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At least 415 records · Page 23Linked to original sources
PubMed2003

The electric field induced in the brain by magnetic stimulation: a 3-D finite-element analysis of the effect of tissue heterogeneity and anisotropy.

We investigate the effect of tissue heterogeneity and anisotropy on the electric field and current density distribution induced in the brain during magnetic stimulation. Validation of the finite-element (FE) calculations in a homogeneous isotropic sphere showed that the magnitude of the total electric field can be calculated to within an error of approximately 5% in the region of interest, even in the presence of a significant surface charge contribution. We used a high conductivity inclusion within a sphere of lower conductivity to simulate a lesion due to an infarct. Its effect is to increase the electric field induced in the surrounding low conductivity region. This boost is greatest in the vicinity of interfaces that lie perpendicular to the current flow. For physiological values of the conductivity distribution, it can reach a factor of 1.6 and extend many millimeters from the interface. We also show that anisotropy can significantly alter the electric field and current density distributions. Either heterogeneity or anisotropy can introduce a radial electric field component, not present in a homogeneous isotropic conductor. Heterogeneity and anisotropy are predicted to significantly affect the distribution of the electric field induced in the brain. It is, therefore, expected that anatomically faithful FE models of individual brains which incorporate conductivity tensor data derived from diffusion tensor measurements, will provide a better understanding of the location of possible stimulation sites in the brain.

Anisotropy↗
PubMedDate not supplied

Finite element analysis of the stress-concentrating effect of fraenal notches in complete dentures.

Based on clinical experience and empiricism, it was postulated that fraenal notches and midline diastemas lead to fracture of complete dentures. This study used finite element stress analysis to investigate the stress-concentrating effect of a fraenal notch with and without a midline diastema. It was found that a large fraenal notch resulted in high stress levels and that these stresses were augmented more by a narrow median diastema than its wider counterpart.

Dental Stress Analysis↗
PubMed1999

[Three-dimensional finite element analysis of stress in supporting bone of mandibular posterior fixed bridge. I. Stress analysis of the surface of alveolar bone around bridge abutments].

OBJECTIVE: Stress levels of the surface of alveolar bone around bridge abutment of posterior fixed bridge were studied. METHODS: The three-dimensional finite element model was developed on the basis of a try adult mandible. 6 vertical and 1 horizontal loadings were applied respectively, and the Maximum stress(epsilon max) and the Minimum stress(epsilon min) of the surface of alveolar bone around bridge abutment were calculated. RESULTS: The stress concentration exhibited at the apical and cervical regions. The stress of the bicuspid was about 2.1 times as high as that of the molar. When transverse loading was applied, the stress level was about 3.5 times as high as that when vertical loading was applied. CONCLUSION: The results were greatly valuable to design of posterior fixed bridge.

Alveolar Process↗
PubMed2005

Finite element analysis of covered microstents.

Currently available neuroendovascular devices are inadequate for effective treatment of many wide-necked or fusiform intracranial aneurysms and intracranial carotid-cavernous fistulae (CCF). Placing a covered microstent across the intracranial aneurysm neck and CCF rent could restore normal vessel morphology by preventing blood flow into the aneurysm lumen or CCF rent. To fabricate covered microstents, our research group has developed highly flexible ultra thin (approximately 150 microm) silicone coverings and elastomerically captured them onto commercially available metal stents without stitching. Preliminary in vivo studies were conducted by placing these covered microstents in the common carotid artery of rabbits. The feasibility of using covered stents was demonstrated. However, the cover affected the deployment pressure and the stents failed occasionally during deployment due to tearing of the cover. Appropriate modeling of covered stents will assist in designing suitable coverings, and help to reduce the failure rate of covered microstents. The purpose of this study is to use the finite element method to determine the mechanical properties of the covered microstent and investigate the effects of the covering on the mechanical behavior of the covered microstent. Variations in the mechanical properties of the covered microstent such as deployment pressure, elastic recoil and longitudinal shortening due to change in thickness and material properties of the cover have been investigated. This work is also important for custom design of covered microstents such as adding cutout holes to save adjacent perforating arteries.

