| Background and objective: Obstructive Hydrocephalus is frequently neurosurgical diseases, many causes can contribute to its appearance. But their common pathogenesis is that the outflow of the cerebrospinal fluid from the ventricle is blocked, which results in the excessive accumulation of the cerebrospinal fluid. Thus, the intraventricular pressure becomes higher and higher, caused pathological ventricular expansion. Along with the progressive expansion of the ventricles, the periventricular brain parenchyma is compressed by the oppressive force, and then series pathophysiological changes followed. Most scholars think that the pathophysiology changes of obstructive hydrocephalus include both primary and secondary. The most prominent primary mechanism is the oppression role of the ventricular pressure on the periventricular brain tissue, and this is the Basis of allpathophysiological changes. Then the periventricular brain tissue will have mechanical response to be suppressed, deformation and displacement occur. Thus, the two interact together in obstructive hydrocephalus has played an important role in the development process. Therefore, to illustrate the biomechanical mechanism will be of important guiding significance in elucidating the secondary pathophysiology of obstructive hydrocephalus, early diagnosis of this disease, and choosing the appropriate treatment. Over the years, many scholars have done a lot of basic and clinical researches. Most of the studies are based on experimental animal models; other methods are experimental corpus method and biological experiments. However, these methods can not directly detect biomechanical response in the brain tissue. Therefore, mathematical simulation methods have been used to solve the problem. There are two methods of mathematical simulation. One is analysis model, but this method is only suitable simple geometry and boundary conditions, and to study the issue of the heterogeneous material; another method is the finite element method. Finite element analysis is a numerical method to solve the mechanical problems by using computers. It discretes continuous medium to finite element, changes infinite freedom degree into finite freedom degree, so as to the numerical analysis will be performed. This method has significant advantages : Analysis of the geometry can be complex, heterogeneous and homogeneous structure, the mechanical response simulation of a variety of constitutive system load and complexity of various boundary conditions; can be obtained by the stress and displacement; Not only numerical results, but can be given automatically by a computer image; High accuracy. Since the human body structure is in the irregular geometry, and heterogeneous material composition of the human body, and there have no possibility by using human body to do mechanical experiments, finite element analysis has been the most commonly biomechanical research tools used on the body without any damage, and has been widely used in the medical field.Biomechanical studies based on the finite element method must first understand the structure of biological material characteristics, then measured to determine the mechanical properties of mechanical parameters. In recent years, with the advances in biomechanical properties and mechanical model research of human brain, much closer to a physiological state of the brain tissue model has been established; Modern medical imaging technology, such as computed tomography (CT) and magnetic resonance imaging (MRI), digital image processing equipment, image information extraction technology profile, CAD and finite element analysis software, provide accurate and simple methods for the construction of human body structure or organ finite element model. Therefore, finite element method has been applied to the brain to study the biomechanical mechanism. Meanwhile, new finite element analysis software continue to emerge. These make it possible to use of the finite element model to study biomechanical mechanism of obstructive hydrocephalus. Thus, despite the irregular ventricular morphology and anatomy of cranial volume fixed, hydrocephalus biomechanical mechanism is extremely complicated; we can still use the powerful computer and the large finite element software to simulate obstructive hydrocephalus, and make a detailed analysis of its biomechanical mechanism.Lateral ventricle is at the top of ventricular system, and it is the origin source of cerebrospinal fluid circulation. Access to any site of its next cycle of obstruction, would lead to excessive accumulation of cerebrospinal fluid, and caused intraventricular pressure increased and ventricle expansion. In other words, as long as obstructive hydrocephalus occurs, the lateral ventricle biomechanical changes will happen. In addition, lateral ventricle has the largest capacity among the ventricular system, and the most complex geometry, the most extensive relations with the surrounding brain tissue structure. Accordingly, to explore the biomechanical mechanism of obstructive hydrocephalus, lateral ventricle and brain parenchyma are the best representative. Moreover, the bearing mechanical model of lateral ventricle and periventricular brain structure can be simplified to the form of plane strain. 