What is the anatomy of the spine?

Anatomy of the Spine

Overview of Spinal Structure

The spine is organized into distinct anatomical regions—cervical, thoracic, lumbar, and sacral—each with unique structural characteristics that determine their specific biomechanical functions and clinical vulnerabilities. 1

Regional Anatomy

Cervical Spine

• The cervical region consists of seven vertebrae (C1-C7) with specialized anatomy at the upper levels 1, 2
• The atlas (C1) transmits the load of the head to the typical cervical vertebrae, while the axis (C2) adapts the suboccipital region to the typical cervical spine 3
• Cervical intervertebral discs have a crescentic (not circumferential) anulus fibrosus that functions as an interosseous ligament in the saddle joint between vertebral bodies 3, 4
• The uncinate processes are unique to the cervical spine and play important roles in limiting lateral flexion and protecting nerve roots 4
• Cervical vertebrae rotate and translate in the sagittal plane, and rotate in the manner of an inverted cone across an oblique coronal plane 3

Thoracic Spine

• The thoracic spine consists of twelve vertebrae (T1-T12) that articulate with the ribs 1
• This region has received less research attention compared to cervical and lumbar regions, though it plays a critical role in trunk stability 3
• The thoracic spine provides structural support and protection for vital organs 5

Lumbar Spine

• The lumbar region typically contains five vertebrae (L1-L5), though anatomical variants with 4 or 6 lumbar vertebrae exist 1
• Lumbar vertebrae are identified by counting from the bottom up, with the iliac crests typically aligned with the L4-L5 intervertebral space 1
• Lumbar discs are well designed to sustain compression loads but rely on posterior elements to limit axial rotation 3
• The lumbar spine differs markedly from the cervical spine in both design and function 3

Sacral Region

• The sacrum is formed by the fusion of five vertebrae and articulates with the pelvis 1
• Sacral fractures account for only 0.16% of all pediatric trauma patients, with most being Denis zone 1 fractures located lateral to neural elements 1

Spinal Cord and Neural Elements

Embryological Development

• The neural plate elevates and folds to form the neural tube, with neural folds converging and fusing in the midline to form the spinal cord from cervical through S2 levels 6
• The cord below S2 and the filum terminale are formed from the caudal cell mass through secondary neurulation 6
• The cutaneous ectoderm separates to form overlying skin during neural tube formation 6

Spinal Cord Anatomy

• The spinal cord is protected within the spinal canal and moves within this canal during physiological spine movements 7
• The cord undergoes associated changes in its cross-sectional area during normal spine motion 7
• MRI provides optimal depiction of intraspinal contents including the epidural space and spinal cord 1

Nerve Roots

• Nerve roots exit the spinal canal at specific anatomical locations that vary by region 4
• The motor (anterior) nerve root has a specific anatomical relationship to surrounding structures 4
• The cauda equina consists of nerve roots below the termination of the spinal cord 5

Supporting Structures

Intervertebral Discs

• Discs consist of a nucleus pulposus surrounded by an anulus fibrosus, with regional variations in structure 3, 4
• The nucleus pulposus characteristics differ between cervical and lumbar regions 4
• Internal disc disruption is the most common basis for chronic low-back pain 3

Ligamentous Complex

• The posterior ligamentous complex (PLC) is the most important factor in determining management of thoracolumbar spine injuries 5
• Spinal ligaments have special characteristics that allow physiological motions while preventing excessive motion between vertebrae and protecting the spinal cord during trauma 7
• The ligaments are arranged systematically to provide stability while permitting necessary movement 7

Zygapophysial (Facet) Joints

• Cervical zygapophysial joints are the most common source of chronic neck pain 3
• These joints contain menisci that play important functional roles 4
• The orientation and structure of facet joints vary by spinal region 3

Musculature

• Spinal muscles are arranged systematically in prevertebral and postvertebral groups 3
• These muscles provide dynamic stability and enable controlled movement 3

Innervation

• The intrinsic elements of the spine are innervated by the dorsal rami of the spinal nerves and by the sinuvertebral nerves 3
• The autonomic nervous system has important anatomical presence and clinical significance in the cervical spine 4

Clinical Relevance

Biomechanical Function

• The spine serves as the main stabilizer and load bearer of the axial skeleton 5
• It provides critical protection for neural structures including the spinal cord, nerve roots, and cauda equina 5
• The experimentally determined physical properties of vertebrae, ligaments, discs, and spinal cord under various loading conditions determine their functional capacity 7

Anatomical Variants

• Lumbosacral transition vertebrae are normal anatomic variants that may require full spine imaging (radiograph, CT, or MRI) for correct identification when specific vertebral labeling verification is needed 1
• Normal variants in young children under 8 years include pseudosubluxation of C2-C3, absence of lordosis, C3 vertebral wedged appearance, widening of the atlantodental interval, prevertebral soft-tissue thickening, intervertebral widening, and pseudo-Jefferson fracture 1
• These variants can be mistaken for pathology, especially in trauma settings 2