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PubMed2007

Determination of suitable sample sizes for multi-patient based finite element studies.

Finite element analysis is used extensively to assess joint replacements, but the majority of these are single sample studies. Recent investigations have suggested that such studies are unable to account for natural inter-patient variation in bone geometry and material property distribution. Recent developments in computer tomography based analyses make multiple sample studies possible; the question remains how many femurs are required to perform a study which accounts for such variations. This work investigates the factors that should be considered in answering this question. It explores sample sizing techniques when comparing strain distribution in the intact and implanted femur and when comparing two or more implant designs in a group of femurs. An example analysis of the effect of femoral head resurfacing was undertaken. Two sample sizing calculations were utilised, one based on achieving the desired precision in results, the other based on determining if a significant difference exists between two designs. The analysis shows that reasonable statistical precision can be achieved with a group of femurs. The study was also able to determine a suitable sample size for the analysis of a statistically significant difference between two groups of femurs with varying design parameters. The study concluded that while sample sizing is recommended for an accurate analysis, consideration must be made for the practicality of such a task.

Biomechanical Phenomena↗
PubMed2006

An experimental and computational study of blunt carotid artery injury.

A carotid artery dissection begins as a tear or defect of the intimal lining of the artery, and can lead to luminal occlusion and ultimately cerebral ischemia. Our aim is to conduct an organ level validation of a finite element model of the carotid artery using an experiment designed to elicit internal layer failure within fluid-filled carotid artery samples. A 2.4-kg beveled guillotine is dropped from three heights (0.3, 0.5 and 0.7 m) onto fluid-filled porcine carotid arteries and resulting damage is recorded. These events are modeled using finite element analysis. Stress, strain and strain rate are correlated to experimental outcome. Internal layer damage is reported in half of the experiments, with damage occurring with 100% frequency at a drop height of 0.7 m. Simulations of this experiment result in maximum principal stress and strain values of 1.43 MPa and 46.2% respectively. The strain level predicted by the model for this impact scenario approaches the strain to intimal failure level for porcine arteries found in the literature. The results of this study represent an important step in validating this finite element carotid artery model at the organ level.

Animals↗
PubMed2005

Stress analysis of carotid plaque rupture based on in vivo high resolution MRI.

Atheromatous carotid plaque rupture is responsible for the majority of ischaemic strokes in the developed world. Plaque rupture has been associated with plaque morphology, plaque components' properties, inflammation and local stress concentration. High resolution multi-spectral magnetic resonance imaging (MRI) has allowed the plaque components to be visualized in vivo. This study combined the recent advances in finite element analysis (FEA) and MRI, and performed stress analysis of five vulnerable carotid plaques based on the geometry derived from in vivo MRI. Image segmentation was based on multi-spectral MRI and co-registered with histology for plaque characterization. Plaque fibrous cap, lipid pool and vessel wall were modelled as isotropic, incompressible hyperelastic materials undergoing large deformation under pulse pressure loading. High stress concentrations were predicted at the shoulders and the thinnest fibrous cap regions of the plaque, and the mean maximal stresses were found to be higher in the ruptured plaques (683.3 kPa) than those in the unruptured plaques (226.9 kPa). The effect of the relative stiffness of fibrous cap to lipid pool on the stress within the cap itself was studied. It was shown that larger relative stiffness of fibrous cap to lipid pool resulted in higher stress within the cap. Thus, it is likely that high stress concentrations in vulnerable plaque may cause plaque rupture and lead to acute ischaemic sequelae. A combination of in vivo high resolution MRI and FEA could potentially act as a useful tool to assess plaque vulnerability and risk stratify patients with carotid atheroma.

Aged↗
PubMed2005

Subject specific finite element modelling of the levator ani.

Understanding of the dynamic behaviour of the levator ani is important to the assessment of pelvic floor dysfunction. Whilst shape modelling allows the depiction of 3D morphological variation of the levator ani between different patient groups, it is insufficient to determine the underlying behaviour of how the muscle deforms during contraction and strain. The purpose of this study is to perform a subject specific finite element analysis of the levator ani with open access magnetic resonance imaging. The method is based on a Mooney-Rivlin hyperelastic model and permits dynamic study of subjects under natural physiological loadings. The value of the proposed modelling framework is demonstrated with dynamic 3D data from nulliparous, female subjects.

Computer Simulation↗
PubMed2005

Accuracy of finite element model-based multi-organ deformable image registration.

