Ústav mechaniky těles, mechatroniky a biomechaniky

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Biomechanical comparison of all-polyethylene total knee replacement and its metal-backed equivalent on periprosthetic tibia using the finite element method
(BMC, 2024-02-23) Apostolopoulos, Vasileios; Boháč, Petr; Marcián, Petr; Nachtnebl, Luboš; Mahdal, Michal; Pazourek, Lukáš; Tomáš, Tomáš
BackgroundTotal knee arthroplasty (TKA) with all-polyethylene tibial (APT) components has shown comparable survivorship and clinical outcomes to that with metal-backed tibial (MBT). Although MBT is more frequently implanted, APT equivalents are considered a low-cost variant for elderly patients. A biomechanical analysis was assumed to be suitable to compare the response of the periprosthetic tibia after implantation of TKA NexGen APT and MBT equivalent.MethodsA standardised load model was used representing the highest load achieved during level walking. The geometry and material models were created using computed tomography data. In the analysis, a material model was created that represents a patient with osteopenia.ResultsThe equivalent strain distribution in the models of cancellous bone with an APT component showed values above 1000 mu epsilon in the area below the medial tibial section, with MBT component were primarily localised in the stem tip area. For APT variants, the microstrain values in more than 80% of the volume were in the range from 300 to 1500 mu epsilon, MBT only in less than 64% of the volume.ConclusionThe effect of APT implantation on the periprosthetic tibia was shown as equal or even superior to that of MBT despite maximum strain values occurring in different locations. On the basis of the strain distribution, the state of the bone tissue was analysed to determine whether bone tissue remodelling or remodelling would occur. Following clinical validation, outcomes could eventually modify the implant selection criteria and lead to more frequent implantation of APT components.
Implant Preference and Clinical Outcomes of Patients with Staged Bilateral Total Knee Arthroplasty: All-Polyethylene and Contralateral Metal-Backed Tibial Components
(MDPI, 2023-11-30) Nachtnebl, Luboš; Apostolopoulos, Vasileios; Mahdal, Michal; Pazourek, Lukáš; Brančík, Pavel; Valoušek, Tomáš; Boháč, Petr; Tomáš, Tomáš
Numerous studies have compared metal-backed components (MBTs) and all-polyethylene tibial components (APTs), but none of them specifically analysed the clinical results and the overall patient preference in patients who had undergone a staged bilateral knee replacement. The purpose of this study is to compare clinical results, perceived range of motion, and overall implant preference among patients who had undergone staged bilateral knee replacement with an APT and contralateral knee replacement with MBTs. A dataset of 62 patients from a single centre who underwent staged bilateral TKA between 2009 and 2022 was selected and retrospectively analysed. Tibial component removal was performed in three knees overall, all of which had MBTs. The mean measured Knee Score (KS) of knees with APTs was 78.37 and that of contralateral knees with MBTs was 77.4. The mean measured Function (FS) of knees with APTs was 78.22, and that of contralateral knees with MBs was 76.29. The mean flexion angle of knees with APTs was 103.8 and that for knees with MBTs was 101.04 degrees. A total of 54.8% of the patients preferred the knee that received APTs over contralateral MBTs. In our cohort, TKA with an APT in one knee and an MBT in the contralateral knee recorded similar clinical results and perceived ranges of motion. Patients in general preferred the knee that received an APT over contralateral knee with an MBT.
Biaxial stretch can overcome discrepancy between global and local orientations of wavy collagen fibres
(ELSEVIER SCI LTD, 2023-11-11) Turčanová, Michaela; Fischer, Jiří; Hermanová, Markéta; Bednařík, Zdeněk; Skácel, Pavel; Burša, Jiří
Most frequently used structure-based constitutive models of arterial wall apply assumptions on two symmetric helical (and dispersed) fibre families which, however, are not well supported with histological findings where two collagen fibre families are seldom found. Moreover, bimodal distributions of fibre directions may originate also from their waviness combined with ignoring differences between local and global fibre orientations. In contrast, if the model parameters are identified without histological information on collagen fibre directions, the resulting mean angles of both fibre families are close to +/- 45 degrees, which contradicts nearly all histologic findings. The presented study exploited automated polarized light microscopy for detection of collagen fibre directions in porcine aorta under different biaxial extensions and approximated the resulting histograms with unimodal and bimodal von Mises distributions. Their comparison showed dominantly circumferential orientation of collagen fibres. Their concentration parameter for unimodal distributions increased with circumferential load, no matter if acting uniaxially or equibiaxially. For bimodal distributions, the angle between both dominant fibre directions (chosen as measure of fibre alignment) decreased similarly for both uniaxial and equibiaxial loads. These results indicate the existence of a single family of wavy circumferential collagen fibres in all layers of the aortic wall. Bimodal distributions of fibre directions presented sometimes in literature may come rather from waviness of circumferentially arranged fibres than from two symmetric families of helical fibres. To obtain a final evidence, the fibre orientation should be analysed together with their waviness.
Influencing the Indentation Curves by the Tilt of the Berkovich Indenter at FEM Modelling of the Steel Nanoindentation
(Sciendo, 2023-11-13) Kovář, Jaroslav; Fuis, Vladimír; Čtvrtlík, Radim; Tomáštík, Jan
The influence of the Berkovich indenter tilt on the results of the FEM modelling of steel nanoindentation is evaluated in this paper. The shape deviations of the indenter have a great impact on the results of the nanoindentation. The main deviations are the indenter bluntness and indenter tilt. The indenter tilt has an impact on the shape of the contact area and the shape of indentation curves. The X5CrNiCuNb 16-4 steel nanoindentation with Berkovich indenter was modelled and the indentation curves were obtained. The Berkovich indenter was blunted and the tilt of the indenter was changed to determine the influence of the indentation curves by the indenter tilt. The plasticity of the steel was modelled with the multilinear model of material, which was fitted to the data from tensile test. The results showed that growing tilt shifts the indentation curves to higher values of indentation forces and makes unloading curves slightly steeper which is in the agreement with larger contact area. The calculated indentation curves were compared with the indentation curves obtained by experiment and the impact of the indenter tilt to the indentation curves was determined by the comparing of the maximal forces and the shapes of the indentation curves.
Influence of flexoelectricity on an interface crack between two dissimilar dielectric materials
(Elsevier B.V., 2023-06-03) Sládek, Ján; Sládek, Vladimír; Hrytsyna, Maryan; Profant, Tomáš
In the present paper, the interface crack between two dissimilar dielectric materials under a mechanical load is investigated with including flexoelectricity effects. Flexoelectricity is a size dependent electro-mechanical coupling phenomenon, where the electric polarization is induced by a strain gradient in dielectrics. The strain gradients may potentially break the inversion symmetry in centrosymmetric crystals and polarization is observed even in all dielectric materials. The polarization is proportional to the strain gradients in the direct flexoelectricity. Layered composite structures are frequently utilized in microelectronics. Due to a poor adhesion of protection layer and basic material, the interface crack can be created there and for the prediction of failure of these structures it becomes essential to investigate distribution of the interfacial stress and strain fields. Governing equations in the gradient theory contain higher-order derivatives than in the standard continuum mechanics. Therefore, a reliable computational tool is required to solve these boundary-value problems. The mixed finite element method (FEM) is developed, where the standard C0 continuous finite elements are utilized for independent approximations of displacements and strains. The constraints between the strain gradients and displacements are satisfied by collocation at Gaussian integration points inside elements. In numerical examples, a parametric study is performed with respect to flexoelectric and elastic coefficients for both material regions. The influence of these parameters on the crack opening displacement is discussed.

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