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PubMed2003

Prevention of mesh-dependent damage growth in finite element simulations of crack formation in acrylic bone cement.

Peak stress levels predicted in finite element analysis (FEA) usually depend on mesh density, due to singular points in the model. In an earlier study, an FEA algorithm was developed to simulate the damage accumulation process in the cement mantle around total hip replacement (THR) implants. It allows cement crack formation to be predicted, as a function of the local cement stress levels. As the simulation is driven by mesh-dependent peak stresses, predicted crack formation rates are also likely to be mesh dependent. The aim of this study was to evaluate the mesh dependence of the predicted crack formation process, and to present a method to reduce the mesh dependence. Crack-propagation experiments were simulated. Experimental specimens, representing transverse slices of cemented THR reconstructions, were subjected to cyclic torsional loading. Crack development around the corners of the stem was monitored. The experiments were simulated using three meshes with increasing levels of mesh refinement. Crack locations and orientations were accurately predicted, and were virtually independent of the level of mesh refinement. However, the experimental crack propagation rates were overestimated considerably, increasing with mesh refinement. To eliminate the effect of stress singularities around the corners of the stem, a stress averaging algorithm was applied in the simulation. This algorithm redistributed the stresses by weighted spatial averaging. When damage accumulation was computed based on averaged stresses, the crack propagation rates predicted were independent of the level of mesh refinement. The critical distance, a parameter governing the effect of the averaging algorithm, was optimized such that the predicted crack propagation rates accurately corresponded to the experimental ones. These results are important for the validity and standardization of pre-clinical testing methods for orthopaedic implants.

Artifacts↗
PubMed2001

The effects of dowel design and load direction on dowel-and-core restorations.

STATEMENT OF PROBLEM: Complications such as loosening of the dowel and core or fracture of the remaining tooth root can be influenced by many factors, including the amount of remaining dentin, direction of the occlusal load, and design of the dowel. Most stress analyses of dowel and cores were conducted without including all aspects of the restorations and supporting structures. PURPOSE: The purpose of this study was to investigate the influence of occlusal stresses on various dowel designs in a restored, endodontically treated maxillary incisor by using a 2-dimensional finite element analysis model. MATERIAL AND METHODS: A 2-dimensional finite element model was constructed in a labiolingual cross-sectional view of a maxillary central incisor, a dowel, a core, and the supporting tissues to investigate stresses in various dowel designs. As a control, a metal-ceramic crown on an endodontically treated tooth without a dowel and core was modeled. A 10-kg force was applied as follows: (1) in a vertical load on the incisal edge, (2) in a horizontal load on the labial surface, and (3) in a 20-degree diagonal load on the lingual surface. RESULTS: The use of a dowel reduced the peak dentinal stress to 75% of the magnitude of the control. When a vertical force was applied, the magnitudes of stress of the various dowel designs were similar; however, when loaded horizontally, the short dowel produced the greatest dentinal stress concentration, and the tapered dowel showed the greatest stress concentration within the cement layer. Greater deflections and higher stresses were generated with horizontal loading. CONCLUSION: The dowel and core provided little reinforcement to the remaining tooth. The direction of the functional load had a greater effect than dowel design on maximum stress and displacement. Parallel-sided dowel and cores with a length of 12 mm distributed the stress widely in the restoration and dentin, resulting in the smallest stresses.

Bite Force↗
PubMed1977

Finite element stress analysis of the human left ventricle whose irregular shape is developed from single plane cineangiocardiogram.

The three-dimensional left ventricular chamber geometrical model is developed from single plane cineangiocardiogram. This left ventricular model is loaded by an internal pressure monitored by cardiac catheterization. The resulting stresses in the left ventricular model chamber's wall are determined by computerized finite element procedure. For the discretization of this left ventricular model structure, a 20-node, isoparametric finite element is employed. The analysis and formulation of the computerised procedure is presented in the paper, along with the detailed algorithms and computer programs. The procedure is applied to determine the stresses in a left ventricle at an instant, during systole. Next, a portion (represented by a finite element) of this left ventricular chamber is simulated as being infarcted by making its active-state modulus value equal to its passive-state value; the neighbouring elements are shown to relieve the "infarcted" element of stress by themselves taking on more stress.

