Cartilage tissue, with its special properties, ensures that joints function smoothly. When the cushioning and elasticity in articular cartilage decrease due to accidents or wear and tear, the importance of articular cartilage becomes noticeable.
What is articular cartilage?
Schematic diagram difference between healthy joint, arthritis and osteoarthritis. Click to enlarge. Cartilage tissue is an essential component of the joint, covering the joint surfaces as articular cartilage (Latin: cartilago articularis). It possesses compressive elastic and flexural properties and is characterized by high tear resistance. For this reason, the term elastic cartilage is used as a synonym for articular cartilage. Together with the adjacent structures, which include the synovium, synovial fluid and joint capsule, it ensures that the joints function properly. Because of its milky and translucent structure, it is called hyaline cartilage (hyalos: glass). This form of cartilage is most commonly found in the body, for example in the hip, knee or ankle.
Anatomy and structure
Articular cartilage is not connected to the vascular system. It is nourished by the joint fluid (synovial fluid). This covers the surface of the articular cartilage and additionally reduces friction during joint movement. The main substance of the cartilage tissue is the gelatinous cartilage substance: the extracellular matrix. It is composed of so-called glycosaminoglycans (GAG), which provide the high elasticity. Glycosaminoglycans are polysaccharides, sugar chains made up of many disaccharides (double sugars). They are able to absorb a lot of water and thus ensure swelling of the matrix. At the same time, the water absorbs nutrients for the cartilage cells embedded in the cartilage tissue, the chondrocytes. Another component of the matrix is collagen. This is a long protein molecule through which the cartilage acquires its shape and stability. The cartilage cells take up five to ten percent of the volume in the cartilage and are responsible for the formation of new collagens and glycosaminoglycans as well as their degradation. Water makes up about 70 percent of the cartilage substance.
Function and tasks
Articular cartilage plays a key role in joint function. The thickness of the cartilage layer differs depending on the size of the joints and the loads to which they are subjected. Finger joints have a cartilage layer 0.5 millimeters thick, while at the knee joint it is five millimeters. The articular cartilage acts as a cushioning system for the bones underneath. The damping properties can be controlled by the amount of water absorbed into the cartilage substance. In this way, the cartilage adjusts to different loads. This ability is supported by the elastic tissue. Forces equivalent to five to seven times the patient’s own body weight can be transmitted by the articular cartilage. With increasing age, the ability to store water and the content of elastic tissue components decrease, and the resilience and pressure elasticity of the cartilage decline. The joint ends close smoothly due to the articular cartilage. In interaction with the synovial fluid, it ensures low-friction sliding of the joint surfaces against each other during movement.
Diseases and complaints
Pathological changes in articular cartilage are called chondropathies. Causes of these diseases include accidents (traumatic causes), wear and tear (degenerative), and inflammation. Because of the absence of nerves and blood vessels, they are initially painless. Any joint can be affected by damage or disease, but cartilage damage is most commonly found in the knee joint, shoulder joint, and hip joint. Acute causes of cartilage damage include accidents in which short-term forces act on the articular cartilage that exceed the load limit. Irreversible cartilage damage can result. Young people are frequently affected. Osteochondrosis dissecans (also: osteochondral lesion) is also diagnosed as a cause of cartilage damage in younger patients, predominantly males. In this disease, bones close to the joint die, damaging the overlying cartilage in the process and can cause osteoarthritis.The knee is affected in 75 percent of cases. Osteoarthritis is a wear and tear disease of the joints that results from overloading and slow wear and tear. If the cartilage is not supplied with sufficient nutrients, arthrosis can also develop. The progressive change in cartilage structure results in joint deformities. In terms of overloading, knock knees or bow legs can place too much stress on the articular cartilage due to uneven load distribution. In chondrocalcinosis (pseudogout), calcium crystals are found in the articular cartilage, primarily in the knee, hand and hip, which can lead to painful inflammation in the joint. In rare cases, cartilage is damaged by infection during surgery. Polychondritis (Greek; poly: much; chondros: cartilage; -itis: for inflammatory) refers to rare inflammatory conditions that can be relapsing or permanent. In chondromalacia, there is a softening of the articular cartilage accompanied by inflammation. It is an autoimmune disease in which the body’s own defenses are mistakenly directed against its own tissues.
