Human Vertebral Column Biomechanics and Anatomy

Spine Structure and Biomechanics

Primary Functions of the Spine

• Support and movement of the trunk.
• Protection of the spinal cord.
• Absorption of shock between the ground and the brain.

Vertebral Column Composition

The vertebral column consists of 33 vertebrae:

• 7 Cervical vertebrae
• 12 Thoracic vertebrae
• 5 Lumbar vertebrae
• 5 Fused Sacral vertebrae
• 4 Fused Coccygeal vertebrae

The Motion Segment

A motion segment consists of two adjacent vertebrae and the intervening soft tissue. Each segment contains three articulations:

• The Intervertebral Disc (a symphysis joint).
• The right and left Facet Joints (gliding synovial joints).

Intervertebral Discs

These are fibrocartilage structures connecting adjacent vertebrae, accounting for 20–25% of the spine's total height.

Annulus Fibrosus

Composed of concentric, cartilaginous rings. It is highly effective at resisting torsion, tension, and shear forces.

Nucleus Pulposus

A central structure composed of cartilage, collagen, proteoglycans, and approximately 90% water. It is highly effective at resisting compression.

Facet Joints (Zygapophyseal Joints)

Formed between the inferior articular process of the superior vertebra and the superior articular process of the inferior vertebra. These gliding synovial joints bear approximately 30% of the compressive load and 80% of the shear and torsional loads.

Spinal Ligaments

• Ligamentum Flavum: Connects the lamina of adjacent vertebrae. It contains less collagen and more elastin. It is in tension when the spine is in the anatomical position, which helps prestress the discs.
• Longitudinal Ligaments: Connect the vertebral bodies.
  • Anterior Longitudinal Ligament (ALL): Strong; resists extension.
  • Posterior Longitudinal Ligament (PLL): Weak; resists flexion.
• Supra- and Intraspinous Ligaments: Connect the spinous processes and resist flexion. They are enlarged in the cervical region.
• Intertransverse Ligaments: Connect adjacent transverse processes, resisting lateral flexion and twisting.

Spinal Shear

Spinal shear describes the movement of one vertebra relative to the vertebra immediately beneath it.

Spinal Curves and Postural Deviations

Primary Curves (Present at Birth)

Located in the thoracic and sacral regions (Kyphosis).

Secondary Curves (Develop Later)

Located in the cervical and lumbar regions (Lordosis).

Postural Deviations

• Lumbar Lordosis: Exaggerated lumbar curvature, often caused by weak muscles (abdominals, hip extensors) or tight muscles (hip flexors, lumbar extensors).
• Thoracic Kyphosis: Exaggerated thoracic curvature, often caused by weak muscles (thoracic extensors) or tight muscles (anterior chest muscles).

Spinal Movements and Musculature

Ranges of Motion (Approximate)

• Sagittal Movement:
  • Flexion: 45–50 degrees
  • Extension: 60–85 degrees
• Transverse Movement:
  • Axial Rotation: 80–90 degrees (each way)
• Frontal Movement:
  • Side Bending (Lateral Flexion): 40–45 degrees (each way)

Cervical Musculature

Anterior Cervical Muscles

• Sternocleidomastoid: Acts in all three planes of motion.
• Scalenes: Bilateral action causes flexion; unilateral action causes side bending toward the active side.
• Deep Cervical Flexors: Bilateral action causes flexion.

Posterior Cervical Muscles

• Splenius (Capitis and Cervicis):
  • Bilateral action causes extension.
  • Unilateral action causes lateral flexion and rotation toward the active side.

Trunk and Spinal Muscles

Posterior Spinal Muscles (Extensors)

• Erector Spinae Group: (Spinalis, Longissimus, Iliocostalis)
• Deep Spinal Extensors

Anterior Spinal Muscles (Flexors)

• Abdominals (Abs): Primarily responsible for spinal flexion and posterior pelvic tilt.