SI Joint Pain or Sciatica? How to Tell the Difference
You have had a dull ache in your lower back and buttock for weeks. Sometimes it shoots down your leg. You searched online and found two likely culprits: sciatica and SI joint pain. They sound different, but the symptoms overlap enough that it is genuinely difficult to know which one you are dealing with.
This matters more than it might seem. SI joint pain and sciatica can feel almost identical in the early stages, but they originate from different structures and respond differently to care. Guessing the cause and following the wrong approach can slow recovery or leave the underlying problem unaddressed.
This article explains how SI joint pain and sciatica differ, what each one typically feels like, and what a proper assessment should look for. At ATLAS, we assess, we do not guess.
What Is SI Joint Pain?
The sacroiliac joint, commonly called the SI joint, sits where the base of your spine meets the pelvis. You have two of them, one on each side, and they connect the sacrum — the triangular bone at the base of your spine — to the ilium of your pelvis.
Their job is primarily load transfer. Every time you walk, climb stairs, or shift your weight, the SI joints absorb and distribute force between your upper body and lower limbs. They are reinforced by strong ligaments and have a relatively small range of motion compared to other joints.
What causes SI joint pain?
When the SI joint becomes irritated, inflamed, or moves abnormally — either too much or too little — it can become a significant source of pain. Research published in the Tzu Chi Medical Journal estimates that the SI joint accounts for 15 to 30 per cent of patients presenting with chronic lower back pain.
Common contributors include:
- A fall, collision, or sudden impact to the pelvis
- Repetitive asymmetrical loading — common in runners, cyclists, and people who carry bags on one shoulder
- Ligament laxity from pregnancy or hormonal changes
- Altered gait mechanics after a leg or knee injury
- Prolonged sitting in a pelvis-loading position
What Is Sciatica?
Sciatica is not a diagnosis in itself — it describes a pattern of symptoms caused by irritation or compression of the sciatic nerve. The sciatic nerve is the longest nerve in the body. It exits the lumbar spine, travels through the pelvis, passes under or through the piriformis muscle in the buttock, and runs down the back of the leg to the foot.
When the nerve is compressed or irritated at any point along its route, symptoms can radiate along its path. The most common cause is a herniated or bulging disc in the lower lumbar spine, typically at L4–L5 or L5–S1.
What does sciatica feel like?
Sciatica typically produces a specific radiating pattern: pain, tingling, or numbness that starts in the lower back or buttock and travels down the back of the thigh, sometimes continuing into the calf, foot, or toes. The sensation is often described as electric, shooting, or burning.
Because the sciatic nerve runs through or near the piriformis muscle, piriformis syndrome — where the muscle irritates the nerve — can also produce sciatica-like symptoms without any disc involvement.
SI Joint Pain or Sciatica? The Key Differences
The overlap between these two conditions is real and clinically relevant. Both can cause lower back pain, buttock pain, and leg symptoms. The differences come down to pain location, quality, pattern, and which specific movements or positions aggravate them.
Where the pain travels
This is one of the clearest differentiators. SI joint pain rarely travels below the knee. The pain typically stays in the lower back, buttock, and posterior thigh. If you have significant calf, ankle, or foot symptoms, sciatica from a lumbar source is more likely.
Sciatica, by contrast, commonly produces symptoms that extend below the knee along the distribution of the sciatic nerve — the back of the thigh, calf, or into the foot and toes.
The quality of the pain
SI joint pain is typically described as a deep, dull ache in the lower back, buttock, or around the posterior superior iliac spine — the bony prominence at the back of the pelvis. It can feel like a constant heaviness or tenderness when pressing on one side of the lower back.
Sciatica tends to feel more electric or burning, often with tingling or numbness that follows the nerve path. The sensation is more specific to a line down the leg rather than a broad aching region.
Which side is affected
SI joint pain is almost always one-sided, though both joints can be involved. Pain is usually felt on one specific side of the lower back and buttock. Sciatica is also typically one-sided but originates centrally from the lumbar spine before tracking down one leg.
How position and movement affect symptoms
SI joint pain often flares with:
- Standing on one leg or transferring weight between legs
- Climbing stairs or stepping up
- Rolling over in bed
- Getting out of a car or rising from a low chair
- Prolonged standing on hard surfaces
Sciatica symptoms often worsen with:
- Prolonged sitting, especially in forward flexion
- Bending forward to pick something up
- Coughing, sneezing, or straining, which increases intraspinal pressure
- Lying flat without the knees supported
One useful self-observation: if sitting relieves your symptoms, an SI joint source is more likely. If sitting makes things significantly worse and walking actually helps, a lumbar disc or nerve root source is more probable.
Is it sciatica or SI joint pain if the pain is only in the buttock?
Isolated buttock pain is one of the most confusing presentations. Both conditions can cause it. SI joint referral commonly produces deep gluteal pain, sometimes mistaken for a hip problem. Piriformis syndrome — sciatic nerve irritation within the muscle — can also produce buttock-dominant symptoms without back pain at all.
This is precisely why isolated buttock pain warrants careful clinical assessment rather than self-diagnosis.
How SI Joint Pain Is Assessed
The SI joint cannot be reliably diagnosed from symptoms alone. Research published in Advances in Orthopedics confirms that history and physical examination alone are insufficient to diagnose SI joint pain — multiple tests must be combined to increase diagnostic validity.
