Vertebrae, Vertebral Column & Thoracic Cage Vocabulary

Vertebral Column: General Overview

• Total vertebrae in adult human: $(33 = 7\;C + 12\;T + 5\;L + 5\;S + 4\;Co)$
  • Cervical $C1–C7$
  • Thoracic $T1–T12$
  • Lumbar $L1–L5$
  • Sacrum (5 fused)
  • Coccyx (3–5 rudimentary, typically 4)
• Primary (kyphotic) curvatures: thoracic & sacral; present at birth
• Secondary (lordotic) curvatures: cervical (appears when infant lifts head) & lumbar (develops with standing/walking)
• Functions
  • Protect spinal cord & spinal nerves
  • Support weight of body superior to pelvis
  • Provide axis & pivot for head
  • Roles in posture & locomotion
• Ethical / clinical context
  • Knowledge crucial for spinal anesthesia, epidural injections, surgery
  • Mis-identification during procedures can cause iatrogenic injury (e.g., vertebral artery laceration)
  • Respect for patient autonomy requires clear explanation of spinal interventions

Atlas (C1)

• Ring-shaped, unique: lacks body & spinous process
• Components
  • Anterior arch with anterior tubercle
  • Posterior arch with posterior tubercle
  • Lateral masses bear:
  • Superior articular facets (kidney-shaped) articulate with occipital condyles → atlanto-occipital joints – permit “yes” (nodding) motion
  • Inferior articular facets articulate with axis ($C2$)
  • Transverse processes are large → leverage for neck muscles
  • Transverse foramina transmit vertebral arteries, veins, sympathetic plexus
  • Facet on internal surface of anterior arch for dens of axis (forms median atlanto-axial joint)
• Clinical pearls
  • Jefferson fracture = burst fracture of C1 by axial loading (e.g., diving injury)
  • No intervertebral disc between $C0/C1$ or $C1/C2$
• Mnemonic / metaphor
  • Like mythical Titan “Atlas” bearing the globe of the head

Axis (C2)

• Strongest cervical vertebra; acts as pivot for head & atlas
• Key feature: Dens (odontoid process)
  • Embryologically body of C1 migrated & fused to C2
  • Held in place against atlas by transverse ligament of atlas; cruciform ligament adds stability
• Other landmarks
  • Large bifid spinous process – palpable in nuchal region
  • Superior articular facets accommodate lateral masses of C1
  • Transverse foramina smaller than $C1$ but still transmit vertebral artery
• Pathology
  • Fracture types: tip, waist, base (high-risk non-union at base)
  • Rupture of transverse ligament → atlanto-axial subluxation → potential spinal cord compression

Typical Cervical Vertebrae ($C3–C6$) & Special Case $C7$

• Shared traits
  • Small, rectangular bodies with uncinate processes
  • Large triangular vertebral foramen
  • Transverse processes pierced by transverse foramen ($C1–C6$ vertebral artery; $C7$ usually transmits only accessory veins)
  • Short bifid spinous processes (except $C7$)
• C7 (Vertebra Prominens)
  • Long, non-bifid spinous process → visible & palpable midline landmark
  • Transitional features: resembles thoracic vertebra (long spinous process) yet retains cervical transverse foramen (often small)
• Applied anatomy
  • Surface landmark for counting vertebrae, locating interspaces for central line placement or epidural
  • Degenerative uncovertebral joints (Joints of Luschka) can cause cervical radiculopathy

Thoracic Vertebra (Example: $T10$ as shown)

• Body: heart-shaped, bears complete &/or demi-costal facets for rib heads
• Vertebral foramen: circular & smaller than cervical/lumbar
• Transverse processes
  • $T1–T10$ possess costal facets for rib tubercles (at $T11–T12$ they disappear)
• Spinous processes: long, slope inferiorly → overlapping “shingle” arrangement
• Articular facets
  • Superior face posteriorly; inferior face anteriorly → limits flex/ext, allows rotation (why thoracic spine relatively rigid)
• Example $T10$
  • Often has single whole facet for 10th rib on body
  • Transitional toward $T11–T12$ (may lack facet on transverse process)
• Clinical notes
  • Compression fractures common with osteoporosis → kyphotic deformity
  • Scoliosis often apexes in thoracic region → cardiopulmonary compromise if severe

Lumbar Vertebrae (Example: $L3$ & $L5$)

