Tomáš Návrat

Pilot study of the wrist orthosis design process

Purpose – The purpose of this paper is to describe a manufacturing methodology for a wrist orthosis. The case study aims to offer new approaches in the area of human orthoses. Design/methodology/approach – The article describes the utilization of rapid prototyping (RP), passive stereo photogrammetry and software tools for the orthosis design process. This study shows the key points of the design and manufacturing methodology. The approach uses specific technologies, such as 3D digitizing, reverse engineering and polygonal-surface software, FDM RP and 3D printing. Findings – The results show that the used technologies reflect the patients requirements and also they could be an alternative solution to the standard method of orthosis design. Research limitations/implications – The methodology provides a good position for further development issues. Practical implications – The methodology could be usable for clinical practice and allows the manufacturing of the perfect orthosis of the upper limb. The usage ...

Finite element analysis for the evaluation of the structural behaviour, of a prosthesis for trans-tibial amputees

The finite element analysis (FEA) has been identified as a useful tool for the stress and strain behaviour determination in lower limb prosthetics. The residual limb and prosthetic socket interface was the main subject of interest in previous studies. This paper focuses on the finite element analysis for the evaluation of structural behaviour of the Sure-flex™ prosthetic foot and other load-bearing components. A prosthetic socket was not included in the FEA. An approach for the finite element modelling including foot analysis, reverse engineering and material property testing was used. The foot analysis incorporated ground reaction forces measurement, motion analysis and strain gauge analysis. For the material model determination, non-destructive laboratory testing and its FE simulation was used. A new, realistic way of load application is presented along with a detailed investigation of stress distribution in the load-bearing components of the prosthesis. A novel approach for numerical and experimental agreement determination was introduced. This showed differences in the strain on the pylon between the experimental and the numerical model within 30% for the anteroposterior bending and up to 25% for the compression. The highest von Mises stresses were found on the foot-pylon connecting component at toe off. Peak stress of 216MPa occurred on the posterior adjusting screw and maximum stress of 156MPa was found at the neck of the male pyramid.

In situ measurements of thin films in bovine serum lubricated contacts using optical interferometry

The aim of this study is to consider the relevance of in situ measurements of bovine serum film thickness in the optical test device that could be related to the function of the artificial hip joint. It is mainly focussed on the effect of the hydrophobicity or hydrophilicity of the transparent surface and the effect of its geometry. Film thickness measurements were performed using ball-on-disc and lens-on-disc configurations of optical test device as a function of time. Chromatic interferograms were recorded with a high-speed complementary metal-oxide semiconductor digital camera and evaluated with thin film colorimetric interferometry. It was clarified that a chromium layer covering the glass disc has a hydrophobic behaviour which supports the adsorption of proteins contained in the bovine serum solution, thereby a thicker lubricating film is formed. On the contrary, the protein film formation was not observed when the disc was covered with a silica layer having a hydrophilic behaviour. In this case, a very thin lubricating film was formed only due to the hydrodynamic effect. Metal and ceramic balls have no substantial effect on lubricant film formation although their contact surfaces have relatively different wettability. It was confirmed that conformity of contacting surfaces and kinematic conditions has fundamental effect on bovine serum film formation. In the ball-on-disc configuration, the lubricant film is formed predominantly due to protein aggregations, which pass through the contact zone and increase the film thickness. In the more conformal ball-on-lens configuration, the lubricant film is formed predominantly due to hydrodynamic effect, thereby the film thickness is kept constant during measurement.

Study of film formation in bovine serum lubricated contacts under rolling/sliding conditions

The aim of this study is to perform a detailed experimental analysis of lubricant film thickness of bovine serum within the contact between the artificial metal and ceramic heads (balls) and the glass disc to analyse the effect of proteins on film formation under various rolling/sliding conditions. Lubricant film observation of bovine serum solutions was carried out using an optical test rig. Chromatic interferograms were recorded with a high-speed CMOS digital camera and evaluated with thin film colorimetric interferometry. Film thickness was studied as a function of time. Under pure rolling conditions, film thickness increases with time as well as with rolling distance for all mean speeds and for both materials of the balls; however the metal ball always forms a thicker lubricating film in comparison to the ceramic ball. Under rolling/sliding conditions, when the disc is faster than the ball, the formation of lubricant film thickness is different compared to pure rolling conditions. At first, film thickness increases rapidly with a rolling/sliding distance for all mean speeds. When maximum film thickness is reached, then this effect is lost and film thickness starts to fall and finally, at the end of the measurement, film thickness drops down to a few nanometres. For the metal ball, maximum values of central film thicknesses are proportional to the mean speed; however this is not observed with the ceramic ball. An absolutely different formation of bovine serum film thickness is observed when the ball is faster than the disc. Under this condition, the protein layer is very thin for both materials of balls, and central film thickness reaches only about a few nanometres. Local protein spots are formed in a very small area of the contact zone and reach the thickness between 20 and 25 nm for the metal ball and 5 nm for the ceramic ball. From the performed experiments under rolling/sliding conditions, it is obvious that the formation of lubricant film thickness is markedly dependent on kinematic conditions acting in the contact, especially on the positive and negative slide-to-roll ratio and the mean speed. In addition, the material of the artificial head has a certain influence on the formation of bovine serum lubricating film.

