Comparison of spinal cord gray matter and white matter softness: measurement by pipette aspiration method

Nonlinear poroplastic model of ventricular dilation in hydrocephalus

Object

The mechanism of ventricular dilation in normal-pressure hydrocephalus remains unclear. Numerical finite-element (FE) models of hydrocephalus have been developed to investigate the biomechanics of ventricular enlargement. However, previous linear poroelastic models have failed to reproduce the relatively larger dilation of the horns of the lateral ventricles. In this paper the authors instead elaborated on a nonlinear poroplastic FE model of the brain parenchyma and studied the influence of the introduction of these potentially more realistic mechanical behaviors on the prediction of the ventricular shape.

Methods

In the proposed model the elasticity modulus varies as a function of the distension of the porous matrix, and the internal mechanical stresses are relaxed after each iteration, thereby simulating the probable plastic behavior of the brain tissue. The initial geometry used to build the model was extracted from CT scans of patients developing hydrocephalus, and the results of the simulations using this model were compared with the real evolution of the ventricular size and shape in the patients.

Results

The authors' model predicted correctly the magnitude and shape of the ventricular dilation in real cases of acute and chronic hydrocephalus. In particular, the dilation of the frontal and occipital horns was much more realistic.

Conclusions

This finding suggests that the nonlinear and plastic mechanical behaviors implemented in the present numerical model probably occur in reality. Moreover, the availability of such a valid FE model, whose mechanical parameters approach real mechanical properties of the brain tissue, might be useful in the further modeling of ventricular dilation at a normal pressure.

A finite element method based deformable brain atlas suited for surgery simulation

Brain Atlases are valuable tools that assist neurosurgeons during the planning of an intervention. Since current printed and digitalized brain atlas have several disadvantages, including lack of physical descriptions of brain structures and incompatibility with finite element analysis, we have developed a meshed brain atlas for finite element analysis of surgical procedures. Our meshed atlas is a multiple-object FE model integrated with knowledge of mechanical properties of brain tissue with detailed anatomical information. A non-linear hyper-viscoelastic mechanical property typically suited for neurosurgery simulation is incorporated. We then partially validated the utility of this model by numerical simulation of a gravity-induced brain deformation scenario. This meshed atlas is able to account for the deformation of both the whole brain and individual structures. It can be applied in large scale finite element simulations and, therefore, offers the possibility of developing bio-mechanical surgical planning and training systems.

Interstitial transport and transvascular fluid exchange during infusion into brain and tumor tissue

A model of convection-enhanced delivery in brain and neoplastic tissue is presented that includes transvascular fluid exchange in addition to interstitial fluid transport. Measured values for the relevant material parameters are compiled from published literature. The transient distributions of interstitial fluid pressure and fluid velocity resulting from infusion into brain tissue and into a tissue-isolated tumor are derived, in addition to the steady-state distribution of interstitial fluid pressure and fluid velocity resulting from infusion into a tumor with a necrotic core. The analytic solutions suggest that (1) the distributions of both pressure and fluid velocity are very sensitive to the ratio of vascular conductivity and hydraulic conductivity; (2) in brain tissue, the convective fluid velocity is significant (within three orders of magnitude of the peak value, for example) for a large number of infusion catheter diameters away from the infusion site, whereas in neoplastic tissue the convective velocity is significant for a small number of infusion catheter diameters away from the infusion site; (3) the presence of a necrotic core substantially increases the convective fluid velocity in its vicinity; and (4) infusion proximal to a tumor is greatly impeded by the outward flow at the tumor's periphery. These and related findings are discussed in terms of their relevance to the treatment of tumors.

Mechanical and morphological characterization of homogeneous and bilayered poly(2-hydroxyethyl methacrylate) scaffolds for use in CNS nerve regeneration

Homogeneous and bilayered macroporous poly(2-hydroxyethyl methacrylate), p(HEMA), hydrogel scaffolds were examined as possible matrices for nerve regeneration in the CNS. An important issue to consider for a CNS scaffold is that it must be able to closely mimic the natural tissue it is replacing, while remaining intact, and mechanically stable enough to allow for regenerating axons to elongate through it. Phase-separated homogeneous and bilayered p(HEMA) scaffolds were fabricated, by varying water, crosslinking, and initiating agents; all of which directly affected the mechanical properties of the polymer. An increase in water concentration resulted in a decrease in the modulus for a given crosslinking and initiating concentration for all homogenous scaffolds, but the same result was not evident in the bilayered scaffolds. The distinct regions within the bilayered scaffolds generate a matrix, containing both a highly porous region with modulus values representative of spinal cord tissue, as well as a nonporous region that provides overall mechanical stability to the entire implant. The overall result is a composite matrix for possible use in CNS nerve regeneration, which mimics the mechanical properties of spinal tissue, but can withstand the forces that it will be subjected to in the injury site.

