Pediatric spinal cord injury with radiographic abnormality: the Beijing experience.

BACKGROUND CONTEXT: Spinal cord injury (SCI) without radiographic abnormality (SCIWORA) is a syndrome that usually occurs in children primarily because of the unique biomechanics of the pediatric spine. We recently found that the histopathological and behavioral effects of SCI with radiographic abnormality (SCIWRA) and SCIWORA are very different from each other in animal models. Although numerous studies were conducted to understand the epidemiological and clinical characteristics of the overall pediatric SCI population and the pediatric SCIWORA population, the characteristics of the pediatric SCIWRA population and their differences from those of the SCIWORA population are poorly understood. PURPOSE: To describe the epidemiology and clinical outcomes of pediatric patients with SCIWRA and their differences from those with SCIWORA. STUDY DESIGN/SETTING: Retrospective study. PATIENT SAMPLE: A total of 47 pediatric SCIWRA patients. OUTCOME MEASURES: Epidemiological characteristics, injury severities, functional deficits, and management and recovery outcomes. METHODS: Review of all cases with SCIWRA at Beijing Children's Hospital between July 2007 and December 2019 and comparison between the present data and our previous SCIWORA data. RESULTS: Of the 187 pediatric SCI patients, 47 had SCIWRA (age: 7.06 ± 3.75 years, male-to-female ratio: 3:2). Main causes of SCIWRA were fall (38%) and traffic accidents (38%). Lesions were often located at multiple levels (62%). Incubation period was 3 ± 18 hours. According to the American Spinal Injury Association impairment scale (AIS), many SCIWRA patients had incomplete impairment (AIS B, 9%; AIS C, 9%; AIS D, 32%). Specifically, many of them had abnormal upper and lower limb muscle powers (55% and 60%), upper and lower limb muscle tones (34% and 49%), sensation (38%), and knee, ankle, and abdominal reflexes (47%, 34%, and 36%). 72% of SCIWRA patients were treated with methylprednisolone, dexamethasone, or both. 81% of them showed neurological improvement before discharge. There was no association between corticosteroid therapy and neurological improvement. Moreover, functional outcomes of their upper and lower limb muscle powers were significantly associated with functional outcomes of their upper and lower limb muscle tones (p < 0.01), respectively. In comparison to the SCIWRA population, the SCIWORA population had a higher ratio of younger and female patients of sports-related thoracic injuries with long incubation period leading to lower-body deficits and complete impairment (p<0.05 or p<0.01). Despite all the differences, their neurological improvement was similar (p>0.05). CONCLUSIONS: Demographic differences exist in the SCIWRA population. Corticosteroids do not appear to be effective in the different types of pediatric SCI. Limb muscle tone may be used to evaluate the functional status of limb muscle power. The epidemiological and clinical characteristics of SCIWRA and SCIWORA are very different from each other. It is important to formulate tailor-made prevention, evaluation, and management strategies for the pediatric population to optimize the SCI outcomes.

In-situmineralized homogeneous collagen-based scaffolds for potential guided bone regeneration.

For guided bone regeneration (GBR) in clinical orthopedics, the importance of a suitable scaffold which can provide the space needed for bone regeneration and simultaneously promotes the new bone formation cannot be overemphasized. Due to its excellent biocompatibility, mechanical strength, and similarity in structure and composition to natural bone, the mineralized collagen-based scaffolds have been increasingly considered as promising GBR scaffolds. Herein, we propose a novel method to fabricate anin-situmineralized homogeneous collagen-based scaffold (IMHCS) with excellent osteogenic capability for GBR by electrospinning the collagen solution in combination with essential mineral ions. The IMHCS exhibited homogeneous distribution of apatite crystals in electrospun fibers, which helped to achieve a significantly higher tensile strength than the pure collagen scaffold (CS) and the scaffold with directly added nano-hydroxyapatite particles (HAS). Furthermore, the IMHCS had significantly better cell compatibility, cell migration ratio, and osteogenic differentiation property than the HAS and CS. Therefore, the IMHCS not only retains traditional function of inhibiting fibroblast invasion, but also possesses excellent osteogenic differentiation property, indicating a robust alternative for GBR applications.

