devices. The present state of corneal TPI is outlined. Finally, we discuss the obstacles that must be overcome and offer perspectives and outlooks to make clinical TPI of the human cornea a reality.

Corneal imaging with blue-light optical coherence microscopy

Corneal imaging is important for the diagnostic and therapeutic evaluation of many eye diseases. Optical coherence tomography (OCT) is extensively used in ocular imaging due to its non-invasive and high-resolution volumetric imaging characteristics. Optical coherence microscopy (OCM) is a technical variation of OCT that can image the cornea with cellular resolution. Here, we demonstrate a blue-light OCM as a low-cost and easily reproducible system to visualize corneal cellular structures such as epithelial cells, endothelial cells, keratocytes, and collagen bundles within stromal lamellae. Our blue-light OCM system achieved an axial resolution of 12 µm in tissue over a 1.2 mm imaging depth, and a lateral resolution of 1.6 µm over a field of view of 750 µm × 750 µm.

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

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

Evaluation of topographic, tomographic, topometric, densitometric, and aberrometric features of cornea with pentacam HR system in subclinical keratoconus

PURPOSE

To investigate topographic, tomographic, topometric, densitometric, and aberrometric parameters in subclinical keratoconus with the Pentacam HR imaging system.

METHODS

Data of 3128 patients were evaluated, finding in 108 patients clinical keratoconus in one eye and subclinical keratoconus in the other. Corneal topographic, tomographic, topometric, densitometric, and aberrometric values obtained using the Pentacam HR imaging system were compared between clinical keratoconus, subclinical keratoconus, and normal eyes.

RESULTS

Comparing eyes with subclinical keratoconus and the control group, while flat K, horizontal coma, horizontal trefoil, and vertical trefoil values were similar (p > 0.05 for each), all other parameters were significantly different (p < 0.05 for each). Densitometry values of eyes with subclinical keratoconus were significantly higher in all layers of the 0-2 mm annular area and in the anterior and central layers of the 2-6 mm annular area compared to the control group (p < 0.05 for each). According to the receiver operating characteristic curve analysis, the densitometry region with the largest area under the curve was the anterior layer of the 0-2 mm annular area. The sensitivity in this region was 79.4% and the specificity 73.2% in distinguishing eyes with subclinical keratoconus from normal eyes when 19.3 GSU was considered the threshold.

CONCLUSION

Corneal densitometry values in the 0-2 and 2-6 mm annular areas, especially in the anterior layers, are parameters that can be used to predict and distinguish subclinical keratoconus from normal eyes.

Femtosecond Lasers in Cornea & Refractive Surgery

Since the introduction of femtosecond laser (FS) systems for corneal flap creation in laser-assisted in-situ keratomileusis there have been numerous applications for FS laser in corneal surgery. This manuscript details the utility of FS lasers in corneal surgical procedures including refractive laser surgeries, intracorneal ring segment tunnels, presbyopic treatments, and FS-assisted keratoplasty. We also review the role of FS lasers in diagnostic procedures such as two photon excitation fluorescence and second harmonic generation.

Collagen ultrastructural symmetry and its malignant alterations in human breast cancer revealed by polarization-resolved second-harmonic generation microscopy

Several specific alterations of the extracellular matrix can be considered a distinctive hallmark of cancer. In particular, a different morphology of the collagen scaffold is frequently found within the peritumoural environment. In this study, we report about a significant difference in the ultrastructural organization of collagen at the supra-molecular level between the perilesional scaffold and the tumour area in human breast carcinoma samples. In particular, we demonstrated that polarization-resolved second-harmonic generation (P-SHG) microscopy is able to link the altered collagen architecture at the ultrastructural level found in perilesional tissue with a different organization of collagen fibrils at the molecular level.