Unraveling the riddle of syringomyelia

The pathophysiology of syringomyelia development is not fully understood. Current prevailing theories suggest that increased pulse pressure in the subarachnoid space forces cerebrospinal fluid (CSF) through the spinal cord into the syrinx. It is generally accepted that the syrinx consists of CSF. The here-proposed intramedullary pulse pressure theory instead suggests that syringomyelia is caused by increased pulse pressure in the spinal cord and that the syrinx consists of extracellular fluid. A new principle is introduced implying that the distending force in the production of syringomyelia is a relative increase in pulse pressure in the spinal cord compared to that in the nearby subarachnoid space. The formation of a syrinx then occurs by the accumulation of extracellular fluid in the distended cord. A previously unrecognized mechanism for syrinx formation, the Bernoulli theorem, is also described. The Bernoulli theorem or the Venturi effect states that the regional increase in fluid velocity in a narrowed flow channel decreases fluid pressure. In Chiari I malformations, the systolic CSF pulse pressure and downward motion of the cerebellar tonsils are significantly increased. This leads to increased spinal CSF velocities and, as a consequence of the Bernoulli theorem, decreased fluid pressure in narrow regions of the spinal CSF pathways. The resulting relatively low CSF pressure in the narrowed CSF pathway causes a suction effect on the spinal cord that distends the cord during each systole. Syringomyelia develops by the accumulation of extracellular fluid in the distended cord. In posttraumatic syringomyelia, the downwards directed systolic CSF pulse pressure is transmitted and reflected into the spinal cord below and above the traumatic subarachnoid blockage, respectively. The ensuing increase in intramedullary pulse pressure distends the spinal cord and causes syringomyelia on both sides of the blockage. The here-proposed concept has the potential to unravel the riddle of syringomyelia and affords explanations to previously unanswered clinical and theoretical problems with syringomyelia. It also explains why syringomyelia associated with Chiari I malformations may develop in any part of the spinal cord including the medullary conus. Syringomyelia thus preferentially develops where the systolic CSF flow causes a suction effect on the spinal cord, i.e., at or immediately caudal to physiological or pathological encroachments of the spinal subarachnoid space.

Mechanically engineered hydrogel scaffolds for axonal growth and angiogenesis after transplantation in spinal cord injury

OBJECT

Spinal cord injury (SCI) is a complex pathological entity, the treatment of which requires a multipronged approach. One way to integrate different therapeutic strategies for SCI is to develop implantable scaffolds that can deliver therapies in a synergistic manner. Many investigators have developed implantable "bridges," but an important property of such scaffolds--that is, mechanical compatibility with host tissues--has been neglected. In this study, the authors evaluated the results of implanting a mechanically matched hydrogel-based scaffold to treat SCI.

METHODS

A nonbiodegradable hydrogel, poly(2-hydroxyethylmethacrylate) (PHEMA), was engineered using thermally initiated free radical solution polymerization. Two groups of 12 adult Sprague-Dawley rats underwent partial cervical hemisection injury followed by implantation of either PHEMA or PHEMA soaked in 1 microg of brain-derived neurotrophic factor (BDNF). Four rats from each group were killed 1, 2, or 4 weeks after induction of the injury. Immunofluorescence staining was performed to determine the presence of scarring, cellular inflammatory responses, gliosis, angiogenesis, and axonal growth in and around the implanted scaffolds.

CONCLUSIONS

The implanted PHEMA with 85% water content had a compressive modulus of 3 to 4 kPa, which matched the spinal cord. Implanted PHEMA elicited modest cellular inflammatory responses that disappeared by 4 weeks and minimal scarring was noted around the matrix. Considerable angiogenesis was observed in PHEMA, and PHEMA soaked in BDNF promoted axonal penetration into the gel. The authors conclude that mechanically engineered PHEMA is well accepted by host tissues and might be used as a platform for sustained drug delivery to promote axonal growth and functional recovery after SCI.

Mechanical properties and function of the spinal pia mater

Object. The pia mater has received little attention regarding its function in the deformation of the spinal cord under compression. In this study the mechanical properties and function of the spinal pia mater were investigated using three methods.

Methods. Spinal cord segments were excised from rabbits. The elastic modulus of the pia mater was measured by performing a tensile test using specimens with the pia mater intact and ones with the pia mater stripped off. The stiffness of the spinal cord was examined by performing a compression test with specimens containing an intact pia mater and ones with a pia mater that was incised at both sides. The cross-sectional area and circumference of the spinal cord were measured on axial views of magnetic resonance images in patients with cervical disc herniations before and after surgery.

The pia mater had an elastic modulus of 2300 kPa, which was 460 times higher than that of spinal cord parenchyma. By covering the parenchyma, it tripled the overall elastic modulus of the spinal cord. The pia mater increased the stiffness of the spinal cord and enhanced its shape recovery after removal of the compression. The cross-sectional area of the spinal cord increased after surgery, whereas the circumference of the spinal cord changed little.

Conclusions. The pia mater firmly covers the spinal cord and has a high elastic modulus; it therefore provides a constraint on the spinal cord surface. It prevents elongation of the circumference and produces a large strain energy that is responsible for shape restoration following decompression.

Mechanism of the spinal cord injury and the cervical spondylotic myelopathy: new approach based on the mechanical features of the spinal cord white and gray matter

OBJECT

The authors have previously investigated the mechanical properties of the white and gray matter in the bovine cervical spinal cord, demonstrating that the gray matter is more rigid, although more fragile, than the white matter. In the present study they conducted additional tensile tests on the bovine cervical spinal cord by changing strain levels and strain rates applied to the white and gray matter.

METHODS

Based on their testing, the authors found the following: 1) Stress within the spinal cord relaxes over time. 2) Intracord stress is related to the strain rates or levels. The finite element method was used to compute the stress distribution within the spinal cord under three compressive loading conditions. Results from the computations showed a different stress distribution in the white and gray matter, where the distribution of stress varied with strain rate, compression volume, and the position of compression.

CONCLUSIONS

These differences in mechanical properties between the white and gray matter constitute different mechanisms contributing to the development of tissue damage and clinical symptoms.