8th. World Congress on Computational Mechanics (WCCM8) 5th European Congress on Computational Methods in Applied Sciences and Engineering (ECCOMAS 2008) June 30 –July 5, 2008 Venice, Italy A SAMPLE-SPECIFIC COMPUTATIONAL MODEL OF ARTICULAR CARTILAGE BASED ON MRI, HISTOLOGY, COMPUTER VISION AND MECHANICAL TESTING *David M. Pierce 1 , Werner Trobin 2 , Horst Bischof 2 , Siegfried Trattnig 3 , Gerhard A. Holzapfel 1 1 Graz University of Technology Institute for Biomechanics Center for Biomed. Engineering Kronesgasse 5-I, A-8010 Graz {pierce|holzapfel}@TUGraz.at www.biomech.tugraz.at 2 Graz University of Technology Institute for Computer Graphics and Vision, Inffeldgasse 16-II A-8010 Graz, Austria {trobin|bischof}@icg.tugraz.at www.icg.tu-graz.ac.at 3 Medical University of Vienna Center of Excellence for High Field MR Department of Radiology Vienna, Austria siegfried.trattnig@univie.ac.at Key Words: Articular Cartilage, Biomechanics, Constitutive Modeling, Finite Element Methods. ABSTRACT In a typical diarthrodial joint, like the human knee joint, the 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 rigid bones in diarthrodial joints, (ii) to provide a long-wearing and resilient surface, and (iii) to distribute the contact pressure to the underlying bone structure. To meet these demands, articular cartilage contains a fluid phase of H 2 O 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 collagen exhibit a high level of structural orga- nization 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. Starting 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 ori- ented. Finally, near the transition to subchondral bone, the 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 colla- gen fibers, articular cartilage may be considered a biphasic fiber reinforced material [1]. Additionally, due in part to the interaction of solid and fluid, articular cartilage exhibits a time-dependent stress-strain response. In light of the complexity of cartilage constitutive modeling, the motivation of our study is as follows: (i) to quantitatively relate the material properties and mechanical response of articular cartilage to the