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L5–S1 Intervertebral disc

The L5–S1 intervertebral disc is the lowest lumbar disc, located between the body of the fifth lumbar vertebra (L5) and the base of the sacrum (S1). It plays a crucial role in transmitting loads from the spine to the pelvis and lower limbs.

This disc is unique because of its location at the lumbosacral junction, where the highly mobile lumbar spine meets the fused sacrum. It bears significant biomechanical stress due to both body weight and shearing forces, making it the most common site of disc degeneration, herniation, and pathology.

Synonyms

  • Lumbosacral disc

  • L5–S1 disc

  • Lumbosacral junction disc

Structure

  • Nucleus pulposus: Gelatinous, hydrated center acting as a shock absorber.

  • Annulus fibrosus: Concentric fibrocartilaginous lamellae providing tensile strength.

  • Endplates: Hyaline cartilage attaching the disc to vertebral bodies, allowing diffusion of nutrients.

Relations

  • Anteriorly: Anterior longitudinal ligament, pelvic retroperitoneal structures

  • Posteriorly: Posterior longitudinal ligament, epidural space, dural sac, cauda equina roots

  • Laterally: Neural foramina containing L5 nerve roots

  • Inferiorly: Sacral promontory

Function

  • Absorbs shock and transmits axial loads

  • Provides controlled mobility at the lumbosacral junction

  • Maintains lumbar curvature and alignment

  • Transfers forces between spine and pelvis

Clinical Significance

  • Herniation: Most frequent level, compressing S1 root → sciatica, lateral foot numbness, weak plantar flexion, absent ankle reflex

  • Degeneration: Early water loss in nucleus → disc height loss and chronic pain

  • Spondylolisthesis: L5–S1 disc degenerates with anterior slippage of L5

  • Discitis: Infection involving disc and endplates

  • Surgical importance: Common site for discectomy, fusion, injections

MRI Appearance

T1-weighted images:

  • Nucleus pulposus: intermediate to low signal

  • Annulus fibrosus: uniformly low signal

  • Degeneration: nucleus becomes darker; fatty endplate changes appear bright

T2-weighted images:

  • Nucleus pulposus: bright (hydrated) in normal disc; darkens with degeneration

  • Annulus fibrosus: dark rim surrounding nucleus

  • Annular tear: focal bright line within annulus

STIR (Short Tau Inversion Recovery):

  • Nucleus pulposus: bright hyperintense due to high water content

  • Annulus fibrosus: dark

  • Endplate edema: also bright

T1 Fat-Sat Post-Contrast:

  • Normal disc: little to no enhancement

  • Degeneration: mild peripheral annular enhancement possible

  • Infection: diffuse disc enhancement ± adjacent endplates

  • Herniated fragment: rim enhancement from granulation tissue

CT Appearance

Non-Contrast CT:

  • Disc shows soft tissue density, not differentiating nucleus from annulus

  • Degeneration: reduced height, vacuum phenomenon, calcifications

  • Herniation: focal posterior or posterolateral protrusion into canal/foramen

Post-Contrast CT:

  • Normal disc: no significant enhancement

  • Infection: diffuse disc and endplate enhancement, paraspinal extension possible

  • Tumors: irregular heterogeneous enhancement

  • Scar vs recurrent disc: scar enhances, recurrent disc does not

MRI image

L5–S1 Intervertebral Disc mri sagittal image

CT image

L5–S1 Intervertebral Disc ct coronal anatomy image-img-00000-00000