Towards a Multi-Scale Model of Cartilage: Coarse-Graining Glycosaminoglycans

Background: Articular cartilage is an avascular tissue that serves to reduce the frictional load between contacting bones in load bearing joints. The tissue consists of a sparse population of chondrocytes (≈ 2% by volume) embedded in an extracellular matrix (ECM). Aggrecan and type-II-collagen are the two primary macromolecules constituting the ECM and play distinct yet complementary biomechanical roles: The swelling pressure induced by the highly charged comb polymer aggrecan is balanced by tensile stresses in the interconnected, fibrillar scaffold formed by collagen [2]. Prior to the onset of osteoarthritis, an incurable and debilitating joint disease that affects more than 30 million Americans, aggrecan undergoes distinct changes in its molecular composition. Primary alterations include changes in the molecular weight and sulfation pattern of the anionic glycosaminoglycans that decorate its protein backbone [4]. Concurrently, the macroscopic biomechanical properties of cartilage are modified, manifested by changes in matrix permeability, viscosity, and elastic moduli [3].