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PubMed2005

Finite element modelling of glenohumeral kinematics following total shoulder arthroplasty.

Due to the shallowness of the glenohumeral joint, a challenging but essential requirement of a glenohumeral prosthesis is the prevention of joint dislocation. Weak glenoid bone stock and frequent dysfunction of the rotator cuff, both of which are common with rheumatoid arthritis, make it particularly difficult to achieve this design goal. Although a variety of prosthetic designs are commercially available only a few experimental studies have investigated the kinematics and dislocation characteristics of design variations. Analytical or numerical methods, which are predictive and more cost-effective, are, apart from simple rigid-body analyses, non-existent. The current investigation presents the results of a finite element analysis of the kinematics of a total shoulder joint validated using recently published experimental data for the same prostheses. The finite element model determined the loading required to dislocate the humeral head, and the corresponding translations, to within 4% of the experimental data. The finite element method compared dramatically better to the experimental data (mean difference=2.9%) than did rigid-body predictions (mean difference=37%). The goal of this study was to develop an accurate method that in future studies can be used for further investigations of the effect of design parameters on dislocation, particularly in the case of a dysfunctional rotator cuff. Inherently, the method also evaluates the glenoid fixation stresses in the relatively weak glenoid bone stock. Hence, design characteristics can be simultaneously optimised against dislocation as well as glenoid loosening.

Arthroplasty, Replacement↗

PubMed2006

[The construction and analysis of three-dimensional finite element model of human unilateral maxillary defect].

PURPOSE: To built a three-dimensional finite element model of the common unilateral maxillary defect, and to study the stress of the traditional maxillofacial prostheses for unilateral maxillary defect. METHODS: The three-dimensional finite element model of unilateral maxillary defect and maxillofacial obturator prosthesis were constructed by means of spiral CT scanning, digital image transfer, self-developed software MedGraphics, UG engineering design program and ANASYS software. Then 3-D finite element stress analysis was carried out, for the prosthesis. RESULTS: The three-finite element models of the unilateral maxillary defect and maxillofacial obturator prosthesis were established. In the traditional obturator prosthesis, the stress was on the posterior teeth of the healthy side; the area of stress concentration was on the palatal base near the position of the anterior teeth. CONCLUSIONS: The three-dimensional finite element models of the unilateral maxillary defect and maxillofacial obturator prosthesis have good similarity in geometry and biology. The obturator prosthesis is in accorded with the biomechanical demands.

Dental Stress Analysis↗

PubMed2003

Analysis of prolapse in cardiovascular stents: a constitutive equation for vascular tissue and finite-element modelling.

The effectiveness of a cardiovascular stent depends on many factors, such as its ability to sustain the compression applied by the vessel wall, minimal longitudinal contraction when it is expanded, and its ability to flex when navigating tortuous blood vessels. The long-term reaction of the tissue to the stent is also device dependant; in particular some designs provoke in-stent restenosis (i.e., regrowth of the occlusion around the stent). The mechanism of restenosis is thought to involve injury or damage to the vessel wall due to the high stresses generated around the stent when it expands. Because of this, the deflection of the tissue between the struts of the stent (called prolapse or "draping") has been used as a measure of the potential of a stent to cause restenosis. In this paper, uniaxial and biaxial experiments on human femoral artery and porcine aortic vascular tissue are used to develop a hyperelastic constitive model of vascular tissue suitable for implementation in finite-element analysis. To analyze prolapse, four stent designs (BeStent 2, Medtronic AVE; NIROYAL, Boston Scientific; VELOCITY, Cordis; TETRA, Guidant) were expanded in vitro to determine their repeating-unit dimensions. This geometric data was used to generate a finite element model of the vascular tissue supported within a repeating-unit of the stent. Under a pressure of 450 mm Hg (representing the radial compression of the vessel wall), maximum radial deflection of 0.253 mm, 0.279 mm, 0.348 mm and 0.48 mm were calculated for each of the four stents. Stresses in the vascular wall were highest for the VELOCITY stent. The method is proposed as a way to compare stents relative to their potential for restenosis and as a basis for a biomechanical design of a stent repeating-unit that would minimize restenosis.

Animals↗

PubMed2005

Anchorage effects of a palatal osseointegrated implant with different fixation: a finite element study.

