Thoracic Wall Anatomy and Physiology Notes

Thoracic Wall

Overview of the Thoracic Wall and Cavity

• Thoracic Cavity: Space enclosed by the thoracic cage, intercostal muscles, and diaphragm. It consists of a superior thoracic aperture and an inferior thoracic aperture.

• Thoracic Wall:

  • Characteristics: Rigid yet flexible, constantly in motion.

  • Components: Skin, subcutaneous tissue, thoracic cage (ribs, costal cartilages, sternum, thoracic vertebrae, intervertebral discs), muscles (intercostals, pectorals), and fascia.

  • Functions:

    • Protection of thoracic and abdominal organs.

    • Respiration.

    • Muscle attachment.

Thoracic Apertures

Superior Thoracic Aperture

• Orientation: Slopes antero-inferiorly.

• Communication: Communicates with the neck and upper limb.

• Boundaries:

  • Posterior: Body of T1 vertebra.

  • Lateral: 1^{st} rib and its cartilage.

  • Anterior: Superior border of the manubrium.

Inferior Thoracic Aperture

• Orientation: Posterior aspect is more inferior than the anterior.

• Size: Larger than the superior aperture.

• Enclosure: Enclosed by the diaphragm.

• Function: Serves as a passageway to/from the abdominal cavity, with structures passing either posterior to or through the diaphragm.

• Boundaries:

  • Body of T12 vertebra.

  • Rib 12 and the distal end of rib 11.

  • Costal cartilages of ribs 7-10.

  • Xiphisternal joint.

• Infrasternal Angle (Subcostal Angle): Formed by the costal margin and the xiphisternal joint.

Bones of the Thoracic Wall

Ribs

• Type: Flat bones.

• Hematopoietic Tissue: Contain active hematopoietic tissue (red bone marrow).

• Classification:

  • True Ribs (Vertebrosternal Ribs): Ribs 1-7; attach directly to the sternum via their own costal cartilages.

  • False Ribs (Vertebrochondral Ribs): Ribs 8-10; attach indirectly to the sternum, usually by joining the cartilage of the rib above.

  • False, Floating Ribs (Free Ribs): Ribs 11, 12; do not attach anteriorly to the sternum or to other costal cartilages.

• Typical vs. Atypical Ribs: This classification is distinct from true/false ribs and refers to specific morphological features.

  • Typical Ribs: Ribs 3-9.

    • Head: Has 2 facets for articulating with 2 adjacent vertebrae and the intervening intervertebral disc (inferior facet articulates with the numerically corresponding vertebra).

    • Neck.

    • Tubercle: Has an articular part (articulates with the transverse process (TP) of the corresponding vertebra) and a non-articular part (attachment site for the lateral costotransverse ligament).

    • Body: Features a costal angle and a costal groove.

  • Atypical Ribs: Ribs 1, 2, 10-12.

    • 1^{st} Rib:

      • Morphology: Widest, nearly horizontal, shortest, and most sharply curved.

      • Facets: Single facet on the head (articulates only with T1 vertebral body).

      • Grooves: Grooves for subclavian vessels.

      • Tubercle: Scalene tubercle (insertion of anterior scalene muscle, separation of subclavian artery/vein).

      • Other attachments: Origin of subclavius, 1^{st} digitation of serratus anterior. T1 nerve root lies beneath the artery.

    • 2^{nd} Rib:

      • Length: Almost twice the length of the 1^{st} rib.

      • Tuberosity: Noticeable tuberosity for serratus anterior (specifically the 2^{nd} digitation) and posterior scalene, middle scalene (insertion).

    • Ribs 10-12:

      • Facets: Ribs 10-12 have a single facet on the head (articulating with their corresponding vertebral body).

      • Ribs 11 and 12: Are short and lack a neck and tubercle (meaning no articulation with transverse processes).

Costal Cartilages

• Function: Provide flexibility to the thoracic cage.

• Attachment to Sternum:

  • Directly: Ribs 1-7.

  • Indirectly: Ribs 8-10 (form the costal margin).

Thoracic Vertebrae (12)

• Features:

  • Bilateral costal facets on the postero-lateral vertebral bodies (superior and inferior costal facets for rib head articulation – can be demifacets or full facets).

  • Costal facets on transverse processes (for articulation with rib tubercle).

• Exceptions: T11 and T12 vertebrae do not have costal facets on their transverse processes.

Sternum

• Type: Flat bone, forming the middle anterior part of the thoracic cage.

• Function: Protects underlying viscera and serves for rib attachment.

• Parts:

  • Manubrium:

    • Vertebral Level: Spans vertebral level T3-4.