As more pretreatment imaging becomes integrated into the treatment planning process and full three-dimensional image-guidance becomes part of the treatment delivery the need for a deformable image registration technique becomes more apparent. A novel finite element model-based multiorgan deformable image registration method, MORFEUS, has been developed. The basis of this method is twofold: first, individual organ deformation can be accurately modeled by deforming the surface of the organ at one instance into the surface of the organ at another instance and assigning the material properties that allow the internal structures to be accurately deformed into the secondary position and second, multi-organ deformable alignment can be achieved by explicitly defining the deformation of a subset of organs and assigning surface interfaces between organs. The feasibility and accuracy of the method was tested on MR thoracic and abdominal images of healthy volunteers at inhale and exhale. For the thoracic cases, the lungs and external surface were explicitly deformed and the breasts were implicitly deformed based on its relation to the lung and external surface. For the abdominal cases, the liver, spleen, and external surface were explicitly deformed and the stomach and kidneys were implicitly deformed. The average accuracy (average absolute error) of the lung and liver deformation, determined by tracking visible bifurcations, was 0.19 (s.d.: 0.09), 0.28 (s.d.: 0.12) and 0.17 (s.d.: 0.07) cm, in the LR, AP, and IS directions, respectively. The average accuracy of implicitly deformed organs was 0.11 (s.d.: 0.11), 0.13 (s.d.: 0.12), and 0.08 (s.d.: 0.09) cm, in the LR, AP, and IS directions, respectively. The average vector magnitude of the accuracy was 0.44 (s.d.: 0.20) cm for the lung and liver deformation and 0.24 (s.d.: 0.18) cm for the implicitly deformed organs. The two main processes, explicit deformation of the selected organs and finite element analysis calculations, require less than 120 and 495 s, respectively. This platform can facilitate the integration of deformable image registration into online image guidance procedures, dose calculations, and tissue response monitoring as well as performing multi-modality image registration for purposes of treatment planning.

Algorithms↗
PubMed2005

Computational biomechanical modelling of the lumbar spine using marching-cubes surface smoothened finite element voxel meshing.

There is a need for the development of finite element (FE) models based on medical datasets, such as magnetic resonance imaging and computerized tomography in computation biomechanics. Direct conversion of graphic voxels to FE elements is a commonly used method for the generation of FE models. However, conventional voxel-based methods tend to produce models with jagged surfaces. This is a consequence of the inherent characteristics of voxel elements; such a model is unable to capture the geometries of anatomical structures satisfactorily. We have developed a robust technique for the automatic generation of voxel-based patient-specific FE models. Our approach features a novel tetrahedronization scheme that incorporates marching-cubes surface smoothing together with a smooth-distortion factor (SDF). The models conform to the actual geometries of anatomical structures of a lumbar spine segment (L3). The resultant finite element analysis (FEA) at the surfaces is more accurate compared to the use of conventional voxel-based generated FE models. In general, models produced by our method were superior compared to that obtained using the commercial software ScanFE.

Biomechanical Phenomena↗
PubMed2005

The role of computational models in the search for the mechanical behavior and damage mechanisms of articular cartilage.

Articular cartilage plays a vital role in the function of diarthrodial joints. Due to osteoarthritis degeneration of articular cartilage occurs. The initial event that triggers the pathological process of cartilage degeneration is still unknown. Cartilage damage due to osteoarthritis is believed to be mechanically induced. Hence, to investigate the initiation of osteoarthritis the stresses and strains in the cartilage must be determined. So far the most common method to accomplish that is finite element analysis. This paper provides an overview of computational descriptions developed for this purpose, and what they can be used for. Articular cartilage composition and structure are discussed in relation with degenerative changes, and how these affect mechanical properties.

Animals↗
PubMed1998

Compression-induced degeneration of the intervertebral disc: an in vivo mouse model and finite-element study.

