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At least 847 records · Page 47Linked to original sources
PubMed2002

An elasto-plastic finite element model for polyethylene wear in total hip arthroplasty.

A new finite element model (FEM) based on an elasto-plastic behavior of ultra high molecular weight polyethylene (UHMWPE) was used to study the wear behavior of UHMWPE acetabular cup, which has a 32 mm diameter femoral head. The model imposed a plastic yield stress of 8 MPa on the UHMWPE so that any stresses beyond this would automatically be redistributed to its neighbor. The FEM model adopted a unique mesh design based on an open cube concept which eliminated the problems of singularities. Wear prediction combined the influences of contact stress, sliding distance and a surface wear coefficient. The new model predicted significantly higher volumetric wear rate (57 mm(3)/yr) well within the average reported clinical values. The model was also used to study the effect of friction and clearance between the acetabular cup and the femoral head. Increase in friction increased the volumetric wear rate but did not appear to affect the linear wear rate, which remained at 0.12 +/- 0.02 mm/yr. The predicted wear was sensitive to clearance. It was found that when the clearance was close to 0 and >0.5mm, severe wear occurred. The best clearance range was between 0.1 and 0.15 mm where the average linear wear rate was 0.1mm/yr and the volumetric wear was 55 mm(3)/yr. The present work indicates the importance of avoiding too tight or too loose a diametrical clearance.

Acetabulum↗
PubMed1997

The distribution of material properties in the equine third metacarpal bone serves to enhance sagittal bending.

The distribution of material properties within the equine third metacarpal bone (MC3), and its possible effect on the mechanics of the structure, was quantitatively evaluated using single-load-to-failure compressive materials testing of specimens from ten horses. Bone samples from six regions within five proximodistal levels of MC3 were milled into right cylinders and compressed at a strain rate of 0.01 s-1. Diaphyseal MC3 bone material was stiffer, stronger, deformed less to yield and failure, and absorbed more energy to yield, than metaphyseal cortical bone material. Lateral and medial MC3 cortical bone material was stiffer and deformed less to yield and failure, than dorsal and palmar material. This distribution of material properties appears to increase the structural compliance in the sagittal plane, and may serve to enhance the predictability of the strain distribution during normal locomotion, as is provided in other bones by a sagittal curvature.

Animals↗
PubMed1997

Development and validation of a series of three-dimensional finite element models of the equine metacarpus.

Three-dimensional finite element (FE) models of the left metacarpi of five adult horses were developed from quantitative computed tomography data, using the algorithms of Keyak et al. (1990, J Biomed. Engng 12, 389-397). The metacarpi were then equipped with 12 rosette strain gauges and loaded non-destructively in a mechanical testing machine. The bones and the models were loaded in axial compression, with the load evenly distributed across the distal row of carpal bones, and with a point load placed mediad to the sagittal midline, to a load equivalent to three times body weight (-15 kN); and in sagittal four-point bending to -2 kN. Maximum and minimum principal strains from the models were compared with those at the strain gauge rosettes. There were significant (p < 0.001) and strong (0.69 < r < 0.90) correlations between predicted and observed surface principal strains, most often resolving as second- or third-order polynomial relationships. In most cases, particularly at extreme strain magnitudes, the models tended to overestimate the observed strain magnitudes. These data suggest that the models are robust and accurate predictors of surface strains. Validation of these models lends further support for the use of this method of automated three-dimensional FE modeling, with its emphasis on accurate, individualized portrayal of structural geometry and material distribution, in research applications, and specifically for the use of these models in the study of the biology and mechanics of the equine metacarpus.

Animals↗
PubMed1997

A three-dimensional parameterized finite element model of the lower cervical spine. Study of the influence of the posterior articular facets.

In this study, we present a three-dimensional geometrical and mechanical finite element model of the complete lower cervical spine. The geometry of the vertebrae is parameterized which allows the model to fit different morphologies of vertebrae. The results obtained with a reduced functional unit model (without posterior arch) and with a complete functional unit model were compared with those obtained from experimental studies, when moments of flexion, extension lateral flexion and axial torque were applied. General agreement was observed. Since the model was parameterized, it was possible to study the influence of some geometrical parameters on the mechanical behavior of the cervical spine. Particularly, we focused on the influence of the posterior articular facets as their geometry is very different from those of the other spinal levels and as large inter-individual variability can be observed. The orientation of the facets with regard to the horizontal plane appeared to have a large influence on the 'coupled rotation to principal rotation' ratio, notably in lateral flexion.

