SI Joint Pain: Causes, Diagnosis, and Treatment Options

The Joint Nobody Thinks About

You have lower back pain. It radiates into your buttock. Maybe down the back of your thigh. Your doctor orders an MRI of your lumbar spine. It shows some "degenerative changes" but nothing that explains your pain. You are told it is just "nonspecific low back pain."

But your pain is very specific — you can point to it. It is right there, at the dimple below your belt line, slightly off-center. It hurts when you get out of a car, climb stairs, roll over in bed, or stand from sitting.

This pattern suggests sacroiliac (SI) joint dysfunction, and it accounts for 15-30% of all chronic low back pain. It is chronically underdiagnosed because lumbar spine MRI does not show it, and many providers do not test for it.

Anatomy Basics

The sacroiliac joints are where your spine connects to your pelvis — specifically where the sacrum (base of spine) meets the ilium (pelvic bone). You have two, one on each side. They are large, strong joints designed to transfer force between your upper body and legs.

Unlike the mobile joints of your spine, SI joints move very little — only 2-4 degrees of rotation and 1-2mm of translation. But when they become inflamed, hypermobile, or hypomobile, the pain can be debilitating.

What Causes SI Joint Pain

Inflammatory

• Ankylosing spondylitis — the classic autoimmune cause. SI joint inflammation is often the first sign. Should be suspected in young patients (under 40) with inflammatory low back pain (worse with rest, better with activity, morning stiffness over 30 minutes).
• Psoriatic arthritis — can involve SI joints unilaterally or bilaterally
• Reactive arthritis — post-infectious SI joint inflammation
• Other spondyloarthropathies

Mechanical/Degenerative

• Pregnancy and postpartum — relaxin hormone loosens SI joint ligaments. Many women develop SI joint pain during pregnancy that persists after delivery.
• Leg length discrepancy — even small differences create asymmetric force on SI joints
• Prior lumbar fusion — increased stress on SI joints is common after spinal fusion surgery (reported in 30-40% of lumbar fusion patients)
• Hypermobility — generalized joint hypermobility (Ehlers-Danlos spectrum) predisposes to SI instability
• Degenerative arthritis — age-related wear, particularly common in women over 50
• Trauma — falls, car accidents, or any significant impact to the pelvis

How to Identify SI Joint Pain

The Pain Pattern

• Location: Over the SI joint (PSIS — posterior superior iliac spine), often described as "right here" with one finger pointing
• Radiation: Into the buttock, posterior thigh, sometimes groin. Rarely below the knee (which helps differentiate from sciatica)
• Aggravating factors: Transitional movements (sit-to-stand, getting in/out of car), stair climbing, single-leg activities, prolonged sitting or standing on one side
• Relief: Changing positions frequently, lying flat

Physical Examination

Specific provocative tests that stress the SI joint:

• FABER test (Patrick's test)
• Gaenslen's test
• Compression/distraction tests
• Thigh thrust
• Sacral thrust

Three or more positive provocation tests strongly suggest SI joint origin (sensitivity 94%, specificity 78%).

Diagnostic Injection

The gold standard: an image-guided anesthetic injection into the SI joint. If your pain decreases by 75% or more after the injection, the SI joint is confirmed as the source. This is both diagnostic and therapeutic.

Imaging

• X-ray: Shows advanced degenerative changes or sacroiliitis
• MRI: Can show bone marrow edema, joint effusion, or inflammatory changes (particularly important for suspected ankylosing spondylitis)
• CT scan: Best for bony detail and chronic degenerative changes
• Labs: HLA-B27, ESR, CRP if inflammatory cause suspected

Treatment Approaches

Conservative Management

Physical therapy is the cornerstone:

• SI joint stabilization exercises (targeting gluteus medius, transversus abdominis, pelvic floor)
• Addressing muscle imbalances (hip flexor tightness, gluteal weakness)
• Manual therapy techniques for joint mobilization
• Pelvic alignment correction

SI joint belts — an external support that compresses and stabilizes the joint. Particularly helpful in pregnancy-related and hypermobility-related SI pain.

Activity modification — avoid asymmetric loading. Distribute weight evenly. Use a pillow between knees when sleeping on your side.

NSAIDs — reduce inflammation. Effective for both inflammatory and mechanical SI joint pain.

Interventional Procedures

SI joint injection (corticosteroid + anesthetic) — provides 3-6 months of relief for many patients. Can be repeated.

Radiofrequency ablation — heats and deactivates the nerves that supply the SI joint (lateral branch nerves). Provides 6-18 months of relief. Can be repeated when nerves regenerate.

Prolotherapy — injection of irritant solution to stimulate ligament healing. Mixed evidence but some patients with ligamentous laxity respond.

PRP (platelet-rich plasma) — emerging evidence for SI joint arthropathy. Theoretical benefit for tissue healing.

Surgical Options

SI joint fusion — for patients who fail comprehensive conservative and interventional treatment. Minimally invasive options (iFuse implant) have shown good outcomes in selected patients. Reserved for severe, confirmed SI joint dysfunction after failed non-operative management.

The Pregnancy Connection

SI joint pain is extremely common during and after pregnancy:

• Estimated 20-30% of pregnant women experience significant SI pain
• Hormonal changes (relaxin) loosen the joint
• Weight gain and postural changes increase SI joint stress
• Often persists postpartum, especially without targeted rehabilitation

If you had a baby and your "low back pain" never went away — especially if it is one-sided, over the belt line, and worsened by single-leg activities — SI joint dysfunction should be evaluated.

When to Worry

Most SI joint pain is mechanical or degenerative. But certain features warrant further investigation:

• Age under 40 with inflammatory symptoms (AS screening)
• Morning stiffness over 30 minutes that improves with activity
• Bilateral SI joint involvement (suggests inflammatory cause)
• Night pain that wakes you
• Associated symptoms: eye inflammation, skin rashes, GI symptoms (associated with spondyloarthropathies)

The Bottom Line

SI joint pain is common, diagnosable, and treatable. The biggest barrier is recognizing it in the first place — it hides behind the label of "nonspecific low back pain" and gets missed when the focus stays on the lumbar spine.

If your back pain is not getting better and it matches the SI joint pattern, bring it up. At Coral, we evaluate lower back pain systematically, including SI joint assessment. [Start your visit](/start) and let us identify what is actually causing your pain.