The purpose of this study was to compare the anchorage effect of the osseointegrated implant with different fixation types using finite element analysis. Three fixation types were investigated. fixation type 1: implant neck in the oral-palatal cortical bone and implant tip in the cancellous bone; fixation type 2: implant neck in the oral-palatal cortical bone and implant tip in the nasal-palatal cortical bone; fixation type 3: implant neck in the oral-palatal cortical bone and implant tip projecting into the nasal cavity. Three finite element models were constructed. Each consisted of two maxillary second premolars, their associated periodontal ligament (PDL), alveolar bones, palatal bone, palatal implant, and a transpalatal arch. Another model without an implant was used to compare with the previous models. The horizontal force (mesial five N, palatal one N) was loaded at the buccal bracket of each second premolar. The stress was calculated in the PDL and implant surrounding bone. The result showed that the palatal implant could significantly reduce von Mises stress (maximum von Mises stress was reduced 30%) and evenly distribute stress in the PDL. The stress magnitude and distribution in the PDL was almost the same in the three implant models. These results suggest that different implant fixation types have almost the same anchorage effects.

Bone Screws↗

PubMed2003

Comparative analysis of torsional and bending stresses in two mathematical models of nickel-titanium rotary instruments: ProTaper versus ProFile.

During root canal instrumentation, nickel-titanium rotary instruments are subjected to continual stresses inside the canal due to its anatomy and the hardness of the dentin they must cut. They must therefore be both stress-resistant and elastic. This study aimed to compare the mechanical behavior of two nickel-titanium rotary instruments (ProTaper and ProFile) by applying the finite element analysis method to produce a numerical evaluation. The nonlinear mechanical behavior of the alloy was taken into account during the study. The distribution of stresses due to torsional and bending moments was compared in the two experimental models. The ProFile model was found to be more elastic than the ProTaper model. Under equal loads, the ProTaper model showed lower and better distributed stresses than the ProFile model.

Dental Alloys↗

PubMedDate not supplied

Fixation of 5-unit implant-supported fixed partial dentures and resulting bone loading: a finite element assessment based on in vivo strain measurements.

PURPOSE: It is believed that implant-supported fixed partial dentures (FPDs) should display passive fit. The objective of this in vivo-based finite element analysis (FEA) was to quantify the magnitude of bone loading occurring on account of the fixation of cemented or screw-retained 5-unit superstructures. MATERIALS AND METHODS: Based on a patient situation with 3 implants, 4 different groups of restorations with 10 samples each were fabricated. Strain gauges on the pontics of the restorations were used for in vivo measurements. Using the values obtained, bone loading in 3-dimensional FE models was simulated as von Mises equivalent stress. RESULTS: The in vivo measured mean strain values ranged from 32 microm/m to 458 microm/m at the different sites. FEA revealed stresses between 5 and 30 MPa in the cortical area, while in trabecular bone values ranging from 2 MPa to 5 MPa were observed. Stress of a similar magnitude was found for axial implant loading with 200 N. DISCUSSION: Assuming that the axial loading of a single implant with 200 N is within the realm of the bone's adaptation ability, it would appear that the amount of stress resulting from the fixation of superstructures alone does not constitute a risk. CONCLUSIONS: The level of precision of fit which can be obtained in superstructure fabrication would appear to suffice to produce restorations that do not cause bone damage.

Bite Force↗

PubMed2004

Coronary stent strut size dependent stress-strain response investigated using micromechanical finite element models.

Cardiovascular stents are metal scaffolds that are used in the treatment of atherosclerosis. These devices are typically composed of very thin struts (< or = 100 microm thickness, for coronary applications). At this size-scale the question arises as to the suitability of using bulk material properties in stent design. This paper investigates the use of finite element analysis to predict the mechanical failure of stent struts, typical of the strut size used in coronary stents. 316 L stainless steel in uniaxial loading was considered. To accurately represent the constitutive behavior of the material at this size-scale, a computational micromechanics approach was taken involving an explicit representation of the grain structure in the steel struts, and the use of crystal plasticity theory to represent the constitutive behavior of the individual grains. The development of the finite element models is discussed and results are presented for the predictions of tensile mechanical behavior as a function of strut thickness. The results showed that using this modelling approach, a size effect, already seen experimentally, is produced. This has significant implications for stent design, especially in the context of the desire to produce smaller stents for small bore neurovascular and peripheral artery applications.

