Vertebral Spine Movement

Motion and Mobility Segment

• The vertebral column consists of passive segments (bony and ligamentous structures) and active segments (mobility features).
• The mobile segment includes intervertebral discs, intervertebral foramina, facet joints, ligamentum flavum, and interspinous ligaments.
• Mobility of this active segment enables vertebral spine movements.
• Functional link between anterior and posterior pillars is formed by pedicles.

Structural Mechanics

• Each vertebra has a trabecular structure forming a first-class lever where articular processes act as the fulcrum.
• Axial compression forces are cushioned directly by intervertebral discs and indirectly by paravertebral muscles.
• Overall cushioning effects combine both passive and active mechanisms.

Available Movements

• Interbody joints permit multiple movements:
  • Gliding: movements include anterior-posterior, medial-lateral, and torsional.
  • Distraction and compression.
  • Rotation in anterior-posterior and lateral directions.
• Mobility includes three translational movements and three angular movements, summing to six degrees of freedom (DoF).

Stability of the Spine

• The stiffness of the vertebral column relates to its capacity to resist loads like axial compression, tension, bending, torsion, and shear stress during various activities.
• Resistance to these forces varies by spinal region and is influenced by loading type, duration, age, posture, and structural condition.

Axial Compression Dynamics

• Caused by gravity, ground reaction forces, and muscle contractions.
• Sustained compressive loading leads to loss of fluid from the nucleus pulposus and annulus fibrosus.
• Fluid reabsorption occurs when compressive forces diminish, explaining height changes from morning to evening.

Effects of Bending

• Forward flexion results in anterior structures being tensioned while the posterior structures face compression.
• Collagen fibers in the posterior anulus fibrosus play a role in providing flexibility and stability during motion.
• In lateral bending, compression occurs on the ipsilateral disc side, while the contralateral side provides resistance against extreme motion.

Prolonged Mechanical Forces

• Prolonged compressive forces can shift loads from the nucleus pulposus to the annulus fibrosus, risking buckling or prolapse.
• Creep leads to elongation of spinal structures, diminishing stability and increasing injury risk.

Torsion and Its Effects

• Torsional forces arise during axial rotation and are highest at the thoracolumbar junction.
• Torsion resistance is mainly provided by the outer layers of vertebral bodies, intervertebral discs, and facet joint orientations.
• The lumbar region plays a crucial role in resisting torsion but is at heightened risk when combined with heavy axial compression and bending forces.

Learn about the mobile and passive segments in the vertical plane of the vertebral spine, including bony and ligamentous structures, intervertebral discs, and facet joints. Understand the mobility of the active segment and its role in spinal movements.