Back, Vertebral Landmarks & Dermatomes
Count the levels, find the root, and read the body's surface map
The vertebral column and craniovertebral junction
Counting the column. The spine has 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and about 4 fused coccygeal. They carry 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal). The thoracic and sacral curves are primary (present at birth), whereas the cervical and lumbar lordoses are secondary curves that develop as the infant holds up its head and begins to walk. C7, the vertebra prominens, has the most easily palpated spinous process at the base of the neck.
Atlas and axis. C1 (the atlas) is a ring of bone with no body and no spinous process. Its articulation with the occipital condyles, the atlanto-occipital joint, allows flexion and extension, the "yes" movement. C2 (the axis) sends the dens (odontoid process) upward against the anterior arch of C1, where the transverse ligament of the atlas holds it in place. The atlantoaxial joint provides about half of all cervical rotation, the "no" movement.
Atlantoaxial instability. When the transverse ligament is lax or destroyed, the dens can slide backward and compress the upper cervical cord. Ligamentous laxity explains the risk in Down syndrome; in rheumatoid arthritis, inflammatory pannus erodes both the ligament and the dens. Warning signs include neck pain, torticollis, gait change, hand clumsiness, and hyperreflexia. In long-standing rheumatoid arthritis, the cervical spine is assessed before intubation, because neck extension during laryngoscopy can injure an unstable junction.
A Jefferson fracture is a burst of the C1 ring from axial loading, as in diving into shallow water; a hangman's fracture is a bilateral fracture of the C2 pars interarticularis from hyperextension.
Nerve roots and disc herniation
Where the roots exit. Because there are eight cervical nerves but only seven cervical vertebrae, the numbering switches at the cervicothoracic junction. Roots C1–C7 exit above their same-numbered vertebra (the C6 root leaves through the C5–C6 foramen), C8 exits between C7 and T1, and from T1 downward every root exits below its vertebra (the L4 root leaves through the L4–L5 foramen).
Disc anatomy. Each intervertebral disc has a gel-like nucleus pulposus, derived from the notochord, surrounded by concentric fibrocartilage lamellae of the annulus fibrosus. With age-related degeneration, fissures in the annulus let the nucleus herniate. Most herniations are posterolateral, because the posterior longitudinal ligament is strongest in the midline and thin and narrow at its lateral edges. Chordoma, a midline tumor of the clivus or sacrum, arises from notochord remnants.
The lower-root rule. A posterolateral herniation compresses the root numbered for the lower vertebra of the pair: C5–C6 affects C6, C6–C7 affects C7, L4–L5 affects L5, and L5–S1 affects S1. In the lumbar spine, the same-numbered root has already exited high in its foramen, above the disc, so the bulge catches the root traversing to the next level. A far-lateral (foraminal) herniation is the exception and compresses the exiting root. L4–L5 and L5–S1 account for the large majority of lumbar herniations, and C5–C6 and C6–C7 for most cervical ones. Lumbosacral radiculopathy classically produces sciatica, pain radiating down the leg that is reproduced by a straight-leg raise.
| Root | Usual disc | Weakness | Reflex | Sensory loss |
|---|---|---|---|---|
| C5 | C4–C5 | Deltoid (abduction), biceps | Biceps (C5–C6) | Lateral shoulder |
| C6 | C5–C6 | Wrist extension, biceps | Biceps, brachioradialis | Lateral forearm, thumb |
| C7 | C6–C7 | Triceps, wrist flexion | Triceps | Middle finger |
| C8 | C7–T1 | Finger flexors, grip | None reliable | Little finger, medial hand |
| L4 | L3–L4 | Quadriceps (knee extension) | Patellar | Knee, medial leg |
| L5 | L4–L5 | Great-toe extension, ankle dorsiflexion, hip abduction | None reliable | Lateral leg, dorsum of foot |
| S1 | L5–S1 | Plantarflexion, eversion | Achilles | Lateral foot, sole |
Foot drop from an L5 radiculopathy can mimic a common fibular (peroneal) nerve palsy, but only the root lesion also weakens foot inversion (tibialis posterior) and hip abduction (gluteus medius).