A histological study of atherosclerotic characteristics in age-related macular degeneration.

This study investigated the pathogenesis of age-related macular degeneration (AMD) using histological methods that are commonly used for atherosclerotic vascular disease (ASVD). 1 normal, 3 early dry AMD, and 1 late dry AMD eyes were obtained from the Lions Eye Bank of Oregon and systematically dissected. They were stained with hematoxylin and eosin, Oil red O, Masson, Elastica van Gieson, Alizarin red, and Prussian blue. Additionally, the normal and late dry AMD eyes were immunostained for a-smooth muscle actin, CD45, and CD68 with Nile red and DAPI. Correlations were found between severity of AMD and lipid accumulation in the deep sclera (+), numbers of drusen between the Bruch's membrane and retinal pigment epithelium (RPE) (+), amount of collagen in the deep sclera (+), and amount of elastin in the deep sclera (-) (P < 0.1). Geographic atrophy, RPE detachment, and abnormal capillary shape and distribution in the choriocapillaris were observed in the fovea of late AMD. There were no stenosis, plaque, hemorrhage, and calcification. Additionally, late AMD tended to have higher smooth muscle thicknesses of the choroidal vascular walls, lower numbers of T lymphocytes in the choroid, and higher numbers of macrophages near the RPE and in the choroid relative to normal (P < 0.1). Macrophages-derived foam cells were detected near the Bruch's membrane in late AMD. Therefore, the present study showed many histological characteristics of ASVD in AMD, which suggests an association between them; however, there were also some histological characteristics of ASVD that were not found in AMD, which indicates that there exist pathogenic differences between them. The results generally support the vascular model of AMD, but some details still need clarification.

Pediatric Spinal Cord Injury without Radiographic Abnormality: The Beijing Experience.

STUDY DESIGN: Retrospective study. OBJECTIVE: To describe the epidemiology and clinical outcomes of pediatric patients with spinal cord injury (SCI) without radiographic abnormality (SCIWORA) in mainland China for the first time. SUMMARY OF BACKGROUND DATA: SCIWORA is a syndrome that often occurs in children mainly due to the unique biomechanics of the pediatric spine. Although there have been numerous retrospective studies on pediatric SCIWORA, and mainland China has more patients with SCI than anywhere else, pediatric patients with SCIWORA in mainland China has not been described in any study. METHODS: Review of all cases with SCIWORA at Beijing Children's Hospital between July 2007 and December 2019. RESULTS: Of the 189 pediatric patients with SCI 140 had SCIWORA (age: 5.65 ±â€Š2.60 years, male-to-female ratio: 2:5). Main causes of injuries were sports (41%, mostly backbend), falls (27%), traffic accidents (10%), and violence (8%). Lesions were located at the thoracic (77%), cervical (10%), multiple (5%), and lumbar (4%) levels. Incubation period was 2 ±â€Š6 hours. Pathological characteristics of SCI were detected in 96% patients by magnetic resonance imaging (MRI). Based on the American Spinal Injury Association impairment scale (AIS), many patients had complete impairment (50% AIS A, 45% AIS B/C/D, 1% AIS E). Particularly, the five patients with normal MRI tended to have mild injury (AIS D) (P < 0.001), but they still showed abnormal reflex. In the one patient who could not be graded at all by AIS, his only functional deficits were abnormal upper and lower limb muscle tones. A total of 59% patients were treated with methylprednisolone, dexamethasone, or both. Out of 76 patients 59 showed neurological improvement before discharge. The only association among age, cause of injury, level of lesion, incubation period, AIS grade, type of corticosteroid therapy, and neurological improvement was between level of lesion and AIS grade (P < 0.001). CONCLUSION: Demographic and clinical differences exist in patients with SCIWORA. MRI and detailed neurological examinations should both be performed for proper diagnosis. There is still a need to develop better treatment strategy for these patients.Level of Evidence: 4.