Biomechanical Phenomena↗
PubMed1997

An improved method for finite element mesh generation of geometrically complex structures with application to the skullbase.

An automated method has been developed to generate finite element meshes of geometrically complex structures from CT images using solely hexahedral elements. This technique improves upon previous voxel-based mesh reconstruction approaches by smoothing the irregular boundaries at model surfaces and material interfaces. Over a range of mesh densities, RMS error in surface Von Mises stress was higher in the unsmoothed circular ring models (0.11-0.24 MPa) than in the smoothed models (0.080-0.15 MPa) at each mesh density. The element-to-element oscillation in surface element stress, as measured by the average second spatial derivative of Von Mises stress along the outer surface of the ring, was higher in the unsmoothed models (11.5-15.0 kPa mm-2) than in the smoothed models (4.0-6.8 kPa mm-2). Similarly, in a human skullbase model, the element-to-element oscillation in surface Von Mises stress was higher in the unsmoothed model (5.52 kPa mm-2) than in the smoothed model (1.83 kPa mm-2). Using this technique, finite element models of complex geometries can be rapidly reconstructed which produce less error at the surface than voxel-based models with discontinuous surfaces.

Biomechanical Phenomena↗
PubMed1997

Leg cycling tracking by dynamic vision.

This study describes a method of tracking of human body limbs from a monocular sequence of perspective images. These objects and the associated articulations must be modelled. The principle of the method is based on the interpretation of image features as the three-dimensional perspective projections points of the object model and an iterative process method to compute the model position in accordance with the analysed image. This attitude is filtered (Kalman filter) to predict the model position relative to the next image of the sequence. The image features are extracted locally according to the computed prediction. Tracking experiments, illustrated in this study by a leg cycling sequence, have been conducted to demonstrate the viability of the approach.

Bicycling↗
PubMed1998

A model for mylohyoid muscle mechanics.

The purpose of the present study was to develop a mathematical model of the mylohyoid muscle allowing to analyze the complex mechanics of the muscle during jaw movement. The model was based on muscle morphology and physiological properties. Bending of fibers was incorporated into the model by pulleys located along the upper lateral border of the anterior belly of the digastric muscle. The dynamical properties of the muscle portions, i.e. force length and force velocity relationships, were related to sarcomere length changes. In addition, the effective force component produced by each portion in the sagittal plane was calculated. The model provided information on the geometrical changes of the muscle portions and the concomitant effect on sarcomere length, dynamical properties and effective force component as a function of jaw opening angle. Muscle configuration changed drastically and non-uniformly during jaw opening. However, sarcomere length changes were relatively small and differed but slightly between the muscle portions. The muscle portions all operated near optimum length regarding to their force-length relationship. In all muscle portions effective muscle force was the smallest (10-20% of maximum isometric force) in the beginning of the opening movement. With an increase of jaw angle effective muscle force gradually increased to 60-70% of maximum isometric force in the posteriormost muscle portion and to 20% in the anteriormost portion. Muscle fiber bending appeared to increase the sagittal plane component of the muscle force substantially.

Biomechanical Phenomena↗
PubMed1998

Pulmonary artery hemodynamics with varying degrees of valvular stenosis: an in vitro study.

