The importance of femur/acetabulum cartilage in the biomechanics of the intact hip: experimental and numerical assessment

Experimental studies have been made to study and validate the biomechanics of the pair femur/acetabulum considering both structures without the presence of cartilage. The main goal of this study was to validate a numerical model of the intact hip. Numerical and experimental models of the hip joint were developed with respect to the anatomical restrictions. Both iliac and femur bones were replicated based on composite replicas. Additionally, a thin layer of silicon rubber was used for the cartilage. A three-dimensional finite element model was developed and the boundary conditions of the models were applied according to the natural physiological constrains of the joint. The loads used in both models were used just for comparison purposes. The biomechanical behaviour of the models was assessed considering the maximum and minimum principal bone strains and von Mises stress. We analysed specific biomechanical parameters in the interior of the acetabular cavity and on femur's surface head to determine the role of the cartilage of the hip joint within the load transfer mechanism. The results of the study show that the stress observed in acetabular cavity was 8.3 to 9.2 MPa. When the cartilage is considered in the joint model, the absolute values of the maximum and minimum peak strains on the femur's head surface decrease simultaneously, and the strains are more uniformly distributed on both femur and iliac surfaces. With cartilage, the cortex strains increase in the medial side of the femur. We prove that finite element models of the intact hip joint can faithfully reproduce experimental models with a small difference of 7%.     

Collision tumor with inflammatory breast carcinoma and malignant phyllodes tumor: a case report and literature review

Background

Pelvic reconstruction after hemipelvectomy can greatly improve the weight-bearing stability of the supporting skeleton and improve patients’ quality of life. Although an autograft can be used to reconstruct pelvic defects, the most suitable choice of autograft, i.e., the use of either femur or tibia, has not been determined. We aimed to analyze the mechanical stresses of a pelvic ring reconstructed using femur or tibia after hemipelvectomy using finite element (FE) analysis.

Methods

FE models of normal and reconstructed pelvis were established based on computed tomography images, and the stress distributions were analyzed under physiological loading from 0 to 500 N in both intact and restored pelvic models using femur or tibia.

Results

The vertical displacement of the intact pelvis was less than that of reconstructed pelvis, but there was no significant difference between the two reconstructed models. In FE analysis, the stress distribution of the intact pelvic model was bilaterally symmetric and the maximum stresses were located at the sacroiliac joint, arcuate line, ischiatic ramus, and ischial tuberosity. The maximum stress in each part of the reconstructed pelvis greatly exceeded that of the intact model. The maximum von Mises stress of the femur was 13.9 MPa, and that of the tibia was 6.41 MPa. However, the stress distribution was different in the two types of reconstructed pelvises. The tibial reconstruction model induced concentrated stress on the tibia shaft making it more vulnerable to fracture. The maximum stress on the femur was concentrated on the connections between the femur and the screws.

Conclusions

From a biomechanical point of view, the reconstruction of hemipelvic defects with femur is a better choice.     

Variability of a three-dimensional finite element model constructed using magnetic resonance images of a knee for joint contact stress analysis

Magnetic resonance (MR) imaging has been widely used to evaluate the thickness and volume of articular cartilage both in vivo and in vitro. While morphological information on the cartilage can be obtained using MR images, image processing for extracting geometric boundaries of the cartilage may introduce variations in the thickness of the cartilage. To evaluate the variability of using MR images to construct finite element (FE) knee cartilage models, five investigators independently digitized the same set of MR images of a human knee. The topology of cartilage thickness was determined using a minimal distance algorithm. Less than 8 percent variation in cartilage thickness was observed from the digitized data. The effect of changes in cartilage thickness on contact stress analysis was then investigated using five FE models of the knee. One FE model (average FE model) was constructed using the mean values of the digitized contours of the cartilage, and the other four were constructed by varying the thickness of the average FE model by +/- 5 percent and +/- 10 percent, respectively. The results demonstrated that under axial tibial compressive loading (up to 1,400 N), variations of cartilage thickness caused by digitization of MR images may result in a difference of approximately 10 percent in peak contact stresses (surface pressure, von Mises stress, and hydrostatic pressure) in the cartilage. A reduction of cartilage thickness caused increases of contact stresses, while an increase of cartilage thickness reduced contact stresses. Furthermore, the effect of variation of material properties of the cartilage on contact stress analysis was investigated. The peak contact stress increased almost linearly with the Young's modulus of the cartilage. The peak von Mises stress was dramatically reduced when the Poisson,s ratio was increased from 0.05 to 0.49 under an axial compressive load of 1,400 N, while peak hydrostatic pressure was dramatically increased. Peak surface pressure was also increased with the Poisson's ratio, but with a lower magnitude compared to von Mises stress and hydrostatic pressure. In conclusion, the imaging process may cause 10 percent variations in peak contact stress, and the predicted stress distribution is sensitive to the accuracy of the material properties of the cartilage model, especially to the variation of Poisson's ratio.     

