Axillary Contents
The axilla is a pyramidal space between the upper arm and chest wall containing critical neurovascular structures, lymph nodes, and adipose tissue that must be understood for surgical procedures, regional anesthesia, and oncologic management.
Anatomical Boundaries
The axilla is defined by:
- Apex: Convergence of clavicle, first rib, and superior scapula
- Base: Axillary skin and fascia
- Anterior wall: Pectoralis major and minor muscles
- Posterior wall: Subscapularis, teres major, and latissimus dorsi muscles
- Medial wall: Serratus anterior and upper ribs
- Lateral wall: Intertubercular groove of humerus
Key Contents
Neurovascular Structures
Axillary Artery and Branches: The axillary artery is divided into three parts by the pectoralis minor muscle, with branches including:
- Superior thoracic artery
- Thoracoacromial artery
- Lateral thoracic artery (LTA)
- Subscapular artery
- Anterior and posterior circumflex humeral arteries
Axillary Vein: Runs medial to the artery, receiving tributaries including the lateral thoracic vein (LTV) 1.
Brachial Plexus: The cords and terminal branches traverse the axilla lateral to the axillary artery. 2 notes this is most easily visualized with ultrasound in the supraclavicular region, but the terminal branches (median, ulnar, radial, musculocutaneous, and axillary nerves) pass through the axilla 3.
Intercostobrachial Nerve (ICBN): Crosses the axilla from the chest wall to innervate the medial upper arm. This nerve shows significant anatomical variation and is frequently encountered during axillary surgery 1.
Medial Cutaneous Nerve of Arm (MCNA): The least commonly identified structure in the axilla (present in only 22% of dissections), showing high variability 1.
Lymph Node Groups
The axillary lymph nodes are traditionally divided into three surgical levels relative to the pectoralis minor muscle 4, 5, 6:
- Level I: Lateral to the lateral border of pectoralis minor (lateral group)
- Level II: Behind the pectoralis minor (central group)
- Level III: Medial to the medial border of pectoralis minor (apical group)
Additional anatomical groupings include medial, posterior, and inferior nodes 7. At least 10 lymph nodes should be removed during level I/II dissection for accurate staging 5, 6.
Muscular Structures
Pectoralis Major and Minor: Form the anterior wall and serve as the primary surgical landmark. The pectoral muscle pedicle (PMP) is the most constant anatomical structure, identified in 100% of axillary dissections 1.
Latissimus Dorsi: Forms the posterior boundary and serves as a key landmark. The anterior border of latissimus dorsi can define the posterior axillary line 8.
Adipose Tissue
Subcutaneous adipose tissue (SAT) overlying the axilla varies significantly (8-60 mm thickness, average 25.9 mm) and correlates with body mass index 1. This variation impacts surgical planning and incision depth.
Clinical Landmarks for Axillary Lines
8 describe the CWIS nomenclature dividing the thorax into three zones using axillary lines:
- Anterior axillary line: Vertical line from the intersection of the posterior border of pectoralis major and the 2nd rib, or at 60° angle from mid-thoracic point
- Posterior axillary line: Vertical line through the tip of the scapula, anterior border of latissimus dorsi, or at 120° angle from mid-thoracic point
Anatomical Variations
Significant variations exist in axillary anatomy 1, 9:
- The MCNA is absent in 78% of cases
- The ICBN, LTV, and LTA show variable entry points, paths, and branching patterns
- Accessory muscle heads (axillary arch, accessory biceps heads) may be present and can compress neurovascular structures 10
- Axillary artery branching patterns vary, with branches occasionally arising from common trunks 9
Imaging Considerations
Ultrasound is the primary modality for axillary evaluation, particularly for lymph node assessment 11, 12. MRI provides superior detail of small vessels and brachial plexus structures compared to CT 13. The arm should be hyperabducted for optimal visualization 13, 14.
Note: I cannot provide a diagram as I can only generate text responses. However, standard anatomy textbooks (Gray's Anatomy, Netter's Atlas) contain excellent illustrations of axillary anatomy showing the pyramidal space with its neurovascular bundle, lymph node levels, and muscular boundaries in multiple planes.