eeing L4-L5 or L5-S1 on a lumbar MRI report can be confusing.
Both levels are located in the lower spine, and both can cause pain that travels into the leg. But they do not always affect the same nerve root, the same part of the foot or the same muscle groups.
One patient may notice pain along the outside of the leg and numbness near the big toe.
Another may experience pain behind the thigh, calf symptoms or numbness along the outer side of the foot.
The important question is therefore not simply:
“Do I have an L4-L5 or L5-S1 disc herniation?”
It is:
Which nerve is being affected, and do the MRI findings match the symptoms and neurological examination?
According to Op. Dr. Fatih Kırar, Neurosurgeon and Spine Surgeon, the disc level written on an MRI report should be interpreted together with pain distribution, sensory changes, muscle strength, reflexes and walking function.
For a broader introduction to lumbar disc disease, see the Lumbar Herniated Disc: Symptoms, Causes and Treatment guide.
What Is the Difference Between L4-L5 and L5-S1 Symptoms?
An L4-L5 disc herniation commonly affects the L5 nerve root and may cause symptoms toward the outer leg, top of the foot or big toe, sometimes with weakness lifting the foot or big toe.
An L5-S1 disc herniation commonly affects the S1 nerve root and may cause pain behind the thigh and calf, numbness toward the heel or outer foot, and weakness when pushing the foot downward or rising onto the toes.
These are common patterns, not absolute rules.
The MRI level should always be correlated with the neurological examination.
Where Are L4-L5 and L5-S1 Located?
The lumbar spine consists of five vertebrae labelled L1 through L5.
Below L5 lies the sacrum.
The L4-L5 disc sits between the fourth and fifth lumbar vertebrae.
The L5-S1 disc sits between the fifth lumbar vertebra and the first sacral segment.
These lower lumbar levels carry substantial mechanical load and are common locations for disc degeneration and herniation.
Because nearby nerve roots travel into the legs and feet, problems at these levels can produce symptoms far away from the lower back.
Why Can a Disc in the Back Cause Pain in the Leg?
The nerves that control sensation and muscle movement in the legs begin in the lower spine.
If disc material irritates or compresses a nerve root, symptoms may follow that nerve into the:
- buttock,
- thigh,
- calf,
- ankle,
- foot,
- or toes.
This is why some patients with lumbar disc herniation experience more leg pain than back pain.
Radiating leg pain related to lumbar nerve irritation is often described as sciatica.
For a detailed explanation of this pattern, see Leg Pain Without Back Pain? When a Herniated Disc May Be the Cause.
What Are the Symptoms of an L4-L5 Herniated Disc?
An L4-L5 herniated disc may affect different structures depending on the location of the disc material.
In a common posterolateral herniation, the L5 nerve root may be affected.
Possible symptoms include:
- pain in the buttock,
- pain along the outer thigh or leg,
- symptoms extending toward the top of the foot,
- numbness or tingling near the big toe,
- weakness lifting the foot,
- weakness lifting the big toe,
- difficulty walking on the heel.
Some people primarily experience pain.
Others notice numbness or weakness more clearly.
The exact pattern varies, so symptoms alone cannot confirm the level.
What Are the Symptoms of an L5-S1 Herniated Disc?
An L5-S1 disc herniation may commonly affect the S1 nerve root.
Possible symptoms include:
- pain in the buttock,
- pain along the back of the thigh,
- calf pain,
- pain toward the heel,
- numbness along the outer side of the foot,
- tingling near the little-toe side,
- weakness pushing the foot downward,
- difficulty repeatedly rising onto the toes.
The ankle reflex may also be affected in some patients.
Again, the pattern should be compared with examination and imaging rather than interpreted from the MRI label alone.
L4-L5 vs L5-S1: Where Does the Pain Usually Travel?
A simplified comparison can be useful.
L4-L5 / L5 Pattern
Symptoms may travel:
buttock → outer leg → top of foot → big toe
L5-S1 / S1 Pattern
Symptoms may travel:
buttock → back of thigh → calf → heel or outer foot
However, real patients do not always follow textbook diagrams perfectly.
Nerve territories overlap, and more than one level can be abnormal at the same time.
