Lumbar Radiculopathy vs Sacroiliac Joint Pain: A Differential Guide

Quick comparison

• Pain location: radiculopathy often runs below the knee in a dermatomal band, with leg pain worse than back pain; SIJ pain sits over the posterior superior iliac spine and buttock, rarely below the knee.
• Symptoms: radiculopathy brings pins and needles, numbness or weakness; SIJ pain is usually aching without neurological change.
• Aggravating factors: radiculopathy worsens with sitting, bending, coughing or sneezing; SIJ pain with single-leg loading, stairs, rolling in bed or getting out of a car.
• Neurological exam: radiculopathy may show reduced reflex, myotomal weakness or sensory change; SIJ pain shows a normal neurological exam.
• Tests: straight leg raise and slump for radiculopathy; the SIJ provocation cluster for SIJ pain.

What is happening in the tissues

Lumbar radiculopathy means a nerve root is irritated or compressed, most often by a disc herniation in younger and middle-aged adults or by foraminal and lateral recess narrowing in older adults. The nerve root becomes sensitive to stretch and pressure, which explains pain that travels down the leg, pins and needles, numbness and, when conduction is affected, reduced reflexes or weakness. Chemical irritation around the root can cause marked pain even without large compression.

The sacroiliac joint links the sacrum to the pelvis and transfers load between the trunk and the legs. Pain from the joint and its strong ligaments is felt deep in the buttock, close to the posterior superior iliac spine, and can refer into the thigh. It is common during pregnancy and after childbirth, after falls onto the buttock, and in inflammatory conditions such as axial spondyloarthritis. Because the joint has no nerve root inside it, true SIJ pain does not produce dermatomal numbness or myotomal weakness.

How to test the SIJ

Single SIJ tests are unreliable. Laslett's cluster uses distraction, thigh thrust, compression, sacral thrust and Gaenslen's test. Three or more positive tests raise the probability of SIJ pain - but only once the lumbar spine has been cleared, for example with repeated movements that do not change the symptoms.

Worked mini-scenario

A 38-year-old has two weeks of back pain with pain down the back of the leg to the outer foot after lifting. Coughing makes the leg pain worse.

Hypotheses: L5/S1 radiculopathy, SIJ pain, non-specific low back pain with referred pain. The ankle reflex is reduced, sensation is altered on the lateral foot, and straight leg raise reproduces leg pain at 40°.

Reasoning: pain below the knee, cough aggravation and neurological signs point to S1 radiculopathy. SIJ provocation adds little here. The plan: safety-net advice for cauda equina symptoms, a neurological baseline to recheck, and graded activity.

A contrasting case: the SIJ patient

A 31-year-old who gave birth four months ago reports deep pain over the right buttock. It is worse on stairs, standing on one leg to dress, turning in bed and getting out of the car. There is no pain below the knee, no pins and needles and no numbness.

Lumbar movements are slightly uncomfortable but do not reproduce her buttock pain, and repeated movements do not change it. The neurological examination is normal and the straight leg raise is negative. Distraction, thigh thrust and compression reproduce her familiar pain; sacral thrust and Gaenslen's test are negative.

Reasoning: buttock pain without neurological signs, provocation by single-leg loading, a lumbar spine that does not change the symptoms and three positive SIJ tests together support pelvic girdle pain from the SIJ. The plan is education, graded hip and trunk strengthening, adjusting provocative tasks and, if helpful, a short-term pelvic belt. She is also screened for postnatal pelvic floor symptoms and mood.

Compare this with the first case. Both patients describe back and buttock pain. The difference is the leg: one has a nerve root signature, the other does not.

Subjective questions that separate them

Ask the patient to draw the pain on a body chart. Radicular pain is often described as sharp, shooting or burning, running in a narrow band down the leg, and frequently worse in the leg than in the back. SIJ pain is usually a deep ache just beside the dimple at the bottom of the back, sometimes spreading into the buttock, groin or back of the thigh.

Ask about neurological symptoms directly, because patients often do not volunteer them: pins and needles, numbness, a foot that catches, or weakness on stairs. Ask what makes it worse. Sitting, bending forward, coughing and sneezing raise pressure on an irritated nerve root. Standing on one leg, climbing stairs, turning in bed and stepping out of a car load the pelvis. Ask about pregnancy, recent childbirth, a fall onto the buttock and inflammatory symptoms such as morning stiffness lasting more than 30 minutes.

• Where exactly is the pain, and is the leg worse than the back?
• Any pins and needles, numbness or weakness? Where?
• Do coughing or sneezing send pain down the leg?
• Is it worse standing on one leg, on stairs or turning in bed?
• Any bladder, bowel or saddle changes? (always ask)

Examination sequence

Begin with the lumbar spine. Watch movement, then use repeated movements, such as repeated extension or flexion, and note whether leg symptoms move toward the back (centralise) or spread further down the leg. Changes with repeated movement point to a lumbar source rather than the SIJ.

Then do a neurological examination: dermatomes, myotomes and reflexes at L4, L5 and S1. Add the straight leg raise and slump test, using ankle or neck movement to check whether symptoms are neural. Only if the lumbar spine does not reproduce or change the pain, and the neurological examination is normal, move to the SIJ provocation cluster. Keep the test sequence the same each session so that you can track change.

How management differs

Most lumbar radiculopathy improves over weeks to months. Management focuses on reassurance, staying active within tolerance, positions and movements that ease leg pain, and progressive exercise. Recheck the neurological examination regularly. Referral is needed for worsening weakness, severe pain that does not respond to treatment, or any signs of cauda equina syndrome.

SIJ pain is managed with load-based rehab: strengthening of the hip and trunk, graded single-leg loading and adjusting provocative activities. Pelvic belts can help some patients in the short term, especially in pregnancy-related pelvic girdle pain. Passive treatments may ease symptoms but should support, not replace, active rehab.

