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Crepitus is "a grating sound or sensation produced by friction between bone and cartilage or the fractured parts of a bone".

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.
• Crepitus of tenosynovitis: From inflammation of the fluid-filled sheath (synovium) that surrounds a tendon.

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 erodes and the surfaces in the joint grind against one another, or when the two fractured surfaces of the broken bones rub together. Crepitus is a common sign of bone fracture.

Crepitus can easily be created and observed 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 force to produce this effect include the hallux (big toes), the knuckles and the neck joints. This phenomenon is caused when the movement of the joint lowers the pressure of its synovial fluid, causing the bubbles to form and burst. A refractory period of about 20 minutes exists where the joint cannot be stimulated to produce crepitus after being cracked until the nitrogen gas dissolves back into the synovial fluid.^[1]

In soft tissues, crepitus can be produced when gas is introduced into an area where it is normally not present.

The term can also be used when describing the sounds produced by lung conditions such as interstitial lung disease; these are also referred to as "rales". Crepitus is often loud enough to be heard by the human ear, although a stethoscope may be needed to detect instances caused by respiratory diseases.

In times of poor surgical practice, post-surgical complications involved anaerobic infection by Clostridium perfringens strains, which can cause gas gangrene in tissues, also giving rise to crepitus.

Subcutaneous crepitus (or surgical emphysema) is a crackling sound resulting from subcutaneous emphysema, or air trapped in the subcutaneous tissues.

• Cracking joints

• Richards RR, McKee MD (October 1989). "Treatment of painful scapulothoracic crepitus by resection of the superomedial angle of the scapula. A report of three cases". Clinical Orthopaedics and Related Research (247): 111–6. doi:10.1097/00003086-198910000-00019. PMID 2791379.
• Jiang CC, Liu YJ, Yip KM, Wu E (1993). "Physiological patellofemoral crepitus in knee joint disorders". Bulletin. 53 (4): 22–6. PMID 8829591.
• Kuhn JE, Plancher KD, Hawkins RJ (1998). "Symptomatic scapulothoracic crepitus and bursitis". The Journal of the American Academy of Orthopaedic Surgeons. 6 (5): 267–73. doi:10.5435/00124635-199809000-00001. PMID 9753753. S2CID 24203563.
• Dennis DA, Kim RH, Johnson DR, Springer BD, Fehring TK, Sharma A (January 2011). "The John Insall Award: control-matched evaluation of painful patellar Crepitus after total knee arthroplasty". Clinical Orthopaedics and Related Research. 469 (1): 10–7. doi:10.1007/s11999-010-1485-3. PMC 3008897. PMID 20706813.
• O'Connor, Anahad (December 15, 2014). "Why Do My Knees Make Noise When I Squat?". Ask Well. The New York Times.

• The dictionary definition of crepitus at Wiktionary