Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Models, Anatomic”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35Linked to original sources
PubMed1987

Analysis of the asymmetrically loaded spine by means of a continuum beam model.

A continuum beam model of the human spine, proposed by Hjalmars (Proceedings of the Fourth International Conference on Continuum Models of Discrete Systems, 1981), and earlier used by the present author as a tool for the analysis of mild functional scoliosis, is here used for the study of a spine, asymmetrically loaded in the frontal plane. In order to validate the theoretical predictions of the model, preliminary experiments are carried out, which show that the model fairly well describes the lateral curvatures that occur when, for various loading conditions, the spine is adjusted into a muscle-relaxed state of equilibrium. Values of the flexural rigidity estimated from the experiments are found to be in reasonable agreement with values, estimated from earlier observations on dead material, reported in the literature. The experiments also indicate that the model may be developed to a tool for estimations of the effective flexural rigidity of the spine in vivo.

Adult↗
PubMed1989

Mathematical modelling of trabecular bone structure: the evaluation of analytical and quantified surface to volume relationships in the femoral head and iliac crest.

The three-dimensional architecture of trabecular bone has structural trends related to physical function as described by Wolff's law. Mathematical modelling provides a means of analysing these structures through the use of simplified representations. A single measure of mineralized bone volume per unit volume of structure (Vv) and the surface area of mineralized bone per unit volume of structure (Sv) does not identify a particular architecture in any detail; the way in which Sv changes in relation to Vv does provide this information as the structure remodels. A series of structures using the elements of plates and rods were created. The rates of change of Sv with respect to Vv for trabecular structures give insight into differences in such models. Structures in the femoral head and iliac crest were analysed by power curve regression. In the principal compressive region, just above the medial cortex, advanced osteoarthritis was associated with a preferential loss of rods from the normal trabecular structure, resulting in a more plate-like architecture. The iliac crest remodelling that takes place in the osteoporotic appears to be the result of a generalised bone loss with some of the thinner elements of the structure being removed completely, resulting in an increase in unit cell dimension. The consequence of changing unit cell size has a major impact on surface availability for osteoblastic and osteoclastic activity. The simple plate model as a basis for the stereological analysis of trabecular structures is therefore limited because of the mixed plate and rod nature of trabecular architecture.

Anthropometry↗
PubMed1989

An iterative procedure to estimate muscle lines of action in vivo.

A method is described to estimate the line of action of muscles in the three-dimensional space from serial images of parallel muscle sections obtained in vivo by means of CT or MRI scanning. The external shape of a muscle, reconstructed from the series of parallel sections, is mathematically divided into a series of imaginary slices directed arbitrarily in the three-dimensional space. The line of action is estimated initially as a regression line through the centroids of these mathematical slices. A new series of mathematical slices is constructed perpendicular to the regression line and a new estimate of the line of action is obtained from their centroids. This procedure is repeated until the estimated line of action is perpendicular to the mathematical slices; it can then be considered as a reliable estimate of the line of action. The accuracy of the method has been tested for various reconstruction parameters and muscle shapes. The results of these tests show that the accuracy is relatively independent of the direction in which the sectional images have been made and that, except for relatively short and thick muscles, the estimated lines of action deviated less than about 2 degrees from the theoretical one. The presented method is a relatively simple mathematical technique which can be used easily for muscles reconstructed in vivo from routinely obtained sectional MRI or CT images.

Biomechanical Phenomena↗
PubMed1989

Effect of prosthetic mitral valve geometry and orientation on flow dynamics in a model human left ventricle.

Pulsatile flow dynamics through bileaflet (St Jude and Duromedics), tilting disc (Bjork-Shiley and Omniscience), caged ball (Starr-Edwards), pericardial (Edwards) and porcine (Carpentier-Edwards) mitral valves in a model human left ventricle (LV) were studied. The model human ventricle, obtained from an in situ diastolic casting, was incorporated into a mock circulatory system. Measurements were made at various heart rates and flow rates. These included the transvalvular pressure drop and regurgitation in percent and cm3 beat-1. The effect of valve geometry and the orientation of the valve with respect to the valve annulus was analyzed using a flow visualization technique. Qualitative flow visualization study indicates certain preferred orientations for the tilting disc and bileaflet valve prostheses in order to obtain a smooth washout of flow in the LV chamber.

