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PubMed2004

Finite element analysis of active Eustachian tube function.

The inability to open the collapsible Eustachian tube (ET) has been related to the development of chronic otitis media. Although ET dysfunction may be due to anatomic and/or mechanical abnormalities, the precise mechanisms by which these structural properties alter ET opening phenomena have not been investigated. Previous investigations could only speculate on how these structural properties influence the tissue deformation processes responsible for ET opening. We have, therefore, developed a computational technique that can quantify these structure-function relationships. Cross-sectional histological images were obtained from eight normal adult human subjects, who had no history of middle ear disease. A midcartilaginous image from each subject was used to create two-dimensional finite element models of the soft tissue structures of the ET. ET opening phenomena were simulated by applying muscle forces on soft tissue surfaces in the appropriate direction and were quantified by calculating the resistance to flow (R(v)) in the opened lumen. A sensitivity analysis was conducted to determine the relative importance of muscle forces and soft-tissue elastic properties. Muscle contraction resulted in a medial-superior rotation of the medial lamina, stretching deformation in the Ostmann's fatty tissue, and lumen dilation. Variability in baseline R(v) values correlated with tissue size, whereas the functional relationship between R(v) and a given mechanical parameter was consistent in all subjects. ET opening was found to be highly sensitive to the applied muscle forces and relatively insensitive to cartilage elastic properties. These computational models have, therefore, identified how different tissue elements alter ET opening phenomena, which elements should be targeted for treatment, and the optimal mechanical properties of these tissue constructs.

Adolescent↗
PubMed1989

Stress distribution in the ulna following a hinged elbow arthroplasty. A finite element analysis.

The failure rates for total elbow arthroplasty, in comparison to those for hip arthroplasty, are quite high, and a precise understanding of the underlying causes still remains elusive. The presence of abnormal stresses is a known factor that accelerates loosening of hip and knee arthroplasties. Although a large number of biomechanical studies have led to a better understanding of elbow joint kinetics, very little is known about the stress distribution in this joint. The implantation of a Coonrad humeral component increases stresses in the bone and cement adjacent to the stem tip and hinge regions. An analysis of implanted ulnar stresses and a comparison of those stresses to implanted humeral stresses would improve our understanding of hinged elbow arthroplasty. For this reason, the distribution of mechanical stresses in the ulna are investigated in this study. Using a specially developed casting and sectioning technique, three-dimensional finite element meshes were obtained from an intact human cadaver ulna and an ulna fitted with a Coonrad prosthesis. The material properties were derived from values presented in the literature. Stress distributions in response to axial compression, axial torque, and anteroposterior (AP) force were computed. The cancellous bone and cement regions adjacent to the stem tip of the prosthesis exhibited higher stresses than those in the same regions of the intact case. The higher stresses in the ulna with an implanted prosthesis, as compared to the intact model, might initiate loosening or failure of the prosthesis. The stresses in the cortical bone region adjacent to the prosthesis head were decreased. This is consistent with the clinical observations of bone atrophy following total elbow arthroplasty.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1997

Mechanical implications of humero-ulnar incongruity--finite element analysis and experiment.

Previous studies show that the humero-ulnar joint is physiologically incongruous [Eckstein et al. (1995a) Anat. Rec. 243, 318-326] and exhibits a bicentric (ventro-dorsal) distribution of subchondral mineralization [Eckstein et al. (1995b) J. Orthop. Res. 13, 286-278]. We therefore asked: (1) Does humero-ulnar incongruity bring about a bicentric distribution of contact pressure? (2) Do tensile stresses occur in the subchondral bone of the trochlear notch that are in the same order of magnitude as the compressive stresses? (3) Do ventral and dorsal maxima of subchondral bone density correlate with a bicentric distribution of strain energy density? To that end, a two-dimensional finite element model was designed. The shape and material properties of the bones were based on CT and the boundary conditions selected to agree with resisted elbow extension at 90 degrees of flexion. The incongruity and contact areas were determined experimentally from casts, and the pressure distribution with Fuji Prescale film. In the model and the experiment contact stresses above 2 MPa were recorded in the ventral and dorsal parts of the joint, and values below 0.5 MPa in the depth of the notch. In the model, tensile stresses of 2.9 MPa were observed in the subchondral bone of the ulna, but not in the humerus. The subchondral strain energy density yielded a bicentric pattern in a model with homogeneous subchondral bone properties. It is shown that humero-ulnar incongruity brings about a bicentric distribution of contact pressure, a tensile stress in the notch that is in the same order of magnitude as the compressive stress, and a distribution of strain energy density that correlates with subchondral density patterns.

