Stretch-Induced Uncrimping of Equatorial Sclera Collagen Bundles.

Stretch-induced collagen uncrimping underlies the nonlinear mechanical behavior of the sclera according to what is often called the process of recruitment. We recently reported experimental measurements of sclera collagen crimp and pressure-induced uncrimping. Our studies, however, were cross-sectional, providing statistical descriptions of crimp with no information on the effects of stretch on specific collagen bundles. Data on bundle-specific uncrimping is necessary to better understand the effects of macroscale input on the collagen microscale and tissue failure. Our goal in this project was to measure bundle-specific stretch-induced collagen uncrimping of sclera. Three goat eyes were cryosectioned sagittally (30 µm). Samples of equatorial sclera were isolated, mounted to a custom uni-axial stretcher and imaged with polarized light microscopy at various levels of clamp-to-clamp stretch until failure. At each stretch level, local strain was measured using image tracking techniques. The level of collagen crimping was determined from the bundle waviness, defined as the circular standard deviation of fiber orientation along a bundle. Eye-specific recruitment curves were then computed using eye-specific waviness at maximum stretch before sample failure to define fibers as recruited. Nonlinear mixed effect models were used to determine the associations of waviness to local strain and recruitment to clamp-to-clamp stretch. Waviness decreased exponentially with local strain (p < 0.001), whereas bundle recruitment followed a sigmoidal curve with clamp-to-clamp stretch (p < 0.001). Individual bundle responses to stretch varied substantially, but recruitment curves were similar across sections and eyes. In conclusion, uni-axial stretch caused measurable bundle-specific uncrimping, with the sigmoidal recruitment pattern characteristic of fiber-reinforced soft tissues.

Role of radially aligned scleral collagen fibers in optic nerve head biomechanics.

Collagen fibers organized circumferentially around the canal in the peripapillary sclera are thought to provide biomechanical support to the sensitive tissues within the optic nerve head (ONH). Recent studies have demonstrated the existence of a family of fibers in the innermost sclera organized radially from the scleral canal. Our goal was to determine the role of these radial fibers in the sensitivity of scleral canal biomechanics to acute increases in intraocular pressure (IOP). Following the same general approach of previous parametric sensitivity studies, we created nonlinear generic finite element models of a posterior pole with various combinations of radial and circumferential fibers at an IOP of 0 mmHg. We then simulated the effects of normal and elevated IOP levels (15 and 30 mmHg). We monitored four IOP-induced geometric changes: peripapillary sclera stretch, scleral canal displacement, lamina cribrosa displacement, and scleral canal expansion. In addition, we examined the radial (maximum tension) and through-thickness (maximum compression) strains within the ONH tissues. Our models predicted that: 1) radial fibers reduced the posterior displacement of the lamina, especially at elevated IOP; 2) radial fibers reduced IOP-induced radial strain within the peripapillary sclera and retinal tissue; and 3) a combination of radial and circumferential fibers maintained strains within the ONH at a level similar to those conferred by circumferential fibers alone. In conclusion, radial fibers provide support for the posterior globe, additional to that provided by circumferential fibers. Most importantly, a combination of both fiber families can better protect ONH tissues from excessive IOP-induced deformation than either alone.

Crimp around the globe; patterns of collagen crimp across the corneoscleral shell.

Our goal was to systematically quantify the collagen crimp morphology around the corneoscleral shell, and test the hypothesis that collagen crimp is not uniform over the globe. Axial longitudinal cryosections (30 µm) of three sheep eyes, fixed at 0 mmHg IOP, were imaged using polarized light microscopy to quantify the local collagen in 8 regions: two corneal (central and peripheral) and six scleral (limbus, anterior-equatorial, equatorial, posterior-equatorial, posterior and peripapillary). Collagen crimp period (length of one wave), tortuosity (path length divided by end-to-end length), waviness (SD of orientation), amplitude (half the peak to trough distance), and conformity (width of contiguous similarly oriented bundles) were measured in each region. Measurements were obtained on 8216 collagen fiber bundles. When pooling measurements across the whole eye globe, the median crimp values were: 23.9 µm period, 13.2 µm conformity, 0.63 µm amplitude, 1.006 tortuosity, and 12.7° waviness. However, all parameters varied significantly across the globe. Median bundle periods in the central cornea, limbus, and peripapillary sclera (PPS) were 14.1 µm, 29.5 µm, and 22.9 µm, respectively. Median conformities were 20.8 µm, 14.5 µm, and 15.1 µm, respectively. Median tortuosities were 1.005, 1.007, and 1.007, respectively. Median waviness' were 11.4°, 13.2°, and 13.2°, respectively. Median amplitudes were 0.35 µm, 0.87 µm, and 0.65 µm, respectively. All parameters varied significantly across the globe. All regions differed significantly from one another on at least one parameter. Regions with small periods had large conformities, and bundles with high tortuosity had high waviness and amplitude. Waviness, tortuosity, and amplitude, associated with nonlinear biomechanical behavior, exhibited "double hump" distributions, whereas period and conformity, representing tissue organization, were substantially different between sclera and cornea. Though the biomechanical implications and origin of the patterns observed remain unclear, our findings of well-defined patterns of collagen crimp across the corneoscleral shell, consistent between eyes, support the existence of mechanisms that regulate collagen characteristics at the regional or smaller levels. These results are experimental data necessary for more realistic models of ocular biomechanics and remodeling.

