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PubMed2004

The apparent increase of the Young's modulus in thin cement layers.

OBJECTIVES: The bond of adhesive luting cements to the tooth tissues and restorative materials is expected to hinder their transverse contraction for the layer thickness applied in dental restorations. It was hypothesized that the hindering of the transverse deformation will influence the relation between stress and strain (the stiffness) in the direction perpendicular to the substrate surface. The aim of this study was to investigate the relation between cement layers with different ratio between bonded and free surface (C-factor) and the stiffness of these layers, i.e. an apparent increase of the Young's modulus of the dental luting cement. METHODS: A commercial luting cement RelyX ARC (3M, St Paul, MN, USA) was used in this study. The 'real" Young's modulus and the Poisson ratio were determined and these values were used in models with layers with different C-factors (0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0) in a three-dimensional Finite Element Analysis program (FEMAP, E.S.P., Maryland Height, MO, USA). The apparent Young's modulus was measured for layers with thicknesses of 0.5 mm (C = 6.0) and 6.0 mm (C = 0.5) and compared with the results of the Finite Element Analysis (FEA) analysis. RESULTS: The apparent Young's modulus in the 0.5 mm layer was 20% higher than the apparent Young's modulus in the 6.0 mm layer. This result was confirmed by the results of the F.E. analysis. For very thin layers the stiffness will be 25% higher than the Young's modulus. SIGNIFICANCE: The hindering of the transverse contraction has to be taken into account studying the mechanical properties of dental luting cements because it influences the behavior of these luting cements in thin layers.

Bisphenol A-Glycidyl Methacrylate↗
PubMed2003

Polyethylene wear and acetabular component orientation.

BACKGROUND: Polyethylene wear contributes substantially to both periprosthetic osteolysis and aseptic loosening after total hip arthroplasty. Acetabular component orientation has been shown to affect the range of motion of the hip as well as contact stresses. A series of studies was designed to test the hypothesis that acetabular component orientation can affect the magnitude and direction of polyethylene wear. METHODS: A finite-element model was used to compute contact stresses during a normal gait cycle. Wear at the end of each gait cycle was calculated with use of the sliding-distance-coupled finite-element formulation. The wear that was calculated with use of finite-element analysis was validated by comparison with the findings of hip wear simulator studies with the acetabular liner oriented to simulate 45 degrees and 55 degrees of abduction. In a clinical study, fifty-six patients who underwent sixty hip arthroplasties with use of a single prosthetic design were followed for as long as five years. Radiographs were analyzed to measure the abduction angle of the acetabular component and polyethylene wear. RESULTS: The finite-element analysis predicted increased peak contact stresses with an increased abduction angle and reduced peak contact stresses with an increased anteversion angle. Linear wear rates ranging from 0.036 to 0.045 mm/million cycles were also predicted, and increased acetabular abduction angles were predicted to be associated with higher linear wear rates. In the hip wear simulator studies, significantly different wear rates were found between the cups with acetabular abduction angles of 45 degrees and 55 degrees (mean, 17.2 compared with 21.7 mg/million cycles; p < 0.01). In the clinical study, radiographic analysis revealed significant correlation between the acetabular abduction angle and the linear polyethylene wear rate. A 40% increase in mean linear polyethylene wear was seen in cups with an abduction angle of >or=45 degrees. The direction of wear was more medial (by 9.4 degrees ) in cups with an abduction angle of <45 degrees. CONCLUSION: All three studies presented here underlined the importance of optimizing the position of the acetabular component. Careful attention to acetabular position may help to minimize wear.

Aged↗
PubMed2007

On the measurement of human osteosarcoma cell elastic modulus using shear assay experiments.

This paper presents a method for determining the elastic modulus of human osteosarcoma (HOS) cells. The method involves a combination of shear assay experiments and finite element analysis. Following in-situ observations of cell deformation during shear assay experiments, a digital image correlation (DIC) technique was used to determine the local displacement and strain fields. Finite element analysis was then used to determine the Young's moduli of HOS cells. This involved a match of the maximum shear stresses estimated from the experimental shear assay measurements and those calculated from finite element simulations.

Biomechanical Phenomena↗
PubMed1997

[A three-dimensional finite element stress analysis of implant-supported prosthesis and its supporting tissue in the edentulous mandible. Part 5. The influence of implant type in complete implant overdenture and its supporting tissue].

