(1/1327) Balloon-artery interactions during stent placement: a finite element analysis approach to pressure, compliance, and stent design as contributors to vascular injury.

Endovascular stents expand the arterial lumen more than balloon angioplasty and reduce rates of restenosis after coronary angioplasty in selected patients. Understanding the factors involved in vascular injury imposed during stent deployment may allow optimization of stent design and stent-placement protocols so as to limit vascular injury and perhaps reduce restenosis. Addressing the hypothesis that a previously undescribed mechanism of vascular injury during stent deployment is balloon-artery interaction, we have used finite element analysis to model how balloon-artery contact stress and area depend on stent-strut geometry, balloon compliance, and inflation pressure. We also examined superficial injury during deployment of stents of varied design in vivo and in a phantom model ex vivo to show that balloon-induced damage can be modulated by altering stent design. Our results show that higher inflation pressures, wider stent-strut openings, and more compliant balloon materials cause markedly larger surface-contact areas and contact stresses between stent struts. Appreciating that the contact stress and contact area are functions of placement pressure, stent geometry, and balloon compliance may help direct development of novel stent designs and stent-deployment protocols so as to minimize vascular injury during stenting and perhaps to optimize long-term outcomes.  (+info)

(2/1327) Diffusion tensor imaging in biomechanical studies of skeletal muscle function.

In numerical simulations of skeletal muscle contractions, geometric information is of major importance. The aim of the present study was to determine whether the diffusion tensor imaging (DTI) technique is suitable to obtain valid input with regard to skeletal muscle fibre direction. The accuracy of the DTI method was therefore studied by comparison of DTI fibre directions in the rat tibialis anterior muscle with fascicle striation patterns visible on high-resolution magnetic resonance imaging (MRI) and with fibre directions in an actual longitudinal section (ALS) through the same muscle. The results showed an excellent qualitative agreement between high-resolution MRI and DTI. Despite less accurate quantitative comparison with ALS, it was concluded that DTI does indeed measure skeletal muscle fibre direction. After the experiment, it was possible to determine an appropriate voxel size (0.9 mm3) that provided enough resolution and acceptable accuracy (5 degrees) to use DTI fibre directions in biomechanical analyses. Muscle deformation during contraction, resulting from a finite element simulation with a mesh that was directly generated from the experimental data, has been presented.  (+info)

(3/1327) Red cell distribution and the recruitment of pulmonary diffusing capacity.

The distribution of red blood cells in alveolar capillaries is typically nonuniform, as shown by intravital microscopy and in alveolar tissue fixed in situ. To determine the effects of red cell distribution on pulmonary diffusive gas transport, we computed the uptake of CO across a two-dimensional geometric capillary model containing a variable number of red blood cells. Red blood cells are spaced uniformly, randomly, or clustered without overlap within the capillary. Total CO diffusing capacity (DLCO) and membrane diffusing capacity (DmCO) are calculated by a finite-element method. Results show that distribution of red blood cells at a fixed hematocrit greatly affects capillary CO uptake. At any given average capillary red cell density, the uniform distribution of red blood cells yields the highest DmCO and DLCO, whereas the clustered distribution yields the lowest values. Random nonuniform distribution of red blood cells within a single capillary segment reduces diffusive CO uptake by up to 30%. Nonuniform distribution of red blood cells among separate capillary segments can reduce diffusive CO uptake by >50%. This analysis demonstrates that pulmonary microvascular recruitment for gas exchange does not depend solely on the number of patent capillaries or the hematocrit; simple redistribution of red blood cells within capillaries can potentially account for 50% of the observed physiological recruitment of DLCO from rest to exercise.  (+info)

(4/1327) Simulation model of an eyeball based on finite element analysis on a supercomputer.

BACKGROUND/AIMS: A simulation model of the human eye was developed. It was applied to the determination of the physical and mechanical conditions of impacting foreign bodies causing intraocular foreign body (IOFB) injuries. METHODS: Modules of the Hypermesh (Altair Engineering, Tokyo, Japan) were used for solid modelling, geometric construction, and finite element mesh creation based on information obtained from cadaver eyes. The simulations were solved by a supercomputer using the finite element analysis (FEA) program PAM-CRASH (Nihon ESI, Tokyo, Japan). It was assumed that rupture occurs at a strain of 18.0% in the cornea and 6.8% in the sclera and at a stress of 9.4 MPa for both cornea and sclera. Blunt-shaped missiles were shot and set to impact on the surface of the cornea or sclera at velocities of 30 and 60 m/s, respectively. RESULTS: According to the simulation, the sizes of missile above which corneal rupture occurred at velocities of 30 and 60 m/s were 1.95 and 0.82 mm. The missile sizes causing scleral rupture were 0.95 and 0.75 mm at velocities of 30 and 60 m/s. CONCLUSIONS: These results suggest that this FEA model has potential usefulness as a simulation tool for ocular injury and it may provide useful information for developing protective measures against industrial and traffic ocular injuries.  (+info)

(5/1327) Abductor weakness and stresses around acetabular components of total hip arthroplasty: a finite element analysis.