Blood Vessel Prosthesis↗
PubMed2005

Finite element analysis of stresses in endodontically treated, dowel-restored teeth.

STATEMENT OF PROBLEM: Endodontically-treated, dowel-restored teeth may experience fracture, but investigations of variables related to fracture are often inconclusive and occasionally contradictory. PURPOSE: The finite element method was used to analyze the stresses in dowel-restored teeth. The variables studied were material, shape, bonding, modulus of elasticity, diameter, and length of the dowel. MATERIAL AND METHODS: The model of the dowel-restored tooth involved dentin, ligament, cortical and trabecular bone, gingiva, and gutta-percha. The dowels were made of glass fiber, titanium, or zirconia and modeled as an approximation of the brands ParaPost Fiber White, ParaPost XH, and Cerapost, respectively. The dowel was cemented with zinc-phosphate cement or with bonded or nonbonded resin luting agents, and an approximation of the material properties of these 2 materials were used in the modeling. The restoration included a composite resin core and a gold crown. Other variables included taper versus parallel-sided posts, modulus of elasticity, diameter, and length of post. The model was axisymmetrical in 3 dimensions. A load of 100 N was applied to the crown at an angle of 45 degrees, and tensile, shear, and von Mises stresses were calculated. RESULTS: The generated stresses decreased with respect to the dowel material in the following order: glass fiber, titanium, and zirconia. Stresses were in general higher with tapered than with parallel-sided dowels. Stresses were reduced by bonding and with an increasing modulus of elasticity, increasing diameter, and increasing length of the dowel. CONCLUSIONS: Within the limitations of this study, it was found that all investigated dowel-related factors influenced the stress field generated in dowel-restored teeth.

Alveolar Process↗
PubMed2005

[Stress distribution on the femoral neck at different abduction angles of the hip joint: a finite element analysis].

OBJECTIVE: To analyze the stress distribution and its changes of the femoral neck at different abduction angles of the hip joint to provide biomechanical evidence for the treatment of hip joint disease. METHODS: A three-dimensional finite element model of the femoral head region was reconstructed on the basis of data available in the dataset of No.1 Virtual Male Chinese. Different abduction angles of the hip joint were simulated under the load of body weight, and the stress distribution on the femoral neck was calculated and compared. RESULTS: With different abduction angles of the hip joint, the stresses on the medial and lateral sides of the femoral neck were obviously greater than those on the anterior and posterior regions. As the abduction angle of the hip joint increased, the stress on the posterior aspect of the femoral neck decreased while that in other regions increased (except for the anterior aspect of the femoral neck which remained unchanged). CONCLUSION: The lateral aspect of the femoral neck withstands the most intense stress of the body weight, which increases with the abduction angle of the hip joint.

Biomechanical Phenomena↗
PubMed2005

Finite-element analysis of biting behavior and bone stress in the facial skeletons of bats.

The wide range of dietary niches filled by modern mammals is reflected in morphological diversity of the feeding apparatus. Despite volumes of data on the biomechanics of feeding, the extent to which the shape of mammal skulls reflects stresses generated by feeding is still unknown. In addition to the feeding apparatus, the skull accommodates the structural needs of the sensory systems and brain. We turned to bats as a model system for separating optimization for masticatory loads from optimization for other functions. Because the energetic cost of flight increases with body mass, it is reasonable to suggest that bats have experienced selective pressure over evolutionary time to minimize mass. Therefore, the skulls of bats are likely to be optimized to meet functional demands. We investigate the hypothesis that there is a biomechanical link between biting style and craniofacial morphology by combining biting behavior and bite force data gathered in the field with finite-element (FE) analysis. Our FE experiments compared patterns of stress in the craniofacial skeletons within and between two species of bats (Artibeus jamaicensis and Cynopterus brachyotis) under routine and atypical loading conditions. For both species, routine loading produced low stresses in most of the skull. However, the skull of Artibeus was most resistant to loads applied via its typical biting style, suggesting a mechanical link between routine loading and skull form. The same was not true of Cynopterus, where factors other than feeding appear to have had a more significant impact on craniofacial morphology.