Which Toes Can Become Numb?
The location of numbness may provide useful clues.
With L5 nerve involvement, sensory changes may occur around:
- the top of the foot,
- the big toe,
- or nearby areas.
With S1 nerve involvement, symptoms may appear around:
- the outer foot,
- heel,
- or little-toe side.
But numbness in a particular toe does not prove which disc is responsible.
Peripheral nerve problems can produce similar symptoms.
L5 Weakness: What Does It Look Like?
L5 nerve dysfunction can affect muscles involved in lifting the foot and toes.
Patients may notice:
- difficulty lifting the big toe,
- difficulty lifting the foot upward,
- difficulty walking on the heel,
- the front of the foot catching on the ground,
- or a feeling that the foot is becoming harder to control.
When weakness becomes significant, foot drop may develop.
For more detail, see Herniated Disc and Foot Weakness: When Does Foot Drop Matter?.
S1 Weakness: What Does It Look Like?
S1 nerve dysfunction can affect the muscles used to push the foot downward.
A patient may notice:
- difficulty standing on tiptoe,
- difficulty repeatedly performing a calf raise,
- reduced push-off while walking,
- or weakness in the calf.
Comparing both sides during examination can help identify subtle motor differences.
Can L4-L5 or L5-S1 Cause Sciatica Without Back Pain?
Yes.
A patient may have relatively mild lower-back discomfort but significant:
- buttock pain,
- leg pain,
- calf pain,
- foot pain,
- numbness,
- or tingling.
This happens because nerve-root irritation can become the dominant symptom.
The absence of severe back pain therefore does not rule out a lumbar disc problem.
Can Both L4-L5 and L5-S1 Be Abnormal on MRI?
Yes.
It is common for lumbar MRI scans to show abnormalities at more than one level.
For example, an MRI may show:
- L4-L5 disc bulging,
- L5-S1 disc protrusion,
- degenerative changes at both levels,
- or varying degrees of foraminal narrowing.
This creates an important diagnostic problem:
Which MRI finding is actually responsible for the symptoms?
The answer depends on matching imaging with:
- the pain pathway,
- numbness distribution,
- muscle strength,
- reflexes,
- and neurological examination.
Does the Bigger Disc Herniation Cause More Symptoms?
Not always.
A larger disc abnormality can sometimes produce limited symptoms if it does not significantly affect a nerve.
A smaller herniation may cause marked pain or weakness if it lies directly against a nerve root in a confined space.
Therefore, the size of the disc alone does not determine clinical severity.
A more useful question is:
Does the location of the disc explain the neurological pattern?
What Does “Nerve Root Compression” Mean on an MRI?
An MRI report may contain terms such as:
- nerve-root contact,
- nerve-root compression,
- foraminal narrowing,
- lateral recess narrowing,
- disc protrusion,
- disc extrusion,
- or sequestration.
These terms describe anatomy.
They do not automatically determine symptoms or treatment.
For a broader explanation of MRI terminology, see Lumbar MRI Report Explained: Disc Bulge, Herniation and Nerve Compression.
Disc Bulge vs Protrusion vs Extrusion: Does the Term Matter?
These terms describe different forms of disc displacement.
A disc bulge generally involves a broader extension of disc tissue.
A protrusion is a more focal displacement in which the base remains relatively broad.
An extrusion refers to disc material extending farther beyond the disc space.
But the terminology alone does not determine how severe the patient's symptoms are.
What matters clinically is:
- where the disc material is,
- whether it contacts or compresses a nerve,
- whether neurological function is changing,
- and whether the finding matches the patient's symptoms.
Can L4-L5 Cause Foot Drop?
Yes, when the L5 nerve root is sufficiently affected.
Foot drop refers to weakness lifting the front of the foot.
Patients may notice:
- the foot catching the floor,
- frequent stumbling,
- dragging the foot,
- the foot slapping during walking,
- or difficulty walking on the heel.
However, lumbar disc herniation is not the only cause of foot drop.
Peripheral nerve disorders and other neurological conditions may produce similar weakness.
New or worsening foot drop warrants neurological assessment.
Can L5-S1 Cause Calf Weakness?
Yes.
If the S1 nerve root is affected, weakness may involve the calf muscles.