The study was to investigate the effects of varying degrees of valvular stenosis on the hemodynamics of the main (MPA), left (LPA), and right (RPA) pulmonary arteries. Particle flow visualization was used to examine the flow patterns in a series of pulmonary artery models manufactured out of glass. These glass models were made based on the geometry of the porcine pulmonary arteries with dilatation in the MPA and LPA. Also, detailed pressure mappings in the models were conducted using a side-hole catheter. As the valve became stenotic, a jet-like flow was observed in the M PA. A higher degree of valvular stenosis corresponded to a narrower jet. This jet-like flow was noted to deflect away from the centerline and impinge on the roof of the dilated MPA. Additionally, a notable pressure gradient across the deflected jet-like flow in the direction of its radius of curvature was seen. Moreover, secondary flows started to appear in the dilated MPA. This suggested that the change in geometry in the MPA, due to its dilatation, had a marked effect on the pulmonary artery hemodynamics. In the LPA and RPA, the strengths of the secondary flows increased as the valve became more stenotic. The flow patterns observed in the LPA appeared to be more disturbed than in the RPA, due to the poststenotic, dilatation present in the LPA. Pressure recovery along the axial direction in the M PA was observed for all the stenotic valves studied. As the degree of valvular stenosis increased, the transvalvular energy loss increased. Moreover, it was observed that the energy loss decreased progressively as the flow traveled downstream. This tendency was consistent with the phenomenon of pressure recovery observed in the pressure measurement. The study demonstrates the importance of analyzing biological flows from a three-dimensional viewpoint.

Algorithms↗
PubMed1998

Optimization-based differential kinematic modeling exhibits a velocity-control strategy for dynamic posture determination in seated reaching movements.

We proposed a velocity control strategy for dynamic posture determination that underlay an optimization-based differential inverse kinematics (ODIK) approach for modeling three-dimensional (3-D) seated reaching movements. In this modeling approach, a four-segment seven-DOF linkage is employed to represent the torso and right arm. Kinematic redundancy is resolved efficiently in the velocity domain via a weighted pseudoinverse. Weights assigned to individual DOF describe their relative movement contribution in response to an instantaneous postural change. Different schemes of posing constraints on the weighting parameters, by which various motion apportionment strategies are modeled, can be hypothesized and evaluated against empirical measurements. A numerical optimization procedure based on simulated annealing estimate the weighting parameter values such that the predicted movement best fits the measurement. We applied this approach to modeling 72 seated reaching movements of three distinctive types performed by six subjects. Results indicated that most of the movements were accurately modeled (time-averaged RMSE < 5 degrees) with a simple time-invariant four-weight scheme which represents a time-constant, inter-joint motion apportionment strategy. Modeling error could be further reduced by using less constrained schemes, but notably only for the ones that were relatively poorly modeled with a time-invariant four-weight scheme. The fact that the current modeling approach was able to closely reproduce measured movements and do so in a computationally advantageous way lends support to the proposed velocity control strategy.

Animals↗
PubMed1999

Abdominal muscles contribute in a minor way to peak spinal compression in lifting.

In lifting, the abdominal muscles are thought to be activated to stabilize the spine. As a detrimental effect, they contribute to spinal compression. The existing literature is not conclusive about the biological relevance of this effect. From biological, mechanical and anatomical considerations it was hypothesised that the relative abdominal contribution to compression would be minor in the beginning of the lift, that the relative and absolute abdominal contribution to compression would rise throughout the lift, and that the obliques would contribute to a larger extent than the rectus abdominis. To investigate these hypotheses, 10 subjects lifted 0.5, 10.5 and 22.5 kg. EMG levels obtained from the rectus abdominis and the obliques were converted into force using normalized EMG, muscle potential and area values, and modulating factors for muscle length and contraction velocity. An anatomical model was applied to compute the abdominal effects on spinal compression in three consecutive phases within a lift. If expressed relative to the total spinal compression, the abdominal contribution for the three weight conditions was 7.1% (SD, 1.7), 10.4% (4.7) and 12.5% (4.4) in the begin and 21.0% (5.8), 19.0% (5.3) and 22.2% (6.6) in the end phase. Thus, the relative abdominal contribution to compression was minor in the beginning and increased towards the end. The absolute abdominal contribution was constant throughout the lift. The contributions could be retraced to the obliques rather than the rectus, while during the lift a shift in activation from the obliquus externus to internus was observed.

Abdominal Muscles↗
PubMed1999

Measuring muscle and joint geometry parameters of a shoulder for modeling purposes.

