P. Vosynek

Analyses of the Shape Deviations of the Contact Cones of the Total Hip Joint Endoprostheses

Complex reliability analyses of the bioimplants are very important today, because the failures of them cause traumatic consequences for the patients. We are interested in the calculations of the reliability of the ceramic heads of hip joint endoprosthesis. Reliability of the component made of the brittle material depends on the material parameters and on the tensile stress distribution in the component (bioimplant – hip joint ceramic head). Tensile stresses in the heads are very significantly influenced by the shape deviations of the cone contact areas of the head and the stem. Concerned are shape deviations from the ideal conical surfaces of the stem and the head of the endoprosthesis. The shape deviations may be modeled at the macro-level - this concerns model shape inaccuracies such as deviation from the nominal degree of taper, at the micro-level - when the stochastic distribution of unevenness on the contact areas is respected. The problem of stress in ceramic heads was solved using the finite element method – system ANSYS under ISO 7206-5 loading. There are presented and analysed the results of solution of the macro shape deviations and micro shape deviations, obtained from measurements made on the cones of stems and heads. The simulations of the loading of the one head pressed on the 5 different stems cones with macro and micro shape deviations (measured) is solved and analysed.

Eccentricity Effect in the Hole-Drilling Residual Stress Measurement

The paper focuses on the analysis of the eccentricity effect in the measurement of the hole-drilling residual stress. Relaxed strains were evaluated by computational simulation of the hole-drilling experiment using the finite element method. Errors induced by eccentricity were estimated for elastic and elastic-plastic states in area around the drilled hole due to the stress concentration. The invariance of the stress change with depth was assumed. The correction of eccentricity and plasticity effects in evaluation of residual stresses was realized within the EVAL 7 software (SINT Technology). The analysis shows that in elastic state the eccentricity and angular position of the drilled hole have a significant effect on relative residual stress errors. Correction according to the HDM method is very effective in this case. If the relative error of 5 % is allowed, which is in engineering practice acceptable, eccentricity of ±0.05 mm could be accepted without correction. When the combination of eccentricity and plasticity occurs, the correction of plasticity is more important in method 13-EXT-UN.

Implementation of American weld connection standards into finite element computations

Presented paper summarizes the weld assessment on the basis of national standards. The approach concerning the static loading has been still not adequately developed in standards. The standard of American Institute of Steel Construction was considered as the best candidate for implementation into the finite element method using post-processing of a particular weld. A comparative analysis of maximum allowable loadings of weld connection models loaded inplane and out-of-plane has been performed. Results obtained by the finite element method and American and Czech standards were compared and the conclusions of applicability and reliability were made.

Biomechanical Testing of Spinal Segment Fixed by Arcofix System on the Swine Spine

Study Design An in vitro biomechanical study. Purpose To evaluate the mechanical properties of the spinal segment in the intact, injured, and stabilized state after fixation by an Arcofix implant. Overview of Literature Several types of thoracolumbar spine injury necessitates anterior instrumentation. The Arcofix plate represents the latest generation of angular stablity systems. The biomechanical properties of these implants have not been sufficiently studied yet. Methods A total of ten porcine specimens (levels Th12-L3) were prepared. The tests were performed for intact, injured, and implanted specimens. In each state, the specimen was subjected to a tension load of a prescribed force, and subsequently, twisted by a given angle. The force load was 200 N. The torsion load had a deformation character, i.e., the control variable was the twisting angle and the measured variable was the moment of a couple. The amplitude of the load alternating cycle was 3°. Another parameter that was evaluated was the area of the hysteresis loop. The area corresponds to the deformation energy which is dissipated during the cycle. Results A statistically significant difference was found between the intact and injured states as well as between the injured and implanted specimens. The statistical evaluation also showed a statistically different value of the hysteresis loop area. In the case of instability, the area decreased to 33% of the physiological value. For the implanted sample, the area increased to 170% of the physiological value. Conclusions The Arcofix implant with its parameters appears to be suitable and sufficiently stable for the treatment of the anterior column of the spine.