Noise reduction and quantification of fiber orientations in greyscale images

Quantification of the angular orientation distribution of fibrous tissue structures in scientific images benefits from the Fourier image analysis to obtain quantitative information. Measurement uncertainties represent a major challenge and need to be considered by propagating them in order to determine an adaptive anisotropic Fourier filter. Our adaptive filter method (AF) is based on the maximum relative uncertainty δcut of the power spectrum as well as a weighted radial sum with weighting factor α. We use a Monte-Carlo simulation to obtain realistic greyscale images that include defined variations in fiber thickness, length, and angular dispersion as well as variations in noise. From this simulation the best agreement between predefined and derived angular orientation distribution is found for evaluation parameters δcut = 2.1% and α = 1.5. The resulting cumulative orientation distribution was modeled by a sigmoid function to obtain the mean angle and the fiber dispersion. A comparison to a state-of-the-art band-pass method revealed that the AF method is more suitable for the application on greyscale fiber images, since the error of the fiber dispersion significantly decreased from (33.9 ± 26.5)% to (13.2 ± 12.7)%. Both methods were found to accurately quantify the mean fiber orientation with an error of (1.9 ± 1.5)° and (2.3 ± 2.1)° in case of the AF and the band-pass method, respectively. We demonstrate that the AF method is able to accurately quantify the fiber orientation distribution in in vivo second-harmonic generation images of dermal collagen with a mean fiber orientation error of (6.0 ± 4.0)° and a dispersion error of (9.3 ± 12.1)%.

Quantification of collagen structural changes during chick corneal development

As the most abundant structural mammalian protein, collagen has been implicated in the pathogenesis of numerous diseases such as osteogenesis imperfecta, and cancer. In the case of cornea, abnormal cornea development can lead to conditions such as agenesis, megalocornea, microcornea, and cornea plana. Therefore, understanding the mechanisms of collagen assembly during development may contribute to the prevention or treatment of corneal diseases. In this study, we applied fast Fourier transform second harmonic generation microscopy to quantify parameters of corneal structures during chick development. Our results show that both the rotational pitch and overall rotational angle of corneal stroma modulate between E9 and E19. In addition, we found that corneal structures between left and right corneas are highly correlated during development.

Self-assembling amyloid-like peptides as exogenous second harmonic probes for bioimaging applications

Amyloid-like peptides are an ideal model for the mechanistic study of amyloidosis, which may lead to many human diseases, such as Alzheimer disease. This study reports a strong second harmonic generation (SHG) effect of amyloid-like peptides, having a signal equivalent to or even higher than those of endogenous collagen fibers. Several amyloid-like peptides (both synthetic and natural) were examined under SHG microscopy and shown they are SHG-active. These peptides can also be observed inside cells (in vitro). This interesting property can make these amyloid-like peptides second harmonic probes for bioimaging applications. Furthermore, SHG microscopy can provide a simple and label-free approach to detect amyloidosis. Lattice corneal dystrophy was chosen as a model disease of amyloidosis. Morphological difference between normal and diseased human corneal biopsy samples can be easily recognized, proving that SHG can be a useful tool for disease diagnosis.

Second harmonic generation imaging reveals asymmetry in the rotational helicity of collagen lamellae in chicken corneas

High tensile strength and optical clarity are unique properties of the cornea. These features are dictated by the three-dimensional architecture of corneal lamellae. Therefore, understanding the microscopic details of the cornea's structural organization may contribute to the development of artificial cornea for the treatment of corneal diseases. In this study, the combination of forward second harmonic generation (SHG) microcopy and fast Fourier-transform based image analysis was used to characterize the depth-dependent superstructure of chicken corneal stroma. Our results show that from the surface, adjacent lamellae of anterior chicken cornea lamella rotate in a counterclockwise direction, and the same rotational helicity is observed in left and right corneas. Furthermore, the overall average rotational pitch of lamellae is 0.92 ± 0.11 degree/µm which persists for 176 ± 14 µm in the anterior stroma. As depth further increased, the rate of lamellar rotation decreases. Upon reaching posterior stroma, lamellar orientation remains constant. Throughout the stroma, collagen lamellae in chicken rotate a total of 169 ± 21 degrees. The lack of lamellar rotation in posterior stroma suggests that packing efficiency cannot be used to explain the helicity of depth-dependent rotation of anterior stroma. In addition, although the right cornea has a higher rotational pitch (0.95 ± 11 vs 0.90 ± 10 degrees/µm) and thinner anterior stroma (173 ± 13 vs 179 ± 14 µm) than the left cornea, the two effects cancel each other out and result in similar total angular rotation of anterior stroma (161 ± 23 and 165 degrees ± 21). Finally, our observation of a total angular rotation of 169 ± 21 degrees shows that within experimental error, chicken cornea lamellae rotate around 180 degrees or half of a complete turn. Additional studies are needed to arrive at an explanation of chicken superstructure in three dimensions.

