If you have pain radiating down your leg, you may immediately think: “I have a pinched nerve in my back.” But sacroiliac (SI) joint dysfunction can also cause pain that radiates down the leg. So, how do you tell the difference?

First, it’s important to understand that sciatica is not a diagnosis: it’s a symptom related to an underlying condition. Sciatica is the sensation of pain, tingling, weakness, and/or numbness in the lower extremities that is typically caused by compression or irritation of a spinal nerve(s) or of the sciatic nerve itself (see diagram). Usually, the nerve compression or irritation occurs in the lumbar spine.

Here are a few subtle differences that he or she may be looking for:

It’s rare for someone with sciatica from an SI joint problem to have real numbness, weakness, or reflex changes. This is because there is rarely a physical compression of the nerve. The L5 and/or S1 nerves are irritated (called radiculitis) when they cross near the SI joint, but these nerves are not compressed.

Your radiating leg pain (sciatica) can be from your spine or from your SI joint. However, it is possible to be diagnosed with problems in both areas. That’s why it’s so important to visit your doctor to truly determine what is causing your low back or leg pain.

SI Joint and Sciatica Definitions

Let’s back up a step and make sure we fully understand the definitions of sciatica and SI joint dysfunction.

What Is Sciatica?

Sciatica is a symptom (radiating leg pain) caused by a problem with the spinal nerve(s) or sciatic nerve, such as compression or irritation, which sends signals of pain, numbness, tingling, or weakness. The sciatic nerve is a made up of several nerves from your lower spine; it extends down the back of your leg to the bottom of your foot. You have one on each side. Sometimes, the compression in the spine affects nerves on both the left and right sides of the body.

The sciatic nerve carries nerve signals down to the muscles and sensation signals up to the spinal cord. These signals tell your muscles to move; when these signals are disrupted, this is why you might sometimes feel weakness or buckling in the knee.

What Is SI Joint Dysfunction?

Sacroiliac (SI) joint dysfunction is caused by trauma or degeneration of the SI joint. The SI joint is where your iliac bone (pelvis) connects to the sacrum (lowest part of the spine above the tailbone).

The SI joint is responsible for transferring the weight from your upper body to your pelvis and legs. Pain caused by SI joint dysfunction can be felt in the lower back or spine, buttocks, pelvis, groin, and sometimes in the legs, which makes it seem like the cause could be nerve compression in the spine.

The L5 and S1 nerves are near the SI joint and studies have shown that SI joint dysfunction can cause pain and other symptoms in the distribution of these nerves.

The SI joint is separate from the sciatic or spinal nerve(s); however, the SI joint can cause sciatica-like symptoms.

Underlying Causes of Sciatica Pain and SI Joint Pain

Oftentimes, it’s difficult to pinpoint the exact cause of pain. Nerve compression in the spine and SI joint dysfunction are two areas that often cause pain running down the back of the leg.

Testing to Determine the Source of Your Leg or Lower Back Pain

Your doctor will likely ask many questions, such as when the pain started, how long it lasts, and what causes it to get worse or better. Answers to these questions will provide clues to which tests you should get first.

For example, if your pain started after a fall on the buttocks or if it extends to the groin area, that might be a clue that it’s SI-joint-related, and you may require physical examination including provocative tests.

Provocative tests help determine whether the pain is caused by the SI joint. A diagnostic injection can help confirm diagnosis. If you are experiencing true muscle weakness, this could indicate that you have a pinched nerve in the spine. When nerves are compressed/irritated in the spine, patients will frequently have a positive passive straight leg raising test.

Your doctor will also likely rule out potential causes of nerve compression in the spine, such as a bulging disc, with an MRI of the spine and other radiological and laboratory testing.

Sometimes patients are misdiagnosed, like in the case of Keith, who was diagnosed with a pinched nerve in the spine but in reality, had SI joint dysfunction. The pain was coming from his SI joint. See Keith’s Sciatica from SI Joint Pain Story.

Treatments to Relieve the Pain

Conservative therapies to treat sciatica from both the spine or the SI joint may include therapeutic injections of steroids, which may offer temporary relief. For sciatica related to the spine, the injection will be targeted in the lumbar spine at the site of the nerve compression. For sciatica related to SI joint dysfunction, the injection will be targeted in the SI joint.

Treatment of spinal conditions may include medications, physical therapy (including exercises specific for sciatic pain), and in extreme cases, surgery to remove the pressure from the pinched nerve(s) in the spine.

SI joint dysfunction treatments also include medications, physical therapy and other non-surgical treatments, and if non-surgical treatments no longer work, minimally invasive SI joint fusion may be an option.

If you suspect your lower back and leg pain is caused by your sacroiliac joint or your spine, visit your doctor with a list of symptoms, including when the pain started, and what makes it worse. If it turns out you need an SI joint specialist, you can find one in your area here.