Animals↗
PubMed1993

The practicability of finite-element analysis for assessing changes in human craniofacial morphology from cephalographs.

The usefulness of a finite-element analytical (FEA) method for studying changes in facial form was investigated. The material was lateral cephalographs of 19 patients who had undergone surgical correction of the chin. FEA was used to assess changes in bony form brought about by surgery. Replicate digitization of cephalographic landmarks was used to minimize measurement error. Random and systematic errors were examined and their influence on the finite-element method was assessed. However, the sensitivity of FEA is such that the level of residual measurement error affects the reliability of the method and makes the technique unsuitable for individual cases. However, for pooled data from a group of cases the effect of measurement error is reduced and the reliability of the analysis is satisfactory for intergroup comparison.

Cephalometry↗
PubMed2006

Finite element analysis of head-neck kinematics during motor vehicle accidents: analysis in multiple planes.

In this study, a detailed three-dimensional head-neck (C0-C7) finite element (FE) model developed previously based on the actual geometry of a human cadaver specimen was used. Five simulation analyses were performed to investigate the kinematic responses of the head-neck complex under rear-end, front, side, rear- and front-side impacts. Under rear-end and front impacts, it was predicted that the global and intervertebral rotations of the head-neck in the sagittal plane displayed nearly symmetric curvatures about the frontal plane. The primary sagittal rotational angles of the neck under direct front and rear-end impact conditions were higher than the primary frontal rotational angles under other side impact conditions. The analysis predicted early S-shaped and subsequent C-shaped curvatures of the head-neck complex in the sagittal plane under front and rear-end impact, and in the frontal plane under side impact. The head-neck complex flexed laterally in one direction with peak magnitude of larger than 22 degrees and a duration of about 130 ms before flexing in the opposite direction under both side and rear-side impact, compared to the corresponding values of about 15 degrees and 105 ms under front-side impact. The C0-C7 FE model has reasonably predicted the effects of impact direction in the primary sagittal and frontal segmental motion and curvatures of the head-neck complex under various impact conditions.

Accidents, Traffic↗
PubMed2004

[Three-dimensional finite element analysis of maxillary canine during the tooth translation movement].

OBJECTIVE: To find the characteristic of the stress distribution in the periodontal tissue during maxillary canine distal translation. METHODS: The model was implemented numerically by means of the 3-dimensional finite element method (FEM) and was used to simulate orthodontic tooth translation movements. The finite element model was loaded with defined force systems. And the stresses at 0, 7, 14, and 21 days of loaded distal force and moment in maxillary canine were calculated. RESULTS: During the whole distal translation of canine, 1. the stress in alveolar bone was higher than that in peridontal ligament; 2. the stresses in the cervical region were higher than those in the apical region of the alveolar bone and periodontal tissues; 3. the stresses in the cervical, medium and apical regions of periodontal tissue were on the decline, and the stress in cervical region declined faster than that in apical region. On the 21st day the stresses in the cervical, medium and apical regions were similar. CONCLUSION: The 3-D FEM analysis revealed that alveolar bone loss could be most likely to happen during the initial stage of the tooth movement, and the data from this study mihgt be helpful to orthodontists in selecting an appropriate device of force systems to control the initial loading during orthodontic treatment.

Cuspid↗
PubMed2006

Biomechanical model for muscular dysfunction of the human pharynx using finite element analysis.