Adult↗
PubMed1990

Three-dimensional finite element analysis of glass-ceramic dental crowns.

Because of the improved esthetic potential of glass-ceramic crowns as dental restorations, they are sometimes preferred over metal-ceramic crowns for restoration of anterior teeth. Because of their relatively high strength, these ceramic crowns are also frequently used for restoration of posterior teeth. However, due to the larger magnitude of biting forces on posterior teeth, intraoral fracture of all-ceramic crowns tends to occur more frequently in posterior crowns (Moffa, 1988). The objective of this study was to determine the relative influence of load orientation and the occlusal thickness of posterior ceramic crowns on the stress distribution which develops under these loading and design conditions. Three-dimensional finite element models for a molar crown were developed to determine the stress distribution under simulated applied loads. Glass-ceramic crowns with occlusal thicknesses of 0.5, 1.5, and 3.0 mm were considered. The largest principal tensile stresses induced in ceramic due to a distributed load of 600 N applied in a cuspal region were approximately 12 and 182 MPa for vertical and horizontal loading orientations, respectively. Stresses which developed in the facial and lingual marginal regions were primarily compressive under vertical loads. However, tensile stresses developed when the load was applied horizontally. Differences in stress distribution within crowns with the three occlusal thicknesses occurred only near the site of loading. Because of the relatively large failure rates of ceramic crowns in the posterior regions, these restorations should be strengthened by improvement in design, composition, and thermal processing conditions. Before any significant progress is made in these areas, these restorations should be used for the anterior teeth. The results of this study suggest that orientation of the applied load has a more important effect on development of large tensile stresses than the occlusal thickness of ceramic.

Bite Force↗
PubMed2001

Finite element analysis of the behavior of the Scanning Microdeformation Microscope.

The Scanning Microdeformation Microscope, as many other scanning probe microscopes developed in the last years, is a kind of ac force microscope using the near-field acoustic interaction. The heart of the system is an electromechanical oscillator made of a silicon cantilever, a diamond or sapphire tip, associated with a bimorph piezoelectric transducer and a specific amplifier. The specificity of the system is the way of detection of the oscillation frequency performed electrically through the admittance of the piezoelectric transducer. In this paper, we describe the technique of detection involved in the microscope. A modelling of the complete behavior of the electromechanical oscillator performed with the finite element method (FEM) (of simulation) is presented. A comparison between experimental and theoretical behavior shows a very good agreement.

Journal Article↗
PubMed1996

All-ceramic resin-bonded bridges. A 3-dimensional finite-element analysis study.

Resin-bonded fixed partial dentures (FPD) with a metal framework have some disadvantages: a grey shimmer of the metal wings through the abutment teeth, a higher corrosion rate, and an allergenic potential of the non-precious alloys used. The Al2O3 ceramic In-Ceram seems to be strong enough to serve as a framework for resinbonded all-ceramic FPDs. Because of the fact that ceramic wings often don't have enough inter-occlusal space, a new preparation design was developed. The objective of this study was to determine the influence of load orientation (45 degrees and 60 degrees) and the design of the interproximal connector on the stress distribution in the bridges. A 3-dimensional finite-element model was developed to simulate the anatomical situation. The biting force was assumed as 250 N and oriented in oro-buccal direction. The loading-point was palatal 1.5 mm beneath the incisal edge. It was found that stress generally increased with an angle of the biting force of 60 degrees. A small interdental connector (3 mm height) and/or strong interdental separation resulted in stresses of up to 455 MPa (45 degrees) or 534 MPa (60 degrees). Less separation with rounded edges and a higher connector (4 mm) reduced the stress to 122 MPa (45 degrees) and 143 MPa (60 degrees). Due to an average tensile strength of In-Ceram at 340-400 MPa, an all-ceramic resin-bonded FPD may only be recommended if the height of the connector could be minimum 4 mm. Rounded edges and little interdental separation are significant for stress reduction.