Finite element analysis of a cemented ceramic femoral component for the assembly situation in total knee arthroplasty]

The femoral components of the total knee replacements are generally made of metal. In contrast, ceramic femoral components promise improved tribological and allergological properties. However, ceramic components present a risk of failure as a result of stress peaks. Stress peaks can be minimised through adequate implant design, proper material composition and optimum force transmission between bone and implant. Thus, the quality of the implant fixation is a crucial factor. The objective of the present study was to analyse the influence of the cement layer thickness on stress states in the ceramic femoral component and in the femur. Two- and three- dimensional finite element analyses of an artificial knee joint with cement layers of different thickness and with an unbalanced cement layer thickness between the ceramic femoral component and the femur were performed. Higher stress regions occurred in the area of force transmission and in the median plane. The maximum calculated stresses were below the accepted tensile strength. Stresses were found to be lower for cement layer thickness of <2.0 mm.     

3 种不同弹性模量钛合金股骨假体在羊股骨置换模型中应力分布的三维 有限元分析

目的:通过三维有限元分析方法来观察并比较3种不同弹性模量钛合金股骨假体在羊股骨置换模型中von-Mises应力分布的情况。方法:采用64排螺旋CT对一健康成年羊的下肢股骨进行全长的CT扫描,扫描层厚为0.5 mm,扫描所得的数据存储为DICOM文件。将得到的DICOM文件导入到CT图像分析软件Mimics 10.0,然后利用Mimics 10.0软件来生成股骨的骨质点云数据,再将生成的骨质点云数据导入到Simpleware分析软件,通过机械加工反求中的复杂曲面造型技术建立起精确的三维实体模型。对三维实体模型进行网格划分,确定了髓腔的形状,并根据羊下肢股骨髓腔的形状设计了作者实验用的羊股骨假体模型,然后在ANSYS 12.1软件中进行网格划分。给予加载缓慢行走载荷以及扭转载荷,分析并比较羊股骨以及3种不同弹性模量钛合金股骨假体在股骨置换模型中von-Mises应力分布的情况。结果:在缓慢行走载荷以及扭转载荷条件下,3种不同弹性模量钛合金股骨假体von-Mises应力分布变化趋势一致,假体的柄颈结合部以及假体柄上1/3为应力集中区域。3种不同弹性模量的最大应力集中点均位于柄颈结合部,60 GPa弹性模量的股骨假体植入后假体的最大应力最小(37.8 MPa、29.1 MPa),股骨的最大应力最大(12.6 MPa、24.5 MPa);80 GPa的次之,假体的最大应力(38.4 MPa、33.4 MPa),股骨的最大应力(12.5 MPa、24.5 MPa);110 GPa的股骨假体植入后假体的最大应力最大(38.9 MPa、38.1 MPa),股骨的最大应力最小(12.3 MPa、24.5 MPa)。60 GPa弹性模量的股骨假体植入后的假体最大位移和相对位移均最小(缓慢行走载荷下假体最大位移为0.551 mm、相对位移为0.008 mm,扭转载荷下假体最大位移为0.730 mm、相对位移为0.011 mm)。结论:较低弹性模量的钛合金股骨假体(60 GPa)由于其弹性模量更接近于骨组织的弹性模量,股骨假体与股骨间的"应力遮挡"效应较小,更有利于应力在股骨假体及股骨间的传递,增加了股骨假体的早期稳定性,延长了其临床寿命。     

Finite element analysis of peri-implant bone volume affected by stresses around Morse taper implants: effects of implant positioning to the bone crest