Common mistakes

• Calling all leg pain 'sciatica' without a neurological examination.
• Using a single SIJ test instead of the cluster.
• Testing the SIJ before clearing the lumbar spine.
• Not recording a neurological baseline to compare against later.
• Forgetting to ask about bladder, bowel and saddle symptoms.

Inflammatory back pain: a third hypothesis

Do not forget axial spondyloarthritis when SIJ pain is suspected, especially in people under 45. Clues include back or buttock pain that began gradually and has lasted more than three months, morning stiffness that lasts longer than 30 minutes, pain that improves with exercise but not with rest, waking in the second half of the night, alternating buttock pain, and a history of psoriasis, inflammatory bowel disease or uveitis. These patients need referral to a rheumatologist, because early medical treatment changes the long-term outcome.

Outcome measures and reassessment

For radiculopathy, record the furthest point of leg symptoms, the straight leg raise angle and the neurological findings at the first session, then recheck them regularly. Symptoms retreating toward the back and an improving straight leg raise are good signs. New or worsening weakness, spreading numbness or any bladder, bowel or saddle changes need urgent action.

For SIJ pain, use the most provocative functional task, such as single-leg stance, stair climbing or the active straight leg raise, and rate the pain or effort each session. A patient-reported measure such as the Oswestry Disability Index or the Pelvic Girdle Questionnaire adds a broader picture of how the problem affects daily life.

Prognosis and what to tell the patient

Many people with radicular leg pain improve substantially within weeks to a few months, and disc herniations often shrink over time. Patients need to know that staying active within tolerance is safe, that bed rest is not recommended, and exactly which symptoms mean they must seek emergency care. Give clear written safety-netting for cauda equina syndrome.

Patients with SIJ pain often worry that the pelvis is 'out of place'. Explain that the joint moves only a tiny amount and that pain is about sensitivity and load, not displacement. Reassure them that strengthening and gradual return to activity are effective, and that pregnancy-related pelvic girdle pain usually improves in the months after birth.

Putting it together

Think of the assessment as testing two competing explanations rather than confirming one. Leg-dominant pain, neurological symptoms, cough or sneeze aggravation, a change with repeated lumbar movements and positive neural tests all add weight to radiculopathy. Buttock pain without neurological change, pain with single-leg loading, a lumbar spine that does not change the symptoms and three or more positive SIJ provocation tests add weight to SIJ pain. Record which findings support each hypothesis, then reassess after two or three sessions: the explanation that best predicts how the patient responds is usually the right one.

How to present this differential in an exam or OSCE

Open with the red-flag screen, because examiners expect cauda equina syndrome to be addressed early in any back and leg pain station. Then name your hypotheses: lumbar radiculopathy, SIJ pain and non-specific low back pain with referred pain, plus inflammatory back pain in younger patients.

Explain how the history separates them: leg-dominant pain below the knee with pins and needles and cough aggravation for radiculopathy, compared with buttock pain on single-leg loading without neurological symptoms for SIJ pain. Describe the physical examination in order: lumbar movements and repeated movements, neurological examination, neural tests and, only after clearing the lumbar spine, the SIJ cluster. State that three or more positive SIJ tests increase the likelihood of SIJ pain.

Finish with a management plan that fits your leading hypothesis, the outcome measures you would repeat and clear safety-netting advice. This shows that your reasoning is organised and safe.

Red flags not to miss

• Saddle numbness, new bladder or bowel change, or sexual dysfunction - cauda equina syndrome needs emergency referral.
• Progressive or widespread weakness.
• Night pain with fever, weight loss or cancer history.
• Pregnancy-related pelvic girdle pain needs its own pathway.

Key takeaways

• Neurological signs and leg-dominant pain below the knee favour radiculopathy.
• SIJ tests are meaningful only as a cluster and after the lumbar spine is cleared.
• Always screen and safety-net for cauda equina syndrome.

Frequently asked questions

Can SIJ pain go below the knee?

Occasionally, but it is uncommon and should not come with neurological signs. Leg-dominant pain below the knee makes a nerve root more likely.

Is a positive straight leg raise enough to diagnose radiculopathy?

No. It is sensitive but not specific. Combine it with pain distribution and the neurological exam.

Do I need an MRI for suspected radiculopathy?

Not routinely. Imaging is considered when serious pathology is suspected, when neurological signs are worsening, or when symptoms persist and the results would change management, such as considering surgery or injection.

Can both problems exist together?

Yes. Some patients have lumbar and pelvic pain at the same time. Test and treat each driver, and track which one changes with treatment.

What is the difference between radicular pain and radiculopathy?

Radicular pain is pain travelling along a nerve root's distribution. Radiculopathy means there is also loss of nerve function, such as numbness, reduced reflexes or weakness. A patient can have radicular pain without radiculopathy, and the distinction matters for monitoring and referral decisions.

Which nerve root is most often affected?

The L5 and S1 roots are the most commonly involved in lumbar disc herniation. L5 problems often affect the big toe extension and the top of the foot, while S1 problems often affect the ankle reflex, calf strength and the outer border of the foot.

When should a patient with SIJ pain be referred?

Refer when inflammatory features suggest axial spondyloarthritis, when there is a history of trauma with severe pain suggesting fracture, when infection is possible, or when symptoms fail to improve with a well-delivered rehab programme over several months.

Is the slump test better than the straight leg raise?

They complement each other. The slump test may be more sensitive for some patients, while the straight leg raise is quicker and easy to repeat. Interpret both with the neurological exam and the pain pattern.

Can SIJ manipulation 'put the joint back in'?

No. The SIJ does not slip out of place. Manual therapy may give short-term relief, but active rehab drives the long-term result.

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