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PubMed1996

Comparison of stress distribution around vertical and angled implants with finite-element analysis.

The distribution of stress around implants placed in the first molar region of the mandible was biomechanically analyzed in a two-dimensional mathematical model. Two types of implants, vertical and angled, were subjected to a vertical load of 100 N and a horizontal load of 50 N in turn. The magnitudes and contours of compressive and tensile stress within the surrounding bone were determined. For the sake of comparison, maximal compressive stress and maximal tensile stress at the surrounding bone were calculated. There were no measurable differences in stress values and contours when a horizontal load was applied to the vertical and angled implants. However, with the vertical loading, the compressive stress values were five times higher around the cervical region of the angled implant than around the same area in the vertical implant.

Compressive Strength↗
PubMed2000

Loosening pattern in a cementless custom-made hip stem: X-ray analysis, finite-elements and photoelasticity measurements.

Thirty-three X-press cementless stems (Depuy) manufactured according to standardized X-rays were inserted from 1992 to 1994. The patients' mean age was 49 (range 15-79) years with a mean follow-up of 32 (+/-6) months. A characteristic radiographic pattern of aseptic loosening with erosion of the medial cortex by the tip of the stem occurred in 28 patients and a valgus shift of the implant in 14 cases. A radiolucent line with increased sclerosis below the tip (zone Gruen 4) was observed in 17 cases. Four stems were revised due to histologically confirmed aseptic loosening. Biomechanical investigation of one of the revised stems with the typical pattern of valgus angulation and medial cortex erosion included photoelasticity and finite-element analysis. The intertrochanteric fit and fill obviously resulted in an unfavorable distribution of contact areas, including peaks of high stress on the medial tip of the stem. These experimental findings are even evident for a postulated rotational stability. The clinical and radiographic results of the cementless X-press stems do not seem to support the fixation concept of intertrochanteric fit and fill of femoral components.

Adolescent↗
PubMed2004

Feasibility of rapid and automated importation of 3D echocardiographic left ventricular (LV) geometry into a finite element (FEM) analysis model.

BACKGROUND: Finite element method (FEM) analysis for intraoperative modeling of the left ventricle (LV) is presently not possible. Since 3D structural data of the LV is now obtainable using standard transesophageal echocardiography (TEE) devices intraoperatively, the present study describes a method to transfer this data into a commercially available FEM analysis system: ABAQUS. METHODS: In this prospective study TomTec LV Analysis TEE Software was used for semi-automatic endocardial border detection, reconstruction, and volume-rendering of the clinical 3D echocardiographic data. A newly developed software program MVCP FemCoGen, written in Delphi, reformats the TomTec file structures in five patients for use in ABAQUS and allows visualization of regional deformation of the LV. RESULTS: This study demonstrates that a fully automated importation of 3D TEE data into FEM modeling is feasible and can be efficiently accomplished in the operating room. CONCLUSION: For complete intraoperative 3D LV finite element analysis, three input elements are necessary: 1. time-gaited, reality-based structural information, 2. continuous LV pressure and 3. instantaneous tissue elastance. The first of these elements is now available using the methods presented herein.

Echocardiography, Three-Dimensional↗
PubMed2006

A pinch elastometer for soft tissue.

A prototype compression elastometer suited to the characterisation of soft tissue is analysed and tested by application to various elastomers. The test material is pinched between two rigid cylinders and the compression force and displacement interpreted to yield a measure of "effective" stiffness or to calibrate a simple non-linear-elastic material model (Neo-Hookean). This deformation suits the testing of bulk soft tissue since it effectively isolates the test material from boundary conditions such as other soft tissue, ligaments and bones. These can be highly variable in the body and can affect results greatly when employing other types of tests to determine the elastic nature of tissue. A simple linear-material analysis, based on established solutions to two-dimensional problems, is extended to take into account various geometrical complexities. This analysis permits immediate inversion of the readings from the device to yield the elastic properties of the material, without the need for complex numerical analysis. Finite element analysis is also employed to determine the range of reliable application of the linear-elastic model. In particular, this analysis permits the extension of the linear-elastic analysis to include simple forms of non-linear-material behaviour. The method is demonstrated using three elastomers having significantly different material properties. A viable range of application of the device is identified in which it yields results with reasonable precision and accuracy. The prototype device was able to measure the effective elastic modulus of the test materials with a maximum error of 13% for three material types (N=25). Repeatability error was less than 7% in all cases. Further refinement of the device and measuring system will reduce this uncertainty.

