The objective of the present work was to determine the dynamic indentation response, stiffness and relaxation curvesfor the shear and the bulk modulus of femoral knee cartilage with no visual damage in cases under unicompartmentalosteoarthritis.A cyclic displacement of 0.5 mm in axial direction was applied with a 3 mm plane-ended cylindrical indenter at specificpoints in the femoral knee cartilage specimens of seven patients with unicompartmental osteoarthritis (UOA). Theindentation force over time was recorded and next the maximum stiffness in all cycles was obtained and compared.Also, the relaxation curves for the shear and the bulk modulus of cartilage were obtained in this work.A decrease in the maximum indentation force was observed comparing between indentation cycles; it was of 6.75 ±0.71% from cycle 1 to cycle 2 and 4.70 ± 0.31% for cycle 2 to cycle 3. Stiffness values changed with a mean of 3.35 ±0.39% from cycle 1 to cycle 2 and 1.40 ± 0.71% from cycle 2 to cycle 3. Moreover, relaxation curves for the shearmodulus and the bulk modulus showed the nonlinear behavior of articular cartilage with UOA.Our results showed that cartilage specimens with no visual damage in UOA preserve a nonlinear viscoelastic behaviorand its stiffness increases through the loading cycles. Our work provides experimental values for generating a morerealistic cartilage behavior than those currently used in computer cartilage models for the study of UOA.
Principal stresses obtained for the first metatarsal with both implants suggest that failure is induced in this bone because, values exceed (up to 136.84% for Swanson model) the tensile strength reported for phalange trabecular bone, which may be related to osteolysis. Stress and strain values obtained in this work suggest that arthroplasty surgery with Swanson implant is more likely to cause postoperative complications versus Tornier implant.
In this study, a methodology that combines artificial neural networks and nonlinear hyperelastic finite element modeling to simulate the temperature-dependent stress response of elastomer solids is presented. The methodology is verified by a discrete model of a tensile test specimen, which is used to generate stress–strain pairs of existent experimental data. The proposed method is also tested with a benchmark problem of a rubber-like cylinder under compression. Three grades of an elastomer used for diverse engineering applications are used throughout the study. On this basis, three neural network architecture with 10 hidden neurons are implemented as constitutive models to reproduce the experimental data of the materials. The validation results show that the proposed methodology can reproduce tensile tests with an error of 5% of less than regarding experimental data for elastomers that present no yielding point. The benchmark problem results were at the range expected for the elastomer materials with no yielding, where it was possible to derive force temperature-dependent responses. These results suggest that the methodology helps the prediction of the material response when only material stress–strain curves at different temperatures exist. Therefore, the presented approach in this contribution helps to simulate the temperature-dependent stress responses of elastomeric solids with no defined yielding point.
Biomechanical properties and dynamic response of soft tissues as articular cartilage remains issues for attention. Currently, linear isotropic models are still used for cartilage analysis in spite of its viscoelastic nature. Therefore, the aim of this study was to propose a nonlinear viscoelastic model for cartilage indentation that combines the geometrical parameters and velocity of the indentation test with the thickness of the sample as well as the mechanical properties of the tissue changing over time due to its viscoelastic behavior. Parameters of the indentation test and mechanical properties as a function of time were performed in Laplace space where the constitutive equation for viscoelasticity and the convolution theorem was applied in addition with the Maxwell model and Hayes et al. model for instantaneous elastic modulus. Results of the models were compared with experimental data of indentation tests on osteoarthritic cartilage of a unicompartmental osteoarthritis cases. The models showed a strong fit for the axial indentation nonlinear force in the loading curve (R2 = 0.992) and a good fit for unloading (R2 = 0.987), while an acceptable fit was observed in the relaxation curve (R2 = 0.967). These models may be used to study the mechanical response of osteoarthritic cartilage to several dynamical and geometrical test conditions.
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