Vertebral column

The vertebral column extends from the skull to the pelvis and consists of 33 vertebrae, which are differentiated into five regions: the cervical spine (C1–C7), the thoracic spine (T1–T12), the lumbar spine (L1–L5), the sacrum (S1–S5; fused in adults), and the coccyx (3–5 fused bones). The functions of the vertebral column include protecting the spinal cord within the vertebral canal; transferring the weight of the upper body to the pelvis; articulating with the skull, ribs, and pelvis; and providing attachment for musculature. It is, furthermore, the main site of hematopoiesis besides the pelvis. The basic vertebra consists of a vertebral body (anterior), a vertebral arch (posterior), and a vertebral foramen, through which runs the spinal cord. More specific morphological features vary according to the region and associated function. Adjacent vertebrae articulate via intervertebral discs and facet joints, and there are specialized joints in the upper cervical region, the thoracic region, and between the sacrum and pelvis for articulation with the head and neck, the ribs, and the hip respectively. Embryologically, the vertebrae are derived from the somites of the paraxial mesoderm, and the nucleus pulposus of the intervertebral discs is derived from the notochord.

Overview ^[1][2]

• Characteristics
  • The vertebral column consists of 33 vertebrae.
  • Differentiated into cervical, thoracic, lumbar, sacral, and coccygeal region
  • Has a double-S shape.
    • Curves convex anteriorly in the cervical and lumbar regions (lordosis)
    • Curves convex posteriorly in the thoracic and sacral regions (kyphosis)
• Function
  • Protection of the spinal cord, spinal nerve roots, and associated vasculature
  • Transfer of weight from the upper body to the pelvis
  • Articulation with the skull, ribs, and pelvis
  • Articulations between vertebrae allow for flexion, extension, rotation, and lateral flexion of the trunk
  • Attachment site for muscles, including the diaphragm and pelvic floor
  • Main site of hematopoiesis besides the pelvis

Vertebrae ^[1][2]

Basic vertebral structure

The basic vertebra consists of a vertebral body anteriorly and vertebral arch posteriorly, which together surround the vertebral foramen.

Vertebral body

• Cylindrical anterior part of the vertebra responsible for weight bearing
• Contains red bone marrow
• Articulates with the vertebral bodies above and below via the intervertebral disc

Vertebral arch

Consists of two pedicles (laterally) and two laminae (posteriorly) with several vertebral processes

• Vertebral pedicles
  • Lateral parts of the vertebral arch, between the vertebral body and the laminae
  • Concavities on the cranial and caudal sides (vertebral notches) form the intervertebral foramina
• Vertebral laminae
  • Posterior part of the vertebral arch, between transverse processes and spinous process
  • Attachment site of the ligamentum flavum from C2/3 to the sacrum
• Intervertebral foramina
  • Lateral space between the pedicles of two adjacent vertebrae, from which spinal nerves and their vessels emerge
    • C1–C7 nerves exit superior to the pedicles of C1–C7.
    • C8 nerves exit inferior to the pedicles of C7 because there is no C8 vertebra.
    • The T1 nerve and the nerves below exit inferior to the pedicle of the corresponding vertebral body.
• Spinous process: single posterior midline process emerging at the junction of the two laminae
• Transverse processes
  • Postero-lateral processes emerging at the junction of the pedicles and laminae on each side
  • The transverse processes of the lumbar vertebrae are also called costal processes.
• Articular processes
  • Flat surfaces at the junction of the pedicles and laminae, two superiorly and two inferiorly, that articulate with the articular processes of the adjacent vertebrae, forming the zygapophyseal (facet) joints
  • The region between the superior and inferior articular processes at the junction of the pedicle and lamina is called the pars interarticularis.

