Introduction
Intercostal space is a space between two adjacent ribs and their costal cartilages. There are 11 pairs of intercostal spaces on each side. The spaces are named according to the rib forming the superior border of the space.
Contents
Each space contains the following structures:
- Three intercostal muscles, viz:
- External intercostal
- Internal intercostal
- Innermost intercostal
- Intercostal nerves
- Intercostal arteries
- Intercostal veins
- Intercostal lymph vessels and lymph nodes
The neurovascular bundle lies between the internal and innermost intercostal muscles. They are arranged in the order: Vein, Artery and Nerve [VAN] from top to bottom in the costal groove.
Intercostal Muscles
These are a group of muscles that are present in the intercostal space and help form the chest wall. There are three intercostal muscles: External, Internal, and Innermost intercostal muscles.
External Intercostal Muscles
- Origin: Lower border of rib above
- Insertion: Upper border (outer lip) of rib below
- Extent: From costochondral junction to the tubercle of rib (anteriorly, it continues as anterior/external intercostal membrane)
- Direction of fibers: Downwards, forwards, and medially
- Action: Elevates the rib during inspiration
Internal Intercostal Muscles
- Origin: Floor of the costal groove of the rib above
- Insertion: Upper border (inner lip) of rib below
- Extent: From the lateral border of the sternum to the angle of rib (posteriorly it continues as posterior/internal intercostal membrane)
- Direction of fibers: Upwards, forwards, and medially
- Action: Depresses the rib during expiration
Innermost Intercostal Muscles
- Form the deepest layer
- An incomplete muscle layer and crosses more than one intercostal surface
- Relation:
- It is related internally to the endothoracic fascia and parietal pleura
- Related externally to the intercostal nerves and vessels
- Parts:
- Innermost intercostal laterally
- Sternocostalis (transversus thoracic) anteriorly
- Subcostalis posteriorly
Neurovasculature
Arterial Supply
Arterial supply is by intercostal arteries derived from the thoracic aorta, subclavian artery, and axillary artery.
Posterior Intercostal arteries
- Origin:
- Superior intercostal artery – intercostal spaces 1 and 2
- Thoracic aorta – remaining intercostal spaces
- Course: Pass between internal and innermost muscles
- Distribution:
- Intercostal muscles
- Overlying skin
- Parietal pleura
Anterior Intercostal Arteries
- Origin:
- Internal thoracic – intercostal spaces 1-6
- Musculophrenic arteries – intercostal spaces 7-9
- Course: Pass between internal and innermost muscles
- Distribution:
- Intercostal muscles
- Overlying skin
- Parietal pleura
Internal Thoracic Arteries
- Origin: Subclavian artery
- Course: Passes inferolateral to the sternum between the costal cartilages and transversus thoracic muscle to divide into superior epigastric and musculophrenic arteries
- Distribution:
- By way of anterior intercostal arteries to intercostal spaces 1-6
- Musculophrenic artery (lateral terminal branch)
Musculophrenic artery
- Origin: Thoracic aorta
- Course: Courses along the inferior border of the 12th rib
Subcostal Artery
- Distribution:
- Muscles of the anterolateral abdominal wall
Venous Drainage
The intercostal veins accompany the intercostal arteries and lie most superior in the costal grooves. There are 11 posterior intercostal veins and one subcostal vein on each side. The posterior intercostal veins anastomose with the anterior intercostal veins. Most posterior intercostal veins (4-11) end in the azygos/hemi-azygos venous system, which drains into the superior vena cava.
- 1st posterior intercostal vein drains directly into the right and left brachiocephalic veins.
- 2nd and 3rd (and occasionally 4th) posterior intercostal veins unite to form the superior intercostal vein.
- Right superior intercostal vein is typically the final tributary of the azygos vein before it enters the SVC.
- Left superior intercostal vein empties into the left brachiocephalic vein. It communicates inferiorly with the accessory hemi-azygos vein.
- The internal thoracic veins are the companions of the internal thoracic arteries.
Nervous Supply
Nervous supply is by 11 pairs of intercostal nerves, which are anterior rami (branches) of thoracic spinal nerves (T1-T11).
- Anterior ramus of nerve T12, coursing inferior to the 12th rib is the subcostal nerve.
- Typical intercostal nerves are those which remain confined to their own intercostal spaces.
Its branches are:
- Rami communicantes
- Muscular branches
- Collateral branches
- Lateral cutaneous branches
- Anterior cutaneous branches
Atypical spinal nerves extend beyond the thoracic wall and partly or entirely supply other organs:
- 1st & 2nd intercostal nerve
- 7th to 11th intercostal nerves
Clinical Correlates
- Intercostal nerve block
- Thoracocentesis