Heart↗
PubMed1990

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.

Bone Density↗
PubMed1991

Application of homogenization theory to the study of trabecular bone mechanics.

It is generally accepted that the strength and stiffness of trabecular bone is strongly affected by trabecular microstructure. It has also been hypothesized that stress induced adaptation of trabecular bone is affected by trabecular tissue level stress and/or strain. At this time, however, there is no generally accepted (or easily accomplished) technique for predicting the effect of microstructure on trabecular bone apparent stiffness and strength or estimating tissue level stress or strain. In this paper, a recently developed mechanics theory specifically designed to analyze microstructured materials, called the homogenization theory, is presented and applied to analyze trabecular bone mechanics. Using the homogenization theory it is possible to perform microstructural and continuum analyses separately and then combine them in a systematic manner. Stiffness predictions from two different microstructural models of trabecular bone show reasonable agreement with experimental results, depending on metaphyseal region, (R2 greater than 0.5 for proximal humerus specimens, R2 less than 0.5 for distal femur and proximal tibia specimens). Estimates of both microstructural strain energy density (SED) and apparent SED show that there are large differences (up to 30 times) between apparent SED (as calculated by standard continuum finite element analyses) and the maximum microstructural or tissue SED. Furthermore, a strut and spherical void microstructure gave very different estimates of maximum tissue SED for the same bone volume fraction (BV/TV). The estimates from the spherical void microstructure are between 2 and 20 times greater than the strut microstructure at 10-20% BV/TV.

Biomechanical Phenomena↗
PubMed1993

A procedure to validate three-dimensional motion assessment systems.

The automation provided by computer-assisted motion-tracking systems allows for three-dimensional motion and force analysis. These systems combined with mathematical modelling are able to analyse quickly the intricate dynamics of human movement. Understanding the limitations of human motion analysis as performed by the present measurement techniques is essential for proper application of the results. It is necessary to validate the analysis system prior to subject testing. This paper provides a validation of an optoelectric motion-tracking system used in a dynamic knee assessment study. While the validation is shown with one particular system only, it is suggested that all systems used in two- or three-dimensional motion analysis should be tested similarly in the actual configuration used. Three simple mechanical representations of the human knee have been used in this validation. The first model provided an understanding of the source and behaviour of the error introduced to the accuracy of defining a vector between the recorded coordinates of two markers. The other two models investigated the effect of processing methods specific to the knee analysis project. Separating the markers by at least 180 mm is recommended to produce stable vectors. Relative joint angles could be calculated in all three planes of rotation. The error in calculating flexion and longitudinal rotation was less than 2.0 degrees, while calculating adduction introduced errors of 4.0 degrees. Force calculations were found to be within 8%. The system behaviour was found to be consistent within the calibrated volume about the force platform. Simple mechanical models combined with straightforward procedures can provide validation in terms of clinically relevant parameters.

Algorithms↗
PubMed1994

A stereophotogrammetric method for determining in situ contact areas in diarthrodial joints, and a comparison with other methods.

Determination of contact areas in diarthrodial joints is necessary for understanding the state of stress within the articular cartilage layers and the supporting bony structures. The present study describes the use of a stereophotogrammetry (SPG) system [Huiskes et al., J. Biomechanics 18, 559-570 (1985) and Ateshian et al., J. Biomechanics 24, 761-776 (1991)] for determining contact areas in diarthrodial joints, using a surface proximity concept similar to the one used by Scherrer et al. [ASME J. biomech. Engng 101, 271-278 (1979)]. This method consists of evaluating the proximity of the articular surfaces to determine joint contact areas using precise geometric models of the joint surfaces obtained from the SPG system, and precise kinematic data, also obtained from SPG. In this study, the SPG method for determining contact areas is compared to other commonly used methods such as dye staining, silicone rubber casting and Fuji film contact measurement techniques which have been often used and reported by other investigators. The bovine glenohumeral joint and the bovine lateral tibiofemoral articulation (without the meniscus) were used to represent congruent and incongruent joints, respectively. While all the methods yielded consistent contact patterns for the incongruent tibiofemoral articulations, the results for the congruent bovine glenohumeral joints showed that the SPG and Fuji film methods were in better agreement than those obtained from the dye staining and silicone rubber casting methods. The advantages of the new SPG method are that it can be used for intact joints, and used repeatedly and quickly thus making contact-area movement analyses possible [Soslowsky et al., J. orthop. Res. 10, 524-534 (1992)]. The results of this comparison study show that the SPG technique is a reliable and versatile method for determining contact areas in diarthrodial joints.