Vertebral foramen

• Central space between the vertebral arch and vertebral body
• All vertebral foramina together form the vertebral canal, containing:
  • The spinal cord
  • Meninges
  • Nerve roots
  • Blood vessels

Specific vertebrae

• C1 (atlas)
  • Has no vertebral body: consists of an anterior and a posterior arch
  • Articulates with the occiput cranially and the axis caudally, see “atlanto-occipital joint” and “atlanto-axial joint” below
• C2 (axis)
  • Characterized by the odontoid process (dens), which projects cranially from the vertebral body into the vertebral foramen of the atlas
  • Articulates with the atlas cranially, see “atlanto-axial joint” below
• Cervical vertebrae
  • The vertebral arteries, veins, and sympathetic fibers pass through the transverse foramina in the transverse processes of C6–C1 before entering the foramen magnum.
  • Have large the largest vertebral foramina to accommodate the cervical enlargement of the spinal cord.
  • C2–C6 have bifid spinal processes.
  • C7 (vertebra prominens) has a large, easily palpable spinal process to which the ligamentum nuchae attaches.
• Thoracic vertebrae
  • Demifacets on the vertebral bodies and transverse costal facets on the transverse processes articulate with the ribs, see costovertebral joints and costotransverse joints below.
  • Have the smallest vertebral foramina, as the spinal cord is thinnest in the thoracic region.
• Lumbar vertebrae
  • Have the largest vertebral bodies to support the weight of the body
  • The spinal cord terminates at approximately the level of L1.
  • The vertebral foramina are larger than in the thoracic region but smaller than in the cervical region.
  • L5 articulates with the sacral promontory.
• Sacrum
  • Formed by 5 fused vertebrae; forms the posterosuperior wall of the pelvis
  • Articulates with the L5 cranially, the ilia laterally, and the coccyx caudally
  • The sacral nerves emerge through the anterior and posterior sacral foramina between the central and lateral masses.
  • The female sacral promontory is smaller and the sacrum is shorter and less curved than the male one, contributing to the larger pelvic inlet required for childbirth.
  • The sacral hiatus is the caudal opening of the vertebral canal on the dorsal surface of the sacrum at the level of S5 medial to the sacral horns (cornua), it is the exit point for the S5 nerves.
• Coccyx
  • 3–5 fused rudimentary vertebrae, articulates with the horns (cornua) of S5
  • The filum terminale attaches to the first segment of the coccyx.
  • Site of attachment for the levator ani muscles and coccygeus muscle, which form the pelvic floor

Specific vertebra levels are associated with key anatomical landmarks: e.g., C4 (bifurcation of common carotid artery), T2 (aortic arch), T4 (bifurcation of trachea), L1 (end of spinal cord in adults), L3 (end of spinal cord in newborns), and L4 (level of iliac crest; bifurcation of aorta).

Joints of the spine ^[1][2]

• Atlanto-occipital joints
  • Synovial joints between the superior articular facets of the atlas and the occipital condyles
  • Allow for flexion and extension of the head
• Atlanto-axial joints
  • Three synovial joints:
    • Two joints between the lateral masses of the atlas and axis
    • One joint between the odontoid of the axis and the anterior arch of the atlas anteriorly and the transverse ligament of the atlas posteriorly
  • Allow for rotation of the atlas (and thus the head) around the odontoid
• Intervertebral joints (C2–S1)
  • Adjacent vertebral bodies are linked by fibrocartilage intervertebral discs, which function as shock absorbers.
  • They consist of:
    • Nucleus pulposus: gelatinous core of the vertebral disk
    • Annulus fibrosus: surrounding fibrocartilaginous lamina
  • The vertebral endplate is a thin layer of hyaline cartilage between the vertebra and the disc.
  • Discs are thickest in the lumbar region and thinnest in the upper thoracic region.
  • There is no intervertebral disk between the atlas and the axis.
• Facet joints (zygapophysial joints)
  • Synovial joints between the articular processes
  • Together these joints allow for flexion, extension, rotation, and lateral flexion, mostly in the cervical and lumbar regions.
• Costovertebral joints
  • Synovial joints between costal facets on the vertebral bodies and the heads of the ribs
    • The superior demifacet articulates with the head of the rib corresponding to the vertebral level.
    • The inferior demifacet articulates with the head of the rib inferior to the vertebral level.
• Costotransverse joints: synovial joints between the transverse costal facets on the transverse processes of T1 to T9/T10 and the rib tubercle of the rib inferior to the vertebral level
• Sacroiliac joints
  • Synovial joints between the articular surfaces of the ilium and sacrum
  • Limited range of movement due to the irregularity of the interlocking surfaces of each bone
  • See “Joints of the pelvis.”

