Serge Cescotto

Effects of platelet-rich plasma (PRP) on the healing of Achilles tendons of rats

Platelet‐rich plasma (PRP) contains growth factors involved in the tissular healing process. The aim of the study was to determine if an injection of PRP could improve the healing of sectioned Achilles tendons of rats. After surgery, rats received an injection of PRP (n = 60) or a physiological solution (n = 60) in situ. After 5, 15, and 30 days, 20 rats of both groups were euthanized and 15 collected tendons were submitted to a biomechanical test using cryo‐jaws before performing transcriptomic analyses. Histological and biochemical analyses were performed on the five remaining tendons in each group. Tendons in the PRP group were more resistant to rupture at 15 and 30 days. The mechanical stress was significantly increased in tendons of the PRP group at day 30. Histological analysis showed a precocious deposition of fibrillar collagen at day 5 confirmed by a biochemical measurement. The expression of tenomodulin was significantly higher at day 5. The messenger RNA levels of type III collagen, matrix metalloproteinases 2, 3, and 9, were similar in the two groups at all time points, whereas type I collagen was significantly increased at day 30 in the PRP group. In conclusion, an injection of PRP in sectioned rat Achilles tendon influences the early phase of tendon healing and results in an ultimately stronger mechanical resistance.

Contact between deformable solids: The fully coupled approach

This paper presents a finite element technique to handle frictional contact between bodies submitted to large deformations. Contact finite elements, based on a penalty method, are derived from the virtual work principle. The originality lies in the fact that the contact conditions (Signorinis conditions and Coulomb friction law) are expressed at some integration points and not at the nodal points. The development of contact introduces a coupling between the Degrees Of Freedom (DOF) of the bodies. In certain cases, this coupling can be neglected in the numerical strategy.

Eccentric training improves tendon biomechanical properties: A rat model

The treatment of choice for tendinopathies is eccentric reeducation. Although the clinical results appear favorable, the biomechanical changes to the tissue are not yet clear. Even if the mechanotransduction theory is commonly accepted, the physiology of tendons is not clearly understood. We aimed to better define the biomechanical and histological changes that affect healthy tendon after eccentric and concentric training. This study compared the effects of two methods of training (eccentric [E] training and concentric [C] training) with untrained (U) rats. The animals were trained over a period of 5 weeks. The tricipital, patellar, and Achilles tendons were removed, measured and a tensile test until failure was performed. A histological analysis (hematoxylin and eosin and Massons trichrome stains) was also realized. There was a significant increase in the rupture force of the patellar and tricipital tendons between the U and E groups. The tricipital tendons in the control group presented a significantly smaller cross‐sectional area than the E‐ and C‐trained groups, but none was constated between E and C groups. No significant difference was observed for the mechanical stress between the three groups for all three tendons. Histological studies demonstrated the development of a greater number of blood vessels and a larger quantity of collagen in the E group. The mechanical properties of tendons in rats improve after specific training, especially following eccentric training. Our results partly explained how mechanical loading, especially in eccentric mode, could improve the healing of tendon.

A fully coupled elastoplastic damage modeling and fracture criteria in metalforming processes

Abstract In this paper a fully coupled elastoplastic damage theory at finite strain is presented. The energy-based Von Mises yield criterion and the damage evolution criterion with two damage variables are postulated through the hypothesis of energy equivalence. The local distributions of variables connected with the history of stress, strain and damage are determined by numerical simulation. These values are then used with six previously published fracture criteria to predict fracture initiation sites and to simulate the propagation of fracture in metalforming processes.

Unified and mixed formulation of the 8-node hexahedral elements by assumed strain method

Abstract In this paper, a class of ‘assumed strain’ mixed finite element methods based on the Hu-Washizu variational principle is presented. Special care is taken to avoid hourglass modes and shear locking as well as volumetric locking. A unified framework for the 8-node hexahedral solid and thermal as well as thermomechanical coupling elements with uniform reduced integration (URI) and selective numerical integration (SI) schemes is developed. The approach is simply implemented by a small change of the standard technique and is applicable to arbitrary non-linear constitutive laws including isotropic and anisotropic material behaviors. The implementation of the proposed SI elements is straightforward, while the development of the proposed URI elements requires ‘anti-hourglass stresses’ which are evaluated by classical constitutive equations. Several numerical examples are given to demonstrate the performance of the suggested formulation, including mechanical problems, heat conduction and thermomechanical problems with emphasis on sheet forming processes.