The posterior eye with age-related macular degeneration has isotropic and nonlinear viscoelastic properties.

Here we characterize and compare the anisotropic and nonlinear viscoelastic properties of the posterior eye of advanced dry age-related macular degeneration (AMD) patients and age-matched normal subjects. Ten normal horizontal, ten normal vertical, ten AMD horizontal, and ten AMD vertical strips of the macular retina and the underlying choroid and sclera were preloaded, preconditioned, and subjected to incremental stress-relaxation tests in body-temperature saline. The stress-relaxation response was characterized by a fully nonlinear viscoelastic formulation in which the relaxation modulus was approximated by a Prony series and a second-order polynomial using the comprehensive viscoelastic characterization method. Normal retina, choroid, and sclera were found to be anisotropic, whereas AMD tissues were isotropic. AMD retina and choroid showed greater stress-relaxation response than normal tissues (p < 0.05), whereas AMD sclera had smaller stress-relaxation response than normal tissue (p < 0.05). The nonlinear viscoelastic stress-strain relationship of the posterior eye is hence describable for the first time.

Walking Function After Cervical Contusion and Distraction Spinal Cord Injuries in Rats.

This study examines and compares the walking function in contusion and distraction spinal cord injury (SCI) mechanisms. Moderate contusion and distraction SCIs were surgically induced between C5 and C6 in Sprague-Dawley male rats. The CatWalk system was used to perform gait analysis of walkway walking. The ladder rung walking test was used to quantify skilled locomotor movements of ladder rung walking. It was found that the inter-paw coordination, paw support, front paw kinematics, hind paw kinematics, and skilled movements were significantly different before and after contusion and distraction. Step sequence duration, diagonal support, forelimb intensity, forelimb duty cycle, forelimb paw angle, and forelimb swing speed were more greatly affected in distraction than in contusion at 2 weeks post-injury, whereas hindlimb stand was more greatly affected in contusion than in distraction at 8 weeks post-injury. After 8 weeks post-injury, diagonal coupling-variation, girdle coupling-variation, ipsilateral coupling-mean, forelimb maximum contact at, forelimb intensity, forelimb paw angle, and number of forelimb misplacements recovered to normal in contusion but not in distraction, whereas step sequence duration, ipsilateral coupling-variation, forelimb stand, forelimb duty cycle, hindlimb swing duration, hindlimb swing speed, and number of forelimb slips recovered to normal in distraction but not in contusion. Some of the behavioral outcomes, but not the others, were linearly correlated with the histological outcomes. In conclusion, walking deficits and recovery can be affected by the type of common traumatic SCI.

Quantification of surviving neurons after contusion, dislocation, and distraction spinal cord injuries using automated methods.

This study proposes and validates an automated method for counting neurons in spinal cord injury (SCI) and then uses it to examine and compare the surviving cells in common types of SCI mechanisms. Moderate contusion, dislocation, and distraction SCIs were surgically induced in Sprague Dawley male rats (n = 6 for each type of injury). Their spinal cords were harvested 8 weeks post injury with 5 normal weight-matched rats. The spinal cords were cut, stained with anti-NeuN antibody and fluorescent Nissl, and imaged in the dorsal and ventral horns at various distances to the epicenter. Neurons in the images were automatically counted using an algorithm that was designed to filter non-soma-like objects based on morphological characteristics (size, solidity, circular pattern) and check the remaining objects for the double-stained nucleus/cell body features (brightness variation, brightness distribution, color). To validate the automated method, some of the images were randomly selected for manual counting. The number of surviving cells that were automatically measured by the algorithm was found to be correlated with the values that were manually measured by 2 observers (P < .001) with similar differences (P > .05). Neurons in the dorsal and ventral horns were reduced after the SCIs (P < .05). Dislocation and distraction, respectively, caused the most severe damage to the ventral horn neurons especially near the epicenter and the most extensive and uniform damage to the dorsal horn neurons (P < .05). Our method was proved to be reliable, which is suitable for studying different types of SCI.