    • Landmarks: Jugular (suprasternal) notch (vertebral level T2), clavicular notches, synchondroses of the 1^{st} rib.

  • Body:

    • Vertebral Level: Spans vertebral level T5-9.

    • Features: Costal notches for ribs 2-7, transverse ridges (lines of fusion, representing the fusion of 4 separate sternebrae).

  • Xiphoid Process:

    • Vertebral Level: T10 (in supine position).

    • Appearance: Varies.

    • Divisions: Level of central tendon of diaphragm, inferior limit of heart, superior limit of liver.

• Important Joints/Landmarks:

  • Manubriosternal Joint (Sternal Angle of Louis): Located at vertebral level T4/5.

  • Xiphisternal Joint: Located at vertebral level T9 (in supine position).

Intercostal Spaces

• Numbering: Numbered based on the superior rib (e.g., 1^{st} intercostal space is between ribs 1 and 2$).

• Quantity: 11 intercostal spaces.

• Contents: Filled by intercostal muscles and membranes. Each space contains 2 sets of intercostal vessels but 1 nerve.

• Subcostal Space: The space below rib 12. Contains the subcostal nerve and vessels.

• Dynamics: Space dimensions change with respiration.

Joints of the Thoracic Wall

All numerical references in parentheses refer to the figures in the original source, not included in this text.

• Intervertebral Joints:

  • Type: Symphysis (secondary cartilaginous joint).

  • Articulations: Adjacent vertebral bodies bound by intervertebral disc.

  • Ligaments: Anterior and posterior longitudinal ligaments.

• Costovertebral Joints (Joints of the Head of Ribs):

  • Type: Synovial plane joint, allows gliding.

  • Articulations:

    • Ribs 2-9: Articulate with the superior costal facet of the corresponding vertebra, the inferior costal facet of the vertebra above, and the intervening intervertebral disc. These have 2 joint cavities separated by an intra-articular ligament and are supported by radiate ligaments.

    • Ribs 1, 10-12: Articulate only with their corresponding vertebral body.

  • Ligaments: Radiate and intra-articular ligaments of the head of the rib.

• Costotransverse Joints:

  • Type: Synovial plane joint, allows gliding.

  • Articulations: Between the rib tubercle and the transverse process of the corresponding vertebra.

  • Ligaments: Lateral and superior costotransverse ligaments.

  • Note: The shape of articulating surfaces of TPs differs between superior and inferior ribs. Ribs 11 and 12 do not articulate with TPs as they lack tubercles.

• Costochondral Joints:

  • Type: Primary cartilaginous joint.

  • Articulations: Lateral end of costal cartilage with the sternal end of the rib.

  • Ligaments: Cartilage and bone; bound together by periosteum. Normally, no movement occurs.

• Interchondral Joints:

  • Type: Synovial plane joint.

  • Articulations: Between costal cartilages of ribs 6-7, 7-8, 8-9.

  • Ligaments: Interchondral ligaments. Articulation between costal cartilages of 9^{th} and 10^{th} ribs is fibrous.

• Sternocostal Joints:

  • Type: Strengthened by radiate sternocostal ligaments.

    • 1^{st}: Primary cartilaginous joint (synchondrosis).

    • 2^{nd}-7^{th}: Synovial plane joints (2^{nd} joint is divided into two compartments by an articular disc).

  • Articulations: Costal cartilages with the sternum.

  • Ligaments: Anterior and posterior radiate sternocostal ligaments.

• Manubriosternal Joint:

  • Type: Secondary cartilaginous joint (symphysis).

  • Articulations: Between manubrium and body of sternum. Often fuses and becomes a synostosis in older people.

• Xiphisternal Joint:

  • Type: Primary cartilaginous joint (synchondrosis).

  • Articulations: Between xiphoid process and body of sternum.

• Sternoclavicular Joint (Upper Extremity Joint):

  • Type: Saddle type of synovial joint.

  • Articulations: Sternal end of clavicle with manubrium and 1^{st} costal cartilage.

  • Ligaments: Anterior and posterior sternoclavicular ligaments; costoclavicular ligament.

Clinical Considerations

• Rib Fracture:

  • Cause: Usually a result of blunt trauma to the thorax.

  • Common Site: Ribs 4-9 are most commonly injured.

  • Complications: Can result in injury to underlying organs (e.g., pulmonary laceration, hemothorax, pneumothorax).

  • 1^{st} Rib Fracture: Not easy to fracture; its fracture is an indicator of injury severity.

• Flail Chest:

  • Nature: Life-threatening condition.

  • Definition: Fracture of 3 or more adjacent ribs at 2 locations.

  • Mechanism: Creates a free segment of the thoracic wall.

  • Symptom: Paradoxical movement of the free segment with respiration (moves inward during inspiration and outward during expiration).

