Thoracic Spine Anatomy Overview

Thoracic Spine Overview

• Most stable vertebral region due to long spinous processes, thoracic cage articulations, numerous ligaments, and facet joints.
• Longest vertebral region, minimal individual segmental mobility, but multiple segments moving together allows for larger ROM.
• Serves as a stable base for head and arms, supporting upper extremity movements.
• Provides protection for the spinal cord and internal organs.

Thoracic Cage

• Composed of thoracic vertebrae, 12 pairs of ribs, and sternum.
• Functions in:
  • Protection of heart and lungs.
  • Flexibility for respiration and spinal motion.
  • Supporting the entire thoracic spine, able to sustain 4x compressive load with intact cage; without cage, T/S vertebrae alone would struggle to support head weight.

Thoracic Spine - Facet Orientation

• Upper to mid thoracic:
  • Between transverse and frontal planes, similar to lower cervical facets.
  • Dominant motion is rotation in upper and mid thoracic region.
  • Ribs limit side bending and rotation.
• Mid to lower thoracic:
  • Oriented more in sagittal plane, similar to upper lumbar facets.
  • Flexion/extension bias.

Thoracic Vertebrae Shape

• Sagittal plane: wedge-shaped, contributing to normal kyphosis.
• Transverse plane: heart-shaped.

Key Thoracic Joints

• Intervertebral discs (IVDs): provide shock absorption and flexibility.
• Costovertebral joints: articulation between ribs and vertebrae, allowing for rib movement during respiration.
• Costotransverse joints: articulation of ribs with transverse processes, supporting rib movement.
• Costosternal joints: articulation of ribs with sternum, contributing to chest expansion during breathing.

Key Support Structures

• Anterior longitudinal ligament (ALL): runs along the front of the vertebral bodies, providing stability and preventing hyperextension.
• Posterior longitudinal ligament (PLL): runs along the back of vertebral bodies, limiting flexion.
• Ligamentum flavum (LF): connects vertebral laminae, providing elastic support.
• Intertransverse ligaments (ITL): connect transverse processes, stabilizing the spine.
• Interspinous ligaments (ISL): connect spinous processes, limiting flexion and rotation.
• Supraspinous ligaments (SSL): connects spinous processes, providing additional support.

Normal Thoracic Movement

• Passive structures are elongated during extension, including the PLL and SSL.
• Vertebrae in frontal plane should be fairly vertical.
• Vertebral motion is coupled, but studies disagree on specific patterns.

Thoracic Scoliosis

• Scoliosis in the frontal plane can affect aorta location and potentially compress it.
• Scoliosis can impact the left intervertebral foramen and left IVD compression, potentially causing pain and muscle spasms.
• Pain on the right side could involve non-contractile tissue and could potentially indicate a nerve issue.

Normal Kinetics

• Thoracic kyphosis is largely due to the wedge-shaped vertebrae.
• The axis of rotation for kyphosis is going backward due to the moment of inertia of the head, increasing the flexion torque created by the muscles.

Kinematics of Ventilation

• Rib Motion
  • Elevation in sagittal plane increases anterior-posterior dimension of the thorax.
  • Elevation in frontal plane increases the transverse dimension of the thoracic cage via lateral rib movement.
  • Both frontal and sagittal plane motion occurs in lower ribs.
  • Torsion allows for energy storage & release during exhalation without muscle contraction, relying on elastic recoil of connective tissues.

Mechanics of Ventilation - The Diaphragm

• Diaphragm descent increases intra-abdominal pressure, stabilizing the central tendon of diaphragm.
• Continued costal fiber contraction creates "bucket handle" action in lower ribs.
• Diaphragm contraction pulls downward and increases the chest cavity volume.
• Increased intra-abdominal pressure necessitates abdominal muscle contraction for stability.

Normal Mechanics of Inspiration

• Normal compliance of the abdominal wall is vital for efficient inspiration.
• Conditions that increase abdominal wall compliance (spinal cord injury, pregnancy, obesity) can make efficient breathing more challenging.
• Conditions that decrease compliance (pregnancy, abdominal obesity) increase tension in abdominal tissues due to over-elongation.

Pulmonary Conditions - Diaphragm Function

• Hyperinflated lungs (COPD) flatten the diaphragm, altering muscle fiber orientation and potentially decreasing thoracic cavity size with diaphragmatic contraction.
• Diaphragmatic fibers are oriented horizontally in chronically hyperinflated lungs.

Intercostal Muscles

• Intercostal muscles are activated from cranial to caudal.
• Lower intercostals are typically activated during deep inspiration.
• Primary function of parasternals is probably stabilizing the rib cage

Scalene Muscles

• Play a role in elevating the first and second ribs and the sternum, creating the “pump handle” motion.
• Anterior and middle scalenes attach to the first rib.
• Posterior scalenes attach to the second rib.
• The scalene interval houses the brachial plexus and subclavian artery.

Conclusion: Thoracic Spine Importance

• Thoracic region has subtle, yet critical, motion for respiration and overall function.
• The entire thoracic cage and its joints contribute to complex motion.
• Abnormal postures can significantly alter forces on the spine.