4.10: Osteology of Thorax

Sternum (171)

Cancellous bone, filed with haemopoietic material through life.

Manubrium has a jugular notch, with investing fascia attached to anterior and posterior borders

- first intercostal attaches to its lateral borders, covered anteriorly by pec major, and gives attachment to sternocleidomastoid, sternohyoid and sternothyroid.

Joints – refer first thorax chapter

Body – articular facets along lateral border for 3-7 Ccs.

- each gap gives rise to anterior intercostal membranes and internal intercostal muscles.

- Pec major arises widely anteriorly (near to midline) and transversus thoracis arises from posterior low down; sternopericardial ligaments also pass from here.

Xiphoid

Typical Rib (171)

Head, neck, and tubercle.

- see start of thorax section for ligaments.

Shaft slopes down and back to the angle, then twists forward. Heads lie higher than anterior ends

- torsion most marked at 7^th-9^th ribs.

- tops are blunt; lower borders are sharp (external intercostals arise here); internal intercostals attach to costal groove; transversus to inner surface of shaft

- neurovascular bundle runs deep to the costal groove.

1^st Rib (171)

Strongest, broadest, flattest, curviest.

Plane of shaft is at 45° in anatomical position (tubercle highest point). When rib laid on table upside down the head lies above the table; distinguishes R from L.

Anterior rami of C8 & T1 clasp the neck before uniting to form lower trunk of BP.

From medial ® lateral:

Sympathetic chain ® supreme intercostal vein ® superior intercostal artery ® 1^st thoracic nerve.

- the cervical dome of pleura and apex of lung hold these vessels and nerves against the rib neck

Upper surface

Grooved by subclavian artery (with it the lower trunk of the BP); anterior to this is the groove for the subclavian vein, with the scalene tubercle for scalenus anterior in between.

Scalenus medius inserts posterior to groove for subclavian artery, and serratus anterior arises lateral to it from outer border.

Inner surface of shaft (concave) gives rise to suprapleural membrane.

Undersurface of shaft is covered by adherent parietal pleura;

Ossification

Sternum: two plates fuse in fidline, variable primary centres at 5^th fetal month

- sternebrae (sternal pieces) fuse between puberty and 25^th year

- incomplete fusion leads to foramina ‘bullet-holes’

Xiphoid: may ossify after 3 years, but often later

Ribs primary centres by 8^th week, at birth pretty-much there. Head and tubercle fuse at ~20 yrs

Costal Cartilages

All ribs 1-12. First short and thick, gives attachment to sternoclavicular disc and costoclavicular ligament. Below this increase in length to 7^th = longest.

- all tend to calcify and even ossify in old age.