THE BIPHASIC POROVISCOELASTIC BEHAVIOR OF ARTICULAR-CARTILAGE - ROLE OF THE SURFACE ZONE IN GOVERNING THE COMPRESSIVE BEHAVIOR

Surface fibrillation of articular cartilage is an early sign of degene rative changes in the development of osteoarthritis. To assess the inf luence of the surface zone on the viscoelastic properties of cartilage under compressive loading, we prepared osteochondral plugs from skele tally mature steers, with and without the surface zone of articular ca rtilage, for study in the confined compression creep experiment. The r elative contributions of two viscoelastic mechanisms, i.e. a flow-inde pendent mechanism [Hayes and Bodine, J. Biomechanics 11, 407-419 (1978 )], and a flow-dependent mechanism [Mow et al. J. biomech. Engng 102, 73-84 (1980)], to the compressive creep response of these two types of specimens were determined using the biphasic poroviscoelastic theory proposed by Mak. [J. Biomechanics 20, 703-714 (1986)]. From the experi mental results and the biphasic poroviscoelastic theory, we found that frictional drag associated with interstitial fluid flow and fluid pre ssurization are the dominant mechanisms of load support in the intact specimens, i.e. the flow-dependent mechanisms alone were sufficient to describe normal articular cartilage compressive creep behavior. For s pecimens with the surface removed, we found an increased creep rate wh ich was derived from an increased tissue permeability, as well as sign ificant changes in the flow-independent parameters of the viscoelastic solid matrix. From these tissue properties and the biphasic porovisco elastic theory, we determined that the flow-dependent mechanisms of lo ad support, i.e. frictional drag and fluid pressurization, were greatl y diminished in cartilage without the articular surface. Calculations based upon these material parameters show that for specimens with the surface zone removed, the cartilage solid matrix became more highly lo aded during the early stages of creep. This suggests that an important function of the articular surface is to provide for a low fluid perme ability, and thereby serve to restrict fluid exudation and increase in terstitial fluid pressurization. Thus, it is likely that with increasi ng severity of damage to the articular surface, load support in cartil age under compression shifts from the flow-dependent modes of fluid dr ag and pressurization to increased solid matrix stress. This suggests that it is important to maintain the integrity of the articular surfac e in preserving normal compressive behavior of the tissue and normal l oad carriage in the joint.