STUDY DESIGN: An in vivo study of the biologic and biomechanical consequences of static compressive loading on the mouse tail intervertebral disc. OBJECTIVES: To determine whether static compression in vivo alters the biologic activity of the disc and leads to diminished biomechanical performance. SUMMARY OF BACKGROUND DATA: Static compressive stress that exceeds the disc's swelling pressure is known to change hydration and the intradiscal stress distribution. Alterations in hydration and stress have been associated with changes in disc cell activity in vitro and in other collagenous tissues in vivo. METHODS: Mouse tail discs were loaded in vivo with an external compression device. After 1 week at one of three different stress levels, the discs were analyzed for their biomechanical performance, morphology, cell activity, and cell viability. A second group of mice were allowed to recuperate for 1 month after the 1-week loading protocol to assess the disc's ability to recover. As an aid to interpreting the histologic and biologic data, finite-element analysis was used to predict region-specific changes in tissue stress caused by the static loading regimen. RESULTS: With increasing compressive stress, the inner and middle anulus became progressively more disorganized, and the percentage of cells undergoing apoptosis increased. The expression of Type II collagen was suppressed at all levels of stress, whereas the expression of aggrecan decreased at the highest stress levels in apparent proportion to the decreased nuclear cellularity. Compression for 1 week did not affect the disc bending stiffness or strength but did increase the neutral zone by 33%. As suggested by the finite-element model, during sustained compression, tension is maintained in the outer anulus and lost in the inner and middle regions where the hydrostatic stress was predicted to increased nearly 10-fold. Discs loaded at the lowest stress recovered anular architecture but not cellularity after 1 month of recuperation. Discs loaded at the highest stress did not recover anular architecture, displaying islands of cartilage cells in the middle anulus at sites previously populated by fibroblasts. CONCLUSIONS: The results of the current project demonstrate that static compressive loading initiates a number of harmful responses in a dose-dependent way: disorganization of the anulus fibrosus; an increase in apoptosis and associated loss of cellularity; and down regulation of collagen II and aggrecan gene expression. The finite element model used in this study predicts loss of collagen fiber tension and increased matrix hydrostatic stress in those anular regions observed to undergo programmed cell death after 1 week of loading and ultimately become populated by chondrocytes after one month of recuperation. This correspondence conforms with the suggestions of others that the cellular phenotype in collagenous tissues is sensitive to the dominant type of tissue stress. Although the specific mechanisms by which alterations in tissue stress lead to apoptosis and variation in cell phenotype remain to be identified, our results suggest that maintenance of appropriate stress within the disc may be an important basis for strategies to mitigate disc degeneration and initiate disc repair.

Aggrecans↗
PubMed2004

Dynamic analysis for clarifying occlusal force transmission during orthodontic archwire application: difference between ISW and stainless steel wire.

The purpose of our study was to utilize the dynamic finite element analysis to clarify the difference between Improved Super-elastic Ti-Ni alloy Wire (ISW) and Stainless Steel Wire (SSW) on occlusal force transmission during orthodontic treatment. ABAQUS/Standard was used to analyze three finite models over a 30-ms period: ISW, SSW, and wireless models; which consisting of premolar, molar, periodontal ligament (PDL), and alveolar bone. Wire model was established by beam element. A Joint C, which exhibits viscoelasticity to buffer occlusal force, was applied between the wire and bracket. The load was applied on the occlusal surface. At load withdrawal point, the average amounts of von Mises stress on PDL in three models were of the same value. However as time progressed, the stress in wireless model became higher than ISW and SSW models. In contrast, as time progressed further, the stress in SSW model became higher than the other two models and maintained its higher level until the end of analysis. Results showed that high damping capacity of ISW had an ability to buffer the transmission of occlusal force to the PDL. Besides, the dynamic analysis demonstrated an advantage to investigate the stress alterative response between models versus time period.

Alveolar Process↗
PubMed2004

Biphasic finite element simulation of the TMJ disc from in vivo kinematic and geometric measurements.

Understanding the biomechanical nature of the degeneration of the temporomandibular joint requires a coupling between experimental measurements and numerical simulation. In this study, geometry measured from MRI, and motion obtained from a specially designed optoelectronic system are fed into a three-dimensional biphasic finite element analysis to generate the spatial and temporal mechanical response of the disc. This study demonstrates how this coupling can effectively predict the biomechanical response of the temporomandibular joint disc to physiological loading. For small jaw opening movements, asymmetries in the load of the disc are found, with especially higher shear stresses in the lateral portion.

Biomechanical Phenomena↗
PubMed2006

An axisymmetric single-path model for gas transport in the conducting airways.