Whole-globe biomechanics using high-field MRI.

The eye is a complex structure composed of several interconnected tissues acting together, across the whole globe, to resist deformation due to intraocular pressure (IOP). However, most work in the ocular biomechanics field only examines the response to IOP over smaller regions of the eye. We used high-field MRI to measure IOP induced ocular displacements and deformations over the whole globe. Seven sheep eyes were obtained from a local abattoir and imaged within 48 h using MRI at multiple levels of IOP. IOP was controlled with a gravity perfusion system and a cannula inserted into the anterior chamber. T2-weighted imaging was performed to the eyes serially at 0 mmHg, 10 mmHg, 20 mmHg and 40 mmHg of IOP using a 9.4 T MRI scanner. Manual morphometry was conducted using 3D visualization software to quantify IOP-induced effects at the globe scale (e.g. axial length and equatorial diameters) or optic nerve head scale (e.g. canal diameter, peripapillary sclera bowing). Measurement sensitivity analysis was conducted to determine measurement precision. High-field MRI revealed an outward bowing of the posterior sclera and anterior bulging of the cornea due to IOP elevation. Increments in IOP from 10 to 40 mmHg caused measurable increases in axial length in 6 of 7 eyes of 7.9 ± 5.7% (mean ± SD). Changes in equatorial diameter were minimal, 0.4 ± 1.2% between 10 and 40 mmHg, and in all cases less than the measurement sensitivity. The effects were nonlinear, with larger deformations at normal IOPs (10-20 mmHg) than at elevated IOPs (20-40 mmHg). IOP also caused measurable increases in the nasal-temporal scleral canal diameter of 13.4 ± 9.7% between 0 and 20 mmHg, but not in the superior-inferior diameter. This study demonstrates that high-field MRI can be used to visualize and measure simultaneously the effects of IOP over the whole globe, including the effects on axial length and equatorial diameter, posterior sclera displacement and bowing, and even changes in scleral canal diameter. The fact that the equatorial diameter did not change with IOP, in agreement with previous studies, indicates that a fixed boundary condition is a reasonable assumption for half globe inflation tests and computational models. Our results demonstrate the potential of high-field MRI to contribute to understanding ocular biomechanics, and specifically of the effects of IOP in large animal models.

Persistent dysbiosis of duodenal microbiota in patients with controlled pediatric Crohn's disease after resolution of inflammation.

Crohn's disease is an inflammatory condition of the intestine characterized by largely unknown etiology and a relapse remission cycle of disease control. While possible triggers have been identified, research is inconsistent on the precise cause of these relapses, especially in the under-researched pediatric population. We hypothesized that patients in remission would have persistent microbial and inflammatory changes in small intestinal tissue that might trigger relapse. To this end, we analyzed intestinal biopsy samples from six patients with pediatric Crohn's disease in remission and a control group of 16 pediatric patients with no evident pathogenic abnormality. We identified compositional microbiota differences, including decreases in the genera Streptococcus and Actinobacillus as well as increases in Oribacterium and Prevotella in patients with controlled Crohn's disease compared to controls. Further, a histologic analysis found that patients with controlled Crohn's disease had increased epithelial integrity, and decreased intraepithelial lymphocytes compared with controls. Additionally, we observed increased peripheral CD4+ T cells in patients with pediatric Crohn's disease. These results indicate that markers of intestinal inflammation are responsive to Crohn's disease treatment, however the interventions may not resolve the underlying dysbiosis. These findings suggest that persistent dysbiosis may increase vulnerability to relapse of pediatric Crohn's disease. This study used a nested cohort of patients from the Bangladesh Environmental Enteric Dysfunction (BEED) study (ClinicalTrials.gov ID: NCT02812615 Date of first registration: 24/06/2016).

