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Topic

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Axilla

The axilla is a pyramidal anatomical space located between the upper limb and the lateral thoracic wall. It serves as a major neurovascular conduit connecting the neck and thorax to the upper extremity and contains critical vessels, nerves, lymph nodes, and adipose tissue.

Because of its complex anatomy and rich contents, the axilla is of major importance in clinical examination, oncologic staging (especially breast cancer), trauma assessment, and cross-sectional imaging.

Synonyms

  • Armpit

  • Axillary fossa

Location

  • Situated between the lateral chest wall and the proximal upper arm

  • Inferior to the shoulder joint

  • Lateral to the thoracic cage

  • Medial to the proximal humerus

  • Extends from the cervico-axillary canal superiorly to the axillary fascia inferiorly

Anatomical components (boundaries)

Apex (cervico-axillary canal):

  • Clavicle (anteriorly)

  • First rib (medially)

  • Superior border of the scapula (posteriorly)

Base:

  • Skin

  • Subcutaneous tissue

  • Axillary fascia

Anterior wall:

  • Pectoralis major

  • Pectoralis minor

  • Clavipectoral fascia

Posterior wall:

  • Subscapularis

  • Teres major

  • Latissimus dorsi

Medial wall:

  • Serratus anterior

  • Upper ribs (1–4)

  • Intercostal muscles

Lateral wall:

  • Intertubercular sulcus of the humerus

  • Coracobrachialis

  • Short head of biceps brachii

Contents

  • Axillary artery and its branches

  • Axillary vein and tributaries

  • Brachial plexus cords and branches

  • Axillary lymph nodes (levels I–III)

  • Fat and connective tissue

Relations

Superiorly:

  • Neck structures via the cervico-axillary canal

Inferiorly:

  • Axillary skin and fascia

Anteriorly:

  • Breast tissue (anteromedially)

  • Pectoral muscles

Posteriorly:

  • Scapular muscles and posterior axillary fold

Medially:

  • Thoracic wall and pleura

Laterally:

  • Proximal humerus and shoulder joint

Function

  • Neurovascular passage: Provides a protected route for vessels and nerves supplying the upper limb

  • Lymphatic drainage: Major drainage basin for upper limb, breast, and thoracic wall

  • Mobility allowance: Fat and loose connective tissue permit free movement of neurovascular structures during shoulder motion

X-ray appearance

Chest or shoulder radiographs:

  • Axilla: Appears as a soft-tissue region lateral to the thoracic cage

  • Fat: Radiolucent axillary fat pad may be visible

  • Bones: Proximal humerus, ribs, and scapula define margins

  • Lymph nodes: Not directly visualized

CT appearance

Soft-tissue window:

  • Axillary fat: Low attenuation background

  • Vessels and lymph nodes: Soft-tissue density structures within fat

  • Muscles: Well-defined margins of axillary walls

Lung window:

  • Axillary fat: Appears very lucent

  • Pleural interface: Clear visualization of adjacent lung apex

  • Utility: Helpful for assessing apical lung extension, pleural disease, and chest wall involvement

MRI appearance

T1-weighted images:

  • Axillary fat: High signal intensity providing excellent contrast

  • Vessels and nerves: Intermediate-to-low signal structures within fat

  • Muscles: Intermediate signal with clear margins

T2-weighted images:

  • Fat: Intermediate-to-high signal

  • Lymph nodes: Intermediate signal with central fatty hilum

  • Vessels: Flow voids or signal loss depending on sequence

STIR:

  • Fat: Suppressed signal

  • Lymph nodes and soft tissues: High signal relative to suppressed fat

  • Utility: Improves detection of edema, inflammation, and subtle soft-tissue changes

X ray image

Axilla x ray

CT image

Axilla CT coronal image