Patients may notice difficulty:
- standing on the toes,
- repeatedly performing heel raises,
- pushing off strongly while walking,
- or using stairs normally.
Weakness is generally more clinically significant than pain intensity alone because it may reflect motor nerve dysfunction.
Pain, Numbness or Weakness: Which Is More Important?
All three matter, but they represent different aspects of nerve function.
Pain may reflect nerve irritation.
Numbness may reflect sensory involvement.
Weakness may indicate motor dysfunction.
New or progressive weakness deserves particular attention.
A person may have severe pain with normal muscle strength.
Another person may have only moderate pain but clear weakness.
The second pattern may carry greater neurological significance despite the lower pain score.
Can Symptoms Switch From Pain to Numbness?
Yes.
Symptoms can change over time.
Some patients initially experience severe shooting pain.
Later, the pain may improve while numbness remains.
This does not automatically mean the nerve is permanently damaged.
Sensory recovery can occur at a different rate from pain improvement.
However, increasing numbness or new muscle weakness should prompt reassessment.
When Does L4-L5 or L5-S1 Become More Concerning?
Prompt evaluation is more important when symptoms include:
- new muscle weakness,
- progressive weakness,
- difficulty lifting the foot,
- foot drop,
- repeated tripping,
- rapidly increasing numbness,
- marked walking difficulty,
- or neurological symptoms affecting both legs.
Emergency assessment is particularly important with:
- new bladder dysfunction,
- new bowel dysfunction,
- numbness around the saddle area,
- or rapidly progressive neurological deterioration.
These symptoms may rarely indicate severe compression of the lower spinal nerves.
When Is a Lumbar MRI Needed?
Not every episode of back or leg pain requires immediate MRI.
MRI becomes more useful when:
- symptoms persist,
- neurological weakness develops,
- numbness progresses,
- walking function changes,
- severe nerve compression is suspected,
- or treatment decisions depend on identifying the affected structure.
MRI can show anatomy, but it does not replace neurological examination.
How Does a Doctor Determine Whether L4-L5 or L5-S1 Is Causing the Symptoms?
The diagnostic process combines several clues.
A specialist may assess:
Pain distribution
Does the pain travel along the outer leg or the back of the leg?
Sensory changes
Which part of the foot feels numb?
Muscle strength
Can the patient lift the foot and big toe normally?
Can the patient rise onto the toes?
Reflexes
Are the knee and ankle reflexes symmetrical?
Walking
Can the patient walk normally on the heels and toes?
MRI
Does the imaging show nerve compression at a level that explains these findings?
The goal is not to choose a diagnosis from the MRI report.
The goal is to identify which anatomical finding corresponds to the neurological problem.
Does Every L4-L5 or L5-S1 Herniated Disc Need Surgery?
No many lumbar disc herniations can be managed without surgery when there is no progressive neurological deficit or urgent indication.
Depending on the individual situation, treatment may involve:
- activity modification,
- medication,
- physical therapy,
- exercise-based rehabilitation,
- injections,
- or other selected non-surgical approaches.
You can explore the available spine and neurosurgical treatment options.
When Does Surgery Become More Relevant?
Surgical evaluation may become more relevant when there is:
- progressive motor weakness,
- foot drop,
- objective neurological loss,
- significant nerve compression matching the symptoms,
- persistent disabling sciatica despite appropriate treatment,
- or an urgent neurological syndrome.
For a focused guide, see When Does a Herniated Disc Require Surgery? Warning Signs to Know.
Endoscopic or Microsurgical Treatment: Does the Disc Level Decide the Technique?
Not by itself.
L4-L5 and L5-S1 can both be treated using different approaches when surgery is actually indicated.
The choice depends on:
- disc location,
- size and direction,
- nerve compression,
- anatomy of the spinal canal,
- associated stenosis,
- previous surgery,
- and the patient's neurological findings.
In selected patients, endoscopic spine surgery or microsurgical decompression may be considered.
The technique should follow the diagnosis—not the other way around.
A More Useful Way to Read Your MRI
Instead of asking only:
“Is my L4-L5 disc worse than my L5-S1 disc?”
ask:
“Which level explains my symptoms?”
