Experimental study of eigenstrains in temporomandibular joint discs using digital image analysis.

The temporomandibular joint is one of the most frequently used joints of the human body. Its malfunction can severely influence patient's well-being. Since the temporomandibular joint disc plays a major role in its functioning, especially in load distribution within the joint, it appears to be a crucial element to understand. This paper aims to improve understanding of the tissues within close in vivo conditions (i.e. hydrated at 37 ° C) by (i) comforting literature by revealing the presence of residual stresses within the temporomandibular joint disc, (ii) quantifying eigenstrains through a relaxation process and finally (iii) evaluating the internal mechanical state in intact temporomandibular joint discs central part, considering the tissue as a thin layer. Both global specimen size measurements and local digital image correlation were used to quantify 6 samples' deformation through a detailed analysis of approximately 30 images, recorded for approximately one hour, per disc. Thanks to a backward time approach combined to an analytical model, eigenstrains were assessed on discs. For the first time, the presence of complex initial strain fields within cylindrical specimens of porcine temporomandibular joint discs was quantified, confirming indications from literature. Digital image analysis revealed the partial internal stress release through specimen self-deformation. Close to zero in central part, it reached approximately 13% radial strain in the outer ring within a characteristic relaxation time close to 530s. The principal strains' distribution agrees with the alignment of the collagen fibers in the central part of the discs revealed in many works. It led to deduce that, in the central area of the discs, the matrix undergoes a radial compression within physiological conditions to compensate the daily loading stresses. Therefore, this work improves understanding of the tissues in vivo conditions highlighting extraction cut effect on temporomandibular joint disc's tissues mechanical state.

Validation of a new fluidic device for mechanical stimulation and characterization of microspheres: A first step towards cartilage characterization.

Articular cartilage is made of chondrocytes surrounded by their extracellular matrix that can both sense and respond to various mechanical stimuli. One of the most widely used in vitro model to study cartilage growth is the model of mesenchymal stromal cells-derived cartilage micropellet. However, mechanical stimulation of micropellets has never been reported probably because of their small size and imperfect round shape. The objective of the study was to develop an original custom-made device allowing both the mechanical stimulation and characterization of cartilage micropellets. The fluidic-based device was designed for the concomitant stimulation or characterization of six microspheres placed into the conical wells of a tank. In the present study, the device was validated using alginate-, collagen- and crosslinked collagen-based microspheres. Different types and ranges of pressure signals (square, sinusoidal and constant) were applied. The mechanical properties of microspheres were equivalent to those determined by a conventional compression test. Accuracy, repeatability and reproducibility of all types of pressure signals were demonstrated even though square signals were less accurate and sinusoidal signals were less reproducible than the others. The interest of this new device lies in the reliability to mechanically stimulate and characterize microspheres with diameters in the range of 900 to 1500 µm. Mechanical stimulation can be performed on six microspheres in parallel allowing the mechanical and molecular characterization of the same group of cartilage micropellets. The device will be useful to evaluate the growth of cartilage micropellets under mechanical stimuli.

Mesenchymal stem cells-derived cartilage micropellets: A relevant in vitro model for biomechanical and mechanobiological studies of cartilage growth.

The prevalence of diseases that affect the articular cartilage is increasing due to population ageing, but the current treatments are only palliative. One innovative approach to repair cartilage defects is tissue engineering and the use of mesenchymal stem/stromal cells (MSCs). Although the combination of MSCs with biocompatible scaffolds has been extensively investigated, no product is commercially available yet. This could be explained by the lack of mechanical stimulation during in vitro culture and the absence of proper and stable cartilage matrix formation, leading to poor integration after implantation. The objective of the present study was to investigate the biomechanical behaviour of MSC differentiation in micropellets, a well-defined 3D in vitro model of cartilage differentiation and growth, in view of tissue engineering applications. MSC micropellet chondrogenic differentiation was induced by exposure to TGFß3. At different time points during differentiation (35 days of culture), their global mechanical properties were assessed using a very sensitive compression device coupled to an identification procedure based on a finite element parametric model. Micropellets displayed both a non-linear strain-induced stiffening behaviour and a dissipative behaviour that increased from day 14 to day 29, with a maximum instantaneous Young's modulus of 179.9 ± 18.8 kPa. Moreover, chondrocyte gene expression levels were strongly correlated with the observed mechanical properties. This study indicates that cartilage micropellets display the biochemical and biomechanical characteristics required for investigating and recapitulating the different stages of cartilage development.

Heterogeneous mechanical hyperelastic behavior in the porcine annulus fibrosus explained by fiber orientation: An experimental and numerical approach.

Our aim is to estimate regional mechanical properties of the annulus fibrosus (AF) using a multi-relaxation tensile test and to examine the relevance of using the transverse dilatations in the identification procedure. We collected twenty traction specimens from both outer (n = 10) and inner (n = 10) sites of the anterior quadrant of the annulus fibrosus of one pig spine. A 1-h multi-relaxation tensile test in the circumferential direction allowed us to measure the force in the direction of traction and the dilatations in all three directions. We performed a specific-sample finite element inverse analysis to identify variations, along the radial position, of material and structural parameters of a hyperelastic compressible and anisotropic constitutive law. Our experimental results reveal that the outer sites are subjected to a significantly greater stress than the inner sites and that both sites exhibit an auxetic behavior. Our numerical results suggest that the inhomogeneous behavior arises from significant variations of the fiber angle taken into account within the hyperelastic constitutive law. In addition, we found that the use of the measured transverse dilatations in the identification procedure had a strong impact on the identified mechanical parameters. This pilot study suggests that, in quasi-static conditions, the annulus fibrosus may be modeled by a hyperelastic compressible and anisotropic law with a fiber angle gradient from inner to outer periphery.