Mechanical Evaluation of Retinal Damage Associated With Blunt Craniomaxillofacial Trauma: A Simulation Analysis.

PURPOSE: To evaluate retinal damage as the result of craniomaxillofacial trauma and explain its pathogenic mechanism using finite element (FE) simulation. METHODS: Computed tomography (CT) images of an adult man were obtained to construct a FE skull model. A FE skin model was built to cover the outer surface of the skull model. A previously validated FE right eye model was symmetrically copied to create a FE left eye model, and both eye models were assembled to the skull model. An orbital fat model was developed to fill the space between the eye models and the skull model. Simulations of a ball-shaped object striking the frontal bone, temporal bone, brow, and cheekbones were performed, and the resulting absorption of the impact energy, intraocular pressure (IOP), and strains on the macula and ora serrata were analyzed to evaluate retinal injuries. RESULTS: Strain was concentrated in the macular regions (0.18 in average) of both eyes when the frontal bone was struck. The peak strain on the macula of the struck-side eye was higher than that of the other eye (>100%) when the temporal bone was struck, whereas there was little difference (<10%) between the two eyes when the brow and cheekbones were struck. Correlation analysis showed that the retinal strain time histories were highly correlated with the IOP time histories (r > 0.8 and P = 0.000 in all simulation cases). CONCLUSIONS: The risk of retinal damage is variable in craniomaxillofacial trauma depending on the struck region, and the damage is highly related to IOP variation caused by indirect blunt eye trauma. TRANSLATIONAL RELEVANCE: This finite element eye model allows us to evaluate and understand the indirect ocular injury mechanisms in craniomaxillofacial trauma for better clinical diagnosis and treatment.

Viscoelastic properties of the posterior eye of normal subjects, patients with age-related macular degeneration, and pigs.

The purpose of this study is to measure, characterize, and compare the viscoelastic properties of the posterior eye of advanced dry age-related macular degeneration (AMD) patients, age-matched normal subjects, and pigs (3 groups). Ten horizontal and ten vertical strips of the macula retina and the underneath choroid and sclera were obtained for each group, respectively. They were examined by incremental stress-relaxation cycles in body-temperature saline. Mechanical response was characterized by the quasi-linear viscoelastic model. All the tissues were shown to be nonlinear viscoelastic. Stiffening and isotropization, increased relaxation, and softening and isotropization were found in AMD retina, choroid, and sclera, respectively, which are the mechanical features of the atherosclerotic process. The patients' medical records were in accordance with epidemiological studies indicating a relationship between the advanced AMD and atherosclerotic vascular disease (ASVD). Moreover, many differences were found between the viscoelastic properties of porcine and normal human retina, choroid, and sclera. The results suggest that AMD is associated with ASVD through a mechanism involving abnormal retinal, choroidal, and scleral mechanics similar to those seen in the atherosclerotic process. Moreover, researchers should be aware of mechanical differences when using porcine posterior eyes as a substitute for human posterior eyes. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2151-2157, 2018.

Sequential therapy of anti-Nogo-A antibody treatment and treadmill training leads to cumulative improvements after spinal cord injury in rats.

Intense training is the most clinically successful treatment modality following incomplete spinal cord injuries (SCIs). With the advent of plasticity enhancing treatments, understanding how treatments might interact when delivered in combination becomes critical. Here, we investigated a rational approach to sequentially combine treadmill locomotor training with antibody mediated suppression of the fiber growth inhibitory protein Nogo-A. Following a large but incomplete thoracic lesion, rats were immediately treated with either anti-Nogo-A or control antibody (2weeks) and then either left untrained or step-trained starting 3weeks after injury for 8weeks. It was found that sequentially combined therapy improved step consistency and reduced toe dragging and climbing errors, as seen with training and anti-Nogo-A individually. Animals with sequential therapy also adopted a more parallel paw position during bipedal walking and showed greater overall quadrupedal locomotor recovery than individual treatments. Histologically, sequential therapy induced the greatest corticospinal tract sprouting caudally into the lumbar region and increased the number of serotonergic synapses onto lumbar motoneurons. Increased primary afferent sprouting and synapse formation onto lumbar motoneurons observed with anti-Nogo-A antibody were reduced by training. Animals with sequential therapy also showed the highest reduction of lumbar interneuronal activity associated with walking (c-fos expression). No treatment effects for thermal nociception, mechanical allodynia, or lesion volume were observed. The results demonstrate that sequential administration of anti-Nogo-A antibody followed in time with intensive locomotor training leads to superior recovery of lost locomotor functions, which is probably mediated by changes in the interaction between descending sprouting and local segmental networks after SCI.