OBJECTIVES: The oropharynx functions to transport food from the oral cavity to the esophagus, as well as to maintain an air passage from the nose to the lungs. By combining data from prior material property experimentation, a 3-dimensional finite element method reconstruction of the pharynx, and the utilization of a optimization process based on an inverse dynamic approach, we can estimate the pressures and associated consecutive pressure gradients created internally when the pharynx functions during swallowing. METHODS: In this study, pharyngeal muscular dysfunction was modeled under 3 scenarios of increasing tissue stiffness. This was done by modifications in the stress-strain relationship material property within the finite element method nodes. This mechanical property was used as a surrogate for clinical changes in muscle function complicating neuromuscular disorders, such as stroke and amyotrophic lateral sclerosis. The pharyngeal tissue and deformation of the cross-sectional area of the pharynx were analyzed while increasing the mechanical stiffness by 25%, 50%, and 75%. RESULTS: Increases in stiffness resulted in modified pressure-area curves predicting diminished movement, primarily in stiffened regions. CONCLUSIONS: These simulation results may act as a clinical index illustrating the association between tissue dysfunction and pharyngeal pressure and movement dysfunction. This type of modeling has the potential to act as an investigational tool, as well as a predictive tool, regarding disease progression, cancer treatment, and perhaps even the effects of aging on swallowing function.

Biomechanical Phenomena↗
PubMed1995

Three-dimensional finite element analysis of the shear bond test.

OBJECTIVES: The purpose of this study was to use finite element analyses to model the planar shear bond test and to evaluate the effects of modulus values, bonding agent thickness, and loading conditions on the stress distribution in the dentin adjacent to the bonding agent-dentin interface. METHODS: All calculations were performed with the ANSYS finite element program. The planar shear bond test was modeled as a cylinder of resin-based composite bonded to a cylindrical dentin substrate. The effects of material, geometry and loading variables were determined primarily by use of a three-dimensional structural element. Several runs were also made using an axisymmetric element with harmonic loading and a plane strain element to determine whether two-dimensional analyses yield valid results. RESULTS: Stress calculations using three-dimensional finite element analyses confirmed the presence of large stress concentration effects for all stress components at the bonding agent-dentin interface near the application of the load. The maximum vertical shear stress generally occurs approximately 0.3 mm below the loading site and then decreases sharply in all directions. The stresses reach relatively uniform conditions within about 0.5 mm of the loading site and then increase again as the lower region of the interface is approached. Calculations using various loading conditions indicated that a wire-loop method of loading leads to smaller stress concentration effects, but a shear bond strength determined by dividing a failure load by the cross-sectional area grossly underestimates the true interfacial bond strength. SIGNIFICANCE: Most dental researchers are using tensile and shear bond tests to predict the effects of process and material variables on the clinical performance of bonding systems but no evidence has yet shown that bond strength is relevant to clinical performance. A critical factor in assessing the usefulness of bond tests is a thorough understanding of the stress states that cause failure in the bond test and then to assess whether these stress states also exist in the clinical situation. Finite element analyses can help to answer this question but much additional work is needed to identify the failure modes in service and to relate these failures to particular loading conditions. The present study represents only a first step in understanding the stress states in the planar shear bond test.

Dental Bonding↗
PubMed2000

Finite Element Analysis of Tibial Implants - Effect of Fixation Design and Friction Model.

A three dimensional nonlinear finite element model was developed to investigate tibial fixation designs and friction models (Coulomb's vs nonlinear) in total knee arthroplasty in the immediate postoperative period with no biological attachment. Bi-directional measurement-based nonlinear friction constitutive equations were used for the bone-porous coated implant interface. Friction properties between the polyethylene and femoral components were measured for this study. Linear elastic isotropic but heterogeneous mechanical properties taken from literature were considered for the bone. The Tensile behaviour of polyethylene was measured and subsequently modeled by an elasto-plastic model. Based on the earlier finite element and experimental pull-out studies, pegs and screws were also realistically modeled. The geometry of every component was obtained through measurement. The PCA tibial baseplate with three different configurations was considered; one with three screws, one with one screw and two short inclined porous-coated pegs, and a third one with no fixation for the sake of comparison. The axial load of 2000N was applied through the femoral component on the medial plateau of articular insert. It was found that Coulomb's friction significantly underestimates the relative micromotion at the bone-implant interface. The lowest micromotion and lift-off were found for the design with screws. Relative micromotion and stress transfer at the bone-implant interface depended significantly on the friction model and on the baseplate anchorage configuration. Cortical and cancellous bones carried, respectively, 10-13% and 65-86% of the axial load depending on the fixation configuration used. The remaining portion was transmitted as shear force by screws and pegs. Normal and Mises stresses as well as contact area in the polyethylene insert were nearly independent of the baseplate fixation design. The Maximum Mises stress in the polyethylene exceeded yield and was found 1-2 mm below the contact surface for all designs.