Biocompatible Materials↗

PubMed2006

Analysis of temperature rise for piezoelectric transformer using finite-element method.

Analysis of heat problem and temperature field of a piezoelectric transformer, operated at steady-state conditions, is described. The resonance frequency of the transformer is calculated from impedance and electrical gain analysis using a finite-element method. Mechanical displacement and electric potential of the transformer at the calculated resonance frequency are used to calculate the loss distribution of the transformer. Temperature distribution using discretized heat transfer equation is calculated from the obtained losses of the transformer. Properties of the piezoelectric material, dependent on the temperature field, are measured to recalculate the losses, temperature distribution, and new resonance characteristics of the transformer. Iterative method is adopted to recalculate the losses and resonance frequency due to the changes of the material constants from temperature increase. Computed temperature distributions and new resonance characteristics of the transformer at steady-state temperature are verified by comparison with experimental results.

Journal Article↗

PubMed1989

[Stress analysis of the periodontal ligament of the human tooth using a three-dimensional finite element method].

Three-dimensional finite element analysis is used to determine the physiological stress values in periodontal ligament (PDL). In comparison with the elastic modulus E of PDL, the E of dentin and bone are much larger. For the estimation of the stresses in PDL, the tooth and the supporting bone can be considered to be rigid. The finite element model of PDL of the mandibular right first molar consists of 210 elements (20-node isoparametric element) and 1612 nodes. Eleven load cases were selected in the study, which included the longitudinal occlusive load, buccal lingual lateral load, mesial-distal lateral load and other load cases to simulate the actual occlusive loads. The results obtained from this study lead to the following conclusions: 1. The loads which go through the physiological center of the tooth, the stresses induced by the loads which are compressive and relatively small. There are no tensile stresses. 2. Among eleven load cases studied in this work, the following three load cases are most harmful. a. The maximum tensile stress was found under mesial-distal lateral load; b. The maximum compressive stress was found under longitudinal occlusive load on the buccal groove or lingual groove; c. The highest stress concentration was found under buccal-lingual lateral load. The clinical significance of the results has also been discussed in the paper.

Biomechanical Phenomena↗

PubMed2006

Patient-specific model of brain deformation: application to medical image registration.

This contribution presents finite element computation of the deformation field within the brain during craniotomy-induced brain shift. The results were used to illustrate the capabilities of non-linear (i.e. accounting for both geometric and material non-linearities) finite element analysis in non-rigid registration of pre- and intra-operative magnetic resonance images of the brain. We used patient-specific hexahedron-dominant finite element mesh, together with realistic material properties for the brain tissue and appropriate contact conditions at boundaries. The model was loaded by the enforced motion of nodes (i.e. through prescribed motion of a boundary) at the brain surface in the craniotomy area. We suggest using explicit time-integration scheme for discretised equations of motion, as the computational times are much shorter and accuracy, for practical purposes, the same as in the case of implicit integration schemes. Application of the computed deformation field to register (i.e. align) the pre-operative images with the intra-operative ones indicated that the model very accurately predicts the displacements of the tumour and the lateral ventricles even for limited information about the brain surface deformation. The prediction accuracy improves when information about deformation of not only exposed (during craniotomy) but also unexposed parts of the brain surface is used when prescribing loading. However, it appears that the accuracy achieved using information only about the deformation of the exposed surface, that can be determined without intra-operative imaging, is acceptable. The presented results show that non-linear biomechanical models can complement medical image processing techniques when conducting non-rigid registration. Important advantage of such models over the previously used linear ones is that they do not require unrealistic assumptions that brain deformations are infinitesimally small and brain stress-strain relationship is linear.

Brain↗

PubMed2006

Prediction of permanent deformation in cast clasps for denture prostheses using a validated nonlinear finite element model.