Spinal cord termination and lumbar puncture
Where the cord ends. The spinal cord tapers into the conus medullaris at L1–L2 in adults. At birth it reaches about L3; the vertebral column then outgrows the cord, so the roots below the conus lengthen into the cauda equina. These roots float in CSF within the dural sac, which continues down to S2, and the filum terminale anchors the cord to the coccyx.
Choosing the level. A lumbar puncture is placed below the conus, at the L3–L4 or L4–L5 interspace. The level is found with Tuffier (intercristal) line, drawn across the tops of the iliac crests, which crosses the L4 spinous process. Flexing the spine, with the patient curled on one side or seated and leaning forward, opens the interspinous spaces. At these levels the needle displaces floating cauda equina roots rather than piercing the cord. In infants, whose cord may reach L3, L4–L5 or L5–S1 is used.
- Skin and subcutaneous fatin the midline, between two spinous processes
- Supraspinous ligamentjoins the tips of the spinous processes
- Interspinous ligamentbetween adjacent spinous processes
- Ligamentum flavumdense and elastic; a sudden loss of resistance as it yields
- Epidural spacefat and the internal vertebral venous plexus; the target for epidural anesthesia
- Dura mater and arachnoid materpierced together to enter the subarachnoid space
- Subarachnoid spaceCSF flows back; the pia mater is never crossed
Epidural versus spinal. Epidural anesthetic is deposited in the epidural space after the loss of resistance at the ligamentum flavum, whereas spinal anesthetic and diagnostic taps enter the CSF. A leak of CSF through the dural hole can cause a positional post-dural-puncture headache that worsens on sitting up.
Vertebral level landmarks
| Level | Surface landmark | Deep structures |
|---|---|---|
| C3 | Hyoid bone | — |
| C4 | Upper border of thyroid cartilage | Common carotid bifurcation (carotid sinus) |
| C6 | Cricoid cartilage | Larynx becomes trachea; pharynx becomes esophagus |
| T4–T5 | Sternal angle (2nd costal cartilage) | Carina; start and end of the aortic arch; azygos arch into the SVC; superior–inferior mediastinal border |
| L1 | Transpyloric plane | Pylorus, neck of pancreas, renal hila, SMA origin; conus medullaris nearby (L1–L2) |
| L4 | Tops of the iliac crests | Aortic bifurcation into the common iliac arteries |
Using the sternal angle. The sternal angle (of Louis) is palpable as a ridge where the manubrium meets the sternal body. The 2nd costal cartilage joins here, making it the reliable starting point for counting ribs and intercostal spaces. The horizontal plane through it reaches the T4–T5 disc and separates the superior from the inferior mediastinum.
The transpyloric plane. The L1 plane lies midway between the jugular (suprasternal) notch and the upper border of the pubic symphysis.
Dermatomes and referred pain
A dermatome is the strip of skin supplied by a single spinal nerve root. Adjacent dermatomes overlap, so loss of one root often produces less sensory loss than expected, whereas pain and paresthesia follow the dermatome more faithfully.
- Upper limb: C5 lateral shoulder; C6 lateral forearm and thumb; C7 middle finger; C8 little finger and medial hand; T1 medial forearm and arm
- Trunk: T4 nipple line; T7 xiphoid; T10 umbilicus; L1 inguinal ligament
- Lower limb: L2–L3 anterior thigh; L4 knee and medial leg; L5 lateral leg and dorsum of the foot, including the great toe; S1 lateral foot, heel, and sole; S2 posterior thigh
- Perineum: S2–S4 saddle area
Referred pain. Visceral afferents enter the cord at particular segments, so visceral pain is felt in the matching dermatome. Early appendicitis produces vague periumbilical pain (T10) that migrates to McBurney point once the parietal peritoneum becomes inflamed and somatic nerves are stimulated. Irritation of the diaphragm, by gallbladder disease or blood from a ruptured spleen, refers to the shoulder through the phrenic nerve (C3–C5).
Herpes zoster. Reactivation of varicella-zoster virus in a single dorsal root ganglion causes a painful vesicular band in one dermatome, most often thoracic; a band at the umbilicus indicates T10.
S2–S4 supply saddle sensation, the external anal and urethral sphincters through the pudendal nerve, and erection. Saddle anesthesia with urinary retention, fecal incontinence, reduced anal tone, and bilateral leg weakness indicates cauda equina syndrome and requires emergency MRI and surgical decompression.