Biomechanical Phenomena↗
PubMed1992

Comparison of stress transmission in the IMZ implant system with polyoxymethylene or titanium intramobile element: a finite element stress analysis.

Using the finite element method, this study modeled a 4.0 x 13.0-mm IMZ implant, restored with a cast gold crown, to examine the influence of the polyoxymethylene (POM) intramobile element (IME) on the transmission of vertical and oblique forces. Stress concentrations in the bone and in components of the implant system were much greater under a 30-degree load than under an equal vertical load. Stress transmission to bone occurred chiefly in the crestal region, and these stresses were not reduced when the IME was modeled in POM rather than in titanium. Maximum stress concentrations occurred in the fastening screw.

Alveolar Process↗
PubMedDate not supplied

Stress distribution in the supraspinatus tendon with partial-thickness tears: an analysis using two-dimensional finite element model.

The purpose of this study was to clarify the stress distribution in the rotator cuff tendon with 3 types of partial-thickness tears by use of 2-dimensional finite element analysis. In the finite element model of the normal human shoulder, a tendon defect was created on the articular surface, on the bursal surface, or in the midsubstance close to the insertion. A tensile force was applied to the proximal end of the tendon, and the stress distribution was calculated. In all 3 types of tears, a high stress concentration appeared around the articular surface at the insertion and at the site of the tear, which extended proximally. The maximum value of the von Mises stress increased with the presence of a partial-thickness tear. With the arm in abduction, a high stress concentration was also observed around the site of the tear. These biomechanical conditions of the supraspinatus tendon may eventually lead to a full-thickness tear at the critical zone.

Finite Element Analysis↗
PubMed1992

Effects of fronto-occipital artificial cranial vault modification on the cranial base and face.

Artificial reshaping of the cranial vault has been practiced by many human groups and provides a natural experiment in which the relationships of neurocranial, cranial base, and facial growth can be investigated. We test the hypothesis that fronto-occipital artificial reshaping of the neurocranial vault results in specific changes in the cranial base and face. Fronto-occipital reshaping results from the application of pads or a cradle board which constrains cranial vault growth, limiting growth between the frontal and occipital and allowing compensatory growth of the parietals in a mediolateral direction. Two skeletal series including both normal and artificially modified crania are analyzed, a prehistoric Peruvian Ancon sample (47 normal, 64 modified crania) and a Songish Indian sample from British Columbia (6 normal, 4 modified). Three-dimensional coordinates of 53 landmarks were measured with a diagraph and used to form 9 finite elements as a prelude to finite element scaling analysis. Finite element scaling was used to compare average normal and modified crania and the results were evaluated for statistical significance using a bootstrap test. Fronto-occipitally reshaped Ancon crania are significantly different from normal in the vault, cranial base, and face. The vault is compressed along an anterior-superior to posterior-inferior axis and expanded along a mediolateral axis in modified individuals. The cranial base is wider and shallower in the modified crania and the face is foreshortened and wider with the anterior orbital rim moving inferior and posterior towards the cranial base. The Songish crania display a different modification of the vault and face, indicating that important differences may exist in the morphological effects of fronto-occipital reshaping from one group to another.

British Columbia↗
PubMed1990

Performance of dental biomaterials: conference report.

On August 13-18, 1989, a dental conference, entitled "Dental Biomaterials: Assessment of Performance Based on Engineering and Statistical Methods", was held in Santa Barbara, California. This conference was sponsored by the Engineering Foundation with co-sponsorship by the United States Army Institute of Dental Research, the United States Naval Dental Research Institute, Block Drug Co., Dentsply International, and the National Institute of Standards and Technology. The objective of this conference was to explore engineering and statistical methods for assessing the resistance of dental biomaterials to structural degradation and failure and for predicting the relative reliability of dental restorations and prostheses under clinical conditions. The conference was designed to evaluate the merits of established and new technology relative to the performance of dental materials. Analytical and experimental methods included fracture mechanics, conventional mechanical tests, adhesion tests, fractographic analysis, finite element analysis, statistical analysis, tribology, image analysis, x-ray diffraction, and scanning tunneling microscopy. The central theme was the development of a systematic approach to predict material performance under clinical conditions based on stress analysis, correlation of clinical data with in vitro data, and fracture surface characterization based on qualitative and quantitative methods. Supported in part by NIDR Grant No. DE06672.

Dental Materials↗
PubMed2004

Mechano-regulation of stem cell differentiation and tissue regeneration in osteochondral defects.