OBJECTIVES: Permanent deformation is one of the most common mechanical complications that affect denture clasps. This can lead to loss of retention and stability of the prosthesis. The purpose of this study was to apply and validate a nonlinear finite element model for permanent deformation prediction in cast denture clasps. Such a model can enhance the process of design optimization and contribute to minimizing the possibility of this problem. METHODS: Cast clasps made from Ti-6Al-7Nb, Co-Cr and Type IV gold alloys were loaded in three different directions (outside, inside and outside inclined 30 degrees ), and the resulting permanent deformation values were recorded. Nonlinear finite element analysis simulations based on the maximum distortion energy criterion for yielding, were conducted for clasp models that were reproduced according to the dimensions of each experimental specimen. Linear regression analysis for the results of the experiment and simulation was performed to verify the validity of the mathematical models. RESULTS: Deflections required to produce specific amounts of permanent deformation were in close agreement with those recorded experimentally. The R2 value for all bending tests was 0.985 and the linear regression equation expressed in micrometers was [DeflectionFEA=0.976 (DeflectionReal)+34]. SIGNIFICANCE: Permanent deformation behavior in the cast clasps with a relatively wide range of deflections (0-2 mm) can be predicted using the proposed model, which shall enhance the design optimization process of cast clasps for denture prostheses.

Dental Alloys↗

PubMed1999

Design and application of combined hip-knee intramedullary joint replacements.

Cases in which there is a total hip arthroplasty and a stemmed total knee arthroplasty in the same femur, with loosening of 1 or both components, with serious endosteal bone loss or even a fracture between the stems present a difficult reconstruction problem. We describe a reconstruction using a combined hip and stemmed knee, designed so that they could be rigidly connected during the surgical procedure. The advantages of this implant design are that the entire femur with its muscle attachments is preserved, and the inherent stability allows for immediate weight bearing. To determine the viability of the connection between the hip and the knee, a stress analysis was carried out using finite element analysis. Guidelines were thus provided for the required metal and cement thicknesses. Three case examples are presented with an average follow-up of 3 years. It was shown that the combined hip-knee implant could provide successful results for these difficult reconstructive problems in appropriately selected cases.

Adult↗

PubMed1990

Application and comparison of techniques for three-dimensional analysis of craniofacial anomalies.

Traditionally, cephalometric analysis has been limited to data determined from two-dimensional (2-D) cephalograms. With imaging facilities such as CT and biplanar radiography now available, the natural extension has been towards the use of three-dimensional (3-D) coordinate positions of landmarks for comparative purposes. While these data have been potentially available for several years, the accurate and reproducible extraction of anatomic landmarks suitable for comparative purposes has been limited. This paper presents results of the application of traditional comparative techniques to well determined 3-D coordinate data acquired from biplanar radiography and CT for a patient with Treacher Collins syndrome and further provides a comparison with the technique of strain analysis, often referred to as finite element analysis, which has been applied recently to craniofacial data. Comparisons of distances and angles between landmarks, landmark coordinate positions, and strains of the patient relative to experimental reference standards reveal that the essential skeletal features of Treacher Collins syndrome have been identified and quantified by the analysis techniques. Further, a measure of the significance of the deviations has been determined by comparisons with the experimental reference standards.

Cephalometry↗

PubMed2003

Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow.

A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway that is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in the computational expenditure when compared to a three-dimensional finite element analysis.

Journal Article↗

PubMed1994

Anterior cranial base morphology in mice with midfacial retrusion.

The role of the anterior cranial base in the morphogenesis of class III malocclusions remains uncertain. This study was conducted to determine whether morphologic deficiencies occur in the anterior cranial base in the Brachyrrhine (Br) mouse mutant showing severe midfacial retrusion, which is characteristic of a class III malocclusion. Crania from three groups of C3H/Hej, 3H1 Br/+, and 3H1+/+ mice, each consisting of 15 animals, were collected at 1, 3, and 5 days of age (total = 135). The anterior cranial base from each specimen was subjected to computerized reconstruction and ten landmarks were digitized from each model. The landmark configurations were compared using Procrustes analysis. Significant differences between models were determined at each age. In order to localize differences between forms, average landmark configurations derived from Procrustes analysis were subjected to finite-element analysis. Size-change values for the 3H1 Br/+ animals showed magnitudes that increased in an anteroposterior direction when compared to the 3H1 +/+ and C3H/Hej animals at all ages. The largest values were located posteriorly along the ossifying front of the presphenoid. In five of six comparisons, the size-change values separated into two distinct clusters. The posterior region of the anterior cranial base was divisible into two subclusters, one located superiorly and the other inferiorly. These data suggest that midfacial retrusion in the Br mouse may be caused, in part, by growth deficiencies in the posterior region of the anterior cranial base, particularly the presphenoidal and sphenoethmoidal regions.

