Cartilage is a type of dense connective tissue. It is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix composed of collagen fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components. Cartilage is found in many places in the body including the articular surface of the bones, the rib cage, the ear, the nose, the bronchial tubes and the intervertebral discs. Its mechanical properties are intermediate between bone and dense connective tissue like tendon. Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are fed by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.
Hyaline cartilage is a rather hard, translucent material rich in collagen and proteoglycan. It covers the end of bones to form the smooth articular surface of joints. It is also found in the nose, the larynx and between the ribs and the sternum. Bones grow via a hyaline cartilage intermediate, a process called Endochondral ossification. Elastic cartilage contains large amounts of elastic fibers (elastin) scattered throughout the matrix. It is stiff yet elastic, and is important to prevent tubular structures from collapsing. Elastic cartilage is found in the pinna of the ear, in tubular structures such as the auditory (Eustachian) tubes and in the epiglottis. Fibrocartilage is the most common form of by weight. It is characterized by a dense network of Type I collagen. It is a white, very tough material that provides high tensile strength and support. It contains more collagen and less proteoglycan than hyaline cartilage. Thus, its properties are closer to those of tendon than hyaline cartilage. It is present in areas most subject to frequent stress like intervertebral discs, the symphysis pubis and the attachments of certain tendons and ligaments.
Medically speaking, the "cartilage" is actually known as the meniscus. The meniscus is a C-shaped piece of fibrocartilage which is located at the peripheral aspect of the joint. The majority of the meniscus has no blood supply. For that reason, when damaged, the meniscus is unable to undergo the normal healing process that occurs in most of the rest of the body. In addition, with age, the meniscus begins to deteriorate, often developing degenerative tears. Typically, when the meniscus is damaged, the torn piece begins to move in an abnormal fashion inside the joint. Because the space between the bones of the joint is very small, as the abnormally mobile piece of meniscal tissue (meniscal fragment) moves, it may become caught between the bones of the joint (femur and tibia). When this happens, the knee becomes painful, swollen, and difficult to move.