A thorough assessment typically includes:
Provocation testing
Specific orthopaedic tests are used to stress the SI joint and reproduce the client’s familiar pain. A single positive test is not conclusive, but when three or more provocation tests are positive, the specificity for SI joint pain increases substantially. Tests commonly used include:
- FABER test (Flexion, ABduction, External Rotation) — places stress on the SI joint and hip
- Gaenslen’s test — stretches the SI joint by positioning one leg in hip extension
- Posterior shear test (thigh thrust) — applies a posterior shear force across the joint
- Compression test — applies lateral pressure to the pelvis
None of these tests should be performed in isolation. The clinical picture is built from multiple findings.
Postural and gait observation
The SI joint does not move much, but how the pelvis loads and shifts during gait gives useful information. Trendelenburg sign, where the pelvis drops on the non-stance side during single-leg standing, can indicate gluteal weakness that increases SI joint stress. Asymmetrical pelvic positioning is also relevant.
Neurological screening
Ruling out a lumbar disc or nerve root problem is part of any SI joint assessment. Reflexes, sensation, and muscle strength should be checked to determine whether the sciatic nerve is involved. If neurological signs are present, the working diagnosis shifts toward a lumbar source.
Imaging and its role
X-ray can identify sacroiliitis — inflammation of the SI joint — when associated with inflammatory conditions such as ankylosing spondylitis. MRI provides more detail about soft tissue and bone marrow changes. However, imaging findings must be matched with clinical symptoms. A structurally normal SI joint can still be a pain source, and imaging changes do not always correlate with symptoms.
How Sciatica Is Assessed Differently
When sciatica is suspected, the focus shifts to the lumbar spine and neural structures. Assessment typically includes:
- Straight Leg Raise — lifting the leg with the knee straight reproduces sciatic symptoms if the nerve is sensitised
- Slump test — a more sensitive neural tension test that reproduces symptoms through spinal flexion combined with leg extension
- Neurological examination — testing reflexes (knee jerk, ankle jerk), sensation in dermatomal patterns, and muscle power in specific myotomes
- Assessment of lumbar mobility and provocation — flexion and extension patterns help identify whether the disc is the likely source
If these tests reproduce the leg symptoms the client reports, and neurological signs are present, a lumbar disc or nerve root cause is the more likely explanation.
Can SI Joint Pain Cause Leg Pain?
Yes, it can — and this is where the confusion with sciatica most commonly arises. The SI joint has a referral pattern that commonly includes the buttock and posterior thigh. It can occasionally produce symptoms that travel toward the knee, though it rarely extends into the calf or foot.
When SI joint referral reaches the thigh, clients often assume they have sciatica. The distinction matters because the assessment and care approach differ. Nerve tension tests like the Straight Leg Raise are typically negative with SI joint pain, whereas they are often positive with true sciatica — this is one of the most useful differentiators in clinical assessment.
Can You Have Both SI Joint Pain and Sciatica at the Same Time?
Yes. This is more common than people expect. A lumbar disc problem can alter how someone moves, which in turn changes pelvic load and contributes to secondary SI joint irritation. Pregnancy is another common scenario where hormonal changes create SI joint laxity while postural shifts increase lumbar disc stress.
When both are present, addressing only one source typically produces partial improvement. This is another reason why thorough assessment is important before a care plan is established.
How ATLAS Assesses Lower Back and Buttock Pain
At ATLAS, lower back and buttock pain is never treated as a single category. The goal is to understand which structures are involved, how the nervous system is responding, and what the most likely driver of symptoms is — before any care plan is discussed.
If you have been told you have sciatica but it has not responded as expected, it is worth asking whether the SI joint has been properly assessed. Conversely, if SI joint dysfunction is suspected, ruling out a lumbar disc source through neurological testing matters before committing to a care direction.
Our assessment for lower back pain and buttock pain typically includes:
- A detailed history covering symptom pattern, onset, aggravating and relieving positions, and how the pain behaves across the day
- Orthopaedic provocation testing for the SI joint using a cluster of validated tests
- Neural tension testing to identify lumbar nerve root involvement
- Neurological screening — reflexes, sensation, and motor function
- Postural and gait observation to assess pelvic mechanics
- Spinal and pelvic mobility assessment
The aim is not to apply a label quickly. It is to build a clear picture so that care is directed at the actual source of symptoms, not a best guess.
When Should You Get Assessed for Lower Back and Buttock Pain?
Some lower back and buttock pain settles within days, particularly after a specific incident like a fall or heavy lift. But there are patterns that warrant proper assessment rather than waiting.
Signs that suggest assessment is worthwhile
- Pain that has been present for more than two to three weeks without meaningful improvement
- Buttock or leg pain that is getting worse rather than better over time
- Tingling, numbness, or weakness in the leg or foot
- Pain that consistently disrupts sleep
- Stiffness that makes it difficult to start moving in the morning
- Symptoms that change markedly with position — particularly if sitting is strongly aggravating or relieving
- A history of previous back or pelvic injury that has been recurring
Red flags that require urgent medical attention
If you experience any of the following, seek medical care promptly rather than booking a chiropractic assessment as a first step:
- Loss of bladder or bowel control
- Numbness in the groin or saddle area
- Progressive weakness in one or both legs
- Symptoms following significant trauma
Sources
Tzu Chi Medical Journal — Diagnosis and interventional pain management options for sacroiliac joint pain
Advances in Orthopedics — Sacroiliitis: A Review on Anatomy, Diagnosis, and Treatment
Cleveland Clinic — Sacroiliac (SI) Joint Pain
NHS — Sciatica: symptoms and causes
Springer — Sacroiliac Joint Dysfunction: anatomy, biomechanics, diagnosis, and treatment