Genome-Wide Association Study of Campylobacter-Positive Diarrhea Identifies Genes Involved in Toxin Processing and Inflammatory Response.

Diarrhea is responsible for the deaths of more than 500,000 children each year, many of whom reside in low-to-middle-income countries (LMICs). Additionally, children with multiple diarrheal infections early in life have increased growth stunting and malnutrition and decreased vaccine efficacy. Two bacteria that contribute to the burden of diarrhea are Campylobacter jejuni and Campylobacter coli, both are endemic in Bangladesh. However, not all children that are exposed to these pathogens, including Campylobacter, will experience diarrhea. We hypothesized that host genetics may influence susceptibility to Campylobacter infections and performed a genome-wide association study in 534 children from two independent birth cohorts in Dhaka, Bangladesh. Infants were monitored for diarrhea for the first 2 years of life and only defined as controls if all diarrheal samples in the first year were negative for Campylobacter jejuni/C. coli. Each cohort was analyzed separately under an additive model and adjusted for length-for-age z-scores at birth and 12 months, sex, water treatment, and ancestry. In a fixed effect inverse-variance weighted meta-analysis of these two cohorts, we identified a genome-wide significant region on chromosome 8 in intron 4 of the rho guanine nucleotide exchange factor 10 gene (ARHGEF10). Individuals with the G allele (rs13281104) had a 2-fold lower risk of having a Campylobacter-associated diarrheal episode than individuals with the A allele (OR 0.41, 95% CI 0.29 to 0.58, P = 3.6 × 10-7). This SNP is associated with decreased expression of the neighboring gene, CLN8, which may be involved in the transport of the cytolethal distending toxin produced by Campylobacter. IMPORTANCE Children in low-to-middle-income countries often suffer from multiple enteric infections in their first few years of life, many of which have the potential for long-lasting effects. These children are already likely to be malnourished and underweight, and chronic intestinal disturbances exacerbate these conditions. Despite public health interventions aimed at improving water, sanitation, and hygiene, enteric infections are still a leading cause of death for children under five. Previous work has included transmission dynamics, pathogen characteristics, and evaluation of interventions. Here, we examined the role of host genetic variation in susceptibility to diarrhea-associated Campylobacter infection. In our meta-analysis of two independent birth cohorts from Dhaka, Bangladesh, we found that children carrying a specific genetic variant (rs13281104, in an intron of ARHGEF10) were half as likely to have a diarrhea-associated Campylobacter infection in their first year of life. This protective effect may be achieved by decreasing gene expression and thereby impacting host-pathogen interactions and host immune response.

Collagen fiber recruitment: A microstructural basis for the nonlinear response of the posterior pole of the eye to increases in intraocular pressure.

Our goal was to quantify and characterize how the collagen fiber crimp waviness of the lamina cribrosa (LC) and peripapillary sclera (PPS) changes with intraocular pressure (IOP). Thirteen sheep (ovine) eyes were immersion and perfusion fixed while maintaining IOP at 0, 10, 15, 20, or 50 mmHg. Coronal optic nerve head (ONH) sections (30 µm) were imaged with polarized light microscopy (PLM) and analyzed for collagen fiber orientation and waviness (SD of fiber orientation). In the LC, the waviness of every LC beam was measured. In the PPS, at least 900 collagen bundles were measured per eye. Using the waviness at 50 mmHg IOP, we defined tissue-specific thresholds to determine the fraction of loaded or recruited fibers. We found that fiber waviness decreased with IOP (P < 0.001). At every IOP, the waviness of the collagen fibers, and the fraction of fibers recruited in the LC were smaller or equal than those of the PPS (P < 0.001). At 15 mmHg IOP, both LC and PPS had ¾ recruited fibers and » left in reserve. The decreased waviness with IOP and associated fiber recruitment is experimental evidence of fiber-based nonlinear biomechanical behavior of the ONH. At all IOPs the PPS had an equal or larger fraction of fibers recruited than the LC. That both LC and PPS had the same fraction of recruited and reserve fibers at normal IOP suggests that this may be an optimal fraction of recruitment for healthy eyes. Whether this extends to human eyes remains unknown. STATEMENT OF SIGNIFICANCE: Collagen fibers exhibit a natural waviness or crimp that largely determine the nonlinear biomechanics of soft tissue. Experimental measurements of crimp morphology in the sheep eye, and how it changes with intraocular pressure (IOP), however, are exceedingly sparse. We quantified how posterior eye crimp changes with increasing IOP. We found that the lamina cribrosa and peripapillary sclera have fundamentally different crimp, and with increasing IOP, have different proportions of fibers that straighten, or get recruited, versus remaining wavy, or in reserve. Interestingly, at physiologic IOP of 15 mmHg, both tissues had about ¾ fibers recruited and » fibers in reserve, indicating there may be an optimal fraction of fibers.