That question shifts attention away from alarming MRI terminology and toward neurological relevance.
For example:
An L4-L5 abnormality is more meaningful when the symptoms and examination support L5 nerve involvement.
An L5-S1 abnormality is more meaningful when the pattern supports S1 nerve involvement.
6 Questions to Ask Before Your Spine Appointment
Before an evaluation, try to answer:
- Where does the pain begin?
- Does it travel below the knee?
- Which part of the foot or toes feels numb?
- Can I lift my foot and big toe normally?
- Can I stand repeatedly on my toes?
- Are my symptoms improving, stable or worsening?
These details can help make the MRI much more clinically meaningful.
When Should You See a Spine Specialist?
Consider specialist assessment when:
- leg pain continues or repeatedly returns,
- pain travels into the foot,
- numbness persists,
- weakness develops,
- the foot starts catching on the ground,
- walking becomes more difficult,
- or an MRI shows abnormalities at multiple levels and it is unclear which level is responsible.
New or progressive weakness deserves earlier assessment than uncomplicated mechanical back pain.
Appointment With Op. Dr. Fatih Kırar
If your MRI shows L4-L5 or L5-S1 disc herniation and you have leg pain, numbness, foot weakness or changes in walking, you can request an appointment with Op. Dr. Fatih Kırar.
MRI images can be reviewed together with the symptom pattern and neurological findings to determine which disc level, if any, is responsible for the nerve symptoms.
Frequently Asked Questions
Which is worse, L4-L5 or L5-S1?
Neither level is automatically “worse.” Clinical importance depends on nerve compression, neurological findings and whether the MRI abnormality matches the patient's symptoms.
Can L4-L5 cause pain in the foot?
Yes. If the L5 nerve root is affected, symptoms may extend toward the top of the foot or big-toe region.
Can L5-S1 cause calf pain?
Yes. S1 nerve-root irritation can produce pain, burning or sensory changes in the calf and outer foot.
Can L4-L5 cause foot drop?
Yes. L5 nerve-root dysfunction can weaken muscles used to lift the foot and toes, potentially contributing to foot drop.
Can L5-S1 cause weakness in the calf?
Yes. S1 involvement may reduce strength used for standing on the toes or pushing off while walking.
Can both L4-L5 and L5-S1 cause sciatica?
Yes. Both levels can contribute to radiating leg pain, but the symptom distribution and neurological findings may differ.
Does an L4-L5 or L5-S1 disc bulge always cause symptoms?
No. Disc abnormalities can appear on MRI without causing corresponding symptoms.
Does a large L5-S1 herniation always need surgery?
No. Disc size alone does not determine treatment. Neurological function, symptom progression and clinical correlation are more important.
Can numbness remain after sciatica improves?
Yes. Sensory symptoms may recover more slowly than pain. Worsening numbness or new weakness should be reassessed.
When is L4-L5 or L5-S1 disc herniation an emergency?
New bladder or bowel dysfunction, saddle-area numbness or rapidly progressive neurological weakness requires urgent assessment.
Key Takeaway
L4-L5 and L5-S1 are neighbouring disc levels, but they may affect different nerve roots and produce different symptom patterns.
A useful simplified distinction is:
L4-L5 → often L5 nerve → outer leg/top of foot/big toe + foot-lifting weakness
L5-S1 → often S1 nerve → back of leg/calf/outer foot + toe-standing weakness
But MRI labels alone do not diagnose the cause of symptoms.
The clinically important question is:
Does the disc level match the pain pattern, sensory changes, muscle weakness and neurological examination?
That correlation is what helps determine whether the next step should be observation, non-surgical treatment, further neurological evaluation or surgical assessment.
For more evidence-focused educational content about lumbar disc herniation, sciatica, MRI findings and spine conditions, visit the Op. Dr. Fatih Kırar Spine and Neurosurgery Blog.
Medical Review: Op. Dr. Fatih Kırar Neurosurgeon and Spine Surgeon
Medical Disclaimer: This article is for general educational purposes and does not replace individual diagnosis, neurological examination or treatment. New bladder or bowel dysfunction, saddle numbness or rapidly progressive weakness requires urgent medical evaluation.