Thorax: Breast & Pectoral Region Anatomy & Clinical Notes

Thorax: Breast and Pectoral Region Notes

Reference Lines on Thoracic Wall

Anterior axillary line: A vertical line passing through the anterior axillary fold.
Posterior axillary line: A vertical line passing through the posterior axillary fold.
Midaxillary line: A vertical line situated midway between the anterior and posterior axillary lines.
Anterior median line: A vertical line running down the midline of the body (midsagittal plane).
Midclavicular lines: Parasagittal vertical lines that pass through the midpoint of each clavicle.
Posterior median (midvertebral) line: A vertical line along the tips of the spinous processes of the vertebrae.
Scapular lines: Vertical lines parallel to the posterior median line, intersecting the inferior angles of the scapulae.

Structure of the Female Breast

Accessory reproductive organ: A modified sweat gland.
Composition: Glandular tissue embedded in fat (connective tissue).
Location: Resides within the superficial fascia, anterior to the deep pectoral fascia.
Extent: Stretches from the anterior axillary line to the sternum, typically spanning ribs $2$ to $6$ .
Axillary process (tail of Spence): An extension of breast tissue extending towards the axilla.
Areola: The pigmented area of skin surrounding the nipple.
- Montgomery tubercles: Sebaceous glands located within the areola.
Nipple:
- Lacks hair, fat, and sweat glands.
- Contains smooth muscle.
- Fissured by the openings of lactiferous ducts.
- Often described as being at the $4^{th}$ intercostal space, but this is not consistently accurate for adult women.
Retromammary space (bursa):
- A loose connective tissue plane located between the superficial and deep fascia.
- Allows for movement of the breast on the pectoral fascia.
Suspensory ligament of Cooper:
- Fibrous tissue that connects the breast to the overlying dermis.
- More developed in the superior parts of the breast.
- Provides support for the gland lobules.

Glandular Tissue Detail

Lobes: Consist of $15-20$ lobes of glandular tissue, which constitute the parenchyma of the breast.
Terminal ductal lobular units: Each lobe contains $20-40$ of these units.
Lactiferous ducts: Each lobe is drained by a single lactiferous duct.
Lactiferous sinus: A dilated portion of the lactiferous duct, located just deep to the nipple.
Support: The glandular tissue is supported by suspensory ligaments of Cooper, which form fibrous septa anchored to the skin and deep fascia.

Vascular Supply of the Breast

Arterial Supply:
- Medial breast: Primarily supplied by perforating branches of the internal thoracic artery (medial mammary arteries).
- Lateral breast: Supplied by branches of:
  - Superior thoracic artery (lateral mammary arteries).
  - Lateral thoracic artery (lateral mammary arteries).
  - Pectoral branch of the thoracoacromial artery.
- Anterior and Posterior intercostal arteries also supply mammary branches.
Venous Drainage: Mirrors the arterial supply, mainly draining into the axillary vein.

Cutaneous Innervation of the Breast

Primary Nerves: The $4^{th}$ – $6^{th}$ intercostal nerves, via their lateral and anterior branches.
Nipple Innervation: Specifically supplied by the $4^{th}$ intercostal nerve (ventral ramus of T4).
Fiber Types: Branches of the intercostal nerves convey:
- Sensory fibers from the skin of the breast.
- Sympathetic fibers to the blood vessels in the breasts and to the smooth muscle in the overlying skin and nipple.

Lymphatic Drainage of the Breast and Cancer Spread

Lymph passes from the lobules of the gland, nipple, and areola to the subareolar lymphatic plexus.
Lymph Nodes of the Breast:
- Axillary lymph nodes: Anterior (pectoral), Posterior (subscapular), Central, Apical.
- Parasternal nodes.
- Interpectoral nodes (Rotter’s nodes).
Lymph Drainage Pathways:
1. Axillary pathway: Drains approximately $75\%$ of lymph, primarily to the anterior axillary nodes. Lymph can also bypass these nodes and drain directly.
2. Internal thoracic (mammary) pathway: Drains around $20\%$ of lymph to the parasternal lymph nodes. This pathway can communicate with lymph vessels of the opposite breast.
3. Retropectoral and Transpectoral pathways: Account for about $5\%$ of drainage, leading to posterior intercostal nodes.
4. Subdiaphragmatic pathway: Drains to the inferior phrenic nodes.
Clinical Significance: Most breast cancers are found in the upper outer quadrant due to the extensive lymphatic drainage in this region, which increases the likelihood of metastasis to axillary nodes.