Review of Pressurized Vessel Structural Design - An Assessment of Total Life

The paper discusses life prediction of pressurized cylindrical container because of its dangerous failure during usage. The fatigue of material and crack propagation were obvious from the fracture surface. Crack initiation was located in the notch root of the thread. Our goal was to review the vessel structural design based on its technical documentation.Two approaches for estimating the crack initiation period were used. As input parameters, stress-strain relations in the notch were obtained from FEM analysis, done in ANSYS Workbench. The first approach makes use of Neuber’s rule based on elastic-plastic shakedown and cyclic stress-strain curve. The second concept (which is described in ČSN EN 13445-3 or AD 2000 Mekblatt S2 respectively) uses elastic shakedown. Included stress-life curves leads to the number of cycles to crack initiation.Finally the subcritical crack growth period and final fracture were estimated = the total life was evaluated.

Response Analysis of Bone Mass of Proximal Femur to Surface Replacement of Hip Joint

Total hip joint replacement is the prevailing surgical treatment of painful joint diseases. In classical THR, both parts of the joint (acetabulum and femoral head) are replaced by prosthesis. Classic THR gives good results as regards the relief of pain and improved mobility of elderly patients (over 65 years). Problematic is the application of the replacement in younger and active patients. Surface replacement has been proposed as one of the possibilities of preventing THR. The correct function of hip joint implant is associated also with correct function of bone mass, where is placed. Therefore the aim of this article is stress-strain analysis of two hip joint variant with stochastic material data properties of computational model. From the output of CT scanner model of geometry were created. Cancellous bone of proximal femur was divided into the 18 regions where were bone modelled by isotropic, linear and elastic model. Material properties were known from literature. Whole model was loaded by force, corresponding to load by standing on one leg. Values of Youngs modulus were assumed by normal distribution in each region. The problem was solved in the ANSYS program system. Stochastic analysis was done with OptiSLang software. From stress-strain analysis of two variants a few points of proximal femur (physiological and resurfaced hip) were picked for stochastic analysis. These points were characterized by maximal and minimal principal stress and by Von misses stress. Thanks to this work we can describe: loading in each region of bone mass, variations of stresses in each points of geometry and sensitivity of input data (material properties, loading) to output data (strain-stress).

Analysis of the Influence of Lumbosacral Fusion on the Adjacent Moving Segment

The term “adjacent segment disease – ASD” is the designation for clinical degeneration of the adjacent active segment to the fused segment. This disease occurs as the subsequent complication of spinal fusion, which could result in more surgery. The fusion spine segment significantly reduces movement which causes load enhancement in adjacent active segments and causes a change of mobility. The changes are probably the primary cause of the occurrence of ASD. The strongest degeneration occurs at the nearest point under or above the fusion. The purpose of this work was to analyze the loading of the adjacent disc to the fused segment for flexion and extension movement of the whole segment. The problem was comprehensively solved using complex experimental and computational modeling. Thanks to experimental measurement we recorded the response of the test specimen to flexion and extension movement. The test sample was composed of three vertebrae and two discs. The specimens were taken from a pig. Two states were modeled: healthy and damaged with a LEGACY implant. In healthy specimens the deformation mainly takes place in the disc. In the case of the fusion of two vertebrae with the same loading the deformation mainly take place on an adjacent disc. One way to determine the state of stress inside discs is by computational modeling. The strain-stress states were done by solving the computational model in the ANSYS system. Two variants were solved: healthy and damaged with a LEGACY implant. The model was loaded by angular displacement according to flexion and extension movement. The most important findings are: the specimen with the fused segment has significantly higher stiffness compared to the healthy specimen. The disc above the fused segment treated with LEGACY is more loaded (higher stresses) in flexion than in the healthy specimen.