Abstract

Objectives: The purpose of the present study was to evaluate the distribution and magnitude of stresses through the bone tissue surrounding Morse taper dental implants at different positioning relative to the bone crest. Materials and Methods: A mandibular bone model was obtained from a computed tomography scan. A three-dimensional (3D) model of Morse taper implant-abutment systems placed at the bone crest (equicrestal) and 2?mm bellow the bone crest (subcrestal) were assessed by finite element analysis (FEA). FEA was carried out on axial and oblique (45°) loading at 150 N relatively to the central axis of the implant. The von Mises stresses were analysed considering magnitude and volume of affected peri-implant bone. Results: On vertical loading, maximum von Mises stresses were recorded at 6-7?MPa for trabecular bone while values ranging from 73 up to 118?MPa were recorded for cortical bone. On oblique loading at the equiquestral or subcrestal positioning, the maximum von Mises stresses ranged from 15 to 21?MPa for trabecular bone while values at 150?MPa were recorded for the cortical bone. On vertical loading, >99.9vol.% cortical bone volume was subjected to a maximum of 2?MPa while von Mises stress values at 15?MPa were recorded for trabecular bone. On oblique loading, >99.9vol.% trabecular bone volume was subjected to maximum stress values at 5?MPa, while von Mises stress values at 35?MPa were recorded for >99.4vol.% cortical bone. Conclusions: Bone volume-based stress analysis revealed that most of the bone volume (>99% by vol) was subjected to significantly lower stress values around Morse taper implants placed at equicrestal or subcrestal positioning. Such analysis is commentary to the ordinary biomechanical assessment of dental implants concerning the stress distribution through peri-implant sites.     

Finite element analysis of a hemi-pelvis: the effect of inclusion of cartilage layer on acetabular stresses and strain

An appropriate method of application of the hip-joint force and stress analysis of the pelvic bone, in particular the acetabulum, is necessary to investigate the changes in load transfer due to implantation and to calculate the reference stimulus for bone remodelling simulations. The purpose of the study is to develop a realistic 3D finite element (FE) model of the hemi-pelvis and to assess stress and strain distribution during a gait cycle. The FE modelling approach of the pelvic bone was based on CT scan data and image segmentation of cortical and cancellous bone boundaries. Application of hip-joint force through an anatomical femoral head having a cartilage layer was found to be more appropriate than a perfectly spherical head, thereby leading to more accurate stress–strain distribution in the acetabulum. Within the acetabulum, equivalent strains varied between 0.1% and 0.7% strain in the cancellous bone. High compressive (15–30 MPa) and low tensile (0–5 MPa) stresses were generated within the acetabulum. The hip-joint force is predominantly transferred from the acetabulum through the lateral cortex to the sacroiliac joint and the pubic symphysis. The study is useful to understand the load transfer within the acetabulum and for further investigations on acetabular prosthesis.     

Development and Physical Validation of a Finite Element Model of Total Hip Dislocation

Component-on-component impingement, followed by levering of the femoral head, is a common mode of dislocation in total hip arthroplasty. While there have been many registry-based studies of dislocation incidence, confounding factors and sources of variability in the clinical domain make it difficult to identify specific parameter influences. A three dimensional nonlinear finite element model has been developed for the purpose of studying the dislocation event, to allow determination of how individual factors such as component design and clinical implantation position affect the propensity for dislocation. Also, a laboratory testing apparatus was constructed to provide physical validation of the computational model. The finite element model correctly predicted the range of motion observed in the physical apparatus to within 1%, and predicted the peak resisting moment to within 2.5%. Under even a light joint load of 200 N, the von Mises stresses developed in the polyethylene insert reached 13 MPa, and the contact stresses rose to as high as 30 MPa. These deleterious elevations occurred not only at the site of neck impingement, but also at the site of head egress from the liner.     

Design considerations for ceramic resurfaced femoral head: effect of interface characteristics on failure mechanisms

Ceramic hip resurfacing may offer improved wear resistance compared to metallic components. The study is aimed at investigating the effects of stiffer ceramic components on the stress/strain-related failure mechanisms in the resurfaced femur, using three-dimensional finite element models of intact and resurfaced femurs with varying stem–bone interface conditions. Tensile stresses in the cement varied between 1 and 5 MPa. Postoperatively, 20–85% strain shielding was observed inside the resurfaced head. The variability in stem–bone interface condition strongly influenced the stresses and strains generated within the resurfaced femoral head. For full stem–bone contact, high tensile (151–158 MPa) stresses were generated at the cup–stem junction, indicating risk of fracture. Moreover, there was risk of femoral neck fracture due to elevated bone strains (0.60–0.80% strain) in the proximal femoral neck region. Stresses in the ceramic component are reduced if a frictionless gap condition exists at the stem–bone interface. High stresses, coupled with increased strain shielding in the ceramic resurfaced femur, appear to be major concerns regarding its use as an alternative material.     