  • Consequences: Indicative of more serious underlying injuries (e.g., pulmonary contusion, cardiac tamponade), leading to respiratory compromise and hypoxia.

Muscles of the Thoracic Wall

• General Role of Intercostal Muscles: The tonus of the intercostal muscles keeps the intercostal spaces rigid, preventing them from bulging out during expiration and from being drawn in during inspiration. Their precise role in rib movement is complex and often considered largely proprioceptive.

Mechanisms of Respiration

• Pulmonary Ventilation: Depends on changes in the volume of the thoracic cavity.

  • Inspiration: Increase in volume $\to$ decrease in pressure, air flows in.

  • Expiration: Decrease in volume $\to$ increase in pressure, air flows out.

Inspiration

• Process: Muscle contraction increases the volume of the thoracic cavity.

• Diaphragm Contraction: Increases vertical dimensions.

• Rib Movements:

  • Pump Handle Movement: Increases the anterior-posterior (AP) diameter.

  • Bucket Handle Movement: Increases the transverse diameter.

• Muscles Recruited (More Effort):

  • External intercostals (elevate ribs).

  • Sternocleidomastoid (SCM).

  • Scalenes.

  • Anterior axio-appendicular muscles.

Expiration

• Process: Muscle relaxation and natural elasticity of tissue decrease the volume of the thoracic cavity.

• Forced Expiration: Requires recruitment of additional muscles.

  • Abdominals.

  • Internal intercostals (depress ribs).

Layers of the Thoracic Wall (Superficial to Deep)

1. Skin
2. Superficial fascia
3. Deep fascia
4. Serratus anterior or other regional muscle.
5. External intercostal muscle.
6. Internal intercostal muscle.
7. Innermost intercostal muscle.
8. Endothoracic fascia (a type of subserous fascia).
9. Parietal pleura.

Neurovasculature of the Thoracic Wall

Innervation (Thoracic Spinal Nerve Branches)

• Posterior (Dorsal) Ramus: Supplies skin over the back.

• Anterior (Ventral) Ramus (Intercostal Nerve):

  • Segmental Distribution:

    • Anterior and lateral cutaneous branches (sensory to skin).

    • Branches to intercostal muscles (motor).

    • Connection with sympathetic chain through rami communicantes.

    • Sensory branches to parietal pleura.

  • Notes:

    • Intercostobrachial Nerve: Formed by the lateral cutaneous branch of the 2^{nd} and/or 3^{rd} intercostal nerves. Supplies the axilla and communicates with branches of the brachial plexus.

    • Thoracoabdominal Nerves: 7^{th}-11^{th} intercostal nerves also supply the abdominal wall.

• Dermatomes:

  • C4/5: Clavicles.

  • T4: Nipples.

  • T10: Umbilicus.

Blood Supply (Arteries)

• Posterior Intercostal Arteries (one in each space):

  • Spaces 1 and 2: Arise from the superior intercostal artery (a branch of the costocervical trunk).

  • Spaces 3-11: Arise from the thoracic aorta.

  • Subcostal Artery: A branch of the thoracic aorta (below rib 12).

• Anterior Intercostal Arteries (one pair in each space):

  • Spaces 1-6: Arise from the internal thoracic artery.

  • Spaces 7-9: Arise from the musculophrenic artery.

  • Spaces 10 and 11: Lack anterior intercostal arteries.

• Other Arteries: Superior and lateral thoracic arteries (branches off the axillary artery) also supply the thoracic wall.

• Internal Thoracic Arteries (Internal Mammary Arteries):

  • Origin: Branches of the subclavian arteries.

  • Course: Descend lateral to the sternum on the interior aspect of the thoracic wall.

  • Branches:

    • Pericardiophrenic artery.

    • Anterior intercostal arteries (for the 1^{st} 6 spaces).

    • Superior epigastric and musculophrenic arteries (terminal branches).

      • Note: The musculophrenic artery gives off anterior intercostal arteries to spaces 7-9.

Intercostal Neurovascular Bundle

• Location: Found between the internal and innermost intercostal muscles, specifically within the costal groove on the inferior border of the superior rib.

• Components: Intercostal Nerve, Posterior/Anterior intercostal arteries, and veins.

• Order (Superior to Inferior): VAN (Vein, Artery, Nerve).

• Collateral Branches: Run on the superior aspect of the inferior rib.

• Subcostal: Below rib 12 are the subcostal nerve and vessels.

Venous Drainage

• Posterior Intercostal Veins: 11$$ posterior intercostal veins and the subcostal vein drain into the azygos venous system.

• Anterior Intercostal Veins: Drain into the internal thoracic and musculophrenic veins.