Radial and Circumferential Collagen Fibers Are a Feature of the Peripapillary Sclera of Human, Monkey, Pig, Cow, Goat, and Sheep.

Purpose: To test the hypothesis that human, monkey, pig, sheep, cow, and goat eyes exhibit circumferential, radial, and interweaving collagen architecture in the posterior sclera. Methods: We analyzed 1,327 cryosections from the posterior poles of 4 human, 4 monkey, 5 pig, 8 sheep, 1 goat, and 2 cow eyes. Images were acquired using polarized light microscopy and processed to obtain polar fiber orientations relative to the center of the canal. Circumferential, radial, and interweaving regions were identified and analyzed for mean fiber orientation and anisotropy and region width and thickness. Results: Every eye exhibited circumferential, radial, and interweaving fibers in consistent locations. Radial fibers extended out from near the canal into the peripapillary and peripheral sclera in the innermost sclera. Circumferential fibers were directly adjacent to the canal and most prevalent in the outermost, posterior sclera. Interweaving fibers were found throughout the sclera thickness. Across all species, median anisotropy in the radial, circumferential, and interweaving regions were 0.95, 0.96, and 0.28, respectively. Conclusions: Regions of radial, circumferential, and interweaving fibers occur in the posterior pole sclera of human, monkey, pig, sheep, cow, and goat eyes. The consistency across species in scleral architecture suggests that they are primary organizational components whose functions should be better understood.

Peripapillary sclera architecture revisited: A tangential fiber model and its biomechanical implications.

The collagen fiber architecture of the peripapillary sclera (PPS), which surrounds the scleral canal, is a critical factor in determining the mechanical response of the optic nerve head (ONH) to variations in intraocular pressure (IOP). Experimental and clinical evidence point to IOP-induced deformations within the scleral canal as important contributing factors of glaucomatous neural tissue damage and consequent vision loss. Hence, it is imperative to understand PPS architecture and biomechanics. Current consensus is that the fibers of the PPS form a closed ring around the canal to support the delicate neural tissues within. We propose an alternative fiber architecture for the PPS, in which the scleral canal is supported primarily by long-running fibers oriented tangentially to the canal. We present evidence that this tangential model is consistent with histological observations in multiple species, and with quantitative measurements of fiber orientation obtained from small angle light scattering and wide-angle X-ray scattering. Using finite element models, we investigated the biomechanical implications of a tangential fiber PPS architecture. We found that the tangential arrangement of fibers afforded better mechanical support to the tissues within the scleral canal as compared to a simple circumferential ring of fibers or a combination of fibers oriented radially and circumferentially. We also found that subtle variations from a tangential orientation could reproduce clinically observed ONH behavior which has yet to be explained using current theories of PPS architecture and simulation, namely, the contraction of the scleral canal under elevated IOP. STATEMENT OF SIGNIFICANCE: It is hypothesized that vision loss in glaucoma is due to excessive mechanical deformation within the neural tissue inside the scleral canal. This study proposes a new model for how the collagen of the peripapillary sclera surrounding the canal is organized to support the delicate neural tissue inside. Previous low-resolution studies of the peripapillary sclera suggested that the collagen fibers are arranged in a ring around the canal. Instead, we provide microscopic evidence suggesting that the canal is also supported by long-running interwoven fibers oriented tangentially to the canal. We demonstrate that this arrangement has multiple biomechanical advantages over a circular collagen arrangement and can explain previously unexplained experimental findings including contraction of the scleral canal under elevated intraocular pressure.

