In adults, long bones surfaces are covered with cartilage made of special cells called chondrocytes (chondros: cartilage, citos: cell). This structure allows the articulation to move, avoiding friction and bone debilitation. In normal conditions, there is a homeostatic balance between the synthesis and the degradation of the cartilage organic matrix components.

The so peculiar biomechanical properties of the cartilage depend on its extracellular matrix or fundamental substance which surrounds it and where the chondrocytes are submerged. Main components of the fundamental substance are water, different types of collagen and proteoglycans.

The cartilage proteoglycans are made of the combination of different types of proteins and polysaccharides or complex sugars such as chondroitin sulfate, dermatan sulfate, glucosamine sulfate, aggrecan and hyaluronic acid. Nature, type and quantity of proteoglycans change with age and diseases.

It is important to underline that these elements not only have a function on the structure but they also stimulate the production of new collagen and proteoglycans in the extracellular matrix of the joint cartilage.