In conventional one-dimensional single-path models, radially averaged concentration is calculated as a function of time and longitudinal position in the lungs, and coupled convection and diffusion are accounted for with a dispersion coefficient. The axisymmetric single-path model developed in this paper is a two-dimensional model that incorporates convective-diffusion processes in a more fundamental manner by simultaneously solving the Navier-Stokes and continuity equations with the convection-diffusion equation. A single airway path was represented by a series of straight tube segments interconnected by leaky transition regions that provide for flow loss at the airway bifurcations. As a sample application, the model equations were solved by a finite element method to predict the unsteady state dispersion of an inhaled pulse of inert gas along an airway path having dimensions consistent with Weibel's symmetric airway geometry. Assuming steady, incompressible, and laminar flow, a finite element analysis was used to solve for the axisymmetric pressure, velocity and concentration fields. The dispersion calculated from these numerical solutions exhibited good qualitative agreement with the experimental values, but quantitatively was in error by 20%-30% due to the assumption of axial symmetry and the inability of the model to capture the complex recirculatory flows near bifurcations.

Biological Transport, Active↗
PubMed2003

Anisotropic properties of tracheal smooth muscle tissue.

The anisotropic (directional-dependent) properties of contracting tracheal smooth muscle tissue are estimated from a computational model based on the experimental data of length-dependent stiffness. The area changes are obtained at different muscle lengths from experiments in which stimulated muscle undergoes unrestricted shortening. Then, through an interative process, the anisotropic properties are estimated by matching the area changes obtained from the finite element analysis to those derived from the experiments. The results obtained indicate that the anisotropy ratio (longitudinal stiffness to transverse stiffness) is about 4 when the smooth muscle undergoes 70% strain shortening, indicating that the transverse stiffness reduces as the longitudinal stiffness increases. It was found through a sensitivity analysis from the simulation model that the longitudinal stiffness and the in-plane shear modulus are not very sensitive as compared to major Poisson's ratio to the area changes of the muscle tissue.

Algorithms↗
PubMed2006

Physiologically based boundary conditions in finite element modelling.

Finite element analysis has been used extensively in the study of bone loading and implant performance, such as in the femur. The boundary conditions applied vary widely, generally producing excessive femoral deformation, and although it has been shown that the muscle forces influence femoral deflections and loading, little consideration has been given to the displacement constraints. It is hypothesised that careful application of physiologically based constraints can produce physiological deformation, and therefore straining, of the femur. Joint contact forces and a complete set of muscle forces were calculated based on the geometry of the Standardised Femur using previously validated musculoskeletal models. Five boundary condition cases were applied to a finite element model of the Standardised Femur: (A) diaphyseally constrained with hip contact and abductor forces; (B) case A plus vasti forces; (C) case A with complete set of muscle forces; (D) distally constrained with all muscle forces; (E) physiological constraints with all muscle forces. It was seen that only the physiological boundary conditions, case E, produced physiological deflections (< 2.0mm) of the femoral head in both the coronal and sagittal planes, which resulted in minimal reaction forces at the constrained nodes. Strains in the mid-diaphysis varied by up to 600 micro-strain under walking loads and 1000 micro-strain under stair climbing loads. The mode of loading, as indicated by the strain profiles on the cortex also varied substantially under these boundary conditions, which has important consequences for studies that examine localised bone loading such as fracture or bone remodelling simulations.

Compressive Strength↗
PubMed2006

Digital diagnostics: Three-dimensional modelling.

Three-dimensional imaging techniques, such as computed tomograms (CT), structured light, and stereophotogrammetry, can be used to capture three-dimensional coordinate data, but comprehensive analysis is required to transform these techniques into powerful diagnostic tools. The object of this review is to highlight analytical functionality using software developed to study three-dimensional digital imaging and communications in medicine (DICOM) based digital data for diagnosis, planning of treatment, and evaluation of craniofacial changes. My specific aim was to apply three-dimensional software routines using geometric morphometrics or conventional measurements. These routines rely on robust algorithms to construct mean objects by manipulating the three-dimensional x, y, and z coordinates of all the objects' vertices. Conventional measurements and statistical tests can then be applied to the changes in the vertices, say, before and after treatment. Using graphical and geometric morphometric techniques such as finite-element analysis and principal components analysis, clinical craniofacial modelling can be used for the localisation and quantification of soft and hard tissue changes; diagnostic modelling can be undertaken for planning of treatment, and data-driven predictive modelling can be undertaken for the planning of many procedures based on the surgeon's own experience, patients, and resources. Three-dimensional modelling of digital data may therefore have added value for clinical diagnosis, and planning and assessment of treatment, including audit.