The stress distribution of implant, implant-bone interface, alveolar ridge surface and the overdenture when supported by cylindrical-type or blade-type implants under three kinds of occlusion, were investigated respectively in this study. The results showed that the extreme principle stress was greater when the denture was supported by blade-type implants than by cylindrical type one. The stress distribution of the system with application of cylindrical type implant is more acceptable.

Alveolar Process↗
PubMed1997

[A three-dimensional finite element stress analysis of implant-supported prosthesis and its supporting tissue in the edentulous mandible. Part 6. The influence of superstructure in mandibular complete implant overdenture and its supporting tissue].

The effect of superstructure, including telescope, conventional-bar and modified-bar, on the stress distribution of mandibular complete implant overdenture and its supporting tissue was investigated in this study. The results demonstrated that bar could lower the extreme principle stress of the implant, but the stress distribution of implant-bone interface was better when the implants were not linked with each other. Modified-bar is not preferred because it increases the stress of implant and its interface.

Alveolar Process↗
PubMed1998

Biomechanical evaluation of a central rod system in the treatment of scoliosis.

OBJECTIVE: To evaluate the efficiency of a central rod system in the treatment of scoliosis. DESIGN: Biomechanical testing and finite element analysis were performed to compare the stiffness and stress distribution of the central rod system with the Cotrel-Dubousset instrumentation. BACKGROUND: During the fixation and derotation processes in the treatment of scoliosis, the Cotrel-Dubousset instrumentation concentrates unilateral stress on the vertebral elements. It may be possible to cause severe injury to the bony tissues especially in the ostaeoporotic patients. A central rod system was developed to reduce the damage to the bony tissue of the vertebrae in the treatment of scoliosis. METHODS: Mechanical non-destructive cyclical tests comparing the compression, flexion, extension and torsional stiffness were carried out on the models of the central rod system and Cotrel-Dubousset instrumentation using a 858 Bionix test system. The stress distribution on the central rod, the Cotrel-Dubousset rod and a vertebral model consisting in four vertebrae each implanted with two screws was evaluated using a general purpose finite element software ANSYS R 5.0a. The SOLID73 element was employed for three-dimensional modelling. RESULTS: Although the central rod system was found to have lower values in the compression, flexion and extension stiffness, its torsional stiffness is comparable to the Cotrel-Dubousset instrumentation. Moreover, the stability of the new design is satisfactory. Finite element analysis indicates that the maximum equivalent stress on the vertebral model with the central rod system is relatively constant. CONCLUSIONS: The central rod system consists of a central rod which applies fixation and derotation through a connector between the screws on both sides of the vertebral column. This new design can not only prevent the effect of unilateral stress concentration, but is also simpler and easier to implant. However, it is necessary to strengthen the material of the rod or the connection between the rod and the screws. RELEVANCE: This study employed the methods of biomechanical testing and finite element analysis to evaluate a newly designed central rod system in the treatment of scoliosis. The same methods can be applied to the evaluation of orthopaedic instrumentations in the pre-clinical stage. The efficiency of the central rod system is comparable to the Cotrel-Dubousset instrumentation, and its implantation is simpler and easier.

Journal Article↗
PubMed2003

[Three dimensional finite element stress analysis on superstructure of the fixed bridge supported by tooth-implant].

OBJECTIVE: To reveal the stress distribution in the superstructure of fixed bridge supported by tooth-implant in the process of mastication for improvement of denture design. METHODS: The stress distribution and displacement of the superstructure were studied and analyzed by means of CT Scan, CAD and three-dimensional finite element when various loads were applied. RESULTS: (1) The stress distribution in abutments under oblique loads at forty-five degrees was uneven and the peak value was 4 - 6 times higher than that under vertical loads. Stress concentration occurred with significant compressive stress. (2) Compressive stress widely distributed in the middle area of occlusal surface of pontic, whose peak value under concentrated loads was significantly higher than that under disperse loads. The loading direction had no effect on the stress concentration. (3) The maximum displacement of implant abutment in medial-distal direction was greater than that of the neck of nature tooth. CONCLUSIONS: The mechanic complications of superstructure could be prevented by reducing oblique loads and concentrated ones. It is certain that the further improvements of curve-resistance of pontics and press-resistance of abutments are available.

Dental Implants↗
PubMed2006

Simulation of physiological loading in total hip replacements.

