Illustrated Encyclopedia of Human Anatomic Variation: Opus V: Skeletal Systems: Thorax

Ribs

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

The number of ribs may be increased by addition at either the cervical or lumbar end of the series. It is extremely rare to find an additional rib or pair of ribs in both the cervical and lumbar regions at the same time. Eight true ribs have been observed. The first rib may be shorter than usual, slender and connected with the second costal cartilage. The number of ribs may also be decreased. The first rib is occasionally absent. It may be absent on one (or both sides) and shortened or hypoplastic on the other side. The association of twelfth rib absence (14-20%) and/or hypoplasia (13%) with myelomeningocele has been reported by Wells et al. A case of incomplete development of the third and fourth ribs, and the absence of ribs eight, nine, and ten has been reported. In another subject, ribs two, three, four, and five were absent.

The twelfth rib is variable in length; it may be markedly reduced in size, or as long as 14 cm.

Differences in length are important considerations in the surgical approach to the kidney. The twelfth rib may be absent on one side and shortened on the other side. Asymmetric ribs are common.

A bicipital rib is seen in relation to the first thoracic rib. It appears to be the result of the fusion of two ribs, either of a cervical and first thoracic or of the first two thoracic ribs. Fusion of the first two ribs is common.

Unusual variations include the following: The costal cartilage and adjacent portions of the body of the rib are occasionally replaced by fibrous tissue. The first rib may be ossified at its extremities but fibrous between. The scapula may be fixed to the first rib by a short costocoracoid ligament. The ligament is a thickening of the clavipectoral fascia and is attached to the coracoid process of the scapula and is inserted onto the conoid tubercle and trapezoid line of the clavicle. The ligament may be cartilagenous and has the potential to ossify. Two adjacent ribs may be completely fused, or the bodies of two or more ribs may be joined by fusion due to small quadrilateral plates between the bones. Ribs bifurcated at their sternal ends are occasionally observed, with the two extremities joined to a bifid costal cartilage. Bifurcated ribs occur in six of 14,000 cases in one study. The tenth rib may be a floating rib ( a frequent observation by Japanese investigators). Two adjacent ribs, often five and six, and six and seven, sometimes articulate at their costal cartilages by a diarthrosis, synarthrosis, or syndesmosis.

Cervical ribs occur in about 0.5-1% of cadavers. They tend to be small but occasionally reach the sternum. The ventral extremity may (a) lie free above or between the scalene muscles, (b) be connected to the sternum by a ligamentous prolongation, (c) articulate with the superior surface of the first thoracic rib, or (d) form a complete rib that articulates with the sternum. Because of their location, cervical ribs may cause a disturbance to the vascular or nerve supply to the arm, forearm, and hand.

Double cervical ribs have been reported.

A cervical rib may pierce m. scalenus medius.

Lumbar ribs are usually quite small and seldom as complete as cervical ribs.

Extra levator costae muscles may be associated with these ribs. Lumbar ribs have been reported to be more numerous than cervical ribs (8% of individuals).

In another study, rib anomalies were found in 0.15% of 38,105 patients; of these, 35% were cervical ribs. Other costal anomalies found in 38,105 chest films included lumbar ribs, dichotomies, forked ribs, and synostoses. In another study of 10,000 subjects, 75% of rib variations were seen in the seventh cervical and first thoracic.

A rib rudiment was reported on the coccyx.

In a study of 6 million chest photofluorograms reviewed for costal variations the incidence was 0.31%. Females were involved more frequently than males, and the right side more commonly than the left.

Radiographic Images of Some Rib Anomalies

Suprasternal Bones. Rib Anomalies. Duplication of Xiphoid Process

Fused First and Second Ribs

Views of Different Vertebrae from above to Illustrate their Homologies and some of their Varieties

Cervical Rib

Cervical Rib

Cervical Ribs

Variations in Lumbar Ribs

References

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