Angiocardiography↗
PubMed2001

Merging of intersecting triangulations for finite element modeling.

Surface mesh generation over intersecting triangulations is a problem common to many branches of biomechanics. A new strategy for merging intersecting triangulations is described. The basis of the method is that object surfaces are represented as the zero-level iso-surface of the distance-to-surface function defined on a background grid. Thus, the triangulation of intersecting objects reduces to the extraction of an iso-surface from an unstructured grid. In a first step, a regular background mesh is constructed. For each point of the background grid, the closest distance to the surface of each object is computed. Background points are then classified as external or internal by checking the direction of the surface normal at the closest location and assigned a positive or negative distance, respectively. Finally, the zero-level iso-surface is constructed. This is the final triangulation of the intersecting objects. The overall accuracy is enhanced by adaptive refinement of the background grid elements. The resulting surface models are used as support surfaces to generate three-dimensional grids for finite element analysis. The algorithms are demonstrated by merging arterial branches independently reconstructed from contrast-enhanced magnetic resonance images and by adding extra features such as vascular stents. Although the methodology is presented in the context of finite element analysis of blood flow, the algorithms are general and can be applied in other areas as well.

Algorithms↗
PubMed2006

Prediction of strength and strain of the proximal femur by a CT-based finite element method.

Hip fractures are the most serious complication of osteoporosis and have been recognized as a major public health problem. In elderly persons, hip fractures occur as a result of increased fragility of the proximal femur due to osteoporosis. It is essential to precisely quantify the strength of the proximal femur in order to estimate the fracture risk and plan preventive interventions. CT-based finite element analysis could possibly achieve precise assessment of the strength of the proximal femur. The purpose of this study was to create a simulation model that could accurately predict the strength and surface strains of the proximal femur using a CT-based finite element method and to verify the accuracy of our model by load testing using fresh frozen cadaver specimens. Eleven right femora were collected. The axial CT scans of the proximal femora were obtained with a calibration phantom, from which the 3D finite element models were constructed. Materially nonlinear finite element analyses were performed. The yield and fracture loads were calculated, while the sites where elements failed and the distributions of the principal strains were determined. The strain gauges were attached to the proximal femoral surfaces. A quasi-static compression test of each femur was conducted. The yield loads, fracture loads and principal strains of the prediction significantly correlated with those measured (r=0.941, 0.979, 0.963). Finite element analysis showed that the solid elements and shell elements in undergoing compressive failure were at the same subcapital region as the experimental fracture site.

Adult↗
PubMed2004

Biomechanics of cantilever fixed partial dentures in shortened dental arch therapy.

PURPOSE: The purpose of the present study was to investigate, by means of 3-dimensional finite element analysis, aspects of the biomechanics of cantilever fixed partial dentures replacing the maxillary canine in shortened dental arch therapy. The null hypothesis was that no differences would be identified by finite element analysis in the mechanical behavior of the 2 designs of cantilever fixed partial denture under different scenarios of occlusal loading. MATERIALS AND METHODS: Single- and double-abutted cantilever fixed partial dentures were modeled and analyzed using the finite element packages PATRAN and ABAQUS. Displacement and maximum principal stresses (magnitude and location) within the fixed partial dentures, supporting structures, and the periodontal ligament/bone and abutment/retainer interfaces were examined under 20 different scenarios of axial and lateral occlusal loading. RESULTS: The results indicate that more displacement occurred in the 2 rather than the 3-unit cantilever fixed partial denture, with the greatest displacement having occurred under lateral loading. The maximum principal stresses observed in the periodontal ligament/bone interfaces were greatest buccocervically, with the highest value being observed in the 2-unit fixed partial denture under lateral loading. The highest maximum principal stresses observed in the retainer/abutment interfaces were located cervically in relation to the distal margin of the retainer of the 2-unit fixed partial denture under axial loading. CONCLUSIONS: It was concluded that in adopting a cantilever fixed partial denture approach for the replacement of a missing maxillary canine in shortened dental arch therapy, there may be merits, in terms of mechanical behavior, in selecting a double-rather than a single-abutment design. Furthermore, prostheses' displacement and functional stresses may be minimized by reducing lateral loading and avoiding pontic only loading.

