Cervical ribs are theribs of theneck in manytetrapods. In most mammals, including humans, cervical ribs are not normally present as separate structures. They can, however, occur as a pathology. In humans, pathological cervical ribs are usually not of clinical concern, although they can cause a form ofthoracic outlet syndrome.

Like other ribs, the cervical ribs form byendochondral ossification.^[1]

The cervical ribs ofsauropoddinosaurs were extended by ossifiedtendons,^[2] and could reach exceptional lengths; a cervical rib ofMamenchisaurus sinocanadorum was 4.2 metres (14 ft) long.^[3]

Inbirds, the cervical ribs are small and completely fused to the vertebrae.

Intherianmammals, the cervical ribs fully fuse with thecervical vertebrae to form part of thetransverse processes, except in rare pathological cases. In contrast,monotremes retain the plesiomorphic condition of having separate cervical ribs.^[4]

Cervical rib
Other names	Neck ribs[5]
Specialty	Thoracic surgery

Acervical rib in humans is anextrarib which arises from the seventhcervical vertebra. Their presence is acongenital abnormality located above the normal first rib. A cervical rib is estimated to occur in 0.2%^[6] to 0.5%^[7] (1 in 200 to 500) of the population.^[8] People may have a cervical rib on the right, left or both sides.^[9][10]

Most cases of cervical ribs are not clinically relevant and do not have symptoms;^[11][12] cervical ribs are generally discoveredincidentally, most often duringx-rays andCT scans.^[7][8][10] However, they vary widely in size and shape,^[7] and in rare cases, they may cause problems such as contributing tothoracic outlet syndrome,^[11] because of pressure on thenerves that may be caused by the presence of the rib.^[11][13]

A cervical rib represents a persistentossification of the C7 lateral costal element.^[11][12] During early development, this ossified costal element typically becomes re-absorbed. Failure of this process results in a variably elongated transverse process or complete rib that can be anteriorly fused with the T1 first rib below.^[14]

On imaging, cervical ribs can be distinguished because their transverse processes are directed inferolaterally, whereas those of the adjacent thoracic spine are directedanterolaterally.^[15]

The presence of a cervical rib can cause a form ofthoracic outlet syndrome due to compression of the lower trunk of thebrachial plexus orsubclavian artery.^[11] These structures become encroached upon by the cervical rib andscalene muscles.

Compression of the brachial plexus may be identified by weakness of the muscles in the hand, near the base of the thumb. Compression of the subclavian artery is often diagnosed by finding a positiveAdson's sign on examination, where the radialpulse in the arm is lost duringabduction andexternal rotation of theshoulder.^[11] A positive Adson's sign is non-specific for the presence of a cervical rib however, as many individuals without a cervical rib will have a positive test. Compression of the sympathetic chain may causeHorner's syndrome.

Recent studies have also found a high percent of cervical ribs in woolly mammoths. It is believed that the decline in mammoth numbers may have forced inbreeding within the species which in turn had increased the number of mammoths being born with cervical ribs. Cervical ribs have been connected withleukaemia in human children, so it has given scientists new evidence to believe that the mammoth's extinction was attributed to the condition.^[16] They have also been interpreted as a sign ofinbreeding depression in other mammals, such asMegaloceros.^[17]

• Supernumerary body parts
• Congenital disorders of musculoskeletal system
• Human anatomy
• Anatomical variations

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