Les distributions de potentiel electrique dans le cartilage articulaire

The main goal of this project was to contribute to the development of a medical device used in conjunction with standard arthroscopic procedures for the functional evaluation of articular cartilage. The diagnostic provided by the device is based on the analysis of the electrical potentials induced during a light manual indentation of the cartilage (TM)surface. This device will soon be commercialized under the trade name Arthro-BST. Its main components are: a computer with acquisition and analysis software, an electrical isolation box, a handle and sterile disposable tips. The first objective was to participate in all development steps of this medical device and in particular to design the sterile disposable tips. The first hypothesis was that it is possible to manufacture an array of microelectrodes on the non-planar surface at the extremity of the indenter.; The second objective was to evaluate the effect of experimental conditions on the measured electric potential distribution. The first hypothesis was that the speed of compression can influence the electromechanical response of the cartilage to spherical indentation. Using a mechanical tester, cartilage samples were indented at compression speeds in the range of 10 to 10000 mum/s and the resulting load and distribution of electric potential were measured. The maximum values of the load and electric potential increased with speed up to 500 mum/s and then leveled off at higher speeds. Load and electric potential revealed similar dependence on speed, supporting poroelastic mechanisms at play in articular cartilage over the entire range of studied speeds. Above 500 mum/s, the cartilage behaved in a purely elastic manner. The second hypothesis was that the salt concentration of the bath can influence the electromechanical response of the cartilage to spherical indentation. Reduction in salt concentration induced a slight increase in the load, but led to more significant increases in electric potential due to reduced electrostatic shielding of fixed charges by fewer mobile ions. These experiments also highlighted certain characteristics of the electric potential distribution at the surface of the indenter. In addition to the expected negative electric potentials due to the presence of fixed negative charge in the cartilage matrix, these measurements also detected positive electric potentials extending at least I mm from the cartilage surface into the external saline bath. The existence of this positive potential has not been mentioned or hypothesized in the literature and our measurements are the first to detect it. The ratio of the maximum positive potential to the maximum negative potential was about 5% in physiological saline concentration, while it reached more than 50% in hypotonic solutions.; The third objective was to develop a simple electromechanical model for the description of the electric potential distributions measured with the instrument. With this goal in mind, the predictions of a forced convection model through hydrated charged membranes were analysed in a generalized fashion. The resulting model also allows the approximation of the local macroscopic electric field once the local interstitial fluid velocity and fixed charge density are known. This uncoupling of the mechanical and electric components of the problem, along with the neglect of the contribution of the ion redistribution and assuming zero total electrical current density, allowed the approximation of the macroscopic electric field to be validated in confined compression before being applied to spherical indentation. Without any fitted parameters, the model correctly predicted measurements (from the literature) of the electric potential difference as a function of frequency and saline concentration in confined compression. Such agreement of experimental measurements with the predictions of a model had not been previously reported. At this point, the macroscopic approximation of the electric variables...