2-D finite element model can be established for biomechanical analysis. 2-D finite element models have the advantages of less difficulty of constructing, quantitative results, studying on the material properties than 3-dimensional model.We constructed two-dimensional finite element model of lateral ventricle accorded to the normal brain MRI. The model is applied to a computer simulation of the development process of obstructive hydrocephalus. Detailed analysis of the changes of ventricular geometry and the biomechanical response were carried out. Combining with clinical images, we discussed the biomechanics of characteristic features of obstructive hydrocephalus and the specific clinical guidance. Our research workmainly included the following three parts:1. Construction of the 2-D finite element model of lateral ventricle andperiventricular brain tissueObjective: Construction of the 2-D finite element model of lateral ventricle andperiventricular brain tissueMethods: According to 3mm thickness T2 weighted MR imaging from a normalyoung male volunteer, we confirmed the reconstruction targets. A slice including theanterior angle, angle and the body of lateral ventricle was selected, scanned byTsinghua UNISCAN M1600, and imported into personal computers, preserved inJPEG format, and edited by using Photoshop 9.0. Only half of the selected image study,given the symmetry of the brain in this region. Boundaries were drawn and key pointswere confirmed. According to MRI-scale, determine the ratio to actual length, this ratiofor digital zooming. 2-Dimension coordinates were picked-up by using Photoshop 9.0coordinate system. Contour coordinates to be recorded and kept in a Microsoft Excelfile, came into being coordinates data files. The anatomic information needed toconstruct the finite-element model for simulation was obtained. Each coordinate wasimported into ANSYS9.0. Taking the bottom-up method, we constructed the geometricmodel.Defined the element type was, chose 8-node quadrilateral element, given themechanical characteristics parameters of brain tissue, set element size of 0.5mm, tookthe free mesh grid, a 2-D finite element model was constructed. The model waspreserved in element (ELIST) and Node (NLIST) format.Results: Construction of a two-dimensional finite element model of lateral ventricleand periventricular brain tissue was completed. The model has a good geometricsimilarity with the actual anatomy. The total number of elements was 2187; the totalnumber of nodes was 6806.Conclusions: Medical imaging data provides a simple and accurate method for the construction of finite element model of the complex structure; Through the use of digital image processing equipment, image information extraction technology profile, CAD and large-scaled finite element analysis software, can improve the degree of automatic construction of finite element model.2. The finite element simulation of obstructive hydrocephalus and biomechanical analysisObjective: A computer simulation of obstructive hydrocephalus based on the 2-D finite element model of the lateral ventricle and periventricular brain tissue was used to make biomechanical analysis.Methods: The 2-D finite element model preserved in element (ELIST) and Node (NLIST) format files was transited into INP format files through the FORTRAN program, then inputted into the large finite element software ABAQUS, which applied to the analysis of consolidation. Brain parenchyma was modeled as a two-phase material composed of a porous elastic matrix saturated by interstitial fluid. In this model, the volume occupied by the solids corresponds to the neurons and neuroglia, whereas the voids correspond to the extracellular space of the tissue saturated by interstitial fluid.Installed consolidation parameters, including Young's modulus (E, MPa), the Poisson's ratio ((?)) and the permeability coefficient (K, cm/s), and the set of initial conditions and boundary conditions, imposed load to simulate solution. Results were extracted from ABAQUS Viewer, numerical value and nephogram of the stress, strain, displacement and pore pressure were showed.Results: The results of the finite-element simulation showed the typical progression of hydrocephalus, a progressive expansion of the ventricles was observed. At every time step, the stress distribution, strain and displacement of the periventricular brain tissue were directly showed. The entire simulation process included a total of 37 time steps. Maximal ventricular distension occurred at t = 345600s, i.e. at the 37th time step, which corresponded to a ventricular pressure of 3.0(22.5mmHg) KPa. With the growth of the time step, anterior, posterior horn gradually inflated, concavity of the body gradually became smaller and leveled. The plot showed that Stress concentrations were heterogeneous, areas of expansion and compression were observed. Expansive (tensile) regions were found surrounding the anterior and posterior horns. In contrast, compressive regions were found near the body and other areas of the brain. The outward movement or displacement of the ventricular wall associated with this distension was heterogeneous. Observing along with the