The determination of biomechanical force systems of implanted femurs to obtain adequate strain measurements has been neglected in many published studies. Due to geometric alterations induced by surgery and those inherent to the design of the prosthesis, the loading system changes because the lever arms are modified. This paper discusses the determination of adequate loading of the implanted femur based on the intact femur-loading configuration. Four reconstructions with Lubinus SPII, Charnley Roundback, Muller Straight and Stanmore prostheses were used in the study. Pseudophysiologic and nonphysiologic implanted system forces were generated and assessed with finite element analysis. Using an equilibrium system of forces composed by the Fx (medially direction) component of the hip contact force and the bending moments Mx (median plane) and My (coronal plane) allowed adequate, pseudo-physiological loading of the implanted femur. We suggest that at least the bending moment at the coronal plane must be restored in the implanted femur-loading configuration.

Computer Simulation↗
PubMed2005

Acromion-fixation of glenoid components in total shoulder arthroplasty.

Successful design of components for total shoulder arthroplasty has proven to be challenging. This is because of the difficulties in maintaining fixation of the component that inserts into the scapula; i.e., the glenoid component. Glenoid components that are fixated to both the glenoid and acromion (a long process extending medially on the dorsal aspect of the scapula) have the possible advantage of greater stability over those that are fixated to the glenoid alone. In this study, a finite element analysis is used to investigate whether or not acromion fixation is advantageous for glenoid components. Full muscle loading and joint reaction forces are included in the finite element model. Reflective photoelasticity of five scapulae is used to obtain experimental data to compare with results from the finite element analysis, and it confirms the structural behaviour of the finite element model. When implanted with an acromion-fixated prosthesis, it is found that high unphysiological stresses occur in the scapula bone, and that stresses in the fixation are not reduced. Very high stresses are predicted in that part of the prosthesis which connects the acromion to the glenoid. It is found that the very high stresses are partly in response to the muscle and joint reaction forces acting at the acromion. It is concluded that, because of the relatively high forces acting at the acromion, fixation to it may not be the way forward in glenoid component design.

Acromion↗
PubMed1999

The implant neck: smooth or provided with retention elements. A biomechanical approach.

A combined three-dimensional and axisymmetric finite element analysis was made of the effect upon the peak interfacial shear stress of providing an axially loaded mandibular dental implant with retention elements all the way up to the crest of the implant as opposed to a smooth neck. The effect of increased wall thickness of the implant and of using bi-cortical fixation as opposed to uni-cortical fixation was also studied. Retention elements at the implant neck were found to bring about a major decrease in the peak interfacial shear stress. Increased wall thickness and bi-cortical fixation also resulted in decreased peak interfacial shear stress but this effect was minor. The interpretation of this was that all these three measures increase the capacity of the implant to carry axial loads. Thus from a biomechanical viewpoint it appears to be advantageous to provide the neck of screw-shaped implants with retention elements, for example a rough surface of suitable micro-architecture and/or a micro-thread. It is furthermore suggested that retention elements at the implant neck will counteract marginal bone resorption in accordance with Wolff's law. This paper is a revision of: Hansson, S. (1997) Some steps to improve the capacity of dental implants to resist axial loads. In: Hansson, S., ed. Towards an optimized dental implant and implant bridge design: A biomechanical approach. Thesis. Göteborg: Chalmers University of Technology.

Alveolar Process↗
PubMed2003

Measuring the elastic modulus of ex vivo small tissue samples.

Over the past decade, several methods have been proposed to image tissue elasticity based on imaging methods collectively called elastography. While progress in developing these systems has been rapid, the basic understanding of tissue properties to interpret elastography images is generally lacking. To address this limitation, we developed a system to measure the Young's modulus of small soft tissue specimens. This system was designed to accommodate biological soft tissue constraints such as sample size, geometry imperfection and heterogeneity. The measurement technique consists of indenting an unconfined small block of tissue while measuring the resulting force. We show that the measured force-displacement slope of such a geometry can be transformed to the tissue Young's modulus via a conversion factor related to the sample's geometry and boundary conditions using finite element analysis. We also demonstrate another measurement technique for tissue elasticity based on quasi-static magnetic resonance elastography in which a tissue specimen encased in a gelatine-agarose block undergoes cyclical compression with resulting displacements measured using a phase contrast MRI technique. The tissue Young's modulus is then reconstructed from the measured displacements using an inversion technique. Finally, preliminary elasticity measurement results of various breast tissues are presented and discussed.