Biomechanical Phenomena↗
PubMed2003

Analysis of complications in fractures of the mandibular angle--a study with finite element computation and evaluation of data of 277 patients.

AIM: The purpose of this study was to compute the load on different osteosynthesis plates in a simplified model using finite element analysis, and to find out whether miniplates were sufficiently stable for application at the mandibular angle. PATIENTS: Data from 277 patients with 293 fractures of the mandibular angle have been evaluated. METHODS: A computation model using finite elements was established in order to compute mechanical stress occurring in osteosynthesis plates used for fixation of fractures of the mandibular angle. In the second part of this study, the data from all in-patients treated for fracture of the mandibular angle were evaluated retrospectively. Age and sex of the patients, cause of fracture, state of dentition, type of therapy as well as complications were noted. RESULTS: In those tests, both the 1.0 mm miniplate and the 2.3 mm module plate were sufficiently stable. The rate of major complications (requiring revisional surgery with general anaesthesia) amounted to approximately 17% in comminuted fractures, or in non-compliant patients in which primary stability with a single miniplate did not appear sufficient, so that other osteosynthesis methods were used in addition. This rate was considerably higher than that in simple mandibular fractures. Simple fractures of the mandibular angle were just treated with one miniplate following Champy's guidelines strictly. In these fractures the rate of major complications was only 2.3%. CONCLUSION: In comminuted fractures and in non-compliant patients, the use of a stronger osteosynthesis material should be considered while in all other cases application of a single 1.0 mm miniplate was regarded as sufficient for fixation using open reduction.

Adolescent↗
PubMedDate not supplied

Anatomic variation of the mechanical properties of the glenoid.

Finite element analysis modeling is an important tool in the design of total joint replacements. However, to use a finite element analysis the material properties of the studied bone must be known. The aim of the study was to measure the elastic properties of the glenoid bone in the axial, coronal, and sagittal planes with an ultrasound transmission technique. The relative density and Houndsfield computed tomography numbers were also assessed. Three pairs of scapulas were obtained from unembalmed human cadavers. Seventy-four cubic cancellous bone specimens of 6 mm were used for ultrasonic measurements. The study showed significant differences with anatomic location. Mechanical properties of cancellous bone were found to be higher near the direction of application of the resultant force, perpendicular to the articular surface of the glenoid. Mechanical properties were found to be significantly higher at the center and posterior edge of the glenoid (p < 0.01). Significant differences were also found in the three planes studied. The lateromedial Young's modulus (E1) was higher than the anteroposterior modulus (E2) and the superoinferior modulus (E3) (E1 = 372 +/- 164 MPa, E2 = 222 +/- 79 MPa, E3 = 198 +/- 75 MPa).

Analysis of Variance↗
PubMed2002

Finite element stress analysis of the effect of short implant usage in place of cantilever extensions in mandibular posterior edentulism.

Although the destructive effect of cantilevered implant supported fixed partial prosthesis is well known, in some cases usage of cantilever extension seems to be inevitable for many clinicians. The purpose of this study was to evaluate the effect of additional placement of a shorter implant in place of a cantilever extension on stress distribution compared with cantilevered fixed prosthesis in mandibular posterior edentulism. A mandibular Kennedy II finite element model was constructed. Six different implant supported fixed partial prosthesis were designed according to two main configurations; anterior and posterior cantilever extensions compared with the placement of additional shorter implant configurations. An oblique occlusal load of 400 N was applied. Tensile and compressive stress values in the cortical bone surrounding the cervical regions of implants and Von Misses stress values in the implants were evaluated. Significant lower stress values were recorded at the shorter implant placement configurations compared with the cantilevered prosthesis. Posterior cantilever extension performed higher stress values than the anterior counterpart. In clinical applications where cantilevered fixed partial prosthesis seems to be inevitable because of anatomical restrictions and/or complications such as loss of implant, an additional placement of a shorter implant should be considered.