Mechanical significance of femoral head trabecular bone structure in Loris and Galago evaluated using micromechanical finite element models

Work on the interspecific and intraspecific variation of trabecular bone in the proximal femur of primates demonstrates important architectural variation between animals with different locomotor behaviors. This variation is thought to be related to the processes of bone adaptation whereby bone structure is optimized to the mechanical environment. Micromechanical finite element models were created for the proximal femur of the leaping Galago senegalensis and the climbing and quadrupedal Loris tardigradus by converting bone voxels from high-resolution X-ray computed tomography scans of the femoral head to eight-noded brick elements. The resulting models had approximately 1.8 million elements each. Loading conditions representing takeoff phase of a leap and more generalized load orientations were applied to the models, and the models were solved using the iterative "row-by-row" matrix-vector multiplication algorithm. The principal strain and Von Mises stress results for the leaping model were similar for both species at each load orientation. Similar hip joint reaction forces in the range of 4.9 x to 12 x body weight were calculated for both species under each loading condition, but the hip reaction values estimated for Loris were higher than predicted based on locomotor behavior. These results suggest that functional adaptation to hip joint loading may not fully explain the differences in femoral head trabecular bone structure in Galago and Loris. The finite element method represents a unique and useful tool for analyzing the functional adaptation of trabecular bone in a diversity of animals and for reconstructing locomotor behavior in extinct taxa.     

Age-related changes in the tensile properties of human articular cartilage: a comparative study between the femoral head of the hip joint and the talus of the ankle joint

Specimens of articular cartilage from the superficial and mid-depth zones of the human femoral head and the talus of the ankle joint were tested in tension in planes parallel to the articular surface and parallel to the predominant orientation of the superficial collagen fibrils. The tensile fracture stress of cartilage from both the superficial and mid-depth zones of the femoral head decreased considerably with age. The superficial zone decreased from 33 MPa at 7 years to 10 MPa by the age of 90 years, while the mid-depth zone decreased from 32 MPa at 7 years to 2 MPa by the age of 85 years. In contrast the fracture stress of both levels of cartilage from the talus of the ankle did not decrease significantly with increasing age. The tensile stiffness at 10 MPa of both the superficial and mid-depth zones of the femoral head decreased with age. That of the superficial zone decreased from 150 MPa at 7 years to 80 MPa at 90 years, while the mid-depth zone decreased from 60 MPa at 7 years to 10 MPa at 60 years. The stiffness of talar cartilage from the superficial zone decreased by 20%, while that of the mid-depth zone showed a slight increase in stiffness at 10 MPa with increasing age. There was no significant decrease in the tensile stiffness at 1 MPa with age for either the femoral head or talar cartilage. Based on the results of previous studies it is possible to conclude that the decrease in tensile properties seen in the femoral head results from a deterioration in the tensile properties of the network of collagen fibrils. It is suggested that progressive fatigue failure, perhaps with associated changes in the structure of cartilage due to altered chondrocyte metabolism, causes the reduction in tensile properties with age. The results offer a potential explanation for the observation that osteoarthritis commonly occurs in the hip and knee joints at an increasing incidence as age increases, while the condition only rarely occurs in the ankle joint except as a secondary event to trauma.     

The use of functionally graded dental crowns to improve biocompatibility: a finite element analysis

In post-core crown restorations, the significant mismatch between stiffness of artificial crowns and dental tissues leads to stress concentration at the interfaces. The aim of the present study was to reduce the destructive stresses by using a class of inhomogeneous materials called functionally graded materials (FGMs). For the purpose of the study, a 3-dimentional computer model of a premolar tooth and its surrounding tissues were generated. A post-core crown restoration with various crown materials, homogenous and FGM materials, were simulated and analyzed by finite element method. Finite element and statistical analysis showed that, in case of oblique loading, a significant difference (p < 0.05) was found at the maximum von Mises stresses of the crown margin between FGM and homogeneous crowns. The maximum von Mises stresses of the crown margin generated by FGM crowns were lower than those generated by homogenous crowns (70.8 vs. 46.3 MPa) and alumina crown resulted in the highest von Mises stress at the crown margin (77.7 MPa). Crown materials of high modulus of elasticity produced high stresses at the cervical region. FGM crowns may reduce the stress concentration at the cervical margins and consequently reduce the possibility of fracture.     