Differential Histopathological and Behavioral Outcomes Eight Weeks after Rat Spinal Cord Injury by Contusion, Dislocation, and Distraction Mechanisms.

The objective of this study was to compare the long-term histological and behavioral outcomes after spinal cord injury (SCI) induced by one of three distinct biomechanical mechanisms: dislocation, contusion, and distraction. Thirty male Sprague-Dawley rats were randomized to incur a traumatic cervical SCI by one of these three clinically relevant mechanisms. The injured cervical spines were surgically stabilized, and motor function was assessed for the following 8 weeks. The spinal cords were then harvested for histologic analysis. Quantification of white matter sparing using Luxol fast blue staining revealed that dislocation injury caused the greatest overall loss of white matter, both laterally and along the rostrocaudal axis of the injured cord. Distraction caused enlarged extracellular spaces and structural alteration in the white matter but spared the most myelinated axons overall. Contusion caused the most severe loss of myelinated axons in the dorsal white matter. Immunohistochemistry for the neuronal marker NeuN combined with Fluoro Nissl revealed that the dislocation mechanism resulted in the greatest neuronal cell losses in both the ventral and dorsal horns. After the distraction injury mechanism, animals displayed no recovery of grip strength over time, in contrast to the animals subjected to contusion or dislocation injuries. After the dislocation injury mechanism, animals displayed no improvement in the grooming test, in contrast to the animals subjected to contusion or distraction injuries. These data indicate that different SCI mechanisms result in distinct patterns of histopathology and behavioral recovery. Understanding this heterogeneity may be important for the future development of therapeutic interventions that target specific neuropathology after SCI.

Discovery of retinal elastin and its possible role in age-related macular degeneration.

Age-related macular degeneration (AMD) etiology is unknown, but its association to atherosclerotic vascular disease (ASVD) has been observed. Since elastin plays an important role in the atherosclerotic process, to understand ASVD and AMD's relationship we examined retinal elastin existence, elastin amount and vessel properties among normal subjects, mild AMD patients, moderate-to severe AMD patients, and ASVD patients (n = 20). One eye per donor was assigned to enzyme-linked immunosorbent assay for quantifying the retinal elastin amount. The rest were assigned to mechanical test for examining the retinal vessel properties. Additionally, two normal human and two porcine eyes were acquired in immunohistochemistry for locating the retinal elastin. We found that elastin presented in the human and porcine retinal vessels at the basement membranes. 3.73 ± 0.55% of the normal retinal tissues were elastin. Elastin decrease, tissue-weight increase, and vessel hardening and in elasticity (p<0.05) were observed in the retina of patients with ASVD and only moderate-to-severe (i.e., not mild) AMD. Most moderate-to-severe AMD patients also happened to have ASVD. The results suggest that ASVD is unlikely the cause of AMD, but it is perhaps a factor that aggravates the condition through mechanism associated with retinal vessel abnormality.

Elastic properties of human posterior eye.

