The axilla is the name given to an area that lies underneath the glenohumeral joint, at the junction of the upper limb and the thorax. It is a passageway by which neurovascular and muscular structures can enter and leave the upper limb.
In this article, we shall examine the anatomy of the axilla – the borders, contents, and any clinical correlations.
The overall 3D shape of the axilla looks slightly like a pyramid. It consists of four sides, an open apex and base:
- Apex – also known as the axillary inlet, it is formed by lateral border of the first rib, superior border of scapula, and the posterior border of the clavicle.
- Lateral wall – formed by intertubercular groove of the humerus.
- Medial wall – consists of the serratus anterior and the thoracic wall (ribs and intercostal muscles).
- Anterior wall – contains the pectoralis major and the underlying pectoralis minor and the subclavius muscles.
- Posterior wall – formed by the subscapularis, teres major and latissimus dorsi.
The size and shape of the axilla region varies with arm abduction. The apex decreases in size most markedly when the arm is fully abducted – leaving the contents of the axilla at risk of compression.
The contents of the axilla region include muscles, nerves, vessels, and lymphatics:
- Axillary artery (and branches) – the main artery supplying the upper limb. It is commonly referred as having three parts; one medial to the pectoralis minor, one posterior to pectoralis minor, and one lateral to pectoralis minor. The medial and posterior parts travel in the axilla.
- Axillary vein (and tributaries) – the main vein draining the upper limb, its two largest tributaries are the cephalic and basilic veins.
- Brachial plexus (and branches) – a collection of spinal nerves that form the peripheral nerves of the upper limb.
- Axillary lymph nodes – they filter lymphatic fluid that has drained from the upper limb and pectoral region. Axillary lymph node enlargement is a non-specific indicator of breast cancer.
- Biceps brachii (short head) and coracobrachialis – these muscle tendons move through the axilla, where they attach to the coracoid process of the scapula.
Passageways Exiting the Axilla
There are three main routes by which structures leave the axilla.
The main route of exit is immediately inferiorly and laterally, into the upper limb. The majority of contents of the axilla region leave by this method.
Another pathway is via the quadrangular space. This is a gap in the posterior wall of the axilla, allowing access to the posterior arm and shoulder area. Structures passing through include the axillary nerve and posterior circumflex humeral artery (a branch of the axillary artery.
The last passageway is the clavipectoral triangle, which is an opening in the anterior wall of the axilla. It is bounded by the pectoralis major, deltoid, and clavicle. The cephalic vein enters the axilla via this triangle, while the medial and lateral pectoral nerves leave.
Thoracic Outlet Syndrome
The apex of the axilla region is an opening between the clavicle, first rib and the scapula. In this apex, the vessels and nerves may become compressed between the bones – this is called thoracic outlet syndrome.
Common causes of thoracic outlet syndrome include:
- Trauma – e.g. fractured clavicle.
- Repetitive movements – seen commonly in occupations that require lifting of the arms.
- Cervical rib – an extra rib which arises from the seventh cervical vertebra.
It often presents with pain in the affected limb (the distribution of pain is dependent on which nerve is compressed), tingling, muscle weakness and discolouration.
Lymph Node Biopsy
Approximately 75% of lymph from the breast drains into the axilla lymph nodes, so can be biopsied if breast cancer is suspected.
If breast cancer is confirmed, the axillary nodes may need to be removed to prevent the cancer spreading. This is known as axillary clearance. During this procedure, the long thoracic nerve may become damaged, resulting in winged scapula.