Animals↗
PubMed1994

Modeling of the wave transmission properties of large arteries using nonlinear elastic tubes.

We propose a new, simple way of constructing elastic tubes which can be used to model the nonlinear elastic properties of large arteries. The tube models are constructed from a silicon elastomer (Sylgard 184, Dow Corning), which exhibits a nonlinear behavior with increased stiffness at high strains. Tests conducted on different tube models showed that, with the proper choice of geometric parameters, the elastic properties, in terms of area-pressure relation and compliance, can be similar to that of real arteries.

Arteries↗
PubMed1996

Validation of a three-dimensional model of the knee.

Three-dimensional mathematical models of the tibio-femoral joint require input of the geometry of articulating surfaces and ligament insertions, and the mechanical properties of cartilage and ligaments. This paper describes a validation of a knee model through a direct specimen-related comparison between the knee model and the kinematics of four knee joint specimens from which the geometry data were used as input of the model. The knee model is quasi-static and is based on equilibrium of forces and moments. The stiffness properties of the ligaments and articular cartilage were estimated on the basis of data reported in the literature. The so-called reference strains in the ligament bundles for the joint in extension, were determined by using an optimization procedure, minimizing the difference between the kinematics of the model and the kinematics of experimentally obtained flexion motions with an internally or an externally rotated tibia (+/- 3 Nm load). A reasonable to good agreement between the model and the experimental kinematics could be obtained for internal-external rotation laxity and the coupled translations and varus-valgus rotation. The disparity between model and experiment varied from knee to knee, average deviations ranging from close to zero to 8 degrees internal rotation deviation and from 5 mm posterior to 3 mm anterior position deviation. The average anterior-posterior laxities at both 20 degrees and 90 degrees flexion were within the variations reported in the literature, although for each individual joint with some underestimation or overestimation. It was concluded that the optimization procedure compensated for the lack of menisci and capsular structures by higher prestrains, thereby overestimating the ligament forces. Despite the gross simplifications relative to the complex anatomy of the knee, the present knee model can realistically simulate the passive motion characteristics of the human knee joint.

Elasticity↗
PubMed1995

Flow instabilities induced by coronary artery stents: assessment with an in vitro pulse duplicator.

An in vitro pulse duplicator system was used to investigate whether coronary artery stents induce downstream flow instabilities. Hot film or electrochemical probes were used to measure wall shear stress before and after deployment of both single and multiple (overlapping) stents in normal and diseased coronary geometries. Left main coronary diameters ranged from 4 to 5 mm, whereas left anterior descending (LAD) and left circumflex (LCX) diameters ranged from 2 to 4 mm. Under resting conditions, all coronary flow waveforms remained laminar, even after stent placement. However, disturbances were found downstream from a stent placed in the proximal LAD under mild exercise conditions. These disturbances were found 5 mm distal to the stent, in both the LAD and the proximal LCX. Turbulence intensities of order 5% were induced by a single slotted stent in a normal LAD geometry. In cases of distal disease, the turbulence intensity was 9% with one stent and 11% with tandem stents. In cases of proximal disease, these values were 19 and 25%, respectively. The shear stress from these disturbances (20-200 dynes cm-2) is sufficient to delay re-endothelialization and promote restenosis. Therefore, the disturbances could contribute to the increased incidence of restenosis reported with multiple stents, and with stents used in cases of diffuse coronary disease.