Adult↗
PubMed2004

An assessment of stress analyses in the theory of abfraction.

Wedged-shaped lesions at the cemento-enamel junction of teeth have been attributed primarily to biomechanical loading forces that cause flexure and failure of enamel and dentin. This theory, termed abfraction, remains controversial. This review examined studies on mechanical properties of enamel and dentin and studies on bite forces and mastication as background information. Abfraction is based principally on a few early finite element analysis and photoelastic models showing stress concentration at the dental cervical area without actually showing enamel and dentin fracture. However, a review of more recent dental stress analyses has been contradictory. Particularly, analyses of the periodontal ligament and alveolar bone, not modeled in previous studies, have shown that those structures may dissipate occlusal loading forces from the cervical areas. In addition, some models may not fully represent intricate dental anatomy and complex occlusal function. Therefore, the key basis of the abfraction theory may be flawed.

Animals↗
PubMed2003

The relationship between cement fatigue damage and implant surface finish in proximal femoral prostheses.

The majority of cemented femoral hip replacements fail as a consequence of loosening. One design feature that may affect loosening rates is implant surface finish. To determine whether or not surface finish effects fatigue damage accumulation in a bone cement mantle, we developed an experimental model of the implanted proximal femur that allows visualisation of damage growth in the cement layer. Five matte surface and five polished surface stems were tested. Pre-load damage and damage after two million cycles was measured. Levels of pre-load (shrinkage) damage were the same for both matte and polished stems; furthermore damage for matte vs. polished stems was not significantly different after two million cycles. This was due to the large variability in damage accumulation rates. Finite element analysis showed that the stress is higher for the polished (assumed debonded) stem, and therefore we must conclude that either the magnitude of the stress increase is not enough to appreciably increase the damage accumulation rate or, alternatively, the polished stem does not debond immediately from the cement. Significantly (P=0.05) more damage was initiated in the lateral cement compared to the medial cement for both kinds of surface finish. It was concluded that, despite the higher cement stresses with debonded stems, polished prostheses do not provoke the damage accumulation failure scenario.

Arthroplasty, Replacement, Hip↗
PubMedDate not supplied

Facial changes following treatment with a removable orthodontic appliance in adults.

The purpose of this study is to determine soft tissue facial changes in adults with mild Class I or Class II malocclusions, treated with a removable orthodontic appliance (Homeoblock '). Standardized intra-oral and frontal facial photographs of 25 adults were scanned and x, y coordinates of 16 maxillary arch and nine facial landmarks were digitized. Mean maxillary arch and facial configurations were subjected to finite-element analysis (FEA). Results showed that the mean pre- and post-treatment maxillary and facial configurations differed statistically (p < 0.05). Specifically, the maxillary arch showed a 30% relative size increase in the mid-palatal region with shape changes consistent with improved dental alignment and maxillary expansion in the transverse direction. For facial changes, a 10% size increase was localized in the nasomaxillary region, and the intercanthal angle changed from -4.6(0) to +4.2(0) (p < 0.05). It is concluded that dental and facial changes associated with the Homeoblock' removable orthodontic appliance are consistent with a broader smile and a more symmetrical facial appearance in non-growing adults.

Adult↗
PubMedDate not supplied

A finite element stress analysis of the transmandibular implant system.

PURPOSE: The aim of this work was a simulation of the onset of osseointegration of the transmandibular implant system in the mandible. This was achieved by imposing joint elements at the implant-bone interface. MATERIALS AND METHODS: The geometric model was derived from the anatomic measurement of several mandibles by tomographic scanning. The spring constants of the joint elements interposed at the implant-bone interface were varied between 1 N/mm and 10(9) N/mm to represent the conditions leading to osseointegration. RESULTS: Increasing the value of the spring constant at the joint elements gradually increases the effective stress in an increasing volume of crestal cortical bone. Additionally, a larger volume of crestal cortical bone assumes a higher stress value as the simulation proceeds. CONCLUSION: This work indicates that considerable changes in stress magnitude and distribution occur in the crestal cortical bone margins as osseointegration is simulated, which may be the necessary stimulus for bone remodeling.