Allergens↗
PubMed2006

Finite element analysis of broadband acoustic pulses through inhomogenous media with power law attenuation.

Acoustic waves in tissues and weakly attenuative fluids often have an attenuation parameter, alpha(omega), satisfying alpha(omega)= alpha0omegay in which alpha0 is a constant, omega is the frequency, and y is between 1 and 2. This power law attenuation is not predicted by the classical thermoviscous wave equation and researchers have proposed different modified viscous wave equations in which the loss term is a convolution operator or a fractional spatial or temporal derivative. In this paper, acoustic waves undergoing power law attenuation are modeled by a modification to the thermoviscous wave equation in which the time derivative of the viscous term is replaced by a fractional time derivative. An explicit time domain, finite element formulation leads to a stable algorithm capable of simulating axisymmetric, broadband acoustic pulses propagating through attenuative and dispersive media. The algorithm does not depend on the Born approximation, long wavelength limit, or plane wave assumptions. The algorithm is validated for planar and focused transducers and results include radiation patterns from a viscous scatterer in a lossless background and signals reflected from a viscous layer. The program can be used to determine scattering parameters for large, strong, possibly viscous scatterers, in either a lossless or viscous background, for which analytic results are scarce.

Journal Article↗
PubMed2000

A new catheter design using needle electrode for subendocardial RF ablation of ventricular muscles: finite element analysis and in vitro experiments.

Radio-frequency (RF) cardiac ablation has been very successful for treating arrhythmias related with atrioventricular junction and accessory pathways with successful cure rates of more than 90%. Even though ventricular tachycardia (VT) is a more serious problem, it is known to be rather difficult to cure VT using RF ablation. In order to apply RF ablation to VT, we usually need to create a deeper and wider lesion. Conventional RF ablation electrodes often fail to produce such a lesion. We propose a catheter-electrode design including one or more needle electrodes with a diameter of 0.5-1.0 mm and length of 2.0-10 mm to create a lesion large enough to treat VT. One temperature sensor could be placed at the middle of the needle electrode for temperature-controlled RF ablation. From finite element analyses and in vitro experiments, we found that the depth of a lesion is 1-2 mm deeper than the insertion depth of the needle and the width increases as we increase the diameter of the needle and the time duration. We showed that a single needle electrode can produce a lesion with about 10-mm width and any required depth. If a wider lesion is required, more than one needle with suggested structures can be used. Or, repeated RF ablations around a certain area using one needle could produce a cluster of lesions. In some cases, a catheter with both conventional electrode and needle electrode at its tip may be beneficial to take advantage of both types of electrode.

Animals↗
PubMed1997

The nonlinear finite element analysis and plantar pressure measurement for various shoe soles in heel region.