Compressive Strength↗
PubMed1983

A comparison of photoelastic and finite element stress analysis in restored tooth structures.

Two methods for determining internal stresses in tooth structures were compared under a specific load condition. The numerical method of the finite element method has a definite number of advantages over the experimental photoelastic method. The numerical results obtained by the first method are in reasonable conformity with those obtained by the latter. In addition the sensitivity of the finite element method towards variation of a number of relevant parameters has been studied.

Dental Restoration, Permanent↗
PubMed2002

The influence of system compliance and sample geometry on composite polymerization shrinkage stress.

The objective of this study was to use finite-element analysis to model tensilometer tests of polymerizing dental composites. A typical sample in polymerization shrinkage stress tests is shaped like a flat disk, that is, has a high aspect ratio (ratio of diameter to height). In the experimental literature it is implied that the induced stress state in the flat disk composite samples is uniaxial. Three published tensilometer tests of curing dental composite samples with similar high aspect ratios (varying from 3 to 5) were modeled, but with test configurations having low, intermediate and high relative compliance (a tenfold variation). With the use of linear elastic finite element analysis, an instantaneous volumetric shrinkage of 1% was applied to the composite via the thermal analogy and the following questions were addressed: 1. Does the numerically predicted state of stress in composite samples tested in this fashion differ substantially from the uniaxial stress state assumed in the experiments?2. How do the numerically predicted stresses compare with the experimentally determined nominal stresses?3. Does compliance of the mountings influence the numerically predicted stress state? The finite-element results predicted a complex triaxial stress state that was strongly influenced by the compliance of the mountings. For the low and intermediate system compliance the model overpredicted the polymerization contraction stress, as would be anticipated due to the inability of the model to account for viscoplastic flow. For high system compliance, the numerical and experimental stress values were in better agreement, mainly because the linear elastic mountings accounted for most of the measured system compliance.

Compliance↗
PubMed2005

Longitudinal structural determinants of atherosclerotic plaque vulnerability: a computational analysis of stress distribution using vessel models and three-dimensional intravascular ultrasound imaging.

OBJECTIVES: This study theoretically examined the longitudinal structural determinants of plaque vulnerability using a color-coded stress mapping technique for several hypothetical vessel models as well as three-dimensional intravascular ultrasound (IVUS) images with use of a finite element analysis. BACKGROUND: It has been shown that an excessive concentration of stress is related to atherosclerotic plaque rupture. However, the local determinants of in-plaque longitudinal stress distribution along the coronary arterial wall remain unclear. METHODS: Using a finite element analysis, we performed a color mapping of equivalent stress distribution within plaques for three-dimensional vessel models as well as longitudinal IVUS plaque images (n = 15). Then, the effects of plaque size, shape, expansive remodeling, calcification, and lipid core on the equivalent stress distribution were examined. RESULTS: The color mapping of vessel models revealed a concentration of equivalent stress at the top of the hills and the shoulders of homogeneous fibrous plaques. Expansive remodeling and the lipid core augmented the surface equivalent stress, whereas luminal stenosis and superficial calcification attenuated the equivalent stress. The location of excessive stress concentration was modified by the distribution of the lipid core and calcification. The thickness of the fibrous cap was inversely related to the equivalent stress within the fibrous cap. However, the color mapping of IVUS plaque images showed that the equivalent stress value at the fibrous cap varied with changes in plaque shape and superficial calcification, even when the thickness of the fibrous cap remained constant. CONCLUSIONS: A distribution analysis of longitudinal stress revealed specific effects of plaque shape, size, and remodeling, as well as effects of the interior distribution of tissue components, on the concentration of stress at the plaque surface. Moreover, fibrous caps of the same thickness did not consistently represent the same vulnerability to rupture.