Structured polarized light microscopy for collagen fiber structure and orientation quantification in thick ocular tissues.

Collagen is a major constituent of the eye and understanding its architecture and biomechanics is critical to preserve and restore vision. We, recently, demonstrated polarized light microscopy (PLM) as a powerful technique for measuring properties of the collagen fibers of the eye, such as spatial distribution and orientation. Our implementation of PLM, however, required sectioning the tissues for imaging using transmitted light. This is problematic because it limits analysis to thin sections. This is not only slow, but precludes study of dynamic events such as pressure-induced deformations, which are central to the role of collagen. We introduce structured polarized light microscopy (SPLM), an imaging technique that combines structured light illumination with PLM to allow imaging and measurement of collagen fiber properties in thick ocular tissues. Using pig and sheep eyes, we show that SPLM rejects diffuse background light effectively in thick tissues, significantly enhancing visualization of optic nerve head (ONH) structures, such as the lamina cribrosa, and improving the accuracy of the collagen fiber orientation measurements. Further, we demonstrate the integration of SPLM with an inflation device to enable direct visualization, deformation tracking, and quantification of collagen fibers in ONHs while under controlled pressure.

Thin Lamina Cribrosa Beams Have Different Collagen Microstructure Than Thick Beams.

Purpose: To compare the collagen microstructural crimp characteristics between thin and thick lamina cribrosa (LC) beams. Methods: Seven eyes from four sheep were fixed at 5 mm Hg IOP in 10% formalin. For each eye, one to three coronal cryosections through the LC were imaged with polarized light microscopy and analyzed to visualize the LC and determine collagen fiber microstructure. For every beam, we measured its width and three characteristics of the crimp of its collagen fibers: waviness, tortuosity, and amplitude. Linear mixed effects models were used to test whether crimp characteristics were associated with the LC beam width. Results: For each eye and over all the eyes, LC beam width was positively associated with crimp waviness and tortuosity, and negatively associated with crimp amplitude (P's < 0.0001). Thin beams, average width 13.11 µm, had average (SD) waviness, tortuosity, and amplitude of 0.27 (0.17) radians, 1.017 (0.028) and 1.88 (1.41) µm, respectively. For thick beams, average width 26.10 µm, these characteristics were 0.33 (0.18) radians, 1.025 (0.037) and 1.58 (1.36) µm, respectively. Conclusions: Our results suggest heterogeneity in LC beam mechanical properties. Thin beams were less wavy than their thicker counterparts, suggesting that thin beams may stiffen at lower IOP than thick beams. This difference may allow thin beams to support similar amounts of IOP-induced force as thicker beams, thus providing a similar level of structural support to the axons at physiologic IOP, despite the differences in width. Measurements of beam-level mechanical properties are needed to confirm these predictions.

Spatial Patterns and Age-Related Changes of the Collagen Crimp in the Human Cornea and Sclera.

Purpose: Collagen is the main load-bearing component of the eye, and collagen crimp is a critical determinant of tissue mechanical behavior. We test the hypothesis that collagen crimp morphology varies over the human cornea and sclera and with age. Methods: We analyzed 42 axial whole-globe sections from 20 normal eyes of 20 human donors, ranging in age from 0.08 (1 month) to 97 years. The sections were imaged using polarized light microscopy to obtain µm-scale fiber bundle/lamellae orientation from two corneal and six scleral regions. Crimp morphology was quantified through waviness, tortuosity, and amplitude. Results: Whole-globe median waviness, tortuosity, and amplitude were 0.127 radians, 1.002, and 0.273 µm, respectively. These parameters, however, were not uniform over the globe, instead exhibiting distinct, consistent patterns. All crimp parameters decreased significantly with age, with significantly different age-related decreases between regions. The crimp morphology of the limbus changed the most drastically with age, such that it had the largest crimp in neonates, and among the smallest in the elderly. Conclusions: Age-related decreases in crimp parameters are likely one of the mechanisms underlying age-related stiffening of the sclera and cornea, potentially influencing sensitivity to IOP. Further work is needed to determine the biomechanical implications of the crimp patterns observed. The comparatively large changes in the crimp morphology of the limbus, especially in the early years of life, suggest that crimp in this region may play a role in eye development, although the exact nature of this is unclear.

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.