Femoral head apparent density distribution predicted from bone stresses

A new theory relating bone morphology to applied stress is used to predict the apparent density distribution in the femoral head and neck. Cancellous bone is modeled as a self-optimizing material and cortical bone as a saturated (maximum possible bone density) response to stress in the bone tissue. Three different approaches are implemented relating bone apparent density to: (1) the von Mises stress, (2) the strain energy density in the mineralized tissue and (3) a defined closed effective stress (spherical stress). An iterative nonlinear three-dimensional finite element model is used to predict the apparent density distribution in the femoral head and neck for each of the three approaches. It is shown that the von Mises stress (an open effective stress) cannot accurately predict bone apparent density. It is shown that strain energy density and the defined closed effective stress can predict apparent density and that they give predictions consistent with the observed density pattern in the femoral head and neck.     

Precision MRI-based joint surface and cartilage density analysis of the knee joint using rapid water-excitation sequence and semi-automatic segmentation algorithm]

The aim of this study was to analyse the precision of three-dimensional joint surface and cartilage thickness measurements in the knee, using a fast, high-resolution water-excitation sequence and a semiautomated segmentation algorithm. The knee joint of 8 healthy volunteers, aged 22 to 29 years, were examined at a resolution of 1.5 mm x 0.31 mm x 0.31 mm, with four sagittal data sets being acquired after repositioning the joint. After semiautomated segmentation with a B-spline Snake algorithm and 3D reconstruction of the patellar, femoral and tibial cartilages, the joint surface areas (triangulation), cartilage volume, and mean and maximum thickness (Euclidean distance transformation) were analysed, independently of the orientation of the sections. The precision (CV%) for the surface areas was 2.1 to 6.6%. The mean cartilage thickness and cartilage volume showed coefficients of 1.9 to 3.5% (except for the femoral condyles), the value for the medial femoral condyle being 9.1%, and for the lateral condyle 6.5%. For maximum thickness, coefficients of between 2.6 and 5.9% were found. In the present study we investigate for the first time the precision of MRI-based joint surface area measurements in the knee, and of cartilage thickness analyses in the femur. Using a selective water-excitation sequence, the acquisition time can be reduced by more than 50%. The poorer precision in the femoral condyles can be attributed to partial volume effects that occur at the edges of the joint surfaces with a sagittal image protocol. Since MRI is non-invasive, it is highly suitable for examination of healthy subjects (generation of individual finite element models, analysis of functional adaptation to mechanical stimulation, measurement of cartilage deformation in vivo) and as a diagnostic tool for follow-up, indication for therapy, and objective evaluation of new therapeutic agents in osteoarthritis.     

A comparison of the influence of global functional loads vs. local contact anatomy on articular cartilage thickness at the knee

Cartilage contact geometry, along with joint loading, can play an important role in determining local articular cartilage tissue stress. Thus individual variations in cartilage thickness can be associated with both individual variations in joint loading associated with activities of daily living as well as individual differences in the anatomy of the contacting surfaces of the joint. The purpose of this study was to isolate the relationship between cartilage thickness predicted by individual variations in contact surface geometry based on the radii of the femur and tibia vs. cartilage thickness predicted by individual variations in joint loading. Knee magnetic resonance (MR) images and the peak knee adduction moments during walking were obtained from 11 young healthy male subjects (age 30.5+/-5.1 years). The cartilage thicknesses and surface radii of the femoral and tibial cartilage were measured in the weight-bearing regions of the medial and lateral compartments of three-dimensional models from the MR images. The ratio of contact pressure between the medial and lateral compartments was calculated from the radii of tibiofemoral contact surface geometries. The results showed that the medial to lateral pressure ratios were not correlated with the medial to lateral cartilage thickness ratios. However, in general, pressure was higher in the lateral than medial compartments and cartilage was thicker in the lateral than medial compartments. The peak knee adduction moment showed a significant positive linear correlation with medial to lateral thickness ratio in both femur (R(2)=0.43,P<0.01) and tibia (R(2)=0.32,P<0.01). The results of this study suggest that the dynamics of walking is an important factor to describe individual differences in cartilage thickness for normal subjects.     