Early evaluation of corneal collagen crosslinking in ex-vivo human corneas using two-photon imaging

The clinical outcome of corneal collagen crosslinking (CXL) is typically evaluated several weeks after treatment. An earlier assessment of its outcome could lead to an optimization of the treatment, including an immediate re-intervention in case of failure, thereby, avoiding additional discomfort and pain to the patient. In this study, we propose two-photon imaging (TPI) as an earlier evaluation method. CXL was performed in human corneas by application of riboflavin followed by UVA irradiation. Autofluorescence (AF) intensity and lifetime images were acquired using a commercial clinically certified multiphoton tomograph prior to CXL and after 2h, 24h, 72h, and 144h storage in culture medium. The first monitoring point was determined as the minimum time required for riboflavin clearance from the cornea. As control, untreated samples and samples treated only with riboflavin (without UVA irradiation) were monitored at the same time points. Significant increases in the stroma AF intensity and lifetime were observed as soon as 2h after treatment. A depth-dependent TPI analysis showed higher AF lifetimes anteriorly corresponding to areas were CXL was most effective. No alterations were observed in the control groups. Using TPI, the outcome of CXL can be assessed non-invasively and label-free much sooner than with conventional clinical devices.

In vivo two-photon microscopy of the human eye

Two-photon (2P) microscopy is a powerful tool for imaging and exploring label-free biological tissues at high resolution. Although this type of microscopy has been demonstrated in ex vivo ocular tissues of both humans and animal models, imaging the human eye in vivo has always been challenging. This work presents a novel compact 2P microscope for non-contact imaging of the anterior part of the living human eye. The performance of the instrument was tested and the maximum permissible exposure to protect ocular tissues established. To the best of our knowledge, 2P images of the in vivo human cornea, the sclera and the trabecular meshwork are shown for the very first time. Acquired images are of enough quality to visualize collagen arrangement and morphological features of clinical interest. Future implementations of this technique may constitute a potential tool for early diagnosis of ocular diseases at submicron scale.

Quantitative Discrimination of Healthy and Diseased Corneas With Second Harmonic Generation Microscopy

Purpose

To analyze the spatial organization of pathological corneas with second harmonic generation (SHG) imaging and to provide a proof of concept to objectively distinguish these from the healthy corneas.

Methods

A custom-built SHG microscope was used to image the anterior stroma of ex vivo corneas, both control and affected by some representative pathologies. The structure tensor (ST) was employed as a metric to explore and quantify the alterations in the spatial distribution of the collagen lamellae.

Results

The collagen arrangement differed between healthy and pathological samples. The former showed a regular distribution and a low structural dispersion (SD < 40°) within the stroma with a well-defined dominant orientation. This regular arrangement drastically turns into a disorganized pattern in pathological corneas (SD > 40°).

Conclusions

The combination of SHG imaging and the ST allows obtaining quantitative information to differentiate the stromal collagen organization in healthy and diseased corneas. This approach represents a feasible and powerful technique with potential applications in clinical corneal diagnoses.

Translational Relevance

The ST applied to SHG microscopy images of the corneal stroma provides an experimental objective score to differentiate control from pathological or damaged corneas. Future implementations of this technique in clinical environments might might be a promising tool in Ophthalmology, not only to diagnose and monitor corneal diseases, but also to follow-up surgical outcome.

Corneal deformation amplitude analysis for keratoconus detection through compensation for intraocular pressure and integration with horizontal thickness profile

BACKGROUND

The Corvis ST provides measurements of intraocular pressure (IOP) and a biomechanically-corrected IOP (bIOP). IOP influences corneal deflection amplitude (DA), which may affect the diagnosis of keratoconus. Compensating for IOP in DA values may improve the detection of keratoconus.

METHODS

195 healthy eyes and 136 eyes with keratoconus were included for developing different approaches to distinguish normal and keratoconic corneas using attribute selection and discriminant function. The IOP compensation is proposed by dividing the DA by the IOP values. The first approaches include DA compensated for either IOP or bIOP and other parameters from the deformation corneal response (DCR). Another approach integrated the horizontal corneal thickness profile (HCTP). The best classifiers developed were applied in a validation database of 156 healthy eyes and 87 eyes with keratoconus. Results were compared with the current Corvis Biomechanical Index (CBI).