Cartilage defects that penetrate the subchondral bone can undergo spontaneous repair through the formation of a fibrous or cartilaginous tissue mediated primarily by mesenchymal stem cells from the bone marrow. This tissue is biomechanically inferior to normal articular cartilage, and is often observed to degrade over time. Whether or not biomechanical factors control the type and quality of the repair tissue, and its subsequent degradation, have yet to be elucidated. In this paper, we hypothesise a relationship between the mechanical environment of mesenchymal stem cells and their subsequent dispersal, proliferation, differentiation and death. The mechano-regulation stimulus is hypothesised to be a function of strain and fluid flow; these quantities are calculated using biphasic poroelastic finite element analysis. A finite element model of an osteochondral defect in the knee was created, and used to simulate the spontaneous repair process. The model predicts bone formation through both endochondral and direct intramembranous ossification in the base of the defect, cartilage formation in the centre of the defect and fibrous tissue formation superficially. Greater amounts of fibrous tissue formation are predicted as the size of the defect is increased. Large strains are predicted within the fibrous tissue at the articular surface, resulting in significant cell apoptosis. This result leads to the conclusion that repair tissue degradation is initiated in the fibrous tissue that forms at the articular surface. The success of the mechano-regulation model in predicting many of the cellular events that occur during osteochondral defect healing suggest that in the future it could be used as a tool for optimising scaffolds for tissue engineering.

Animals↗
PubMedDate not supplied

Mechanical environment of the supraspinatus tendon: a two-dimensional finite element model analysis.

We performed 2-dimensional finite element model analysis to estimate the mechanical environment of the supraspinatus tendon. The geometric shape of the finite element model was determined by magnetic resonance imaging of a normal human shoulder obtained at 0 degrees, 30 degrees, and 60 degrees of abduction, whereas the histologic location of noncalcified and calcified fibrocartilage was determined from a cadaveric specimen. The supraspinatus tendon was pulled proximally with the force of 10 N at 0 degrees, 53 N at 30 degrees, and 115 N at 60 degrees of abduction. The area of high principal stress maximum was observed on the articular side of the supraspinatus tendon, which shifted toward the insertion as the arm was abducted. High stress concentration on the articular side of the supraspinatus tendon near its insertion during arm elevation may explain the frequent occurrence of rotator cuff tears at this site.

Adult↗
PubMed1991

Regional myocardial stress distribution from magnetic resonance image-based mathematical models.

The instantaneous regional stress distribution within the myocardium, which cannot be directly measured, has been estimated using improved numerical methods and nonaxisymmetric biventricular geometry. To do this, we have employed computer-aided solid mathematical modeling to generate a three-dimensional representation for an ex vivo canine biventricular unit using magnetic resonance imaging. A two-dimensional transverse section was isolated from the solid mathematical model for regional stress analysis using p-version finite element analysis. Loading conditions and material property descriptions were taken from published reports. Analyses showed the maximum principal stresses to range from -1.76 X 10(5) to 8.52 X 10(5) dynes/cm2 during systolic loading, and from -3.85 X 10(4) to 1.13 X 10(5) dynes/cm2 during diastolic loading. This study demonstrates that magnetic resonance image-based solid mathematical biventricular models are suitable for regional stress analysis using p-version finite element analysis. p-Version finite element analysis using magnetic resonance image-based cardiac representations facilitates in vivo stress-strain analyses and may allow the clinical estimation of regional myocardial stress.

Animals↗
PubMed2003

How do tissues respond and adapt to stresses around a prosthesis? A primer on finite element stress analysis for orthopaedic surgeons.

Joint implant design clearly affects long-term outcome. While many implant designs have been empirically-based, finite element analysis has the potential to identify beneficial and deleterious features prior to clinical trials. Finite element analysis is a powerful analytic tool allowing computation of the stress and strain distribution throughout an implant construct. Whether it is useful depends upon many assumptions and details of the model. Since ultimate failure is related to biological factors in addition to mechanical, and since the mechanical causes of failure are related to load history, rather than a few loading conditions, chief among them is whether the stresses or strains under limited loading conditions relate to outcome. Newer approaches can minimize this and the many other model limitations. If the surgeon is to critically and properly interpret the results in scientific articles and sales literature, he or she must have a fundamental understanding of finite element analysis. We outline here the major capabilities of finite element analysis, as well as the assumptions and limitations.

Arthroplasty, Replacement↗