The thoracic cage

 

The thoracic cage is formed by the sternum and costal cartilages in

front, the vertebral column behind and the ribs and intercostal spaces

laterally.

It is separated from the abdominal cavity by the diaphragm and communicates

superiorly with the root of the neck through the

thoracic

inlet

(Fig. 1.1).

The ribs

(Fig. 1.1)

• Of the 12 pairs of ribs the first seven articulate with the vertebrae posteriorly

and with the sternum anteriorly by way of the costal cartilages

(

true ribs).

• The cartilages of the 8th, 9th and 10th ribs articulate with the cartilages

of the ribs above (

false ribs).

• The 11th and 12th ribs are termed ‘floating’ because they do not articulate

anteriorly (

false ribs).

Typical ribs (3rd–9th)

These comprise the following features (Fig. 1.2):

• A

head which bears two demifacets for articulation with the bodies

of: the numerically corresponding vertebra, and the vertebra above

(Fig. 1.4).

• A

tubercle which comprises a rough non-articulating lateral facet as

well as a smooth medial facet. The latter articulates with the transverse

process of the corresponding vertebra (Fig. 1.4).

• A

subcostal groove: the hollow on the inferior inner aspect of the

shaft which accommodates the intercostal neurovascular structures.

Atypical ribs (1st, 2nd, 10th, 11th, 12th)

• The

1st rib (see Fig. 63.2) is short, flat and sharply curved. The head

bears a single facet for articulation. A prominent tubercle (

scalene

tubercle

) on the inner border of the upper surface represents the insertion

site for scalenus anterior. The subclavian vein passes over the 1st

rib anterior to this tubercle whereas the subclavian artery and lowest

trunk of the brachial plexus pass posteriorly.

A cervical rib is a rare ‘extra’ rib which articulates with C7 posteriorly

and the 1st rib anteriorly. A neurological deficit as well as vascular

insufficiency arise as a result of pressure from the rib on the lowest

trunk of the brachial plexus (T1) and subclavian artery, respectively

(Fig. 1.3).

• The

2nd rib is less curved and longer than the 1st rib.

• The

10th rib has only one articular facet on the head.

• The

11th and 12th ribs are short and do not articulate anteriorly.

They articulate posteriorly with the vertebrae by way of a single facet

on the head. They are devoid of both a tubercle and a subcostal groove.

The sternum

(Fig. 1.1)

The sternum comprises a manubrium, body and xiphoid process.

• The

manubrium has facets for articulation with the clavicles, 1st

costal cartilage and upper part of the 2nd costal cartilage. It articulates

inferiorly with the body of the sternum at the

manubriosternal joint.

• The

body is composed of four parts or sternebrae which fuse between

15 and 25 years of age. It has facets for articulation with the lower part

of the 2nd and the 3rd to 7th costal cartilages.

• The

xiphoid articulates above with the body at the xiphisternal joint.

The xiphoid usually remains cartilaginous well into adult life.

Costal cartilages

These are bars of hyaline cartilage which connect the upper seven ribs

directly to the sternum and the 8th, 9th and 10th ribs to the cartilage

immediately above.

Joints of the thoracic cage

(Figs 1.1 and 1.4)

• The

manubriosternal joint is a symphysis. It usually ossifies after the

age of 30.

• The

xiphisternal joint is also a symphysis.

• The

1st sternocostal joint is a primary cartilaginous joint. The rest

(2nd to 7th) are synovial joints. All have a single synovial joint except

for the 2nd which is double.

• The

costochondral joints (between ribs and costal cartilages) are primary

cartilaginous joints.

• The

interchondral joints (between the costal cartilages of the 8th, 9th

and 10th ribs) are synovial joints.

• The

costovertebral joints comprise two synovial joints formed by the

articulations of the demifacets on the head of each rib with the bodies of

its corresponding vertebra together with that of the vertebra above. The

1st and 10th–12th ribs have a single synovial joint with their corresponding

vertebral bodies.

• The

costotransverse joints are synovial joints formed by the articulations

between the facets on the rib tubercle and the transverse process

of its corresponding vertebra

.