The most influential factor contributing to foot and shoe comfort is underfoot cushioning. The shock absorbing ability of footwear in the heel area is of particular importance in reducing the impact load during athletic activities and in therapeutic footwear prescribed for heel pain. Furthermore, foot care for foot problem patients is an important part of treatment and educational programs. Therefore, a well-designed sport shoe which can provide comfort and protection is essential. In order to design a functional shoe, biomechanics and other new technologies should be considered, and the design process should be examined in the biomechanics laboratory over and over. The design process requires too much time and effort since the entire experimental and test work can only be done after the prototype is manufactured. Therefore, this study tried to introduce the Finite Element Method (FEM) into the shoe design process by building a three-dimensional FE model with various shoe soles and loading conditions. The material properties of shoe materials were tested using an Instron Testing Machine. An in-shoe pressure insole was used to measure the plantar pressure in different ambulation conditions with various shoe constructions. The subject for this study was a healthy young male without any foot problem. The average plantar pressures obtained from approximately 50 steps in the heel region for each of the various conditions were collected. The results showed that the mean peak plantar pressure of the running situation was significantly higher than that of the walking situation as predicted, and that the insole could provide better cushioning compared to the other shoe constructions. The stress strain relationship for shoe materials was approximated better by a second-order nonlinear curve according to the Instron test. The results of the finite element method suggested that only the second-order nonlinear stress strain curve could correctly describe the shoe material, which also confirmed a potential valuable role for FEM in designing functional shoes.

Biomechanical Phenomena↗
PubMed2003

In-air and underwater performance and finite element analysis of a flextensional device having electrostrictive poly(vinylidene fluoride-trifluoroethylene) polymers as the active driving element.

A flextensional transducer, in which the electrostrictive poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer was used as the active driving element, was fabricated and characterized. The results show that transducers of several millimeters thick can produce an axial displacement of more than 1 mm in air along the thickness direction, and a transmitting voltage response of 123 dB re 1 microPa/V at 1 m in water at frequencies of several kilohertz. A finite element code (ANSYS, Inc., Canonsburg, PA) was used to model the in-air and underwater responses of the flextensional transducer over a broad frequency range. The calculated resonance frequencies and transmitting voltage response spectra show good agreement with the experimental data. In addition, the performance of both the in-air actuator and underwater transducer was analyzed for different design parameters of the flextensional structure. These results show that the performance of the flextensional transducer could be tailored readily by adjusting the parameters of the flextensional metal shell.

Journal Article↗
PubMed1997

Factors influencing stability at the interface between a porous surface and cancellous bone: a finite element analysis of a canine in vivo micromotion experiment.

Several factors contribute to the success of stable bony ingrowth into the porous coated surfaces of orthopaedic implants used in hip arthroplasty. Despite having good bony apposition, bony ingrowth might not occur if the relative motion between bone and implant is large. Therefore, determining the limiting micromotion value that inhibits stable bony ingrowth is important. From a previous canine in vivo micromotion study performed at our laboratory, this limiting value was found to be 20 microns. Initially, cementless orthopaedic implants are stabilized only by frictional forces at the bone-implant interface. Therefore, other parameters such as the coefficient of friction and the compressive force normal to the interface should be considered as important factors which stabilize the interface along with micromotion. The purpose of this analytical study was to elucidate how the stability at the bone-implant interface is influenced by various factors, namely, motion of the implant, the coefficient of friction, the degree of pres fit, and the modulus of the surrounding cancellous bone in determining the stability of the bone-implant interface. Nonlinear and linear finite element models which simulated the immediate postsurgical condition and the end point of the canine in vivo micromotion experiment, respectively, were used to this end. From the results of the finite element models it was possible to identify the displacement magnitude for which the implant slipped relative to the bone as the motion of the implant was increased incrementally. This was done for combinations of the coefficient of friction, press fit, and Young's modulus of cancellous bone. This was used as an indicator of the limiting implant motion value beyond which bony ingrowth will be inhibited. The stress distribution in the surrounding cancellous bone bed was also obtained from the results of the finite element analyses for different press-fit conditions. The results of the study indicated that under slight press-fit conditions, the implant slipped relative to bone for implant motions as low as 20 microns. For higher degrees of press fit and reasonable values for the coefficient of friction, no slip occurred for implant motions as much as 100 microns. Although higher degrees of press fit were theoretically conducive to better implant stability, the concomitant high stresses in the adjacent cancellous bone will tend to compromise the integrity of the press fit. This was also evident when the results of an analytical model with a lower degree of press fit correlated well with those of the canine in vivo experiment in which a higher press fit was used, suggesting a possibility of achieving a less than desired press fit during the process of implantation. Through this study the importance of factors other than implant motion was emphasized. The results of the study suggest that the limiting value of implant motion that inhibits bone ingrowth might vary with the degree of press fit for reasonable coefficients of friction.