Aging↗

PubMed1996

Morphological deficiency in the prenatal anterior cranial base of midfacially retrognathic mice.

The role of the anterior cranial base in the establishment of midfacial retrognathia remains unclear. The purpose of this study was to determine whether morphological deficiencies occur in the developing anterior cranial base of the retrognathic Brachyrrhine (3H1 Br/+) mouse mutant shortly after overt cartilaginous differentiation and to localise any malformations. Crania from 2 groups of 3H1 Br/+ and +/+ mice, each consisting of 15 animals, were collected at gestational days 15, 17, and 19 (Theiler stages 23, 25, 27). The anterior cranial base from each specimen was subjected to computerised reconstruction and 8 homologous anatomical landmarks were digitised on each model. The landmark configurations were subjected to Procrustes analysis and significant differences between models were determined at each age. In order to localise differences between forms, average landmark configurations derived from Procrustes analysis were subjected to finite-element analysis. Two cluster models were generated based on size-change values. One cluster was located anteriorly and superiorly while the second was located posteriorly and inferiorly within the anterior cranial base. Results indicate that the size-change values for the posterior and inferior cluster increased more rapidly compared with the anterior and superior region over the age range tested. These data indicate that the midfacial retrognathia in Br/+ mice is associated with abnormal growth activity in the presphenoid component of the presumptive anterior cranial base. In addition, the deficiency is present in the presphenoid at the time of overt cartilaginous differentiation.

Animals↗

PubMed1998

Design of prosthetic cantilever bridgework supported by osseointegrated implants using the finite element method.

OBJECTIVES: The aim of the present work was to establish a design procedure for fixed metal prostheses supported by osseointegrated implants in order to prevent permanent deformation and hence failure following loading. Previously, the cantilever cross-sectional shape in the buccal lingual plane has been based on clinical experience and subjectivity. METHODS: This work has relied on the use of linear elastic finite element analysis in order to generate a maximum effective stress at which permanent deformation commences on loading. A number of different cross-sectional shapes were investigated, both of conventional design as well as new innovative possibilities. Both straight and curved cantilever beams 26 mm long were examined. RESULTS: The design failure chosen was based on a von Mises plastic collapse principle by comparing the calculated effective stresses with the yield stress of the metal in simple tension. It was found that the "L" shaped design was more rigid than other designs for a given mass, while a framework based on an open "I" section offers good possibilities particularly when used as curved shapes. SIGNIFICANCE: Assuming a failure criterion based on the von Mises principle, then "L" shaped Co/Cr or stainless steel frameworks, typically 26 mm of cantilever span, undergo permanent deformation at end loadings between 130 and 140 N depending on section curvature. Since it is known biting loads can exceed these values, good design is critical if such failures are to be avoided.

Dental Alloys↗

PubMed2002

Combination of topological parameters and bone volume fraction better predicts the mechanical properties of trabecular bone.

Trabecular bone structure may complement bone volume/total volume fraction (BV/TV) in the prediction of the mechanical properties. Nonetheless, the direct in vivo use of information pertaining to trabecular bone structure necessitates some predictive analytical model linking structure measures to mechanical properties. In this context, the purpose of this study was to combine BV/TV and topological parameters so as to better estimate the mechanical properties of trabecular bone. Thirteen trabecular bone mid-sagittal sections were imaged by magnetic resonance (MR) imaging at the resolution of 117 x 117x 300 microm(3). Topological parameters were evaluated in applying the 3D-line skeleton graph analysis (LSGA) technique to the binary MR images. The same images were used to estimate the elastic moduli by finite element analysis (FEA). In addition to the mid-sagittal section, two cylindrical samples were cored from each vertebra along vertical and horizontal directions. Monotonic compression tests were applied to these samples to measure both vertical and horizontal ultimate stresses. BV/TV was found as a strong predictor of the mechanical properties, accounting for 89-94% of the variability of the elastic moduli and for 69-86% of the variability of the ultimate stresses. Topological parameters and BV/TV were combined following two analytical formulations, based on: (1) the normalization of the topological parameters; and on (2) an exponential fit-model. The normalized parameters accounted for 96-98% of the variability of the elastic moduli, and the exponential model accounted for 80-95% of the variability of the ultimate stresses. Such formulations could potentially be used to increase the prediction of the mechanical properties of trabecular bone.

Adult↗