An extensive set of muscle and joint geometry parameters was measured of the right shoulder of an embalmed male. For all muscles the optimal muscle fiber length was determined by laser diffraction measurements of sarcomere length. In addition, tendon length and physiological cross-sectional area were determined. The parameter set was needed to enhance the reliability of a computer model of the shoulder (Van der Helm, 1994a,b Journal of Biomechanics 27, 527-550, 551-569). With the model, an abduction of the arm was simulated in seven positions, at 30 degrees intervals. In each of the simulated arm positions, actual sarcomere lengths were calculated from the lengths of 104 muscle elements, distributed over 16 shoulder muscles. For most muscle elements, the simulated abduction appeared to take place within the sarcomere length range in which the muscle elements can exert force. The muscle elements can then act on the ascending limb as well as on the plateau and on the descending limb of the relative force-length curves of sarcomeres. The produced data set is not only important for the refinement of shoulder modeling, but also for functional analyses of shoulder movements in general.

Cadaver↗
PubMed2000

Improving the local solution accuracy of large-scale digital image-based finite element analyses.

Digital image-based finite element modeling (DIBFEM) has become a widely utilized approach for efficiently meshing complex biological structures such as trabecular bone. While DIBFEM can provide accurate predictions of apparent mechanical properties, its application to simulate local phenomena such as tissue failure or adaptation has been limited by high local solution errors at digital model boundaries. Furthermore, refinement of digital meshes does not necessarily reduce local maximum errors. The purpose of this study was to evaluate the potential to reduce local mean and maximum solution errors in digital meshes using a post-processing filtration method. The effectiveness of a three-dimensional, boundary-specific filtering algorithm was found to be mesh size dependent. Mean absolute and maximum errors were reduced for meshes with more than five elements through the diameter of a cantilever beam considered representative of a single trabecula. Furthermore, mesh refinement consistently decreased errors for filtered solutions but not necessarily for non-filtered solutions. Models with more than five elements through the beam diameter yielded absolute mean errors of less than 15% for both Von Mises stress and maximum principal strain. When applied to a high-resolution model of trabecular bone microstructure, boundary filtering produced a more continuous solution distribution and reduced the predicted maximum stress by 30%. Boundary-specific filtering provides a simple means of improving local solution accuracy while retaining the model generation and numerical storage efficiency of the DIBFEM technique.

Algorithms↗
PubMed2000

A new method for the representation of articular surfaces using the influence surface theory of plates.

The traditional approach to the representation of an articular surface is by using piecewise polynomial functions with a limited continuity to fit the surface from ordered data points. In this study, we introduce a new method, which is based on the influence surface theory of plates, for the representation of articular surfaces. The most significant advantage of this method is that it can effectively represent an articular surface from non-ordered data points. The effectiveness of the present method was shown by reconstruction of a human femoral surface and a mathematical cone.

Algorithms↗
PubMed1982

Force transmission by indirect retainers when a unilateral dislodging force is applied.

Within the experimental design of this study, the following conclusions were drawn: A unilateral vertical dislodging force applied to a distal-extension removable partial denture is transmitted to the supporting structures with a vertical and horizontal component. The presence of indirect retainers transmits the vertical component of force to both indirect retainer abutment teeth while the horizontal component is transmitted to the indirect retainer and primary abutments on the side opposite the loading force. The presence of indirect retainers reduces the stress to the primary abutment teeth. The presence of indirect retainers distributes stress to more supporting teeth.

Dental Abutments↗
PubMed1996

Effect of splinting on load distribution of extracoronal attachment with distal extension prosthesis in vitro.

Previous reports have suggested that the restoration of distal extension edentulous areas with an extracoronal precision attachment partial denture necessitates splinting of abutments. However, the number of splinted teeth remains controversial. In this study, strain gauge technology was used to monitor stresses induced around the abutment and in the distal extension area under different loads as a function of the number of splinted abutments. Three strain gauges were installed in the outer buccal, lingual, and distal surfaces of the abutment to record stresses in the three planes of motion, and another gauge was placed under the distal extension base. The reduction of the number of splinted units from three to two resulted in an insignificant increase of stresses recorded, whereas reduction of the number from two to one resulted in a significant increase of stresses nine times greater than the three-unit splint values. On the basis of this in vitro model, at least two teeth on each side should be splinted when extracoronal distal extension attachment prostheses are used.

Bite Force↗