L4–L5 disc !

The L4-L5 disc is the cushion between L4 and L5 in the lower back; with L5-S1 it carries the most load and herniates most often.

Official name: L4-L5

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What it is

The cushion between L4 and L5: a tough outer ring (the annulus fibrosus) made of crossing layers of fibres around a soft gel centre.

What it does

Absorbs shock, spreads load across the vertebrae and lets the spine bend and twist, while holding the bones together.

Key facts

Where: Between L4 and L5 bodies; thick lumbar disc, taller in front (lumbar lordosis)
Attaches: Annulus fibrosus anchored to the endplates and ring apophyses of L4 and L5; anterior and posterior longitudinal ligaments
Function: Absorbs and spreads load; posterolateral herniation usually compresses the L5 root (far-lateral: L4); one of the two commonest herniation levels

Relations

In front: The aortic bifurcation and the common iliac arteries and veins; the left common iliac vein crosses in front, where an instrument pushed through the front of the annulus during discectomy can injure it
Behind: Posterior longitudinal ligament, the dural sac with the cauda equina, and the ligamentum flavum, which thickens and buckles as the disc degenerates
Lateral: Psoas major; the L4 root exits through the foramen at this level while the L5 root runs behind the disc in the lateral recess

How to find it

At or just below the Tuffier line. Before the incision for surgery, the level is marked on a lateral X-ray.

Examination

Straight leg raise (passive hip flexion with the knee straight): leg pain below the knee at roughly 30–70° suggests tension on the L5 or S1 root
Crossed straight leg raise: lifting the healthy leg brings on pain in the affected one; less sensitive but more specific for herniation
Great-toe extension against resistance: weakness is an early sign of L5 root compression

On imaging

Sagittal and axial T2 MRI show the fragment and the L5 root in the lateral recess. Standing lateral X-rays, with flexion and extension views, show whether L4 slips forward on L5.

Common problems

Herniation (slipped disc): the gel pushes through a tear in the ring and presses on a nerve root, causing sciatica down the leg.
Degeneration with age: the disc dries out, loses height and can bulge, sometimes with back pain.
Tears in the ring (annular fissures) that can cause back pain.

How it is diagnosed

History and exam (the straight-leg raise test, strength, reflexes and feeling in the leg). MRI shows the disc and which nerve is pressed; it is usually done only if pain lasts more than about 6 weeks or there are warning signs.

Treatment

Most herniations improve within weeks to months: staying active, physical therapy, pain medicine, sometimes an epidural steroid injection.
Severe or lasting leg pain, or weakness: microdiscectomy to remove the herniated piece.
Urgent surgery if there are signs of cauda equina syndrome (numbness in the saddle area, bladder or bowel changes).
Selected cases of disc collapse with instability: fusion with a cage and bone graft.

Clinical notes

Microdiscectomy: through a small midline incision and a small window in the lamina, the ligamentum flavum is opened, the L5 root is moved medially and the fragment removed
Lumbar spinal stenosis at this level causes neurogenic claudication: leg pain on walking that eases with sitting or bending forward, as over a shopping cart

Anatomical variants

Sacralised L5 (an L5 joined to the sacrum on one or both sides) makes L4–L5 the lowest moving disc; it then carries more load and tends to degenerate earlier

Development

The rim of each vertebral body has a ring-shaped secondary ossification centre (ring apophysis) that appears around puberty and fuses in early adulthood. In teenagers the back edge of this ring can break off with the disc at L4–L5 or L5–S1, mimicking a herniation; CT shows the bone fragment.

Test yourself

Think of the answer, then tap the question to see it.

What is the L5 root picture from an L4–L5 disc herniation?

Weak great-toe extension (extensor hallucis longus), ankle dorsiflexion and hip abduction (gluteus medius); numbness of the outer leg and top of the foot; no reliable reflex.

How is foot drop from an L5 root lesion told apart from a common fibular nerve palsy?

In an L5 root lesion hip abduction and foot inversion (tibialis posterior) are also weak; the common fibular nerve supplies neither.

What does the straight leg raise test look for in an L4–L5 or L5–S1 disc herniation?

Leg pain below the knee at roughly 30–70° of passive hip flexion with the knee straight, which suggests tension on the L5 or S1 root.

Which vessel in front of the L4–L5 disc can be injured during discectomy?

The left common iliac vein, which crosses in front of the disc; an instrument pushed through the front of the annulus can injure it.

What symptom does lumbar spinal stenosis cause at the L4–L5 level, its most common site?

Neurogenic claudication: leg pain on walking that eases with sitting or bending forward.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): discus intervertebralis L4-L5 TA2 1684
Official Spanish (DeCS): Disco L4-L5 DeCS D007403
English (TA2): L4-L5
Also called: fibrous ring · fibrocartilago intervertebralis

Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Back, Intervertebral Discs.

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Representative model, for educational purposes only.

L4–L5 disc highlighted on the Durgical 3D model