Articular cartilage, consisting of proteoglycan (PG) molecules and collagen fibrils as well as interstitial fluid, is a soft tissue covering the articulating surfaces of bones in diarthrodial joints. It plays a vital role in joint articulation by providing a nearly frictionless bearing mechanism with excellent mechanical durability, which can be attributed to the tissue's viscoelastic characteristics. It has been well documented that abnormal mechanical stresses in diarthrodial joints can cause mechanical damage in the structural integrity of articular cartilage. Once mechanically damaged, the cartilage usually fails to heal and recover its mechanical function due to a lack of a primary spontaneous repair mechanism, thus leading to degenerative joint disease or osteoarthritis (OA). An indication of such degenerative disease is usually manifested by a rupture of the dense collagen meshwork, a decrease in PG content of the tissue, and an increase in interstitial water content. As a result, articular cartilage becomes softer and loses its normal mechanical properties, resulting in further deterioration. Understanding the mechanical characteristics of articular cartilage is critically important in order to improve our knowledge about the normal and abnormal behaviors of articular cartilage as well as the treatment of disabilities caused by damaged cartilage. Therefore, the long-term objective of this research is to understand the mechanical characteristics of articular cartilage using an experimentally validated mathematical model.
The mechanical characteristics of articular cartilage depend upon the structural integrity of its molecular constituents and their interactions. There have been significant efforts in mathematical modeling of articular cartilage reported in the literature, which accounted for such interactions between the constituents of the tissue. The current paradigm in cartilage biomechanics is based on the model assumption that articular cartilage is a binary mixture of a porous, purely elastic solid matrix (PG and collagen) and an inviscid interstitial fluid. This assumption postulates that the viscoelastic behavior of articular cartilage is solely governed by a diffusive friction caused by the relative motion between the fluid and solid phases, which is termed the "fluid flow-dependent viscoelastic mechanism". However, there has recently been strong evidence that articular cartilage exhibits significant viscoelastic behaviors in the absence of the apparent flow of interstitial fluid within the tissue matrix, which is termed the "fluid- flow-independent viscoelastic mechanism". Furthermore, the investigator's recent preliminary studies have indicated that the overall mechanical behavior of articular cartilage is largely governed by complex coupling phenomena of the fluid flow-dependent and fluid flow-independent viscoelastic mechanisms. Despite its strong evidence, however, the role of such coupled viscoelastic mechanism has not been fully explored, nor appreciated. Therefore, the short-term objective of the present proposal is to understand the two distinct viscoelastic mechanisms - i.e., the fluid flow-dependent and fluid flow-independent viscoelastic mechanisms - of articular cartilage under various loading conditions using extensive mathematical/ computational modeling and its experimental validation.
Understanding the relative contribution of each (fluid flow-dependent or fluid flow-independent) viscoelastic mechanism is essential in developing a fundamental mechanical theory of articular cartilage, which will eventually help to develop various analytical tools for normal and pathological articular cartilage. The outcome of the proposed research will also be useful for accurate understanding of mechanical behaviors and damage tolerance of normal and pathological cartilage. This information will improve our knowledge about the underlying mechanisms resulting in the degeneration of articular cartilage and lead us toward an improved treatment for damaged cartilage. Furthermore, the proposed study will help to advance current knowledge of the biomechanics of other soft tissues including tendon, ligament, muscle, brain, etc., since most of the soft tissues in the animal body share a similar structural composition. They are composed of cells, porous viscoelastic ground substance (extracellular matrix or EMC) and interstitial fluid.