RESULTS

The best biomechanical approach used the DA values compensated by IOP (Approach 2) using a linear discriminant function and reached AUC 0.954, with a sensitivity of 88.2% and a specificity of 97.4%. When thickness horizontal profile data was integrated (Approach 4), the best function was the diagquadratic, resulting in an AUC of 0.960, with a sensitivity of 89.7% and a specificity of 96.4%. There was no significant difference in the results between approaches 2 and 4 with the CBI in the training and validation databases.

CONCLUSIONS

By compensating for the IOP, and with the horizontal thickness profile included or excluded, it was possible to generate a classifier based only on biomechanical information with a similar result to the CBI.

Morpho-mechanics of human collagen superstructures revealed by all-optical correlative micro-spectroscopies

In every biological tissue, morphological and topological properties strongly affect its mechanical features and behaviour, so that ultrastructure, composition and mechanical parameters are intimately connected. Overall, it is their correct interplay that guarantees the tissue functionality. The development of experimental methods able to correlate these properties would open new opportunities both in the biological and the biomedical fields. Here, we report a correlative study intended to map supramolecular morphology, biochemical composition and viscoelastic parameters of collagen by all-optical microscopies. In particular, using human corneal tissue as a benchmark, we correlate Second-Harmonic Generation maps with mechanical and biochemical imaging obtained by Brillouin and Raman micro-spectroscopy. The study highlights how subtle variations in supramolecular organization originate the peculiar mechanical behavior of different subtypes of corneal lamellae. The presented methodology paves the way to the non-invasive assessment of tissue morpho-mechanics in biological as well as synthetic materials.

Quantitative Analysis of the Corneal Collagen Distribution after In Vivo Cross-Linking with Second Harmonic Microscopy

Corneal cross-linking (CXL) is a surgical procedure able to modify corneal biomechanics and stabilize keratoconus progression. Although it is known that CXL produces changes in corneal collagen distribution, these are still a topic of discussion. Here we quantitatively compare the corneal stroma architecture between two animal models four weeks after in vivo conventional CXL treatment, with second harmonic generation (SHG) imaging microscopy and the structure tensor (ST). The healing stage and the stroma recovery were also analyzed by means of histological sections. Results show that the CXL effects depend on the initial arrangement of the corneal collagen. While the treatment increases the order in corneas with a low level of initial organization, corneas presenting a fairly regular pattern are hardly affected. Histological samples showed active keratocytes in anterior and middle stroma, what means that the recovery is still in progress. The combination of SHG imaging and the ST is able to objectively discriminate the changes suffered by the collagen arrangement after the CXL treatment, whose effectiveness depends on the initial organization of the collagen fibers within the corneal stroma.

Collagen fiber crimping following in vivo UVA-induced corneal crosslinking

The purpose of this study was to measure collagen fiber crimping (CFC) using nonlinear optical imaging of second harmonic generated (SHG) signals to determine the effects of UVA-riboflavin induced corneal collagen crosslinking (UVA CXL) on collagen structure. Two groups, four rabbits each, were treated in the right eye with standard UVA CXL. In vivo confocal microscopy was performed at 1, 2, and 4 weeks after treatment for the first group and up to three months for the second group to measure epithelial/stromal thickness and corneal haze during recovery. Rabbits were sacrificed at one and three months, respectively, and their corneas fixed under pressure. Regions of crosslinking were identified by the presence of collagen autofluorescence (CAF) and then collagen structure was imaged using SHG microscopy. The degree of CFC was determined by measuring the percentage difference between the length of the collagen fiber and the linear distance traveled. CFC was measured in the central anterior and posterior CXL region, the peripheral non-crosslinked region in the same cornea, and the central cornea of the non-crosslinked contralateral eye. No change in corneal thickness was detected after one month, however the stromal thickness surpassed its original baseline thickness at three months by 25.9 μm. Corneal haze peaked at one month and then began to clear. Increased CAF was detected in all CXL corneas, localized to the anterior stroma and extending to 42.4 ± 3.4% and 47.7 ± 7.6% of the corneal thickness at one and three months. There was a significant (P < 0.05) reduction in CFC in the CAF region in all eyes averaging 1.007 ± 0.006 and 1.009 ± 0.005 in one and three month samples compared to 1.017 ± 0.04 and 1.016 ± 0.06 for controls. These results indicate that there is a significant reduction in collagen crimping following UVA CXL of approximately 1%. One possible explanation for this loss of crimping could be shortening of the collagen fibers over the CXL region.

