Structure
A typical rib (ribs 3 to 9) has a head, neck, tubercle and a long, thin, curved shaft that twists along its length.
- Head: wedge-shaped, with two articular facets separated by a crest. The lower facet meets the body of its own vertebra; the upper facet meets the vertebra above. The crest is tied to the intervertebral disc by the intra-articular ligament.
- Neck: the short narrow part between head and tubercle, lying in front of the transverse process.
- Tubercle: on the outer surface at the neck and shaft junction. Its articular facet meets the transverse process of its own vertebra; its rough part takes the lateral costotransverse ligament.
- Angle: the point of greatest curvature, a short distance lateral to the tubercle, where the shaft turns forward. It marks the lateral edge of the erector spinae muscles.
- Shaft: its lower inner border carries the costal groove, which shelters the posterior intercostal vein, artery and nerve, in that order from above down.
Classification
True ribs (1 to 7) reach the sternum through their own costal cartilage. False ribs (8 to 10) join the cartilage above to form the costal margin. Floating ribs (11 and 12) end freely in the posterior abdominal wall.
Atypical ribs
Ribs 1, 2, 10, 11 and 12 differ from the typical pattern in ways examiners expect you to recognise.
| Rib | Distinguishing features |
|---|---|
| 1st | Shortest, broadest and most curved; flat upper and lower surfaces; single facet on the head; scalene tubercle on the inner border for scalenus anterior; groove for the subclavian vein in front of the tubercle and for the subclavian artery and lower trunk of the brachial plexus behind it |
| 2nd | Thinner and longer than the 1st; a rough tuberosity on its outer surface for serratus anterior |
| 10th | Single facet on the head, articulating with T10 only |
| 11th and 12th | Single head facet; no neck or tubercle, so no costotransverse joint; short and tapering; the 12th has no angle and may be very short |
Joints and movements
Each rib moves at two posterior synovial joints and is linked in front by cartilage, so the chest expands in two planes during breathing.
- Joints of the head of rib: synovial plane joints with the vertebral bodies; ribs 1, 10, 11 and 12 meet one vertebra only.
- Costotransverse joints: synovial plane joints between the tubercle and the transverse process, absent on ribs 11 and 12.
- Costochondral joints: primary cartilaginous joints between rib and costal cartilage.
- Sternocostal joints: the 1st is a primary cartilaginous joint (synchondrosis); the 2nd to 7th are usually synovial.
- Interchondral joints: synovial joints between the cartilages of ribs 6 to 10.
Upper ribs rise in a pump handle movement that increases the anteroposterior diameter. Lower ribs swing out in a bucket handle movement that increases the transverse diameter.
Relations and neurovascular supply
Each rib carries its intercostal space's muscles and neurovascular bundle, and the lower ribs overlie upper abdominal organs.
The external, internal and innermost intercostal muscles fill the spaces between ribs. The posterior intercostal vessels and intercostal nerve run between the internal and innermost layers in the costal groove, with smaller collateral branches along the upper border of the rib below. The ribs are supplied by the posterior intercostal arteries and the nutrient arteries they give off. The lower ribs protect the liver on the right, the spleen on the left, and the kidneys behind; the 12th rib crosses the kidney and the pleura dips below it posteriorly.
Clinical relevance
Rib anatomy guides the safe placement of chest drains and explains the injuries that accompany rib fractures.
- Rib fractures: usually occur near the angle, the weakest point. Fractures of the 1st rib signal high-energy trauma and possible subclavian or brachial plexus injury; fractures of ribs 9 to 11 raise concern for liver, spleen or kidney injury.
- Flail chest: two or more adjacent ribs broken in two or more places create a segment that moves paradoxically with breathing.
- Chest drains and intercostal blocks: the needle or drain passes just above the upper border of the rib below, away from the main neurovascular bundle in the costal groove. The standard drain site lies in the safe triangle, at about the 5th intercostal space just anterior to the mid-axillary line.
- Cervical rib: an extra rib from C7 may compress the lower trunk of the brachial plexus or the subclavian artery, causing thoracic outlet syndrome.
- Counting ribs: start from the 2nd costal cartilage at the sternal angle; the 1st rib is hard to feel under the clavicle.