The knee joint is covered by cartilage, a resilient and smooth elastic tissue that protects the ends of long bones at the joints. It serves as a frictionless surface during movement. Cartilage is composed by a three-dimensional network of extracellular macromolecules called the extracellular matrix.
The macromolecules that compose this matrix are proteins adorned with sugars (the proteoglycans), collagen fibrils, that give structure, shape and tensile strength to the tissue, and water. The chondrocytes, the cartilage cells, are imbedded in this matrix, producing, maintaining, and renewing it. Optimal conservation of the matrix composition is crucial to preserve the biomechanical properties of articular cartilage, such as support, load distribution and to provide lubrication in the joints.