Comparison of Artificial Anterior Chamber Internal Pressures and Cutting Systems for Descemet's Stripping Automated Endothelial Keratoplasty

Purpose

To optimize methods of preparing donor cornea tissue for Descemet's stripping automated endothelial keratoplasty (DSAEK), we compared five experimental conditions with different internal pressures and cutting systems.

Methods

The artificial anterior chamber internal pressure (IP) was set at 100 or 200 mm Hg. The microkeratome cut was performed with or without an artificial chamber pressurizer (ACP), using a CBm turbine (CBm) or one use-plus automated (OUP-A). Thirty human research corneas were divided into five groups, and compared after the cut with donor tissue quality parameters, including cutting depth, graft uniformity, cell evaluation, and smoothness of the stromal surface.

Results

The smallest variation in mean cut depth was observed in the condition, which had IP of 200 mm Hg used ACP and OUP-A. In experimental groups cut using CBm, significantly more consistent thicknesses were made at an IP of 200 than 100 mm Hg. There were no statistically significant differences among the groups in either endothelial cell density or cell viable assay results after cuts. Using an IP of 200 mm Hg with ACP and CBm produced the roughest stromal surface, and the roughness grading scores showed a positive correlation with the percentage of cut depth.

Conclusions

An IP of 200 mm Hg was the best setting for DSAEK grafts with high predictability of cut depth and uniformity of graft thickness without endothelial cell damage.

Translational Relevance

For successful DSAEK, it is recommended that a set internal pressure of 200 mm Hg be used during microkeratome cutting for donor tissue preparation.

Collagen fiber changes related to keratoconus with secondary corneal amyloidosis

We describe the histological changes in the collagen fibers of a 50-year-old male who presented keratoconus with secondary corneal amyloidosis. Corneal tissue from the patient was obtained following a penetrating keratoplasty and was subjected to histochemical analysis using Masson’s trichrome staining, Congo red staining, anti-lactoferrin antibody, and anti-transforming growth factor-beta-induced protein (TGFBIp) antibody. A Congo red-positive region was detected in the anterior half of the stroma in the center and inferior cornea. Although hemotoxylin and eosin staining revealed irregularity in the Congo red-positive region, other parts of the stroma did not show any abnormalities. Positive staining both by anti-TGFBIp and anti-lactoferrin antibodies was observed in the Congo red-positive region. Interestingly, all the layers of the corneal stroma, including the peripheral region, were positively stained by anti-TFGBIp antibody, even in the Congo red-negative area. Masson’s trichrome staining also showed irregular staining throughout the corneal stroma, even outside of the Congo red-positive region. Additionally, Bowman’s layer, which consists of collagen type IV, was damaged. TGFBIp was strongly expressed and Masson’s trichrome staining was reduced throughout the entire keratoconic stroma. The constant qualitative changes in keratoconic collagen fibers, along with the observed abnormality in the Bowman’s membrane, might point to the pathogenesis of secondary corneal amyloidosis in keratoconus.

Polarized light microscopy for 3-dimensional mapping of collagen fiber architecture in ocular tissues

Collagen fibers play a central role in normal eye mechanics and pathology. In ocular tissues, collagen fibers exhibit a complex 3-dimensional (3D) fiber orientation, with both in-plane (IP) and out-of-plane (OP) orientations. Imaging techniques traditionally applied to the study of ocular tissues only quantify IP fiber orientation, providing little information on OP fiber orientation. Accurate description of the complex 3D fiber microstructures of the eye requires quantifying full 3D fiber orientation. Herein, we present 3dPLM, a technique based on polarized light microscopy developed to quantify both IP and OP collagen fiber orientations of ocular tissues. The performance of 3dPLM was examined by simulation and experimental verification and validation. The experiments demonstrated an excellent agreement between extracted and true 3D fiber orientation. Both IP and OP fiber orientations can be extracted from the sclera and the cornea, providing previously unavailable quantitative 3D measures and insight into the tissue microarchitecture. Together, the results demonstrate that 3dPLM is a powerful imaging technique for the analysis of ocular tissues.

Axial mechanical and structural characterization of keratoconus corneas

PURPOSE

Previous studies indicate that there is an axial gradient of collagen lamellar branching and anastomosing leading to regional differences in corneal tissue stiffness that may control corneal shape. To further test this hypothesis we have measured the axial material stiffness and quantified the collagen lamellar complexity in ectatic and mechanically weakened keratoconus corneas (KC).