Algorithms↗
PubMed2004

New methods for assessing cartilage contact stress after articular fracture.

Progress in reducing the incidence and severity of posttraumatic arthritis depends in part on avoiding deleterious stress levels at residual local incongruities. Systematic efforts to elucidate factors adversely influencing cartilage's mechanical environment in turn depend on the availability of suitable modalities to assess intraarticular contact stresses. This has been and remains a challenging biomechanical problem. Technologic approaches used in the past have included mathematical analyses and indwelling physical sensors, each with advantages and limitations. Two emerging, mutually complementary capabilities show promise of dramatically altering the state of the art in this important field. The first of these methodologies, voxel-based contact finite element analysis, provides accurate computational estimates of cartilage stress on a patient-specific basis, and does so while accommodating arbitrarily idiosyncratic patterns of local articular incongruity. The second methodology, instrumentational, involves transient pressure distribution recordings using specially designed piezoresistive array sensors. Operational considerations for both of these new assessment technologies are described, and promising directions for future development are outlined.

Biomechanical Phenomena↗
PubMed2001

A surgical simulation system of skin sutures using a three-dimensional finite element method.

OBJECTIVE: To establish a surgical simulation system of skin sutures using a three-dimensional finite element method. DESIGN: Three-dimensional finite element models were developed from point data obtained with a rapid three-dimensional surface-measuring device and postoperative profiles were evaluated using these models. BACKGROUND: Since suturing a wound may result in undesirable skin extrusion, it is important to make the extrusion as inconspicuous as possible. We have investigated a means of determining appropriate suture methods to decrease the extrusion. METHODS: Affected body parts were measured non-invasively with a rapid three-dimensional surface-measuring device. Finite element models were prepared, and an appropriate method for reducing skin extrusion was evaluated by attempting various suturing methods. RESULTS: Two kinds of finite element models were prepared: a conventional spindle model and a modified S-shape model. The height of the extrusion of the modified S-shape model was decreased by 40% in comparison with that of the spindle model. These results agreed with clinical findings. CONCLUSIONS: Due to this surgical simulation system of skin sutures, with a rapid three-dimensional surface-measuring device and three-dimensional finite element analysis, it was possible to design an appropriate suturing method and to evaluate the postoperative skin profiles. The modified S-shape suture method would be a recommendable method. RELEVANCE: Using this surgical simulation system of skin sutures, a surgeon can evaluate an appropriate suturing method before operation. It is expected that this system will reduce a surgeon's labor.

Computer Simulation↗
PubMed2003

Micro X-ray computed tomography analysis for the evaluation of asymmetrical condylar growth in the rat.

OBJECTIVES: To investigate the influence of forced lateral bite on mandibular growth, micro X-ray computed tomography (CT) was used for the purpose evaluating condylar cartilage and cancellous bone formation in 10 male Wister rats (3 weeks of age). SETTINGS AND SAMPLE POPULATION: The rats were divided into two groups--experimental and control. In experimental group, an inclined crown was cemented onto the maxillary incisors to produce 2.5 mm shift toward the left side during mastication. Right-left differences in whole mandibular length, mandibular height, condylar size, trabecular structure of the condylar head and three-dimensional (3-D) finite element analysis were assessed using 3-D images reconstructed from micro X-ray CT scans when the mice had reached 21 weeks. MEASUREMENTS AND RESULTS: Asymmetrical growth was found in the experimental group, in which the left condylar head became thicker and shorter than the right condylar head during development. When comparing the left and right condyles of the experimental animals, histomorphometric analysis from micro X-ray CT showed that the bone volume (BV) of the cancellous bone, the surface area of the cancellous bone (BS), the BS/BV ratio, the BV fraction (BV/TV), and the trabecular thickness and trabecular number were less for the right condyle than for the left condyle. CONCLUSIONS: These findings suggested that artificial changes in the mastication do influence the growth of condylar head, condylar bone trabecular structure, and mineralization.

Animals↗
PubMed1998

[The use of isoparametric element of arbitrary hexahedron for solving 3-D thoracic potential distribution].

Isoparametric element of arbitrary hexahedron plays an important role in finite element analysis. This article devotes itself to establishment of mathematical model and combination of derivative of shape function of such element type. In addition, the article also discusses the usage of isoparametric element for solving 3-D thoracic potential distribution in electrical field.

Algorithms↗