Animals↗
PubMed1983

A finite element analysis of the influence of surgical herniation on the viscoelastic properties of the intervertebral disc.

The purpose of this paper was to study the influence of surgical herniation on the viscoelastic properties of the intervertebral disc. The work was broken down into two phases: experimentation and analysis. In the first step of experimentation, five specimens were excised from the L1-L2 level of young adult rhesus monkeys. The prepared specimen consisted of the intervertebral joint and the two adjacent vertebral centrums without either the posterior elements or associated soft tissue. The specimens were subjected to a constant compressive load for eight hours followed by a sixteen hour relaxation time. During all the load cycles, displacement of the specimens was measured with an LVDT. After the first compressive test, the specimens were herniated and then the compressive test repeated. In the second phase of the work, a viscoelastic axisymmetric finite element model was used to quantify the experimental data. A three parameter Kelvin solid was employed in the finite element model. The results of this paper will be used to construct a dynamic model for the vertebral column.

Animals↗
PubMed1993

Finite element analysis of airflow in the nasal valve.

We digitized the outline of the nasal cavity from images obtained after applying contrast material to the nose. A computer-aided simulation of flow was undertaken in the sagittal plane in the anterior nasal cavity. Previous work on the nasal valve was reviewed. Our results showed that the nasal valve is an oblique structure bounded laterally by the caudal end of the upper lateral cartilage, medially by the septum, and ventrally by the inferior rim of the piriform aperture. We found that this rim, projecting from the floor of the nose, produces an uneven distribution of airflow through the valve with most of the flow occurring in the ventral segment. Removal of this rim should result in a more even distribution of flow across the valve.

Biophysical Phenomena↗
PubMed2003

Finite-element analysis of magnetic field driven transport at inlaid platinum microdisk electrodes.

We describe a computer-assisted analysis of three-dimensional magnetohydrodynamic (MHD) fluid flow resulting from the passage of electrochemically generated charge through a uniform magnetic field. Magnetic field driven molecular transport in electrochemical systems offers a number of emerging opportunities in research and technology. For instance, electrochemical microfluidic transport and molecule trapping using magnetic fields and field gradients have been demonstrated in recent reports from this laboratory. A key limitation of these investigations is the difficulty in analyzing magnetic field driven flow and transport, due to the complexity of the governing equations of fluid mechanics, electrochemical molecular transport, and magnetic forces. In general, quantitative expressions describing the distribution and fluxes of electroactive species under the influence of a magnetic field cannot be obtained by a direct analytical solution.

Journal Article↗
PubMed1998

Some factors that influence mechanical behavior of the left ventricle of the human heart in late systole: a feasibility study using finite element analysis.

In systole the left ventricle of the heart behaves mechanically in two modes simultaneously, passive and active. When in the former mode, the ventricle has to carry and react to the pressure increase within the cavity, while in the latter, force is generated within the myocardium itself through the contraction, i.e., active self-shortening, of the muscle fibers. When the deformations of these two opposing modes balance, isovolumic contraction occurs. After this phase, when the aortic valve has opened, the active mode dominates. Many models of the left ventricle under passive internal pressure have been reported, usually for analyzing the situation in diastole. Only a few attempts have been made to incorporate the self-activation effect present in systole. In this paper, a model for systole is described in which the active component has been treated by analogy to thermal stress analysis methods common in dealing with conventional engineering structures. The model was applied to the pressure and volume data for the ventricles of four patients with cardiac disease. A parametric study was then undertaken to investigate the influence of some of the mechanical factors on ventricular behavior. It was found that fiber angle has a very significant effect on the deformation of the ventricle and also that for ventricles with stiff myocardia, the passive component to the reaction of the ventricle was negligible.

Biomechanical Phenomena↗