METHODS

Acoustic radiation force elastic microscopy (ARFEM) was used to probe the axial mechanical properties of the cone region of three donor KC buttons. 3 Dimensional second harmonic generation microscopy (3D-SHG) was used to qualitatively evaluate lamellar organization in 3 kC buttons and quantitatively measure lamellar branching point density (BPD) in a separate KC button that had been treated with epikeratophakia (Epi-KP).

RESULTS

The mean elastic modulus for the KC corneas was 1.67 ± 0.44 kPa anteriorly and 0.970 ± 0.30 kPa posteriorly, substantially below that previously measured for normal human cornea. 3D-SHG of KC buttons showed a simplified collagen lamellar structure lacking noticeable angled lamellae in the region of the cone. BPD in the anterior, posterior, central and paracentral regions of the KC cornea were significantly lower than in the overlying Epi-KP lenticule. Additionally, BPD in the cone region was significantly lower than the adjacent paracentral region in the KC button.

CONCLUSIONS

The KC cornea exhibits an axial gradient of mechanical stiffness and a BPD that appears substantially lower in the cone region compared to normal cornea. The findings reinforce the hypothesis that collagen architecture may control corneal mechanical stiffness and hence corneal shape.

An Early Finding of Keratoconus: Increase in Corneal Densitometry

PURPOSE

To investigate the corneal densitometry in subclinical keratoconus with normal elevation and pachymetric parameters.

METHODS

Patients with clinical keratoconus in one eye and subclinical keratoconus in the fellow eye were identified. The study group was selected from patients with subclinical keratoconus who showed normal results from topographic and Belin-Ambrósio Enhanced Ectasia Display III (BAD) analysis (Kmean <47.2 diopters, inferior-superior asymmetry <1.4 diopters, and KISA% <60%, elevation <5 μm, PPIaverage <1.06, PPImaximum <1.44, ARTaverage <414 μm, ARTmaximum <339 μm, and final D <1.6). The control group was selected from candidates for refractive surgery. The densitometric analyses were performed through the Pentacam HR (Oculus, Germany).

RESULTS

The medical records of 3474 patients with keratoconus were examined, and 116 (3.3%) subclinical keratoconus cases were detected. Normal BAD analysis results were obtained from 38 patients (1.1%). The control group also consisted of 38 patients. There were no significant differences between the eyes with subclinical keratoconus and those of the control in corrected distance visual acuity and topographic, topometric, and tomographic parameters (P > 0.05). In all layers of the 0- to 2-mm zone and in the anterior and central layers of the 0- to 6-mm zone, corneal densitometry was significantly higher in the subclinical keratoconus than the control (P < 0.001). In discriminating eyes with subclinical keratoconus from normal, the anterior layer in the 0- to 2-mm zone showed the highest area under the curve (0.883; cutoff: 19.7; sensitivity: 75%; specificity: 90%) in a receiver operating characteristic analysis.

CONCLUSIONS

The increase in densitometry in the central zone could be useful in detecting subclinical keratoconus.

High-resolution, label-free two-photon imaging of diseased human corneas

The diagnosis of corneal diseases may be improved by monitoring the metabolism of cells and the structural organization of the stroma using two-photon imaging (TPI). We used TPI to assess the differences between nonpathological (NP) human corneas and corneas diagnosed with either keratoconus, Acanthamoeba keratitis, or stromal corneal scars. Images were acquired using a custom-built five-dimensional laser-scanning microscope with a broadband sub-15 femtosecond near-infrared pulsed excitation laser and a 16-channel photomultiplier tube detector in combination with a time-correlated single photon counting module. Morphological alterations of epithelial cells were observed for all pathologies. Moreover, diseased corneas showed alterations to the cells' metabolism that were revealed using the NAD(P)H free to protein-bound ratios. The mean autofluorescence lifetime of the stroma and the organization of the collagen fibers were also significantly altered due to the pathologies. We demonstrate that TPI can be used to distinguish between NP and diseased human corneas, based not only on alterations of the cells' morphology, which can also be evaluated using current clinical devices, but on additional morphological and functional features such as the organization of the stroma and the cells' metabolism. Therefore, TPI could become an efficient tool for diagnosing corneal diseases and better understanding the biological processes of the diseases.