Clinical Manifestations of Breast Cancer

Peau d’orange appearance: Refers to the orange-peel-like appearance of the skin, caused by obstruction of superficial lymphatic drainage, leading to edema and accentuation of hair follicles.
Skin dimpling and nipple retraction: Occurs when tumors invade through the retromammary space and deep fascia, causing breast fixation and shortening of the suspensory ligaments of Cooper, pulling the skin inwards.

Fascia of the Pectoral Region

Pectoral fascia: Covers the anterior and posterior surfaces of the pectoralis major muscle. It becomes the axillary fascia, forming the floor of the axilla.
Clavipectoral fascia:
- Attaches superiorly to the clavicle.
- Envelops the subclavius and pectoralis minor muscles.
- Pierced by the cephalic vein, thoracoacromial artery, and lateral pectoral nerve.
- Below the pectoralis minor, it is referred to as the suspensory ligament of the axilla.
- Connects to and supports the axillary fascia.

Pectoral Region Anterior Axio-Appendicular Muscles

These muscles connect the pectoral girdle and humerus to the axial skeleton, primarily functioning in moving the pectoral girdle and humerus.

Pectoralis Major

Heads: Consists of $2$ heads: clavicular and sternocostal.
Anterior Axillary Fold: Forms the anterior border of the axilla.
Proximal Attachment: Medial half of the clavicle, anterior surface of the sternum, superior $6$ costal cartilages, and the aponeurosis of the external oblique muscle.
Distal Attachment: Lateral lip of the intertubercular sulcus of the humerus.
Innervation: Lateral and medial pectoral nerves (Clavicular head: C5, C6; Sternocostal head: C7, C8, T1).
Main Actions: Adducts, medially rotates, and flexes the humerus (clavicular head); extends the humerus from a flexed position (sternocostal head); draws the scapula anteriorly and inferiorly.

Pectoralis Minor

Location: Lies deep to the pectoralis major.
Important Landmark: Serves as a key landmark in the axilla, with relations to the axillary artery and brachial plexus.
Subcoracoid Space: The space deep to the pectoralis minor contains parts of the brachial plexus and axillary vessels.
Proximal Attachment: $3^{rd}$ to $5^{th}$ ribs near their costal cartilages.
Distal Attachment: Medial border and superior surface of the coracoid process of the scapula.
Innervation: Medial pectoral nerve (C8, T1); Lateral pectoral nerve (variable).
Main Actions: Stabilizes the scapula by drawing it inferiorly and anteriorly against the thoracic wall (lateral angle moves inferiorly). It can also function as an accessory muscle of respiration.

Subclavius

Proximal Attachment: Junction of the $1^{st}$ rib and its costal cartilage.
Distal Attachment: Inferior surface of the middle third of the clavicle.
Innervation: Nerve to subclavius (C5, C6).
Main Actions: Anchors and depresses the clavicle; stabilizes the sternoclavicular joint. It affords some protection to the subclavian vessels and the superior trunk of the brachial plexus if the clavicle fractures.

Serratus Anterior

Appearance: Has a characteristic serrated (saw-like) appearance.
Nickname: Known as the “boxer’s muscle” due to its role in protracting the scapula, essential for punching.
Proximal Attachment: External surfaces of the lateral parts of the $1^{st}$ – $8^{th}$ (sometimes $9^{th}$ ) ribs.
Distal Attachment: Anterior surface of the medial border of the scapula, including the superior and inferior angles.
Innervation: Long thoracic nerve (C5, C6, C7).
Main Actions: Protracts the scapula and holds it against the thoracic wall; rotates the scapula (especially for arm abduction above $90$ degrees) by rotating the glenoid cavity superiorly.

Long Thoracic Nerve Injury

Origin: A branch of the brachial plexus.
Location: Has a superficial course, making it vulnerable to injury.
Common Causes: Mastectomy, knife injury, chest tube insertion, or other trauma to the lateral thoracic wall.
Consequences: Weakness or paralysis of the serratus anterior muscle.
Results in:
- Winging of the scapula: The medial border of the scapula protrudes posteriorly from the thoracic wall.
- Difficulty with arm elevation (abduction): Particularly above $90$ degrees, as serratus anterior cannot effectively rotate the scapula superiorly. From the chest.