Journal Article↗
PubMed1994

Dynamic response of the human head to impact by three-dimensional finite element analysis.

The impact response of the human head has been determined by three-dimensional finite element modeling. This model represents the essential features of a 50th percentile human head. It includes a layered shell closely representing the cranial bones with the interior contents occupied by an inviscid continuum to simulate the brain. A thin fluid layer was included to represent the cerebral-spinal fluid. To validate the model, its response was obtained by applying a sine-squared pulse of 6.8 kN in magnitude and 10 ms in duration. The load was applied to a freely supported head on the frontal bone in the midsagittal plane. The computed pressure-time histories at 5 locations within the brain material compared quite favorably with previously published experimental data from cadaver experiments and provided a reasonable level of confidence in the validation of the model. A parametric study was subsequently conducted to identify the model response when the impact site (frontal, side, occipital) and the material properties of the head were varied. Interestingly, the model predicted higher contre-coup pressure in the frontal lobe (from occipital impact) than that predicted in the occipital region from frontal impact. This finding supports clinical findings of contre-coup injury being more likely to result from occipital impact than from frontal impact.

Biomechanical Phenomena↗
PubMed1998

A parametric study of acetabular cup design variables using finite element analysis and statistical design of experiments.

To isolate the primary variables influencing acetabular cup and interface stresses, we performed an evaluation of cup loading and cup support variables, using a Statistical Design of Experiments (SDOE) approach. We developed three-dimensional finite element (FEM) models of the pelvis and adjacent bone. Cup support variables included fixation mechanism (cemented or noncemented), amount of bone support, and presence of metal backing. Cup loading variables included head size and cup thickness, cup/head friction, and conformity between the cup and head. Interaction between and among variables was determined using SDOE techniques. Of the variables tested, conformity, head size, and backing emerged as significant influences on stresses. Since initially nonconforming surfaces would be expected to wear into conforming surfaces, conformity is not expected to be a clinically significant variable. This indicates that head size should be tightly toleranced during manufacturing, and that small changes in head size can have a disproportionate influence on the stress environment. In addition, attention should be paid to the use of nonmetal backed cups, in limiting cup/bone interface stresses. No combination of secondary variables could compensate for, or override the effect of, the primary variables. Based on the results using the SDOE approach, adaptive FEM models simulating the wear process may be able to limit their parameters to head size and cup backing.

Acetabulum↗
PubMed1997

Finite element analysis of the cranial base in subjects with Class III malocclusion.

The association between cranial base morphology and Class III malocclusion is poorly understood. This study analyses local shape- and size differences in cranial base configurations of Class I and Class III subjects, employing finite element (FEM) analysis. Seventy-three prepubertal European-American children with Class III malocclusion were compared to their counterparts with a normal, Class I molar occlusion. Lateral cephalographs were traced, checked and subdivided into age- and sex-matched groups. Thirteen points on the cranial base were identified and digitized, providing a geometrical cranial base representation. Average cranial geometries were scaled to an equivalent size and a FEM analysis, capable of depicting and quantifying local shape- and size-change, employed to highlight regionalized, morphological differences. While the anterior cranial base was more homogeneous for shape-change, significant, localized anisotropy in the posterior regions of the cranial base and around sella turcica was evident. For size-change, areas of negative allometry were located posteriorly, but dilations in the mid- and anterior cranial base also were apparent. It is concluded that morphological alterations within the petro-occipital complex accompanied by changes in the sphenoidal and ethmoidal regions induce deviation from a normal cranial base configuration to one associated with deficient orthocephalization and an appearance of Class III malocclusion.

Age Factors↗