Abductor weakness, and the resulting Trendelenburg gait, after total hip arthroplasty is believed to be associated with a poor long-term outcome. We have constructed a two-dimensional finite element analysis using load cases to mimic this abductor weakness. The finite element analysis demonstrates slightly increased stresses, particularly at the bone-cement interface in the DeLee-Charnley zone I, which does not seem sufficient to explain the adverse effect of abductor weakness.  (+info)

(6/1327) Determination of the centre of resistance in an upper human canine and idealized tooth model.

The purpose of this investigation was to analyse the influence of geometric and material parameters of a human canine on initial tooth mobility, and the stress and strain profiles in the periodontal ligament. While the material parameters of tooth and bony structures are known within an uncertain limit of approximately a factor of 10, values reported for the elasticity parameters of the periodontal ligament differ significantly. In the course of this study, bilinear behaviour was assumed for the mechanical property of the periodontium. The finite element model of an elliptical paraboloid was created as an approximation to the geometry of a human canine to reduce calculation time and to determine influences of the geometry on numerical results. The results were compared with those obtained for a realistic human canine model. The root length of both models was 19.5 mm. By calculating pure rotational and pure tipping movements, the centre of resistance (CR) was determined for both models. They were located on the long axis of the tooth approximately 7.2 mm below the alveolar crest for the idealized model and 8.2 mm for the canine model. Thus, the centre of resistance of a human canine seems to be located around two-fifths of the root length from the alveolar margin. Using these results, uncontrolled tipping (1 N of mesializing force and 5 Nmm of derotating momentum), as well as pure translation (additionally about 10 Nmm of uprighting momentum) were calculated. Comparing the idealized and the realistic models, the uncontrolled tipping was described by the parabolic-shaped model within an accuracy limit of 10 per cent as compared with the canine model, whereas the results for bodily movement differed significantly showing that it is very difficult to achieve a pure translation with the realistic canine model.  (+info)

(7/1327) Wall stress distribution on three-dimensionally reconstructed models of human abdominal aortic aneurysm.

PURPOSE: Abdominal aortic aneurysm (AAA) rupture is believed to occur when the mechanical stress acting on the wall exceeds the strength of the wall tissue. Therefore, knowledge of the stress distribution in an intact AAA wall could be useful in assessing its risk of rupture. We developed a methodology to noninvasively estimate the in vivo wall stress distribution for actual AAAs on a patient-to-patient basis. METHODS: Six patients with AAAs and one control patient with a nonaneurysmal aorta were the study subjects. Data from spiral computed tomography scans were used as a means of three-dimensionally reconstructing the in situ geometry of the intact AAAs and the control aorta. We used a nonlinear biomechanical model developed specifically for AAA wall tissue. By means of the finite element method, the stress distribution on the aortic wall of all subjects under systolic blood pressure was determined and studied. RESULTS: In all the AAA cases, the wall stress was complexly distributed, with distinct regions of high and low stress. Peak wall stress among AAA patients varied from 29 N/cm(2) to 45 N/cm(2) and was found on the posterior surface in all cases studied. The wall stress on the nonaneurysmal aorta in the control subject was relatively low and uniformly distributed, with a peak wall stress of 12 N/cm(2). AAA volume, rather than AAA diameter, was shown by means of statistical analysis to be a better indicator of high wall stresses and possibly rupture. CONCLUSION: The approach taken to estimate AAA wall stress distribution is completely noninvasive and does not require any additional involvement or expense by the AAA patient. We believe that this methodology may allow for the evaluation of an individual AAA's rupture risk on a more biophysically sound basis than the widely used 5-cm AAA diameter criterion.  (+info)

(8/1327) Removal of the subchondral plate in acetabular preparation.

Retention of the subchondral plate during acetabular preparation in total hip replacement is believed to be an important part of modern cementing techniques. We have constructed a two-dimensional finite element analysis to assess the effect of retention and removal of this relatively stiff structure. The finite element analysis demonstrates increased stiffness and stress concentrations at the bone-cement interface that may have an adverse effect. Although further study is required, it may be that subchondral bone retention is not advantageous.  (+info)