Spine Anatomy — Cervical Through Sacrum/Coccyx

Overview of the Vertebral Column

The vertebral column is the central bony axis of the body. It consists of 33 vertebrae organized into five regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused into the sacrum), and 4 coccygeal (fused into the coccyx). On a radiograph, each region has a distinctive appearance that allows you to orient yourself quickly. Understanding normal anatomy is the foundation of recognizing pathology and positioning patients correctly.

Parts Common to Most Vertebrae

Every vertebra, regardless of region, shares a core set of structures:

• Vertebral body — the anterior, weight-bearing block of bone. On an AP radiograph it appears as a rectangular density; on a lateral it looks like a square or rectangle with distinct superior and inferior end-plates.
• Pedicles — two short, stout bony columns that project posteriorly from the body. On an AP projection they appear as oval "eye-like" densities on either side of the midline — sometimes called the "owl eyes" sign.
• Laminae — flat plates of bone that extend from each pedicle toward the midline, fusing to form the posterior arch.
• Spinous process — the most posterior projection, formed by the joining of the two laminae. Palpable through the skin.
• Transverse processes — lateral projections, one on each side, arising at the junction of pedicle and lamina.
• Superior and inferior articular processes (facets) — paired projections that form the zygapophyseal (facet) joints with adjacent vertebrae.
• Vertebral foramen — the opening enclosed by the body anteriorly and the arch posteriorly; collectively these foramina form the vertebral (spinal) canal.
• Intervertebral foramina — openings between adjacent pedicles through which spinal nerve roots exit. Best seen on lateral cervical and lateral lumbar projections.

Cervical Region (C1–C7)

Cervical vertebrae are the smallest and are unique in having a transverse foramen in each transverse process for the vertebral artery. The atlas (C1) has no body and no spinous process; it forms two lateral masses that articulate with the occipital condyles above (atlanto-occipital joints) and the axis below. The axis (C2) has the dens (odontoid process), a superior peg-like projection that acts as a pivot for the atlas. On the AP open-mouth (odontoid) view, the dens projects between the upper incisors and the base of the skull. C3–C6 have bifid spinous processes (split tips). C7, called the vertebra prominens, has the longest, non-bifid spinous process and is easily palpable at the base of the neck.

Standard projections: AP, lateral, and AP open-mouth for C1–C2. Lateral cervical is the most informative single view, showing vertebral bodies, disc spaces, and intervertebral foramina.

Thoracic Region (T1–T12)

Thoracic vertebrae are identified by costal facets (demifacets) on the sides of the bodies and on the transverse processes for articulation with the ribs. These are unique to this region. Bodies are heart-shaped on a superior axial view and increase in size from T1 to T12. Spinous processes are long and angled sharply downward, which is why tube angulation is required on AP projections to open the disc spaces.

Standard projections: AP and lateral thoracic spine.

Lumbar Region (L1–L5)

Lumbar vertebrae are the largest of the movable vertebrae, built to bear the greatest mechanical load. Vertebral bodies are large and kidney-shaped on axial view. Transverse processes are slender; mammillary processes — small bony knobs on the posterior surface of the superior articular processes — are a distinctive lumbar feature. The spinous process is broad and hatchet-shaped on a lateral view.

The lumbosacral junction (L5–S1) is a common site of pathology. The angle between the lumbar spine and sacrum is the lumbosacral angle. Standard projections: AP and lateral lumbar spine. Oblique projections demonstrate the zygapophyseal joints and, on properly exposed obliques, the classic "Scottie dog" sign — the dog's eye is the pedicle, ear is the superior articular process, front leg is the inferior articular process, body is the lamina, and neck is the pars interarticularis.

Sacrum and Coccyx

The sacrum is formed by the fusion of five sacral segments. It is wedge-shaped and articulates with L5 above and the two iliac bones laterally at the sacroiliac joints. The anterior surface has four pairs of anterior sacral foramina for nerve passage. The sacral canal is the continuation of the vertebral canal.

The coccyx consists of three to five fused rudimentary vertebrae and articulates with the sacral apex at the sacrococcygeal joint.

Standard projections for sacrum and coccyx are AP and lateral. Because the sacrum angles posteriorly, a cephalad tube angle on AP is used; for the coccyx, a caudad angle is used to project the structure without superimposition.

Intervertebral Discs

Fibrocartilaginous discs lie between adjacent vertebral bodies from C2–C3 through L5–S1. Each disc has a tough outer anulus fibrosus and a gelatinous inner nucleus pulposus. Discs are radiolucent (dark) on conventional radiographs; the space they occupy is called the intervertebral disc space. Narrowing of a disc space is a radiographic finding that warrants clinical evaluation.

Safety and Scope of Practice Notes

Always confirm the correct patient and correct region before exposure. Proper positioning reduces retakes and unnecessary radiation to the patient. Never force a patient into position if they report pain or resist — notify the supervising radiologist or physician. The cervical spine of any trauma patient must be treated as potentially unstable until cleared clinically; follow your facility protocol and never remove immobilization devices without authorization.