It is helpful to visualise the axilla as a pyramid.
The boundaries are formed by the:
- The apex of the axilla is formed by the junction of the medial third of the clavicle and the first rib. This junction is formed by the costoclavicular ligament, which is also known as Halsted’s ligament.
- The anterior border of the axilla is formed by the clavipectoral fascia. This fascia encases the clavicular portion of the pectoralis major, pectoralis minor and subclavius muscles.
- The medial border of the axilla is formed by the serratus anterior muscle, which overlies the chest wall. Serratus anterior arises as a series of ‘fingers’ from the lateral aspects of the upper eight ribs and fascia. It runs as a large, flat muscle over the medial chest wall. Serratus anterior inserts onto the medial aspect of the costal surface of the scapula, from the superior to the inferior angle. Its function is to protract and laterally rotate the scapula.
- The posterior border of the axilla is formed by three muscles: the latissimus dorsi, subscapularis and teres minor. The latissimus dorsi arises from the spines and supraspinous ligaments of the lower six thoracic vertebrae (deep to trapezius). It is attached via the thoracolumbar fascia to all of the lumbar and sacral vertebrae, the posterior superior iliac crest and the lower three to four ribs. Latissimus dorsi inserts into the bicipital groove of the humerus.
- The lateral border of the axilla is the small area of the bicipital groove of the humerus that lies between the attachments of the muscles that form the anterior and posterior walls of the axilla
- The base of the axilla is formed by the axillary fascia and skin. It extends from the lower border of pectoralis major (anteriorly) to the lower border of latissimus dorsi behind (posteriorly).
There are a number of important structures within the axilla that a surgeon must be aware of. These include some that will serve as landmarks when performing axillary lymphadenectomy and others where the intention is to avoid encountering them.
The structures include:
- Long thoracic nerve
- Thoracodorsal nerve
- Medial and lateral pectoral nerves
- Intercostobrachial nerves
- Axillary artery and vein
- Thoracodorsal artery and vein
- Lateral thoracic artery and vein
Long Thoracic Nerve
The long thoracic nerve is also known as the ‘nerve of Bell’.
The nerve emerges from roots of the fifth to seventh cervical nerves and runs down on the surface of the middle scalene muscle to pass between the first rib and axillary artery.
Having entered the axilla it continues down parallel to the chest wall to supply the serratus anterior muscle. It is closely adherent to the fascia overlying serratus anterior.
Damage to the nerve causes a ‘winged scapula’ deformity.
The thoracodorsal nerve arises from roots of C6-8 via the posterior cord of the brachial plexus. It runs through the axilla with the thoracodorsal artery and the thoracodorsal vein. Together, these three structures form the thoracodorsal pedicle, which supplies the latissimus dorsi muscle.
Damage to the nerve causes denervation of the latissimus dorsi, leading to muscle atrophy. There is loss of volume of the posterior axillary fold and weakness of shoulder abduction and internal rotation.
Medial Pectoral Nerve
The medial pectoral nerve arises from the roots of C8 to T1 via the medial cord of the brachial plexus. It runs between the axillary artery and vein.
The medial pectoral nerve runs caudally with a series of thoracoacromial vessels until it enters the deep surface of the pectoralis minor muscle, which it supplies. Two or three branches pierce the muscle and supply the pectoralis major muscle.
Damage to the medial pectoral nerve causes wasting, predominantly of the pectoralis minor and the sternocostal portion of the pectoralis major. The muscular atrophy is most noticeable in the infraclavicular region.
Lateral Pectoral Nerve
The lateral pectoral nerve (C5 to C7) arises from the lateral cord of the brachial plexus, or just before the union of the anterior divisions of the upper and middle trunks to form the lateral cord.
It pierces the clavipectoral fascia to supply the pectoralis major. It also supplies the pectoralis minor via a branch to the medial pectoral nerve.
Remember that the medial and lateral pectoral nerves refer to the emergence of these nerves from the brachial plexus. In fact, within the chest, the lateral pectoral nerve runs medial to the pectoral muscles and the medial pectoral nerve runs lateral to them.
Damage to the nerve causes atrophy of the sternoclavicular fibres of the pectoralis major, with infraclavicular wasting and loss of the anterior axillary fold.
These sensory nerves are important because they run transversely across the axilla, dividing it into deep and superficial compartments. In contrast, virtually all other important structures run vertically in the axilla.
