Theoretical Models of Micro-cracked Continua: Discontinuity of Scalar and Vector Fields

In order to represent discontinuities in the deformation of a material and its consequences on the energy dissipation during micro-crack kinetics, a model of micro-cracked continuum is derived. The Micro-crack density is represented in terms of the non-metric connection on a manifold. Torsion and curvature of the nonmetric connection represent a non-topological deformation and explicitly include mesoscopic discontinuities. The developed model includes both the non-equilibrium thermodynamic processes of micro-crack creation and the micro-crack growth. This approach contrasts to the empirical methodology of continuum mechanics that seeks a phenomenological description. An illustrative example of the model application is presented for the uniaxial vibration test, providing representations for wave propagation within a micro-cracked solid. The result of this example highlights the importance of rigorously revisiting the conservation laws in the framework of nonmetric connected manifolds

Non-linear viscoelastic laws for soft biological tissues

The development of a conceptual framework to test different viscoelastic constitutive laws is presented. This framework has the advantage of satisfying a priori the thermodynamic restrictions and is valid for large deformations. In addition, the different mechanical contributions are separated according to the time scale of their effects. As an illustration of its ability to model the immediate, short time memory and long time memory contributions, the framework is used to identify mechanical tests performed on human patellar tendons. The resulting viscoelastic law is then proposed to model the soft biological tissues as these tissues present an important viscoelastic behaviour. (C) 2000 Editions scientifiques et medicales Elsevier SAS

Can the increase of bone mineral density following bisphosphonates treatments be explained by biomechanical considerations?

OBJECTIVE: We hypothesized that bone mineral density increase following bisphosphonates treatments may be explained by the influence of the drug on the mechanical bone remodeling parameters. BACKGROUND: Patients treated with bisphosphonates continuously increase their bone mineral density. This increase is explained in the first 12-18 months following the treatment by the filling of the transient remodeling deficit. Recently, results of a clinical study of alendronate treatment over 7 years still show a continuous increase of bone mineral density. These results raised several questions regarding our understanding of bisphosphonates mode of action. METHODS: Bone remodeling is influenced by different factors including mechanical forces. In the present study, we propose then to consider the effect of bisphosphonates also under biomechanical considerations. RESULTS: Identification of the model with the clinical data showed that daily treatment of 10 and 20 mg alendronate decreased the bone turnover rate by 2% and 11%, respectively, in comparison with the 5 mg alendronate treatment. Moreover, the alendronate treatments decreases the resorption threshold stimulus by 19% (25%, 28%) for the 5 mg (10 and 20 mg, respectively) compared to placebo. CONCLUSIONS: The increase of bone mineral density following bisphosphonates treatment may then be explained by biomechanical considerations. Based on this description, bisphosphonates treatment may indeed change the susceptibility of bone to its biomechanical environment decreasing the mechanical threshold where bone should undergo resorption

On the independence of time and strain effects in the stress relaxation of ligaments and tendons

The hypothesis of variables separation, namely the time and the strain separation in the relaxation function, is widely used in soft tissue biomechanics. Although this hypothesis is central to several biomechanical models, only few experimental works have tried to verify it. From these studies, contradictory results have been found. Moreover, it has recently been noted that no such experimental verification has been performed for ligament tissues. In this paper, an experimental method is developed to test the hypothesis of variables separation. This method is then used with human cruciate ligaments and patellar tendons. It is shown that the use of the variables separation hypothesis is justified at least for strain values lower than 16% for anterior cruciate ligament, lower than 12% for posterior cruciate ligament and lower than 6% for patellar tendon. The method presented in this paper could be used to verify the validity of variables separation for other tissues

Biphasic constitutive laws for biological interface evolution

A model of tissue differentiation at the bone-implant interface is proposed. The basic hypothesis of the model is that the mechanical environment determines the tissue differentiation. The stimulus chosen is related to the bone-implant micromotions. Equations governing the evolution of the interfacial tissue are proposed and combined with a finite element code to determine the evolution of the fibrous tissue around prostheses. The model is applied to the case of an idealized hip prosthesis

Strain rate effect on the mechanical behavior of the anterior cruciate ligament-bone complex

Traction tests were performed on the bovine anterior cruciate ligament-bone complex at seven strain rates (0.1, 1, 5, 10, 20, 30, 40%/s). Corresponding stress-strain curves showed that, for a given strain level, the stress increased with the augmentation of the strain rate. This phenomenon was important since the stress increased by a factor of three between the tests performed at the lowest and highest strain rates. The influence of the strain rate was quantified with a new variable called the "supplemental stress". This variable represented the percentage of total stress due to the effect of strain rate. It was observed that at a strain rate of 40%/s, more than 70% of the stress in the ligament was due to the strain rate effect. In fact, the strain rate strongly affected the toe region, but did not influence the linear part of the stress-strain curves. The use of the linear tangent moduli was then not adequate to describe the strain rate effect in the anterior cruciate ligament-bone complex. This study showed that the "supplemental stress" was a synthetic and convenient variable to quantify the effect of the strain rate on the entire stress-strain curves. This quantification is especially important when comparing the mechanical behavior between anterior cruciate ligament and tissues used as ligament graft. (C) 1999 IPEM. Published by Elsevier Science Ltd. All rights reserved

The fixation of the cemented femoral component - Effects of stem stiffness, cement thickness and roughness of the cement-bone surface

After cemented total hip arthroplasty (THA) there may be failure at either the cement-stem or the cement-bone interface. This results from the occurrence of abnormally high shear and compressive stresses within the cement and excessive relative micromovement. We therefore evaluated micromovement and stress at the cement-bone and cement-stem interfaces for a titanium and a chromium-cobalt stem. The behaviour of both implants was similar and no substantial differences were found in the size and distribution of micromovement on either interface with respect to the stiffness of the stem. Micromovement was minimal with a cement mantle 3 to 4 mm thick but then increased with greater thickness of the cement. Abnormally high micromovement occurred when the cement was thinner than 2 mm and the stem was made of titanium. The relative decrease in surface roughness augmented slipping but decreased debonding at the cement-bone interface. Shear stress at this site did not vary significantly for the different coefficients of cement-bone friction while compressive and hoop stresses within the cement increased slightly

Effect of micromechanical stimulations on osteoblasts: development of a device simulating the mechanical situation at the bone-implant interface

Many experimental models have been developed to investigate the effects of mechanical stimulation of cells, but none of the existing devices can simulate micromotions at the cellular-mechanical interface with varying amplitudes and loads. Osteoblasts are sensitive to mechanical stimuli, so to study the bone-implant interface it would be important to quantify their reaction in a situation mimicking the mechanical situation arising at that interface. In this study, we present the development of a new device allowing the application of micromotions and load on cells in vitro. The new device allowed the cells to be stimulated with sinusoidal motions of amplitudes comprised between +/- 5 and +/- 50 microm, frequencies between 0.5 and 2 Hz, and loads between 50 and 1000 Pa. The device, with a total length of 20 cm, was designed to work in an incubator at 37 degrees C and 100% humidity. Expression of various bone important genes was monitored by real-time RT-PCR. Micromotions and load were shown to affect the behavior of osteoblasts by down-regulating the expression of genes necessary for the creation of organic extracellular matrix (collagen type I) as well as for genes involved in the mineralization process (osteocalcin, osteonectin). The developed device could then be used to simulate different mechanical situations at the bone-implant interface