Atherosclerosis↗
PubMed1981

Finite element temperature analysis of a total hip replacement and measurement of PMMA curing temperatures.

Analytical and experimental studies were performed to examine some characteristics of poly(methyl methacrylate) bone cement in the context of total hip replacement. Proceeding from fundamental principles of balance of energy and heat conduction, three simulation models of total hip replacement, using the finite-element method of numerical analysis, were used to predict the temperature response in the femoral prosthesis, cement, cancellous bone, and bone-cement interface. When cement collars are a centimeter or less in thickness, the models predict little likelihood of permanent bone thermal necrosis. In addition, laboratory experimental results indicate that peak temperatures attained in Simplex-P and Palacos-R cement specimens are nearly identical and that adding 0.5 and 1.0 g of the antibiotic gentamicin to Palacos-R has negligible influence on peak temperature. Experimental results also indicate that cooling cement specimens to 8 degrees C extends the set time, which in turn could significantly increase the likelihood of stem-cement loosening during surgery.

Bone Cements↗
PubMedDate not supplied

Analysis of dentinal stress distribution of maxillary central incisors subjected to various post-and-core applications.

This study evaluated the stress distribution on an endodontically treated maxillary central incisor restored with different post-and-core systems by using a three-dimensional finite element analysis model. Seven three-dimensional finite element models were created. Each model contained cortical bone, cancelous bone, periodontal ligament, 3 mm apical root canal filling, post-and-core and all-ceramic crowns. Two different pre-fabricated zirconia ceramic post systems, a glass fiber-reinforced post system and a titanium post system were modeled. As a control, an all-ceramic crown on an endodontically treated maxillary central incisor without a post-and-core was modeled. Each model received a 45 degrees oblique occlusal load at a constant intensity of 100 N. In each model, the ratio of Von Mises stress distribution was compared. The greatest stresses were observed in the coronal third of the roots on facial surfaces. The ratio of Von Mises stress distribution in dentin for the zirconia ceramic post (CosmoPost) and ceramic core (Cosmo Ingot), zirconia ceramic post (CosmoPost) and composite core (Tetric Ceram), glass fiber-reinforced post (FRC Postec) and composite core (Tetric Ceram), titanium post (Er post) and composite core (Tetric Ceram), zirconia ceramic post (Cerapost) and ceramic core (Cosmo Ingot), zirconia ceramic post (Cerapost) and composite core (Tetric Ceram) and the control group were 0.886, 0.889, 0.988, 0.924, 0.889, 0.893 and 1, respectively. The stress concentrations in dentin created by two different zirconia ceramic post systems were nearly the same. The zirconia ceramic post systems created slightly less stress concentration in dentin than the glass fiber-reinforced and titanium posts.

Bite Force↗
PubMed2006

A short plate compression screw with diagonal bolts--a biomechanical evaluation performed experimentally and by numerical computation.

INTRODUCTION: Decreasing the length of the side plate of the dynamic hip screw would theoretically allow a smaller surgical incision, a shorter surgical time, decreased operative blood loss and minimal periosteal stripping. A new design of a very short plate dynamic hip screw based on two diagonal screws has been developed. Our study compares the new design and the four-hole side plate in respect to mechanical properties and bio-mechanical outcomes utilizing the Finite Element Analysis method. METHODS: Four pairs of fresh frozen cadaveric femora were extracted from male corpses aged 25-43 years (mean 34.8). One femur of each pair was fixated by means of the new system and the other by means of the conventional design. Mechanical loading was applied to all four pairs. The decline which occurred during the periodical loadings and the breakage loads of fixated bones were measured. Mechanical performance and probability of failure was assessed by conducting a mathematical analysis using the finite element method. FINDINGS: The average deflection under excessive cyclic loading was 33% higher in the bones fixated with the very short plate-dynamic hip screw device than in those fixated with the conventional dynamic hip screw. The average load failure during the collapse-loading test was 3120N for the very short plate-dynamic hip screw as compared to 4160N for the regular device. Mechanical testing did not provide decisive results regarding failure. The mathematical analysis performed indicated that the maximal stress in the very short plate-dynamic hip screw reached values 3-4-fold higher than in the regular dynamic hip screw. INTERPRETATION: Although the new design offers a minimally invasive approach to subtrochanteric femur fracture fixation, it was found to have insufficient biomechanical performance resulting in high probability of mechanical failure. The authors believe that the finite element method may have the potential to serve as an additional clinical tool for performing surgical preplanning and assist in decision making.