Aorta↗
PubMed1975

Modification of stresses in alveolar bone induced by a tilted molar.

Stresses induced in the supporting bone by a tilted molar tooth under load have been investigated by both the photoelastic and the finite element model systems. The following conclusions were reached. 1. Altering the angle of the load applied to the unsupported molar from 0 (axial) to 30 degrees resulted in a fourfold increase in compressive stress in the supporting bone mesial to the tooth. 2. Increasing the load from 30 to 90 pounds while maintaining a 30 degree angle of application resulted in a linear increase in the shear stress on the supporting bone mesial to the tooth. 3. Following the placement of a fixed partial denture, the induced stress at a point on the mesial aspect of the molar tooth, subjected to a 60 pound load at 30 degrees to the long axis, was reduced from 241 to 43 p.s.i. 4. The introduction of a fixed partial denture resulted in a decrease in the compressive stress in the bone adjacent to the apex of the mesial root of the molar from 481 to 174 p.s.i. 5. A distributed 120 pound load applied over the length of the fixed partial denture compared against individual tooth loadings of 60 pounds revealed that placement of the fixed partial denture favored the tilted molar at the expense of the premolar.

Alveolar Process↗
PubMed1976

Open-cast technique for metal molds used in constructing facial prostheses.

A new (open-cast) technique for the fabrication of metal molds has been presented. Its simplicity relies on the use of gummed asbestos tape as a boxing agent for the investment models. This technique has the following advantages over the "lost wax" technique: (1) it requires approximately one-quarter the amount of investment material, (2) it eliminates the use of investing rings or wax patterns, (3) it eliminates the boil-out step for the wax pattern and reduces the total time required to make a metal mold, and (4) it affords a high degree of visibility. The only disadvantage is that the larger prostheses require large amounts of Lino-type metal which is very heavy. However, this has not restricted the use of this technique since these metals cast well and can be easily placed in large ovens. This technique, because of its advantages, offers more efficient use of laboratory time and personnel.

Asbestos↗
PubMed1977

Evaluation of photoelastic stress patterns produced by various designs of bilateral distal-extension removable partial dentures.

The results of this study showed that: 1. The design of a retainer with a mesial rest in conjunction with a buccal I-bar or a wrought-wire and cast lingual arm exhibited the most favorable distribution of vertically applied forces. 2. Retainer designs with a distal rest tend to move the clinical crown distally and the root mesially at the apex, resulting in horizontal forces in the bone. 3. Placing rests of distal-extension removable partial dentures more anteriorly provides an axis of rotation that directs applied forces in a more vertical direction. 4. The distal rest in conjunction with circumferential retainers developed greater horizontal forces within the supporting structures.

Chromium Alloys↗
PubMed1978

Photoelastic comparison of posterior denture occlusions.

The development of stress patterns was observed within a photoelastic model under complete mandidular dentures. Characteristic stress patterns were related to type of tooth material. Plastic teeth transmitted less force than porcelain teeth. Cusp teeth induced concentrated regions of force within the model that contrasted with the uniform distribution of force observed with flat teeth. The dentist should be aware of the biomechanical problems associated with any posterior denture occlusion in relation to the residual alveolar ridge.

Alveolar Process↗
PubMed1979

Root retention and removable partial denture design.

The position of a retained root for preservation of a ridge requires modification of the design of a distal-extension removable partial denture on the side of the retained root. A model with silicone rubber over the ridges and simulated retained roots was fabricated. A framework with mesial rests, distal proximal plates, and I-bar clasp arms was fitted to it. A pencil lead was attached to the framework to record the arcs of rotation of the I-bar clasp arm. The movements and the fulcrums of the prosthesis were changed by the presence of simulated retained roots posterior to the distal abutment.

Dental Abutments↗
PubMed1981

Fabricating moisture chamber spectacles with a sectional facial moulage.

A technique for the construction of acrylic resin moisture chambers has been presented. The utilization of a sectional facial moulage allows for the construction of the chambers to be carried out in the laboratory saving valuable patient contact time. The necessary materials are readily available, and the techniques are common to dental practice.

Eyeglasses↗