Lumbar Vertebrae (L1–L5)¶

Definition¶

The lumbar spine consists of five large vertebrae (L1–L5) located between the thoracic spine and the sacrum. It bears the greatest axial load of any spinal segment and is the most common site of degenerative disc disease, disc herniation, and spinal stenosis. The lumbar vertebrae are the largest movable vertebrae, reflecting their weight-bearing function.

Anatomy¶

General Features¶

Lumbar vertebrae are distinguished by their robust size and characteristic morphology:

Large, kidney-shaped vertebral bodies — wider in the transverse dimension; the largest of all vertebral bodies
Short, thick pedicles — originate from the upper posterolateral aspect of the body
Broad, flat laminae — form the posterior arch
Short, horizontal spinous processes — thick and quadrangular, directed posteriorly
Long transverse processes — sometimes called "costal processes" as they represent vestigial ribs
Sagittally oriented facet joints — approximately 90° to the axial plane, allowing flexion/extension but limiting rotation
Triangular vertebral foramen — larger than thoracic, accommodating the cauda equina below L1–L2

Lumbar Pedicles and Foramina¶

The neural foramina are clinically critical structures:

Each foramen is bounded by the pedicle above, pedicle below, disc and vertebral body anteriorly, and facet joint posteriorly
The exiting nerve root passes beneath the pedicle of the same-numbered vertebra (e.g., the L4 nerve root exits beneath the L4 pedicle)
The traversing nerve root (one level below) crosses the disc space and is vulnerable to posterolateral disc herniations

Clinical Pearl

Understanding the relationship between disc herniations and nerve roots is essential. A posterolateral L4–L5 disc herniation typically compresses the traversing L5 nerve root, while a far lateral (foraminal) herniation at the same level compresses the exiting L4 nerve root.

Imaging Findings¶

Radiography¶

AP view — vertebral body alignment, disc space heights, pedicle spacing (interpedicular distance), scoliosis
Lateral view — vertebral body heights, disc spaces, alignment, spondylolisthesis, pars defects
Oblique views — evaluate the pars interarticularis ("Scotty dog" appearance); a defect in the pars indicates spondylolysis

Measurement	Normal Value
Lumbar lordosis (L1–S1)	40°–60°
Disc height (L4–L5)	10–15 mm
Interpedicular distance	Gradual widening from L1 to L5
AP canal diameter	≥ 15 mm (< 12 mm = stenosis)

CT¶

CT excels at evaluating:

Bony anatomy, including facet joints and pars interarticularis
Spinal canal and foraminal dimensions
Disc calcification and vacuum phenomenon
Post-surgical hardware positioning

MRI¶

MRI is the primary modality for lumbar spine evaluation:

** Marrow Signal**: | Age | T1 Signal | T2 Signal | Notes | |-----------|-----------|-----------|-------| | Infant/young child | Hypointense| Hypointense | signal relative to discs | | Adult | Hyperintense| Intermediate | reflecting predominant fatty marrow |

** Disc Signal: ** | Structure | T1 Signal | T2 Signal | Notes | |-----------|-----------|-----------|-------| | Normal disc (young) | Intermediate | Bright | High T2 signal = normal hydration | | Degenerated disc | Intermediate | Dark | Loss of T2 signal = desiccation | | Normal nerve roots | Intermediate | Intermediate | Well-defined within thecal sac | | Epidural fat | Bright | Bright | Outlines the thecal sac | | CSF (cauda equina) | Dark | Bright | Nerve roots visible as filling defects |

Lumbar MRI Protocol

Standard lumbar MRI includes sagittal T1, sagittal T2, and axial T2 sequences through the disc levels. Post-contrast imaging is added for suspected infection, tumor, or post-operative evaluation to differentiate scar from recurrent disc herniation.

Key Points¶

The lumbar vertebrae (L1–L5) are the largest movable vertebrae and bear the greatest axial load
Sagittally oriented facet joints permit flexion/extension but restrict rotation
The exiting nerve root passes below the pedicle of the same level
Posterolateral disc herniations affect the traversing (next-level) nerve root
MRI is the primary imaging modality; T2 signal loss in discs indicates degeneration

References¶

Waxenbaum JA, Reddy V, Williams C, Futterman B. Anatomy, Back, Lumbar Vertebrae. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2026. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459278/
Pereira Duarte M, Camino Willhuber GO. Pars Interarticularis Injury. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2026. Available from: https://www.ncbi.nlm.nih.gov/books/NBK545191/
Steurer J, Roner S, Gnannt R, Hodler J. Quantitative radiologic criteria for the diagnosis of lumbar spinal stenosis: a systematic literature review. BMC Musculoskelet Disord. 2011;12:175. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC3161920/
Bartynski WS, Lin L. Lumbar root compression in the lateral recess: MR imaging, conventional myelography, and CT myelography comparison with surgical confirmation. AJNR Am J Neuroradiol. 2003;24(3):348-360. Available from: https://pubmed.ncbi.nlm.nih.gov/12637281/
Standaert CJ, Herring SA. Spondylolysis: a critical review. Br J Sports Med. 2000;34(6):415-422. Available from: https://pubmed.ncbi.nlm.nih.gov/11131228/
Leone A, Cianfoni A, Cerase A, Magarelli N, Bonomo L. Lumbar spondylolysis: a review. Skeletal Radiol. 2011;40(6):683-700. Available from: https://pubmed.ncbi.nlm.nih.gov/20440613/
Radiopaedia.org. Spondylolysis. Available from: https://radiopaedia.org/articles/spondylolysis