Collagen Architecture of the Posterior Pole: High-Resolution Wide Field of View Visualization and Analysis Using Polarized Light Microscopy.

Purpose: The purpose of this study was to leverage polarized light microscopy (PLM) to visualize the collagen fiber architecture of posterior pole and optic nerve head with micrometer-scale resolution and to identify and quantify major organizational components. Methods: Eight sheep posterior poles were cryosectioned and imaged using PLM. Collagen fiber orientation was determined by using custom scripts, and the resulting orientation maps were inspected and quantified to identify major structural elements and tested for differences in mean fiber orientation and anisotropy, using linear mixed effect models. Results: Images revealed an intricate organization of collagen fibers in the posterior pole. In the lamina cribrosa, interweaving fibers formed large knots and wrapped around nerve fiber pores, with beam insertions into the scleral canal wall that were either narrow and straight or wide. In the peripapillary sclera, three significantly different (P < 0.0001) components were identified: fibers oriented circumferentially proximal to the canal, radially in the innermost sclera, and unaligned with interweaving fibers. The radial fibers were between 60 and 180 µm thick, extending at least 3 mm from the canal. Conclusions: PLM revealed structural aspects of the lamina cribrosa and sclera that may have important biomechanical roles but that were previously unreported or not characterized quantitatively. In the lamina cribrosa, these roles included wide and narrow beam insertions and details of collagen fibers interweaving and wrapping around the pores. In the sclera, we described regions of circumferential, radial, and unaligned "random" fibers. Although there is consensus that circumferential fibers protect neural tissues by resisting canal expansion, the role of the radial fibers remains unclear.

Formalin Fixation and Cryosectioning Cause Only Minimal Changes in Shape or Size of Ocular Tissues.

Advances in imaging have made it increasingly common to study soft tissues without first embedding them in plastic or paraffin and without using labels or stains. The process, however, usually still involves fixation and cryosectioning, which could deform the tissues. Our goal was to quantify the morphological changes of ocular tissues caused by formalin fixation and cryosectioning. From each of 6 porcine eyes, 4 regions were obtained: cornea, equatorial and posterior sclera, and posterior pole containing the optic nerve head. Samples were imaged using visible light microscopy fresh, 1-minute and 24-hours post-fixation, and post-cryosectioning. Effects were assessed by 14 parameters representing sample size and shape. Overall, formalin fixation and sectioning caused only minimal changes to the ocular tissues, with average percentage parameter differences of 0.1%, 1%, and 1.2% between fresh and post-fixing by 1 minute, 24 hours, and post-cryosectioning, respectively. Parameter changes were not directional, and were only weakly dependent on the duration of fixation and the region of the eye. These results demonstrate that formalin fixation and cryosectioning are good choices for studying ocular tissue morphology and structure, as they do not cause the large tissue shrinkage or distortions typically associated with other, more complicated, techniques.

Lamina Cribrosa Pore Shape and Size as Predictors of Neural Tissue Mechanical Insult.

Purpose: The purpose of this study was to determine how the architecture of the lamina cribrosa (LC) microstructure, including the shape and size of the lamina pores, influences the IOP-induced deformation of the neural tissues within the LC pores using computational modeling. Methods: We built seven specimen-specific finite element models of LC microstructure with distinct nonlinear anisotropic properties for LC beams and neural tissues based on histological sections from three sheep eyes. Changes in shape (aspect ratio and convexity) and size (area and perimeter length) due to IOP-induced hoop stress were calculated for 128 LC pores. Multivariate linear regression was used to determine if pore shape and size were correlated with the strain in the pores. We also compared the microstructure models to a homogenized model built following previous approaches. Results: The LC microstructure resulted in focal tensile, compressive, and shear strains in the neural tissues of the LC that were not predicted by homogenized models. IOP-induced hoop stress caused pores to become larger and more convex; however, pore aspect ratio did not change consistently. Peak tensile strains within the pores were well predicted by a linear regression model considering the initial convexity (negative correlation, P < 0.001), aspect ratio (positive correlation, P < 0.01), and area (negative correlation, P < 0.01). Significant correlations were also found when considering the deformed shape and size of the LC pores. Conclusions: The deformation of the LC neural tissues was largely dependent on the collagenous LC beams. Simple measures of LC pore shape and area provided good estimates of neural tissue biomechanical insult.

Microstructural Crimp of the Lamina Cribrosa and Peripapillary Sclera Collagen Fibers.

