Chest wall

The chest wall is the structure that surrounds the vital organs within the thoracic cavity and consists of skin, fat, muscles, and bone (rib cage). It furthermore supports breathing and stabilizes the shoulder girdle and upper arms during movement. Anatomical landmarks that play an important role in clinical examination and thoracic surgery include the midsternal line, the midclavicular line, and the midaxillary line. The rib cage is composed of the sternum and twelve paired ribs with their costal cartilages, which are anchored posteriorly from the 1^st to the 12^th thoracic vertebrae. Muscles that comprise the chest wall include the external, the internal and innermost intercostal muscles, the subcostal muscles, and the transverse thoracic muscles, all of which are innervated by the intercostal nerves. These muscles are involved in the movement of the rib cage during inspiration and expiration. There are also muscles that attach to the chest wall but are not inherently part of it (e.g., pectoralis major, serratus anterior, latissimus dorsi), which help with respiration and upper limb movement. The neurovascular bundle (intercostal artery, vein, and nerve) runs along the inferior surface of the rib within the costal groove. To prevent injury of the intercostal nerves and vessels during surgery, chest tubes are, therefore, placed close to the upper margin of a rib. Conditions that affect the chest wall include costochondritis, pectus excavatum, and thoracic outlet syndrome.

The chest wall is composed of the thoracic skin, fat, muscles, and skeleton .

• Important functions
  • Protection of thoracic organs
  • Stabilization of shoulder girdles
  • Dynamic expansion and recoil during respiration (chest wall dynamics)
• Reference lines
  • Anterior chest
    • Midsternal line
    • Anterior axillary lines
    • Midclavicular lines
  • Posterior chest
    • Vertebral line
    • Midscapular lines
  • Axilla
    • Anterior axillary lines
    • Midaxillary lines
    • Posterior axillary line
• Surface anatomy: See “Intercostal surface projections.”
• Vasculature and innervation: neurovascular bundles
  • Each bundle is composed of an intercostal vein (most superior), intercostal artery , and intercostal nerve (most inferior).
  • They course along the costal groove at the inferior border of each rib.

Below each rib parks a VAN: Vein, Artery, Nerve (intercostal bundles from superior to inferior).

Chest tubes are placed along the upper margin of a rib to prevent injury of the intercostal nerves and vessels, which run along the lower margin.

The thorax is bounded by 12 sets of ribs that wrap around from the T1–T12 vertebrae of the thoracic spine and connect to the sternum and/or costal cartilages.

Sternum

• Composed of 3 flat bones
  • Manubrium: cartilages of 1^st ribs articulate here
  • Sternal body: cartilages of 3^rd–7^th ribs attach here
  • Xiphoid process: small cartilaginous extension at the 7^th dermatome that ossifies in adults
• Suprasternal notch: visible and palpable U-shaped depression above the manubrium between the two clavicles
• Sternal angle
  • Palpable connection between manubrium and sternal body
  • Marks level of the 2^nd ribs, level of intervertebral discs T4/5, the aortic arch, and the bifurcation of the trachea

Ribs

There are 12 pairs of ribs separated by intercostal spaces (ICSs)

• Parts of a typical rib
  • Head of the rib
  • Tubercle of the rib: Small bump near the head on the posterior rib surface.
  • Body of the rib (shaft)
• Types
  • True ribs (ribs 1–7): Connect directly to the sternum via costal cartilage.
  • False ribs (ribs 8–10): Connect indirectly to the sternum via the costal arch.
  • Floating ribs (ribs 11–12): Do not connect to the sternum.
  • Accessory ribs: Anatomical variant in up to 0.5% of the population that usually arises from the seventh cervical vertebra (cervical rib).
• Articulations
  • Anteriorly
    • Costochondral joints: Connect the ribs to their costal cartilages.
    • Chondrosternal joints: Connect the costal cartilage of the true ribs with the sternum.
  • Posteriorly
    • Costovertebral joints: Connect the rib heads to the vertebral bodies.
      • Rib heads have two facets.
        • Lower facet connects to upper costal facet of the vertebra that determines that rib's number.
        • Upper facet connects to lower costal facet of the superior vertebral body.
    • Costotransverse joints: Connect the rib tubercle to the transverse processes of the vertebral bodies.

Intercostal surface projections

• Nipples in men: ∼ 5^th ICS, can be lower in women
• Dome of the diaphragm and liver: right 5^th ICS at the midclavicular line
• Lung: see pulmonary examination
• Heart
  • Right margin: extends from right 3^rd costal cartilage to right 6^th costal cartilage
  • Left margin: extends from left 2^nd ICS to the apex at the midclavicular line of the 5^th ICS
  • Anterior interventricular sulcus: 2.5 cm lateral of the midline at 3^rd left ICS
  • For the projection of the heart valves and the auscultation of the heart sounds, see cardiovascular examination.

The first rib is not palpable because the clavicle overlies it. When counting the ribs, one starts at the palpable manubriosternal junction (sternal angle), where the second rib attaches.

A cervical rib is usually asymptomatic but can cause compression of the C8/T1 roots, the lower trunk of the brachial plexus, and/or the subclavian artery (thoracic outlet syndrome).

References:^[1]

The chest wall is comprised of 5 muscles, which are all innervated by the intercostal nerves:

SALT: the Serratus Anterior muscle is innervated by the Long Thoracic nerve.

• Trunk develops from paraxial and lateral plate mesoderm
• Failure of the fusion of the sternal bars results in a cleft sternum
• Ectopia cordis
  • A rare congenital condition in which the heart is completely or partially located outside the thoracic cavity
  • Impaired migration of lateral mesoderm to the midline results in incomplete fusion of the anterior chest wall
  • Associated with omphalocele, congenital diaphragmatic hernia, sternal cleft, and congenital heart defects
• Xiphoid process does not ossify until after birth, unlike the rest of the sternum
• Excessive depression and rotation of the sternum:
  • Can result in pectus excavatum, the most common congenital chest wall deformity.
  • Usually occurs sporadically, but is associated with connective tissue disorders (e.g., Marfan syndrome).

• Pectus excavatum
• Rib fracture
• Thoracotomy
• Chest tube
• Examination of the lungs
• Cardiovascular examination
• Cervical rib and thoracic outlet syndrome