This study examines the elastic properties of the human posterior retina, choroid, and sclera. Twenty-four human eyes from 30- to 74-year-old donors were obtained from an eye bank. Vertically and horizontally oriented tissue strips of the retina, choroid, and sclera (ideally n = 12 in each group) were harvested from the posterior eyes. Their thicknesses were estimated optically. The samples were stretched at 1 mm/s in 37°C saline. Stress and strain were obtained from the mechanical tests, and then the transition stress, transition strain, toe modulus, and heel modulus were calculated. Statistical analysis was performed for comparison between groups. Linear regression analyses were used to explore the relationship between the mechanical parameters and age. We found that the stress-strain relationship of the retina, choroid, and sclera were nonlinear. Except for the retinal transition strain (p = 0.0124), no statistical difference was found between the vertical and horizontal meridian in the mechanical parameters (p > 0.05). Furthermore, weak relationship was observed between some of the mechanical parameters and the donors' age. Our results suggest that there is significant anisotropy in the retina, and mechanical properties of each layer may change with age.

Relationship between vitreous temperature and posterior vitreous detachment.

The aim of this study is to examine the relationship between vitreous temperature and posterior vitreous detachment (PVD). 30 porcine eyes were hemisected to form eyecups. In 10 eyes, the vitreous was warmed to 37 ± 1°C for 1 min and cooled down to room temperature. In 10 eyes, the vitreous was warmed to 44 ± 1°C for 1 min and cooled down to room temperature. 10 eyes were kept at room temperature. The vitreous was then removed surgically by forceps. In the eyes that were kept at room temperature (n=10), and in the eyes that the vitreous was warmed to 37 ± 1°C (n=10), the vitreous removal caused significant retinal damage, detachment and fold, and choroidal detachment. In the eyes that the vitreous was warmed to 44 ± 1°C (n=10), the vitreous removal did not cause any observable changes in the retina and choroid. Therefore, the vitreous separates from the retina after warming to 44 ± 1°C. The effect is likely irreversible through vitreous liquefaction. Thermal effect might have caused PVD in some patients. Since cell death occurs at above 55-58°C, the effect can potentially be safe and beneficial in surgical separation of the vitreous. Evidence is provided to show that this technique could be safely performed in vivo in humans, and that it would lead to safer removal of the vitreous during vitrectomy with fewer post-operative complications.

Mechanical characteristics of the porcine retina in low temperatures.

BACKGROUND: We previously observed that the stiffness of the porcine retina was significantly higher when deforming at room temperature than at body temperature. The present study further investigates this phenomenon by examining the mechanical properties of the retina in saline at temperature lower than room temperature. METHODS: Tensile testing was performed on a total of 15 retinal strips dissected from pig eyes. Equal amount of strips from the dissection were tested at 37.0 ± 0.3°C, 26.1 ± 0.1°C, and 7.8 ± 1.2°C. Their transition modulus, stress, and strain were measured for statistical analysis. RESULTS: The transition modulus, the transition stress, and the transition strain of the retinal strips were found to be 11.12 ± 6.10 kPa, 0.12 ± 0.07 kPa, and 0.016 ± 0.001, respectively, at 37.0°C. These values were 111.25 ± 88.16 kPa, 1.11 ± 0.85 kPa, and 0.016 ± 0.001 at 26.1°C, and 125.13 ± 63.61 kPa, 1.30 ± 0.50 kPa, and 0.017 ± 0.003 at 7.8°C, respectively. The differences of the transition modulus and the transition stress at between 37.0°C and 26.1°C and at between 37.0°C and 7.8°C were statistically significant (P < 0.05). There were no significant differences in these values at between 26.1°C and 7.8°C. CONCLUSION: These findings suggest that lowering the temperature of the retina from body temperature is potentially useful to decrease retinal damage in posterior eye surgeries by increasing the resistance of the retina to mechanical deformation.

Mechanical properties of orbital fat and its encapsulating connective tissue.