Bone Remodeling↗
PubMed2005

Multi-factorial retainer design analysis of posterior resin-bonded fixed partial dentures: a finite element study.

OBJECTIVES: Biomechanical aspects related to prosthesis design of RBFPD have been proposed as the predominant contributor to unpredictable clinical retention. The aim of this study was to investigate the biomechanics of multiple posterior retainer designs and their interactions using three-dimensional finite element analysis. METHODS: To understand the interactions among the retainer design factors, three values of retainer thicknesses (1.2, 0.8 and 0.4mm), heights (100, 75 and 50% of the distance from 2mm above the CEJ to the occlusal surface) and angle of the axial surface extensions (150, 180 and 210 degrees ) were selected as the design parameters. Twenty-seven RBFPD FE models with three retainer-design parameters were created by image processing, contour stacking, and mapping mesh procedures. The maximum principal and von Mises stresses in remaining tooth and prosthesis, respectively, were recorded in 54 FE analyses (27 FE models with axial and lateral occlusal loadings). RESULTS: The simulated results showed that the averaged stress values of the remaining tooth and prosthesis decreased with greater retainer thickness and height as a result of increasing prosthesis stiffness and maximizing of the bonding area between the enamel and the retainer, respectively. However, no significant stress differences were found according to the angle of the retainer extension because stress transmission was concentrated at the connectors. CONCLUSIONS: The stress elevation rate (termed as VSR-stress elevation by decreasing one unit volume of the remaining tooth) addresses that the height was the most influential factor for the remaining tooth structure, followed by retainer thickness and extension angle.

Dental Prosthesis Retention↗
PubMed2006

Composite shrinkage stress as a function of specimen dimensions and compliance of the testing system.

OBJECTIVES: Verify the influence of specimen dimensions on composite shrinkage stress in testing systems of known compliance and in situations where axial strain of bonding substrates was suppressed. Stress distribution was evaluated using finite element analysis (FEA). METHODS: A chemically activated composite (Bisfill 2B, Bisco) was inserted between the flat surfaces of two glass rods (2, 4 or 6mm diameter, D) attached to a universal testing machine. Specimen height (H) was defined by adjusting the distance between the rods (0.5, 1, 2 or 4mm). An extensometer was used to monitor the distance between them. Maximum force after 30min of polymerization was divided by the cross-sectional area of the rod to obtain nominal stress (sigma(n)). Mathematical equations were employed in order to estimate the 'corrected nominal stress' (sigma(cor)) that would be obtained in ideally rigid systems. Data were analyzed by two-way ANOVA/Tukey test (alpha=0.05) and regression analysis (stress versus 'C factor' and stress versus specimen volume). Axysimetrical 2D models were used to evaluate X-component stress distribution (sigma(x)) in the zero compliance condition. RESULTS: The interactions between D and H were significant for sigma(n) and sigma(cor) (p<0.001). For D=2mm, height influenced only sigma(cor) values. A direct correlation was found between stress and C factor (sigma(n):R(2)=0.959; sigma(cor):R(2)=0.923), but not between stress and volume. FEA evidenced the effect of boundary restraints on sigma(x). SIGNIFICANCE: Specimen dimensions influenced test results, mostly regarding sigma(cor). Stress values strongly increased with the confinement of the specimen probably due to stress concentration adjacent to the bonded interface.

Compliance↗
PubMed2006

Optimizing cardiac material parameters with a genetic algorithm.

Determining the unknown material parameters of intact ventricular myocardium can be challenging due to highly nonlinear material behavior. Previous studies combining a gradient-search optimization procedure with finite element analysis (FEA) were limited to two-dimensional (2D) models or simplified three-dimensional (3D) geometries. Here we present a novel scheme to estimate unknown material parameters for ventricular myocardium by combining a genetic algorithm (GA) with nonlinear finite element analysis. This approach systematically explores the domain of the material parameters. The objective function to minimize was the error between simulated strain data and finite element model strains. The proposed scheme was validated for a 2D problem using a realistic material law for ventricular myocardium. Optimized material parameters were generally within 0.5% of the true values. To demonstrate the robustness of the new scheme, unknown material parameters were also determined for a realistic 3D heart model with an exponential hyperelastic material law. When using strains from two material points, the algorithm converged to parameters within 5% of the true values. We conclude that the proposed scheme is robust when estimating myocardial material parameters in 2D and 3D models.

Algorithms↗