Vertebral column

Plan of a typical vertebra

• A typical vertebra has a body in front (weight bearing) and a vertebral arch behind, made of two pedicles and two laminae. Together they enclose the vertebral foramen; stacked foramina form the vertebral canal.
• The arch carries seven processes: one spinous, two transverse, and four articular (superior and inferior pairs) forming the facet joints.
• The pars interarticularis is the narrow bar of lamina between the superior and inferior articular processes; a stress fracture here is spondylolysis.
• Notches above and below each pedicle form the intervertebral foramen, through which the spinal nerve leaves. Its anterior wall is the disc and its posterior wall is the facet joint, so disc bulges and facet osteophytes both narrow it.

Regional and atypical vertebrae

• The sacral hiatus exists because the laminae of S5 (often also S4) fail to fuse in the midline. It lies below the end of the dural sac (S2), so a needle passed through it enters the epidural space: this is the route of a caudal epidural block in children and for perineal analgesia.

Development and transitional anomalies

• Vertebrae form from the sclerotomes of somites. Each vertebral body forms from the caudal half of one sclerotome fused with the cranial half of the next (resegmentation), which is why spinal nerves lie between vertebrae.
• Klippel–Feil syndrome: failure of segmentation of cervical sclerotomes gives fused cervical vertebrae, a short neck, a low posterior hairline and limited movement. It often accompanies renal, cardiac (septal defects) and hearing anomalies, so these must be checked.
• Sacralisation of L5: an enlarged transverse process of L5 fuses with the sacrum on one or both sides, leaving four mobile lumbar vertebrae. Lumbarisation of S1 is the reverse (six apparent lumbar vertebrae). Counting vertebrae from C2 downward avoids surgery at the wrong level.
• Spina bifida occulta: unfused laminae, often with a tuft of hair or dimple over the lower lumbar spine; usually harmless.

Joints and ligaments

• Atlanto-occipital joint: flexion and extension (nodding, "yes"). Atlantoaxial joints: rotation ("no"), about half of all neck rotation.
• Intervertebral disc (a secondary cartilaginous joint, symphysis): outer annulus fibrosus of fibrocartilage and inner nucleus pulposus, a gel derived from the notochord. Discs form about a quarter of the column length. There is no disc between C1 and C2 or between skull and atlas.
• Facet (zygapophyseal) joints are synovial. Their orientation decides movement: thoracic facets allow rotation; lumbar facets lie nearly sagittally and limit rotation.

Upper cervical instability and dens fracture

• Rheumatoid arthritis erodes the synovial joint around the dens and weakens the transverse ligament, causing atlantoaxial subluxation. Neck flexion (as during laryngoscopy) can push the dens into the cord. Long-standing rheumatoid patients need flexion-extension films before intubation, even when they have no symptoms. Down syndrome causes the same risk through ligamentous laxity.
• Dens fracture is common in the elderly after a simple fall. A fracture through the base of the dens (type II) often fails to unite because the base is a watershed zone between apical and basal blood supply, and the fracture surface is small and largely cortical.
• Jefferson fracture: burst of the C1 ring from axial loading (diving). Hangman's fracture: bilateral fracture of the C2 pedicles (pars) from hyperextension.

Disc prolapse and root levels

• The nucleus herniates through a weakened annulus, usually posterolaterally beside the narrow posterior longitudinal ligament. Commonest at L4/5 and L5/S1, and at C5/6 and C6/7 in the neck.
• In the lumbar region a posterolateral disc misses the root exiting at its own level (which leaves high in the foramen) and compresses the root traversing to the level below: L4/5 disc affects the L5 root; L5/S1 affects S1.
• In the cervical region roots exit above their numbered vertebra (C1–C7), so the C5/6 disc affects C6 and C6/7 affects C7.

• Straight leg raise stretches L5 and S1 roots; the femoral stretch test detects L3 and L4 irritation.
• Cauda equina syndrome from a large central disc: saddle anaesthesia, urinary retention, bilateral leg signs, reduced anal tone. It needs urgent MRI and decompression.
• Whiplash (sudden flexion-extension) mostly injures soft tissues around C5/C6; a root lesion here weakens elbow flexion and shoulder abduction.

Lumbar puncture

• The spinal cord ends at L1/L2 in adults (about L3 at birth); the dural and arachnoid sac ends at S2.
• LP is done at L3/4 or L4/5. The intercristal (Tuffier's) line joining the highest points of the iliac crests crosses the L4 spine or the L4/5 space.
• Layers in order: skin, subcutaneous fat, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura, arachnoid, subarachnoid space (CSF).

Spinal disease and injury

• Spondylolisthesis: forward slip of one vertebra on the one below. The isthmic type follows a pars defect from repeated hyperextension (gymnasts, fast bowlers), usually L5 on S1. On oblique films the pars defect is a "collar on the Scottie dog". The degenerative type is commoner at L4/5 in older women and has an intact pars.
• Chance fracture: horizontal fracture through spinous process, laminae, pedicles and body, usually at the thoracolumbar junction, from flexion–distraction over a lap belt. Associated bowel and mesenteric injury is common.
• Spinal tuberculosis (Pott's disease): lower thoracic and upper lumbar spine. Infection starts at the anterior margin of the body, spreads under the anterior longitudinal ligament to the adjacent vertebra and destroys the disc, causing collapse, an angular gibbus, paravertebral or psoas abscess and paraplegia.
• Metastases usually spare the disc and destroy the pedicle ("winking owl" sign). Prostate, breast, lung, kidney and thyroid cancers spread to the spine through Batson's vertebral venous plexus, a valveless network linking pelvic, breast and abdominal veins with the vertebral veins. Raised abdominal pressure reverses flow into it.
• Ankylosing spondylitis: HLA-B27, sacroiliitis, syndesmophytes and a "bamboo spine"; morning stiffness easing with exercise.

Vertebral levels used as landmarks

Remember Lumbar discs hit the root below; cervical discs hit the root numbered by the lower vertebra; infection eats the disc, cancer spares it.

Exam trap A caudal block through the sacral hiatus is epidural, not spinal, because the dural sac ends at S2, above the hiatus.

Exam trap Do not assume a rheumatoid neck is safe because it is painless: atlantoaxial subluxation is often silent until intubation.

How FCPS asks about this

• Prostate or breast cancer in vertebrae: spread through Batson's valveless vertebral venous plexus.
• Two adjacent vertebrae destroyed with lost disc and gibbus: spinal tuberculosis (Pott's disease).
• Scottie dog with a collar, L5 slipped on S1 in a gymnast: isthmic spondylolisthesis from a pars defect.
• Fused cervical vertebrae, short neck, low hairline: Klippel–Feil syndrome, failure of segmentation of cervical sclerotomes.
• Dens base fracture not uniting in the elderly: watershed blood supply at the base of the dens.
• Rheumatoid patient before anaesthesia: exclude atlantoaxial subluxation.
• L4/5 disc prolapse: L5 root; weak big toe extension, numb dorsum of foot.