A SAMPLE-SPECIFIC COMPUTATIONAL MODEL OF ARTICULAR CARTILAGE BASED ON MRI, HISTOLOGY, COMPUTER VISION AND MECHANICAL TESTING

In a typical diarthrodial joint, like the human knee joint, t he opposing bones are covered with a layer of dense connective tissue known as articular cartilage, which provides articulating surfaces. Articular cartilage has a composition and mechanical structure well-suited to the required functions: (i) to provide a compliant, low-friction surface between the relatively r igid bones in diarthrodial joints, (ii) to provide a long-wearing and resilient surface, and (iii) to distribu te the contact pressure to the underlying bone structure. To meet these demands, articular cartilage cont ains a fluid phase of H 2O and electrolytes (approximately 68% to 85%), and a solid phase composed of chondrocytes, type I and II collagen fibers, proteoglycans and other glycoproteins (cf. [1]). Within the cartilage, fibers of predominantly type II collag en exhibit a high level of structural organization and provide tensile reinforcement to the solid phase, a proteoglycan gel. The collagen fibers support only tension and accommodate essentially no resistance to compression. Within the cartilage, three basic zones of collagen fiber orientation exist. Start ing from the surface, the superficial tangent zone (comprising approximately 10-20% of the total thickness) has fibers which are tangential to the articular surface. Next, the middle zone (40-60%) has fibers which are isotropically distributed and oriented. Finally, near the transition to subchondral bone, th e deep zone (approximately 30%) has fibers which are oriented perpendicular to the aforementioned surface. Clearly some simplifying assumptions are required to facilitate computational modeling and numerical simulation. By considering only two main components; a fluid and a solid embedded with type II collagen fibers, articular cartilage may be considered a biphasic fiber reinforced material [1]. Additionally, due in part to the interaction of solid and fluid, articular ca rtilage exhibits a time-dependent stress-strain response.