THE EFFECTS OF A SUBCHONDRAL BONE PERFORATION ON THE LOAD SUPPORT MECHANISM WITHIN ARTICULAR-CARTILAGE

Impact loading of diarthrodial joints of skeletally mature animals can crack the zone of calcified cartilage and subchondral bone (ZCC-SB), often without immediate disruption to the joint surface. A common sequ elae to transarticular impact is the development of osteoarthritis (OA ) in the traumatized joint. An understanding of how and why damage to cartilage occurs and progresses in deep regions of cartilage would the refore increase our understanding of the etiology of this type of trau matic OA. In this study, a 2D linear biphasic finite element analysis was performed to determine articular cartilage deformations, and inter stitial fluid flow and pressurization, under both uniform and non-unif orm hydrodynamic pressure distributions. The crack in the ZCC-SB was m odelled by a ''moderate-size'' perforation at which there is no load s upport or fluid pressurization (i.e. a free surface under a free-drain ing condition). The effects of such a defect on the cartilage solid ma trix stresses and strains, and interstitial fluid flows and pressures have been calculated. Results show that in normal cartilage, consisten t with our previous contact analysis (G.A. Ateshian, W.M. Lai, W.B. Zh u and V.C. Mow, A biphasic model for contact in diarthrodial joints, A dv. Bioeng., ASME, 22 (1992) 191-194), interstitial fluid pressurizati on plays a dominant role in providing support when cartilage is loaded . Hence, normally, the solid matrix of cartilage is shielded from the high stresses of joint loading. In cartilage with a perforation in the ZCC-SB, however, the solid matrix stresses and strains are significan tly increased as a consequence of a diminished fluid pressurization in the region of the defect. Thus, to maintain local equilibrium, there is a transfer of internal load support from interstitial fluid pressur e to solid matrix stress defeating the normal stress-shielding effect. This stress transfer suggests a mechanism for progressive solid matri x damage, thus increasing the likelihood of cartilage failure followin g a single episode of impact.