There is an increasing need to understand the mechanical properties of human orbital fat and its encapsulating connective tissue (OFCT), but such knowledge is not available in the current literature. The purpose of the present study is to examine the mechanical properties of the OFCT. From 5 pairs of 76- to 92-year-old Caucasian human eyes and 33 5- to 7-month-old porcine eyes, 5 human and 11 porcine OFCT samples were dissected at the posterior pole or adjacent to the pole in the vertical, horizontal, and radial directions. Sample dimensions were fixed or measured. Tensile tests were performed on the samples in body-temperature saline. The stress-strain relationship was first approximately linear and then became nonlinear. The linear, the neo-Hookean, and the Mooney-Rivlin constants are reported in Tables 1 and 2. No statistical difference was found among their properties in the different directions in either the human or the porcine samples. Statistical differences were found between the human and the porcine material constants in the horizontal and radial directions. Among our material models, only the Mooney-Rivlin model was able to capture the mechanical properties of the OFCT in large deformation properly. The Mooney-Rivlin model was especially adaptive to the human data. This is the first time the mechanical properties of the human and porcine OFCT have been examined in the literature. We believe our data will provide valuable information to others regarding designing implant biomaterials in orbital treatments and developing computer models to study orbital biomechanics.

Anisotropic and inhomogeneous mechanical characteristics of the retina.

We studied contribution of blood vessels to the mechanical properties of retina. Previous studies revealed anisotropic and inhomogeneous retinal mechanical characteristics. To examine different vessel types and sizes, 3 strips of retinal samples were dissected in each of 5 pig eyes. One strip contained the superior-temporal vein in the axial direction, one strip contained the superior-temporal artery in the axial direction, and one strip did not contain any visible vessel. To examine different vessel orientations, 2 strips of retinal samples were dissected in each of 5 other pig eyes. One strip contained the superior-temporal vein in the axial direction, and one strip contained the superior-temporal vein in the circumferential direction. Tensile testing was performed on the samples. The venous and arterial samples were found to be significantly stiffer than the no-visible-vessel samples. No significant difference was observed neither between the venous and arterial samples, nor between the venous samples in the axial versus circumferential directions. We therefore conclude that the blood vessels contribute significantly to the stiffness of the retina, and the vessel size is the only determining factor that governs the anisotropic and inhomogeneous characteristics of the retina.

Elastic properties of porcine ocular posterior soft tissues.

We examine the thickness and mechanical properties of the porcine posterior retina, choroid, and sclera in different environments and surface directions. Vertical and horizontal samples were surgically obtained. Uniaxial experiments were performed in room-temperature air, room-temperature saline, and body-temperature saline. Sample thicknesses were estimated optically. Thickness of all layers was found to vary significantly among the samples; thickness standard deviation of the mean was 24, 19, and 19% for the retina, choroid, and sclera, respectively. Transition stresses and heel moduli of all layers were consistently higher in saline than air. The retinal stress-strain relationship in air was typically linear with significantly lower horizontal transition strain. Transition stresses and moduli of all layers were consistently lower in body than room temperature and the differences in the transition stresses and heel moduli of the retina and sclera were significant. Also, the sclera had significantly lower transition strains in body temperature. These results illustrate the importance of testing the tissues at conditions like those found in the body. In body-temperature saline, all layers behaved nonlinearly, but only the retina exhibited surface anisotropy between the vertical and horizontal directions.

Characterization of the highly nonlinear and anisotropic vascular tissues from experimental inflation data: a validation study toward the use of clinical data for in-vivo modeling and analysis.

We study whether an inverse modeling approach is applicable for characterizing vascular tissue subjected to various levels of internal pressure and axial stretch that approximate in-vivo conditions. To compensate for the limitation of axial-displacement/pressure/diameter data typical of clinical data, which does not provide information about axial force, we propose to constrain the ratio of axial to circumferential elastic moduli to a typical range. Vessel wall constitutive behavior is modeled with a transversely isotropic hyperelastic equation that accounts for dispersed collagen fibers. A single-layer and a bi-layer approximation to vessel ultrastructure are examined, as is the possibility of obtaining the fiber orientation as part of the optimization. Characterization is validated against independent pipette-aspiration biaxial data on the same samples. It was found that the single-layer model based on homogeneous wall assumption could not reproduce the validation data. In contrast, the constrained bi-layer model was in excellent agreement with both types of experimental data. Due to covariance, estimations of fiber angle were slightly outside of the normal range, which can be resolved by predefining the angles to normal values. Our approach is relatively invariant to a constant or a variable axial response. We believe that it is suitable for in-vivo characterization.