Adult↗
PubMed2003

The effect of occlusal rest size and shape on yield strength.

STATEMENT OF PROBLEM: Occlusal rests, essential for conventional removable partial dentures, may fracture and cause failure of the prosthesis. It is unknown whether alterations in the size or shape of occlusal rests might improve their performance. PURPOSE: In this in vitro study, 3-dimensional finite element analysis was used to evaluate the effects of occlusal rest size and shape on yield strength. MATERIAL AND METHODS: An occlusal rest and a minor connector as part of a clasp assembly were modeled for 3-dimensional finite element analysis with 3 variations of width (2.0, 3.0, and 4.0 mm), thickness (0.7, 1.0, and 1.5 mm) and length (2.0, 3.0, and 4.0 mm). A concentrated vertical load of 100 N was applied at the most protruded point of the occlusal rest base. Maximal principal stress and yield strength were calculated. In addition, the following parameters were altered: radius of curvature at the inner connection (r), thickness of the minor connector (m), inclination of horizontal axis of occlusal rest (i), and thickness at the most protruded point (t). RESULTS: Yield strength increased with increased width (183% to 242% from 2.0 to 4.0 mm) and thickness (141% to 230% from 0.7 to 1.5 mm). Alterations in length had a lesser effect on yield strength (120% to 178% from 4.0 to 2.0 mm). All other parameters except thickness at the most protruded point (t) affected yield strength. CONCLUSION: The results of this in vitro study suggest basic principles for optimizing the size and shape of occlusal rests. In general, increased width and thickness are desirable, whereas overrounded or sharp line angles and excessive rest inclination should be avoided. These principles may be useful in removable partial denture design.

Computer Simulation↗
PubMed1994

Noninvasive determination of bone stiffness for distraction osteogenesis by quantitative computed tomography scans.

Apparent density of canine tibial specimens was correlated to measured quantitative computed tomography (QCT) numbers, and a conversion equation was derived. Finite element analysis models of canine tibiae were constructed using spatial coordinates of converted QCT data. Two QCT-derived finite elemental analysis models were constructed for a pair of matched tibiae: one lengthened 15% by distraction osteogenesis and one contralateral control. Two equations relating the modulus of elasticity to apparent density were used. These models were then verified mechanically by measuring strains of the two specimens under applied load. Equation I correlated best with measured strains in the less-dense regenerate bone of the distracted tibia (R2 = 0.914, p = 0.0028, n = 6); Equation II performed best in the denser control tibia (R2 = 0.820, p = 0.0129, n = 6). These results demonstrate that the stiffness of tubular bones, including cortical and heterogenous cancellous structure, can be accurately predicted by a noninvasive QCT scan.

Animals↗
PubMed1999

The effectiveness of element downsizing on a three-dimensional finite element model of bone trabeculae in implant biomechanics.

More validity of finite element analysis in implant biomechanics requires element downsizing. However, excess downsizing needs computer memory and calculation time. To investigate the effectiveness of element downsizing on the construction of a three-dimensional finite element bone trabeculae model, with different element sizes (600, 300, 150 and 75 microm) models were constructed and stress induced by vertical 10 N loading was analysed. The difference in von Mises stress values between the models with 600 and 300 microm element sizes was larger than that between 300 and 150 microm. On the other hand, no clear difference of stress values was detected among the models with 300, 150 and 75 microm element sizes. Downsizing of elements from 600 to 300 microm is suggested to be effective in the construction of a three-dimensional finite element bone trabeculae model for possible saving of computer memory and calculation time in the laboratory.

Biomechanical Phenomena↗