Purpose: Although collagen microstructural crimp is a major determinant of ocular biomechanics, no direct measurements of optic nerve head (ONH) crimp have been reported. Our goal was to characterize the crimp period of the lamina cribrosa (LC) and peripapillary sclera (PPS) at low and normal IOPs. Methods: ONHs from 11 sheep eyes were fixed at 10-, 5-, or 0-mm Hg IOP and crimp periods measured manually from coronal cryosections imaged with polarized light microscopy (PLM). Using linear mixed-effect models, we characterized the LC and PPS periods, and how they varied with distance from the scleral canal edge. Results: A total of 17,374 manual collagen crimp period measurements were obtained with high repeatability (1.9 µm) and reproducibility (4.7 µm). The periods were smaller (P < 0.001) and less variable in the LC than in the PPS: average (SD) of 13.8 (3.1) µm in the LC, and 31.0 (10.4) µm in the PPS. LC crimp period did not vary with distance from the scleral canal wall (P > 0.1). PPS period increased with the square root of the distance to the canal (P < 0.0001). Conclusions: Small, uniform crimp periods within the sheep LC and immediately adjacent PPS may indicate that these tissues are setup to prevent large or heterogeneous deformations that insult the neural tissues within the canal. An increasing more variable period with distance from the canal provides a smooth transition of mechanical properties that minimizes stress and strain concentrations.

In-vivo effects of intraocular and intracranial pressures on the lamina cribrosa microstructure.

There is increasing clinical evidence that the eye is not only affected by intraocular pressure (IOP), but also by intracranial pressure (ICP). Both pressures meet at the optic nerve head of the eye, specifically the lamina cribrosa (LC). The LC is a collagenous meshwork through which all retinal ganglion cell axons pass on their way to the brain. Distortion of the LC causes a biological cascade leading to neuropathy and impaired vision in situations such as glaucoma and idiopathic intracranial hypertension. While the effect of IOP on the LC has been studied extensively, the coupled effects of IOP and ICP on the LC remain poorly understood. We investigated in-vivo the effects of IOP and ICP, controlled via cannulation of the eye and lateral ventricle in the brain, on the LC microstructure of anesthetized rhesus monkeys eyes using the Bioptigen spectral-domain optical coherence tomography (OCT) device (Research Triangle, NC). The animals were imaged with their head upright and the rest of their body lying prone on a surgical table. The LC was imaged at a variety of IOP/ICP combinations, and microstructural parameters, such as the thickness of the LC collagenous beams and diameter of the pores were analyzed. LC microstructure was confirmed by histology. We determined that LC microstructure deformed in response to both IOP and ICP changes, with significant interaction between the two. These findings emphasize the importance of considering both IOP and ICP when assessing optic nerve health.

Use and Misuse of Laplace's Law in Ophthalmology.

PURPOSE: Laplace's Law, with its compactness and simplicity, has long been employed in ophthalmology for describing the mechanics of the corneoscleral shell. We questioned the appropriateness of Laplace's Law for computing wall stress in the eye considering the advances in knowledge of ocular biomechanics. METHODS: In this manuscript we recapitulate the formulation of Laplace's Law, as well as common interpretations and uses in ophthalmology. Using numerical modeling, we study how Laplace's Law cannot account for important characteristics of the eye, such as variations in globe shape and size or tissue thickness, anisotropy, viscoelasticity, or that the eye is a living, dynamic organ. RESULTS: We show that accounting for various geometrical and material factors, excluded from Laplace's Law, can alter estimates of corneoscleral wall stress as much as 456% and, therefore, that Laplace's Law is unreliable. CONCLUSIONS: We conclude by illustrating how computational techniques, such as finite element modeling, can account for the factors mentioned above, and are thus more suitable tools to provide quantitative characterization of corneoscleral biomechanics.

MAPS - a Magic Angle Positioning System for Enhanced Imaging in High-Field Small-Bore MRI.

The "magic angle" MRI effect can enhance signal intensity in aligned collagenous structures oriented at approximately 55° with respect to the main magnetic field. The difficulty of positioning tissue inside closed-bore scanners has hampered magic angle use in research and clinics. An MRI-conditional mechatronic system has been developed to control sample orientation inside a 9.4T small bore MRI scanner. The system orients samples to within 0.5° and enables a 600% increase in tendon signal intensity.