Experimental verification of brain tissue incompressibility using digital image correlation

For decades, incompressibility has been a major assumption in the mechanical study of brain tissue. This assumption is based on the hydrated nature of the biological tissues and the incompressibility of fluids. In this paper, an experimental validation of this assumption using digital image correlation is presented. Unconfined compression tests, relaxation tests and cyclic tests were performed on cylindrical samples of swine brains at loading rates suitable for neurosurgical applications. Digital image correlation was used to evaluate the evolution of the volume ratio throughout the tests. The preparation of the samples is described and it is demonstrated that it causes no statistically significant change of their mechanical properties. The results indicate that the brain tissue incompressibility assumption is verified.

An 8-node brick element with mixed formulation for large deformation analyses

Abstract An 8-node brick element with a mixed formulation has been developed in this paper. This element uses a reduced integration scheme (RI) and the hourglass control technique with the aid of mixed formulations. It has the correct rank of the element stiffness matrix so that the hourglass-modes can be avoided completely. It passes large strain patch tests and guarantees stability and convergence. With the aid of Hu-Washizu principle and “assumed strain” method, it gives no volumetric locking for (nearly) incompressible materials and by setting some shear parameters β ij to about zero, “shear locking” can also be reduced or even eliminated for thin bodies in pure bending. It is economical since only ore-point quadrature is used to evaluate the element stiffness matrix. It is robust, accurate and efficient in numerical simulations for almost all kind of problems. It can provide good results not only for non-linear solids but also for thin bodies (plates and shells). At the end of this paper, several examples concerning the patch tests, large deflection of beams and plates as well as some metal forming processes are used to show the performance of this element.

Automatic adaptive remeshing for numerical simulations of metal forming

Abstract An automatic, adaptive remeshing scheme with a new, independent mesh has been developed to enable finite element simulations of forming processes with complicated die geometries. The different features of this technique are presented with one industrial example of forging. We especially focus on the fully automatic aspect of the procedure. The results are obtained with 8 node quadratic, isoparametric elements and 4 node mixed elements. For some practical applications, the implementation of this scheme is necessary to perform a complete analysis.

A mixed element method in gradient plasticity for pressure dependent materials and modelling of strain localization

This paper presents a mixed strain element method in gradient plasticity for pressure dependent materials at large strains and its application to the modelling of strain localization. The two yield strength parameters of strain hardening/softening materials not only depend on the internal state variable but also on its Laplacian. The evaluation of the Laplacian is based on a least square polynomial approximation of the internal state variable around each integration point. The present non-local approach allows to satisfy exactly the consistency condition in a pointwise fashion but in the non-local sense and at each iteration of a load step. To derive the consistent element formulations in the context of mixed finite element with one point quadrature and hourglass control and the non-local pressure dependent elasto-plasticity, a natural coordinate system in the stress space and a new definition of internal state variable are introduced. The capability of the present approach to preserve ellipticity as strain softening behaviour is incorporated into a computational model is demonstrated by the numerical results, which illustrate that pathological mesh dependence has been overcome.

Contaminant transport with non‐equilibrium processes in unsaturated soils and implicit characteristic Galerkin scheme

A non-equilibrium sorption—advection—diffusion model to simulate miscible pollutant transport in saturated–unsaturated soils is presented. The governing phenomena modelled in the present simulation are: convection, molecular diffusion, mechanical dispersion, sorption, immobile water effect and degradation, including both physical and chemical non-equilibrium processes. A finite element procedure, based on the characteristic Galerkin method with an implicit algorithm is developed to numerically solve the model equations. The implicit algorithm is formulated by means of a combination of both the precise and the traditional numerical integration procedures. The stability analysis of the algorithm shows that the unconditional stability of the present implicit algorithm is enhanced as compared with that of the traditional implicit numerical integration procedure. The numerical results illustrate good performance of the present algorithm in stability and accuracy, and in simulating the effects of all the mentioned phenomena governing the contaminant transport and the concentration distribution. Copyright