Vertebral Column: Structure, Function, and Requirements

Mechanical Changes in the Disk

• Prolonged forces can cause mechanical changes in the disk, resulting in creep loading, which affects the neural arch and ligaments, especially the apophyseal joints.
• This leads to large compressive and bending forces on the neural arch and ligaments.

Effects of Creep Loading

• Creep loading causes a shift in load from the nucleus pulposus to the annulus fibrosus, especially the posterior aspects.
• This increased load can cause buckling or prolapse of the annulus fibrosus.
• Elongation of the supporting structures due to creep leads to a lack of stability and places the vertebral structures at risk of injury.

Torsional Forces

• Torsional forces are created during axial rotation, which is a part of the coupled motions.
• The highest torsional stiffness is found at the thoracolumbar junction.
• Torsional stiffness is provided by the outer layers of both the vertebral bodies and intervertebral disks, and by the orientation of the facets.
• Half of the annulus fibrosus fibers resist clockwise, and the other half resist counterclockwise rotations.

Shear Forces

• Shear forces act on the midplane of the disk and tend to cause each vertebra to move anteriorly, posteriorly, or from side to side in relation to the inferior vertebra.
• In the lumbar spine, the apophyseal joints resist some of the shear force, while the disks resist the remainder.

The Vertebral Column

• The vertebral column requires both rigidity and plasticity to function.
• Rigidity is necessary for maintaining structure and stability, while plasticity allows for adaptation and movement.
• The vertebral column can be thought of as a ship's mast, with ligamentous and muscular tighteners acting as stays to maintain its shape.

Coupling Movements

• Movement of the vertebral column in one plane is usually associated with an automatic and nearly imperceptible movement in another plane.
• This phenomenon is called spinal coupling.
• Pure lateral flexion and pure rotation do not occur in any region of the spine.
• Coupling patterns involve an association between lateral flexion and ipsilateral axial rotation in the middle and lower cervical spine.

Ligamentous Support

• Ligaments provide support to the spine by resisting distraction, translation, and rotation of vertebral bodies.
• Different ligaments have specific functions and regions of influence, including:
  • Anulus fibrosus (outer fibers)
  • Anterior longitudinal ligament
  • Posterior longitudinal ligament
  • Ligamentum flavum
  • Posterior atlantoaxial ligament
  • Supraspinous ligaments
  • Interspinous ligaments
  • Ligamenta flava

Conective Tissues that Limit Motions

• Flexion is limited by ligamentum nuchae, interspinous and supraspinous ligaments, ligamenta flava, capsule of the apophyseal joints, posterior annulus fibrosus, and posterior longitudinal ligament.
• Extension is limited by cervical viscera (esophagus and trachea), anterior annulus fibrosus, and anterior longitudinal ligament.
• Lateral flexion is limited by intertransverse ligament (lumbar region), contralateral annulus fibrosus, and contralateral capsule of the apophyseal joints.
• Axial rotation is limited by alar ligament (craniocervical region), annulus fibrosus, and capsule of the apophyseal joints