Abstract
Cartilage and intervertebral disc (IVD) are examples of charged hydrated soft tissues consisting of three phases: a water phase, a solid phase (e.g., the collagen-proteoglycan matrix) and an ion phase (e.g., NaCl). It has been known for many years that the mechanical and swelling behaviors of these tissues are affected by their water content. For example, both theoretically and experimentally, we know that the hydraulic permeability and the aggregate modulus are related to the tissue porosity, density, and fixed charge density [1–6]. These intrinsic tissue properties will also influence other mechanical behaviors of the tissue, such as creep and stress-relaxation. To quantify these types of functional dependence, a triphasic constitutive law was developed to describe the mechanical and physicochemical behaviors of charged-hydrated soft tissues [6]. In this theory, it was postulated that the Helmholtz free energy functions depend on the apparent densities of the constituents which are related to the porosity of the tissue and true density of the constituents. However, the true porosity (water volume per unit volume of tissue) and the true density of the solid matrix (solid mass per unit volume of solid) have never been measured Thus the objective of this investigation is to develop an accurate method to measure the volumes of such tissues and of its solid matrix. From these measured volumes the true porosity and true density of the solid matrix can be calculated.