Crepitus/ˈkrɛpɪtəs/ (also termed crepitation) is a medical term to describe the grating, crackling or popping sounds and sensations experienced under the skin and joints or a crackling sensation due to the presence of air in the subcutaneous tissue.
Various types of crepitus that can be heard in joint pathologies are:
Bone crepitus: This can be heard when two fragments of a fracture are moved against each other.
Joint crepitus: This can be obtained when the affected joint is passively moved with one hand, while the other hand is placed on the joint to feel the crepitus.
Crepitus of bursitis: This is heard when the fluid in the bursa contains small, loose fibrinous particles.
Look up crepitus in Wiktionary, the free dictionary.
The sound can be created when two rough surfaces in an organism's body come into contact—for example, in osteoarthritis or rheumatoid arthritis when the cartilage around joints has eroded away and the surfaces in the joint start to grind against one another, or when the fracture surfaces of two broken bones rub together. Crepitus is a common sign of bone fracture.
Crepitus can easily be introduced by exerting a small amount of force on a joint, thus 'cracking it'. This is caused by bubbles of nitrogen forming in the synovial fluid bursting. Almost every joint in the body can be 'cracked' in this way, but the joints which require the least amount of effort include the hallux, knuckles and neck joints.
In soft tissues, crepitus can be produced when gas is introduced into an area where it normally is not present.
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Jiang CC, Liu YJ, Yip KM, Wu E (1993). "Physiological patellofemoral crepitus in knee joint disorders". Bulletin53 (4): 22–6. PMID8829591.