Reliability of the Ceramic Head of the Total Hip Joint Endoprosthesis Using Weibull’s Weakest-link Theory

The paper deals with calculation of the parameters of ceramic material from a set of destruction tests of ceramic heads of total hip joint endoprosthesis. The standard way of calculation of the material parameters consists in carrying out a set of 3 or 4 point bending tests of specimens cut out from parts of the ceramic material to be analysed. In case of ceramic heads, it is not possible to cut out specimens of required dimensions because the heads are too small (if the cut out specimens were smaller than the normalised ones, the material parameters derived from them would exhibit higher strength values than those which the given ceramic material really has). On that score, a special testing jig was made, in which 40 heads were destructed. From the measured values of circumferential strains of the head’s external spherical surface under destruction, the state of stress in the head under destruction was established using the final elements method (FEM). From the values obtained, the sought for parameters of the ceramic material were calculated using Weibull’s weakest-link theory

COMPUTATIONAL SIMULATION OF CROSS ROLL STRAIGHTENING

Abstract. The paper deals with numerical analysis of the process of cross roll straightening of circular bars in a seven-roll straightening machine. In this machine, initially curved bar rotates along its axis as it progresses through laterally staggered rolls, being loaded by a fluctuating bending moment beyond its elastic limit. To simulate the process efficiently, special program was developed, based on a beam-type finite element, Euler scheme of material flow along the straightened bar and nonlinear iterative solution of the elasto plastic material behavior. Based on the input data, process characteristics like roller loading, product deflection, curvature and plasticization can be quickly obtained. With reliable solution of the direct problem, optimal intermeshing of rollers for given input data can be found in an iterative process. The paper describes main features of the algorithm and examples of its application.

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.

Comparison of the Resistance to Bending Forces of the 4.5 LCP Plate-rod Construct and of 4.5 LCP Alone Applied to Segmental Femoral Defects in Miniature Pigs

The study deals with the determination of mechanical properties, namely resistance to bending forces, of flexible buttress osteosynthesis using two different bone-implant constructs stabilizing experimental segmental femoral bone defects (segmental ostectomy) in a miniature pig ex vivo model using 4.5 mm titanium LCP and a 3 mm intramedullary pin (“plate and rod” construct) (PR-LCP), versus the 4.5 mm titanium LCP alone (A-LCP). The “plate and rod” fixation (PR-LCP) of the segmental femoral defect is significantly more resistant ( p < 0.05) to bending forces (200 N, 300 N, and 500 N) than LCP alone (A-LCP). Stabilisation of experimental segmental lesions of the femoral diaphysis in miniature pigs by flexible bridging osteosynthesis 4.5 mm LCP in combination with the “plate and rod” construct appears to be a suitable fixation of non-reducible fractures where considerable strain of the implants by bending forces can be assumed. These findings will be used in upcoming in vivo experiments in the miniature pig to investigate bone defect healing after transplantation of mesenchymal stem cells in combination with biocompatible scaffolds. Fracture fixation, comminuted fracture, ostectomy, buttress osteosynthesis, implant failure

Development of Experimental Devices for Testing of the Biomechanical Systems

The paper is focused on the description of the development of single-purpose devices for determining me- chanical properties of the segments of human body. The re- search were focused on the development and design of a device for establishing the abrasion wear of the THR of the hip joint, and for testing spinal segments. On the basis of the gained knowledge, the attention has also been focused on the devel- opment and design of a multi-purpose experimental biome- chanical device. Its basic idea was derived from Stewart plat- form, for the modification of which an animation 3D model was developed.

Stress-Deformation Analysis of an Elbow Articulation with Radial Head Replacement

this contribution speaks about the progress of forming a geometric elbow model, generating the FEM network in the created volumes; stress-deformation analysis of the Final Element Model (contact task) and suggests possible geometric models of partial alloplasty of the radius’s proximal part.