X-axis, displacements of the body were most notable, displacements of anterior and posterior horns were less than the body's. This differential displacement resulted in a change in shape from the original ventricular configuration. The horns became "inflated" at the time that the body was effectively displaced and flattened in the radial direction.Conclusions: When Hydrocephalus occurs, complex stress,strain and displacement appear around the periventricular brain tissue due to the irregular geometry of the lateral ventricle. The outward movement or displacement of the ventricular wall associated with this distension is heterogeneous; the type and distribution of the stress and strain are heterogeneous also. All these induce the changes of the void ratio and pore pressure. These biomechanical responses have played an important role in primary and secondary pathophysiological changes of obstructive hydrocephalus.3. The clinical assessment and application of finite element analysis of obstructive hydrocephalus(1) Define the biomechanics mechanism why brain edema is most prominent surround the hornsAccording to the results of finite element analysis, there is biomechanical mechanism why periventricular edema is most prominent in the anterior and posterior horns when obstructive hydrocephalus occurs. Increased intraventricular pressure produces periventricular stress concentration. But, because of the convex/concave geometry of the ventricular wall, the type and distribution of stress concentration were heterogeneous. Expansive (tensile) regions were found surrounding the anterior and posterior horns. In contrast, compressive regions were found near the body and areas in the rest of the brain. Expansive stress concentrations stretch the ependyma around the ventricular horns, result in separation among the ependymal cells, increase extracellular spaces of tissue. Thus, when intraventricular pressure increases, the cerebrospinal fluidEpendymal easily penetrate through the ependyma and accumulate in the extracellular spaces, brain edema around the anterior and posterior horns appears. Finite element analysis results consistent with the clinical imaging findings.(2) The sensitivity analysis of linear measurement methods of lateral ventricle volumeIn the course of diagnosis and treatment of acute obstructive hydrocephalus, serial CT or MRI scans are usually compared with each other. But subtle changes in ventricular volume often make subjective assessment difficult. Therefore, many methods had been developed to measure or estimate the ventricular volume. While volumetric technique is accurate estimate of true ventricular volume, the calculation of it is impractical in most clinical circumstances, and linear measurements may be the only practicable methods. They mainly include Evans' ratio, the CVI, the FO ratio and the DVD ratio. The results of the finite-element simulation showed the typical progression of hydrocephalus, a progressive expansion of the ventricles was observed. The outward movement or displacement of the ventricular wall associated with this distension was heterogeneous. Maximal displacement occurred at the body of the ventricle, and the ventricular span at the body (VBS) increased most obviously. The outward movement of the anterior and posterior horns and other portions of the ventricular wall was less than that of the body. Gray matter nuclei are much more resistant to structural distortion than the white matter is. So that the under part of the body of lateral ventricle bordered by the thalamus and basal nuclei is slower to enlarge; conversely, the upper part of the body is apt to dilate because of the limited resistance of the surrounding white matter. As far as the material properties are concerned, Periventricular brain tissue around the upper part of the body are more homogeneous, and more close to what the finite element model required, so that we chose a slice 60mm above the otic-canthus line.In order to verify the simulated results, a clinical study was carried out. Sixty CT scans from 20 patients with the evaluation of obstructive hydrocephalus from various causes were measured. The scans of 60mm above the OM line that including the upper part of the body and the anterior and posterior horns of lateral ventricle were chose. Ventricular volume was measured by determining the ventricular area in each slice of a CT image using an image analysis system (MCID). Linear measurements included the Evans' ratio, the CVI, the FO ratio, the DVD ratio and the proposed VBS ratio. Ventricular size estimates were then compared using the Spearman's rank order correlation coefficient (using the Statistical Analysis System) and correlations were compared using the Z test statistic. The results of the statistical analyses showed that the VBS ratio was the best correlate of ventricular volume for our group of patients among all the above mentioned linear measurements. Therefore we conclude that this ratio is the most sensitive linear measurement method of the ventricle size in patients with acute obstructive hydrocephalus. The result was consistent with that of the computer simulation based on the finite-element method.
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