An analytical model of joint contact

The stress distribution in the region of contact between a layered elastic sphere and a layered elastic cavity is determined using an analytical model to stimulate contact of articulating joints. The purpose is to use the solution to analyze the effects of cartilage thickness and stiffness, bone stiffness and joint curvature on the resulting stress field, and investigate the possibility of cracking of the material due to tensile and shear stresses. Vertical cracking of cartilage as well as horizontal splitting at the cartilage-calcified cartilage interface has been observed in osteoarthritic joints. The current results indicate that for a given system (material properties mu and nu constant), the stress distribution is a function of the ratio of contact radius to layer thickness (a/h), and while tensile stresses are seen to occur only when a/h is small, tensile strain is observed for all a/h values. Significant shear stresses are observed at the cartilage-bone interface. Softening of cartilage results in an increase in a/h, and a decrease in maximum normal stress. Cartilage thinning increases a/h and the maximum contact stress, while thickening has the opposite effect. A reduction in the indenting radius reduces a/h and increases the maximum normal stress. Bone softening is seen to have negligible effect on the resulting contact parameters and stress distribution.     

A modified human head model for the study of impact head injury

A recently published finite element (FE) head model is modified to consider the viscoelasticity of the meninges, the spongy and compact bone in the skull. The cerebrospinal fluid (CSF) is simulated explicitly as a hydrostatic fluid by using a surface-based fluid modelling method, which allows fluid and structure interaction. It is found that the modified model yields smoother pressure responses in a head impact simulation. The baseline model underestimated the peak von Mises stress in the brain by 15% and the peak principal stress in the skull by 33%. The increase in the maximum principal stress in the skull is mainly caused by the updation of the material's viscoelasticity, and the change in the maximum von Mises stress in the brain is mainly caused by the improvement of the CSF simulation. The study shows that the viscoelasticity of the head tissue should be considered, and that the CSF should be modelled as a fluid, when using FE analysis to study head injury due to impact.     

Hemiarthroplasty of hip joint: An experimental validation using porcine acetabulum

Biphasic properties of articular cartilage allow it to be an excellent bearing material and have been studied through several simplified experiments as well as finite element modelling. However, three-dimensional biphasic finite element (FE) models of the whole joint are rare. The current study was carried out to experimentally validate FE methodology for modelling hemiarthroplasty. Material properties such as equilibrium elastic modulus and permeability of porcine acetabular cartilage were initially derived by curve-fitting an experimental deformation curve with that obtained using FE. These properties were then used in the hemiarthroplasty hip joint modelling. Each porcine acetabular cup was loaded with 400N using a 34mm diameter CoCr femoral head. A specimen-specific FE model of each acetabular cup was created using μCT and a series of software processes. Each model was analysed under conditions similar to those tested experimentally. Contact stresses and contact areas predicted by the model, immediately after loading, were then compared with the corresponding experimentally measured values. Very high peak contact stresses (maximum experimental: 14.09MPa) were recorded. A maximum difference of 12.42% was found in peak contact stresses. The corresponding error for contact area was 20.69%. Due to a fairly good agreement in predicted and measured values of contact stresses and contact areas, the integrated methodology developed in this study can be used as a basis for future work. In addition, FE predicted total fluid load support was around 80% immediately after loading. This was lower than that observed in conforming contact problems involving biphasic cartilage and was due to a smaller local contact area and variable clearance making fluid exudation easier.     

Tibiotalar cartilage stress corresponds to T2 mapping: application to barefoot running in novice and marathon-experienced runners

Abstract

A 3D anatomically-based finite-element foot model was adopted for predicting von Mises stresses within tibiotalar cartilage following 5?km barefoot running. To compare this predicted stress with T2 maps, magnetic resonance scans of the right ankle and plantar pressure were obtained from ten novices and ten marathon-experienced runners before and after running. Following running, tibiotalar cartilage stress was decreased in experienced runners. This corresponded with T2 values that did not change between pre- and post-running suggesting no increase in cartilage fluid levels. In contrast, novices maintained the same level of von Mises stress and this corresponded with a significant T2 increase in tibiotalar cartilage.     

A modified human head model for the study of impact head injury

A recently published finite element (FE) head model is modified to consider the viscoelasticity of the meninges, the spongy and compact bone in the skull. The cerebrospinal fluid (CSF) is simulated explicitly as a hydrostatic fluid by using a surface-based fluid modelling method, which allows fluid and structure interaction. It is found that the modified model yields smoother pressure responses in a head impact simulation. The baseline model underestimated the peak von Mises stress in the brain by 15% and the peak principal stress in the skull by 33%. The increase in the maximum principal stress in the skull is mainly caused by the updation of the material's viscoelasticity, and the change in the maximum von Mises stress in the brain is mainly caused by the improvement of the CSF simulation. The study shows that the viscoelasticity of the head tissue should be considered, and that the CSF should be modelled as a fluid, when using FE analysis to study head injury due to impact.