L5–S1 disc !

The L5-S1 disc is the lowest disc, between L5 and the sacrum; with L4-L5 it carries the most load and herniates most often.

Official name: L5-S1

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What it is

The cushion between L5 and the sacrum, the lowest disc. Together with L4-L5 it carries the most load and is the most often herniated.

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 L5 body and sacrum (S1); most wedge-shaped disc, forms the lumbosacral angle
Attaches: Annulus fibrosus anchored to the endplates and ring apophyses of L5 and S1; anterior and posterior longitudinal ligaments
Function: Absorbs and spreads load; posterolateral herniation usually compresses the S1 root (far-lateral: L5); one of the two commonest herniation levels

Relations

In front: Between the diverging common iliac arteries and common iliac veins; the median sacral artery and the superior hypogastric plexus lie directly on the disc
Behind: The dural sac, which ends around S2, and the S1 roots running in the lateral recess behind the disc
Lateral: The L5 root leaving through the L5–S1 foramen, under the L5 pedicle and near the sacral ala and the iliolumbar ligament

How to find it

The dimples over the posterior superior iliac spines mark about S2; the L5–S1 interspace is the last gap felt in the midline above the sacrum, below the Tuffier line.

Examination

Toe walking or repeated single-leg heel raises: early tiring on one side points to S1 weakness
Ankle (Achilles) reflex: reduced or absent with S1 root compression
Slump test (seated, back slumped, neck bent, knee straightened): leg pain that eases when the head is lifted suggests neural tension, as with S1 root irritation

On imaging

Lateral X-ray and sagittal MRI show the lumbosacral angle and any slip of L5 on S1; oblique X-rays show a pars defect as a «collar» on the «Scottie dog».

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

Anterior lumbar interbody fusion: the vessels are moved aside; injury to the superior hypogastric plexus can cause retrograde ejaculation
Microdiscectomy at L5–S1 is helped by the wide interlaminar window; the S1 root is gently retracted in the lateral recess to reach the fragment

Anatomical variants

Lumbosacral transitional vertebrae (a sacralised L5 or a lumbarised S1) are common and change how levels are numbered; counting from C2 down on whole-spine images helps avoid wrong-level surgery

Development

The L5–S1 disc is the most wedge-shaped of all, much taller in front, and builds the angle between the lumbar spine and the sacrum. The pars defect of isthmic spondylolisthesis is not present at birth: it appears in childhood or adolescence as a stress fracture, more often in sports with repeated arching of the back.

Memory aid

Reflex rhyme: «1, 2, buckle my shoe» (S1–S2, ankle jerk), «3, 4, kick the door» (L3–L4, knee jerk).

Test yourself

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

What is the S1 root picture from an L5–S1 disc herniation?

Weak plantar flexion and foot eversion, a reduced or absent ankle (Achilles) reflex, and numbness of the lateral foot, sole and little toe.

Why is the L5–S1 disc the easiest lumbar disc to reach from the front?

It lies below the aortic bifurcation, between the diverging common iliac vessels, which suits anterior lumbar interbody fusion.

Which complication follows injury to the superior hypogastric plexus during anterior fusion at the L5–S1 disc?

Retrograde ejaculation; the plexus lies directly on the front of the disc.

Which type of spondylolisthesis is most common at the L5–S1 disc, and which root can it trap?

Isthmic spondylolisthesis, from a pars defect in L5; the slip can trap the L5 root in its foramen.

Why do lumbosacral transitional vertebrae matter at the L5–S1 disc?

A sacralised L5 or lumbarised S1 is common and changes how levels are numbered; counting down from C2 on whole-spine images helps avoid wrong-level surgery.

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

Its names

Official Latin (TA2): discus intervertebralis L5-S1 TA2 1684
Official Spanish (DeCS): Disco L5-S1 DeCS D007403
English (TA2): L5-S1
Also called: fibrous ring · fibrocartilago intervertebralis

Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Back, Intervertebral Discs; Anatomy of the Human Body (20th ed.): Articulations of the Vertebral Column (Lea & Febiger (Gray's Anatomy, 1918), 1918) · adapted, Public domain.

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Representative model, for educational purposes only.

L5–S1 disc highlighted on the Durgical 3D model