Atlanto-axial instability is the loss of ligamentous stability between atlas (C1) and axis (C2), which may cause the odontoid process to compress the spinal cord, medulla, or vertebral arteries when the neck is flexed. It is most commonly caused by Down syndrome, rheumatoid arthritis, or trauma. ^[3]

Degeneration of the intervertebral discs with age is common and may lead to back pain, radiculopathy, or cauda equina syndrome.

Ligaments of the spine

The narrowing of the posterior longitudinal ligament leaves the annulus fibrosus without support in the posterolateral region, increasing the likelihood of disk herniations in this region, which may cause compression of the spinal nerve at the level below (e.g., herniation of the L4/5 disk compresses the L5 nerve)!

In ankylosing spondylitis, calcification of the spinal ligaments and intervertebral discs causes fusion and immobility of the spine!

During lumbar puncture, the needle pierces the following structures in order before it reaches the subarachnoid space: skin, subcutaneous tissue, supraspinal ligament, interspinal ligament, ligamentum flavum, epidural space, dura mater, and arachnoid mater.

Muscles

• The intrinsic muscles of the back are innervated by dorsal rami of the spinal nerves and enable extension, rotation, and lateral flexion of the head and spine.
• For other muscles attaching to the vertebrae, see:
  • Deep muscles of the neck
  • Muscles of the shoulder joint
  • Chest wall musculature
  • Crura of the diaphragm
  • Muscles of the posterior abdominal wall
  • The gluteal region
  • Pelvic floor

Vasculature and lymphatics

The lack of valves of the venous plexuses facilitates spread of malignant cells (e.g., from prostate cancer) and bacteria in the bloodstream to the vertebrae, leading to metastases and osteomyelitis.

Innervation

• Vertebrae, facet joints, superficial ligamenta flava, and overlying skin: dorsal rami of the spinal nerves
• Walls of the vertebral canal, dura mater, outer annulus of intervertebral discs, and deep ligamenta flava: sinuvertebral nerves
• Atlanto-occipital and atlanto-axial joints: ventral rami of the C1 and C2 nerves

See “Bone tissue,” “Bone marrow,” and “Connective tissue.”

The vertebral column is derived from the somites of the paraxial mesoderm and begins developing in the 4^th week.

• Vertebral bodies
  • Sclerotome cells migrate from the somite toward the midline and surround the notochord, forming the vertebral body.
  • The vertebral bodies are each derived from parts of two sclerotomes
    • The caudal part of the upper sclerotome
    • The cranial part of the lower sclerotome
  • Sclerotome migrates dorsally to form the vertebral arch, surrounding the neural tube
  • Ossification starts in 8^th week of embryonic development
• Intervertebral discs
  • The notochord develops into the nucleus pulposus of each disc
  • Mesenchyme at the center of the sclerotome develops into the annulus fibrosus
• Development of spinal curvatures
  • In utero, the spine is C-shaped with thoracic and sacral kyphoses.
  • Cervical lordosis develops when a child begins to hold their head up (from 1 month of age).
  • The lumbar lordosis develops when a child begins to walk (from 12 months).

• Congenital and developmental abnormalities
  • Spina bifida
  • Craniovertebral junction anomalies
  • Idiopathic scoliosis
  • Scheuermann juvenile kyphosis
  • VACTERL association
  • Atlanto-axial instability in Down syndrome
• Degenerative disorders
  • Degenerative disk disease
  • Osteoporosis
  • Spinal stenosis
• Infectious diseases
  • Vertebral osteomyelitis
  • Tuberculosis (Pott disease)
• Inflammatory conditions
  • Ankylosing spondylitis
  • Seronegative spondyloarthropathies
  • Rheumatoid arthritis
• Trauma
  • Vertebral fractures
  • Spondylolisthesis
• Neoplasia
  • Bone metastasis
  • Primary bone neoplasms
    • Osteoblastoma
    • Chordoma
  • Hematological malignancies
    • Multiple myeloma
    • Acute leukemia
    • Chronic myeloid leukemia
    • Chronic lymphocytic leukemia
• Neurological disorders
  • Radiculopathy
  • Spinal cord compression
  • Conus medullaris syndrome
  • Cauda equina syndrome
• Medical procedures
  • Spinal anesthesia
  • Epidural anesthesia
  • Lumbar puncture