• Largest vertebrae – bear weight of upper body
• Body: large, kidney-shaped
• Vertebral foramen: triangular
• Spinous processes: short, sturdy, hatchet-shaped, project posteriorly
• Transverse processes: long & slender; $L5$ processes shorter but massive
• Mammillary processes on posterior aspect of superior articular processes – attachment for multifidus & intertransversarii
• Orientation of articular facets (medial/lateral) → flexion-extension permitted, rotation limited
• $L5$
  • Largest body; wedge-shaped → lumbosacral angle (sacral promontory)
  • Common site of spondylolysis (pars interarticularis fracture) & spondylolisthesis (forward slip)
• Clinical relevance
  • Lumbar puncture performed between $L3–L4$ or $L4–L5$ to avoid spinal cord (terminates at $L1/L2$ in adults)

Sacrum

• Formed by fusion of $5$ sacral vertebrae; triangular, wedged between hip bones
• Base articulates with $L5$ (lumbosacral joint)
• Apex articulates with coccyx
• Landmarks
  • Sacral promontory: anterior projecting edge of $S1$ body; obstetric landmark
  • Ala (wing) = fused costal & transverse elements
  • Auricular surface → synovial part of sacro-iliac joint
  • Anterior & posterior sacral foramina transmit ventral/dorsal rami of sacral nerves
  • Sacral canal continuous with vertebral canal; ends at sacral hiatus (site for caudal epidural anesthesia)

Coccyx

• Typically 4 small vertebrae (first may be separate, remaining fused)
• Provides attachment for pelvic floor muscles (coccygeus, levator ani)
• Trauma (fall onto seated position) → coccydynia

Vertebral Column Curvatures (Fig. 12-13)

• Cervical lordosis: $\approx 20–40^{\circ}$
• Thoracic kyphosis: $\approx 20–45^{\circ}$
• Lumbar lordosis: $\approx 40–60^{\circ}$
• Sacral kyphosis fixed
• Abnormalities
  • Kyphosis (humpback)
  • Lordosis (swayback)
  • Scoliosis (lateral & rotational deviation)
• MRI sagittal view shows alignment, intervertebral discs, spinal cord termination

Intervertebral Foramina & Discs

• Foramen formed between adjacent pedicles – exit of spinal nerves
• Disc components
  • Annulus fibrosus (concentric fibrocartilage)
  • Nucleus pulposus (gelatinous remnant of notochord)
• Disc thickness increases inferiorly → contributes to lumbar lordosis
• Herniation typically posterolateral where annulus thin & PLL narrow → compresses nerve roots one level below (e.g., $L4/L5$ herniation affects $L5$ root)

Thoracic Cage (Fig. 12-15)

• Components
  • Sternum: manubrium, body, xiphoid process
  • Ribs: $12$ pairs
  • True ribs $1–7$: vertebro-sternal (direct costal cartilage)
  • False ribs $8–10$: vertebro-chondral (indirect via common cartilage)
  • Floating ribs $11–12$: vertebral; no anterior attachment
  • Costal cartilages confer elasticity
• Sternal angle (Angle of Louis)
  • Junction manubrium/body; level of $2^{nd}$ costal cartilage & $T4/T5$ intervertebral disc
  • Landmark for transverse thoracic plane (bifurcation of trachea, start/end of aortic arch)
• Clinical notes
  • CPR hand placement over lower half of sternal body to avoid xiphoid fracture & liver injury
  • First rib rarely fractured (protected) – suspect severe trauma if broken

Biomechanics & Real-World Relevance

• Load transmission: $\text{Ground reaction force} \rightarrow \text{hip} \rightarrow \text{sacrum} \rightarrow \text{lumbar} \rightarrow \ldots \rightarrow \text{skull}$
• Flexion/extension greatest in cervical & lumbar regions; rotation greatest in cervical & thoracic
• Occupational hazards: improper lifting → disc herniation (lumbar), neck strain (cervical)
• Athletic context: axial compression (diving) → C-spine fractures; rotational loads (golf, baseball) stress lumbar facets
• Ageing: loss of disc hydration $\Rightarrow$ height loss, increased kyphosis
• Philosophical/ethical: balancing mobility vs. protection exemplifies evolutionary compromise; underscores duty to maintain spinal health through ergonomics, exercise, & preventive care