The intercostobrachial nerves supply sensation to the inner aspect of the arm.
It is desirable, but not essential, to preserve these nerves during axillary dissection, because damage will cause a noticeable loss of sensation on the inner aspect of the arm.
The axillary vein marks the superior limit for axillary dissection and is therefore a key landmark. After the basilic vein has crossed the lower border of teres minor it is renamed the axillary vein. The axillary vein runs medially and superiorly through the axilla, medial and slightly inferior to the axillary artery. On reaching the outer border of the first rib it is renamed the subclavian vein. During surgery, the axillary vein can be identified by retraction of pectoralis major. The vein can be revealed with a combination of sharp and blunt dissection. The axillary artery lies deep and slightly superior to the vein. Its position can be located by palpation but it is rarely exposed during surgery. The axillary vein may be seen to collapse with inspiration which can be a useful identification tool. It is important to know that the axillary vein is commonly bifid.
In a similar manner to the axillary vein, the subclavian artery is renamed the axillary artery beyond the outer border of the first rib. It is then renamed the brachial artery at the lower border of teres minor.
The axillary artery is classically considered to consist of three parts:
As described earlier, the thoracodorsal vessels and nerve form the thoracodorsal pedicle.
This lies deep to the lateral thoracic vessels and the intercostobrachial nerves and just inferior to the axillary vein. It can be found by dissecting in the mid-axilla inferior to the axillary vein
Lateral Thoracic Artery
The lateral thoracic artery (also known as the external mammary artery) arises from the second part of the axillary artery. It runs across the first intercostal space to supply the upper chest wall and breast.
The lateral thoracic vein helps to drain the upper chest wall into the axillary vein. Within the axilla the vein can be found lateral to the artery and deep to the axillary vein.
The lateral thoracic vessels are in the superficial axillary compartment because they lie superficial to the intercostobrachial nerve.
From a purely anatomical perspective, there are five groups of axillary lymph nodes:
- Anterior (pectoral)
- Posterior (subscapular)
These drain the arm, chest and breast and form a lymphatic plexus around the axillary vein.
However, the lymph node anatomy of the axilla is better understood from a functional perspective, by considering the different levels of axillary lymph node dissection:
- Level I nodes lie lateral to the outer border of the pectoralis minor
- Level II nodes lie underneath the pectoralis minor
- Level III nodes lie medial to the pectoralis minor and inferior to the lower border of the clavicle
- The patient is placed supine with the arm abducted to 90 ° at the shoulder. This provides good exposure of the axilla without placing undue traction on its contents (especially the brachial plexus). The surgeon can choose either a longitudinal or transverse incision.
- The superficial fat and fascia is divided until the lateral border of the pectoralis major is identified.
- Next, the clavipectoral fascia, which surrounds the pectoralis minor, is identified and incised under tension to open up access to the axillary contents.
- Before any structures are excised or ligated the axillary vein must be identified to define the superior limit of the dissection. With retraction of the pectoral muscles, careful dissection through the axillary fat should reveal the vein.
- The superficial tissues are dissected off the axillary vein and the lateral thoracic vein can be found joining the anterior surface of the axillary vein. The lateral thoracic artery can be found medial to the lateral thoracic vein and deep to the axillary vein. Once clearly identified, the lateral thoracic artery and vein should be ligated and cut. It is important not to confuse these vessels with the thoracodorsal vessels, which lie deep to the intercostobrachial nerve.
- Running transversely, inferior to the axillary vein and deep to the lateral thoracic vessels, is the intercostobrachial artery. This separates the superficial and deep axillary compartments.
- Lying deep to this is the thoracodorsal pedicle, whose vessels run infero-laterally to the latissimus dorsi. The thoracoepigastric vein is often encountered. This is a superficial venous tributary that drains into the axillary vein, just anterior to the thoracodorsal pedicle. This vein should only be divided after identifying the neurovascular pedicle.
- The long thoracic nerve is held against the chest wall by the serratus anterior fascia. This should be kept intact to reduce the risk of damage to the nerve.
- This is the limit of a level I dissection. To access level II, the pectoralis minor is retracted and the tissue underneath is dissected, in continuity with level I contents, along the inferior border of the axillary vein.
- A level III dissection is the continuation of the dissection to the costoclavicular ligament (Halsted’s ligament). Some surgeons divide the pectoralis minor to access level III.