What is the anatomy of the spine?

Anatomy of the Spine

The spine consists of 33 vertebrae organized into five distinct regions: cervical (7), thoracic (12), lumbar (5), sacral (5 fused), and coccygeal (4 fused), forming a protective column for the spinal cord while enabling mobility and load-bearing.

Vertebral Structure and Regional Characteristics

Cervical Spine (C1-C7)

• Atlas (C1): Ring-shaped vertebra that transmits the load of the head to the typical cervical vertebrae 1
• Axis (C2): Features the odontoid process (dens) that adapts the suboccipital region to the typical cervical spine 1
• C3-C7: Typical cervical vertebrae with unique features:
  • Bifid spinous processes
  • Foramen in each transverse process for vertebral arteries
  • Uncinate processes that form uncovertebral joints
  • Crescentic anulus fibrosus in intervertebral discs (not circumferential) 1
• Allows greatest range of motion in the spine, with rotation across an oblique coronal plane 1

Thoracic Spine (T1-T12)

• Distinguishing features:
  • Facets for articulation with ribs (costovertebral joints)
  • Long, downward-angled spinous processes
  • Limited mobility due to rib cage attachment
• Primarily allows rotation with limited flexion/extension 1

Lumbar Spine (L1-L5)

• Largest vertebral bodies designed to bear weight
• Thick, rectangular spinous processes
• Strong, well-developed discs designed to sustain compression loads 1
• Relies on posterior elements to limit axial rotation 1
• The conus medullaris (end of spinal cord) typically ends at the L1-L2 disc space by 2 months after birth 2

Sacrum (S1-S5)

• Five fused vertebrae forming a triangular structure
• Articulates with the ilium at sacroiliac joints
• Contains sacral foramina for nerve roots
• Sacral fractures account for only 0.16% of pediatric trauma cases 2

Coccyx

• 3-5 fused rudimentary vertebrae
• Serves as attachment for pelvic floor muscles

Spinal Canal and Neural Elements

Spinal Cord

• Extends from the foramen magnum to the conus medullaris (typically at L1-L2 level)
• Protected within the spinal canal
• Gives rise to 31 pairs of spinal nerves:
  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccygeal

Nerve Roots

• Exit through intervertebral foramina
• Cervical nerve roots exit above their corresponding vertebrae (except C8)
• Thoracic and lumbar nerve roots exit below their corresponding vertebrae
• Compression of nerve roots in foraminal stenosis causes radicular pain, sensory changes, and motor weakness in the affected limb 3

Supporting Structures

Intervertebral Discs

• Composed of:
  • Nucleus pulposus (gelatinous center)
  • Anulus fibrosus (fibrous outer ring)
• Function as shock absorbers and allow movement
• Cervical discs: crescentic anulus fibrosus serves as an interosseous ligament 1
• Lumbar discs: well-designed to sustain compression loads 1
• Internal disc disruption is the most common basis for chronic low-back pain 1

Ligaments

• Anterior longitudinal ligament: runs along anterior vertebral bodies
• Posterior longitudinal ligament: runs along posterior vertebral bodies
• Ligamentum flavum: connects laminae
• Interspinous ligaments: connect spinous processes
• Supraspinous ligament: connects tips of spinous processes
• These ligaments allow physiological motions while preventing excessive movement 4

Spinal Muscles

• Arranged systematically in prevertebral and postvertebral groups 1
• Local system: muscles with insertion or origin at lumbar vertebrae
• Global system: muscles with origin on pelvis and insertions on thoracic cage 5

Biomechanical Considerations

Spinal Stability

• Defined as the ability of the spine to maintain its alignment during physiologic loads 6
• Depends on:
  • Bony structures
  • Ligamentous restraints
  • Muscular support
  • Neural control
• Damage to any spinal structure can lead to instability 6

Regional Biomechanics

• Cervical spine: allows flexion, extension, lateral bending, and rotation
• Thoracic spine: limited mobility due to rib attachments
• Lumbar spine: primarily allows flexion, extension, and limited lateral bending
• The global muscle system handles different distributions of outer forces, while the local system maintains posture 5

Clinical Relevance and Pathology

Congenital Malformations

• Split cord malformations: spinal cord splits into two separate "hemicords" 2
• Myelocystocele: dilated terminal spinal cord within a dilated distal dural sac 2
• Dermal sinus tracts: midline congenital epithelial-lined tracts 2

Traumatic Injuries

• CT is the gold standard for identifying fractures of the thoracolumbar spine (sensitivity 94-100%) 2
• MRI is superior for evaluating ligamentous injuries and spinal cord damage 2
• Children may have cartilaginous injuries better detected with MRI than radiographs 2

Degenerative Conditions

• Spinal stenosis: narrowing of the spinal canal or neural foramina
• MRI without IV contrast is the first-line imaging modality for evaluating foraminal stenosis 3

Imaging Considerations

• Radiography: limited sensitivity (49-62% for thoracic spine fractures, 67-82% for lumbar spine fractures) 2
• CT: excellent for bone detail (94-100% sensitivity for thoracolumbar fractures) 2
• MRI: best for soft tissue, ligaments, discs, and neural elements 2, 3
• DXA scanning: used to assess bone mineral density in specific regions (L1-L4, hip) 2

Understanding the complex anatomy and biomechanics of the spine is essential for diagnosing and treating spinal disorders, as well as for developing effective surgical approaches and rehabilitation strategies.