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L4–L5 Intervertebral Disc

The L4–L5 intervertebral disc lies between the fourth and fifth lumbar vertebrae and is one of the most mobile and clinically significant discs in the lumbar spine. It is a common site for degeneration and herniation, second only to L5–S1, due to high mechanical stress at the lower lumbar region.

This disc plays a central role in weight transmission, spinal mobility, and stabilization, and is often involved in lumbar radiculopathy syndromes affecting the L5 nerve root.

Synonyms

  • Lumbar intervertebral disc (L4–L5)

  • L4–L5 disc space

  • Lower lumbar disc

Structure

  • Nucleus pulposus: Central gelatinous structure, rich in proteoglycans and water, providing compressibility and shock absorption.

  • Annulus fibrosus: Outer concentric lamellae of fibrocartilage, providing tensile strength and stability.

  • Cartilaginous endplates: Hyaline cartilage layers attaching the disc to L4 and L5 vertebral bodies, facilitating nutrient diffusion.

Relations

  • Anteriorly: Anterior longitudinal ligament and abdominal aorta (more proximally)

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

  • Laterally: Exiting L4 nerve roots in foramina

  • Inferiorly: L5 vertebral body and adjacent psoas major tendon laterally

Function

  • Acts as a shock absorber between L4 and L5 vertebrae

  • Allows flexion, extension, lateral bending, and rotation of lumbar spine

  • Maintains lumbar lordosis and spinal alignment

  • Distributes load and shear forces across the lower lumbar spine

Clinical Significance

  • Disc herniation: Common at L4–L5; may compress L5 nerve root → weakness of dorsiflexion, toe extension, sensory loss on dorsum of foot.

  • Degenerative disc disease: Causes chronic low back pain, narrowing of disc height.

  • Spondylolisthesis: Forward slippage of L4 over L5 often involves disc degeneration.

  • Infections (spondylodiscitis): L4–L5 disc frequently affected in tuberculous and pyogenic infections.

  • Surgical target: Frequently treated with discectomy, decompression, or fusion.

MRI Appearance

T1-weighted images:

  • Nucleus pulposus: intermediate-to-low signal (darker than fat) due to water-rich composition

  • Annulus fibrosus: very low signal, sharply demarcating the nucleus

  • Degeneration: nucleus loses hydration, becoming uniformly dark

T2-weighted images:

  • Nucleus pulposus: normally bright (hyperintense) due to high water content

  • Annulus fibrosus: dark rim (hypointense) encasing the bright nucleus

  • Degeneration: decreased nucleus brightness (“dark disc” sign)

  • Tears: annular fissures appear as bright hyperintense clefts within dark annulus

STIR (Short Tau Inversion Recovery):

  • Normal disc: nucleus appears bright, annulus dark

  • Degeneration or desiccation: overall darker disc

  • Pathology: endplate edema, discitis, or inflammatory changes appear bright

T1 Fat-Sat Post-Contrast:

  • Normal disc: no significant enhancement

  • Herniated disc: peripheral annular enhancement may be seen

  • Infection: diffuse disc and endplate enhancement

  • Tumor infiltration: irregular enhancement crossing margins

CT Appearance

Non-Contrast CT:

  • Disc appears as homogeneous soft tissue density

  • Degeneration: disc space narrowing, vacuum phenomenon, calcifications in annulus

  • Herniation: focal protrusion compressing canal or foramina

Post-Contrast CT:

  • Normal disc: no enhancement

  • Infection: enhancing disc with endplate involvement and paraspinal collections

  • Tumors: irregular enhancing soft tissue replacing or extending into disc space

MRI image

L4–L5 Intervertebral Disc mri anatomy image

CT image

L4–L5 Intervertebral Disc CT sagittal anatomy image-img-00000-00000