Ankylosing spondylitis (AS) is a type of arthritis that primarily affects the spine. However, it can also affect the hips and sacroiliac (SI) joint.

The SI joint is the joint that connects the bottom of the spine to the pelvis.

People with SI joint pain, or sacroiliitis, may feel stiffness or pain in the buttocks, lower back, or down one or both legs.

SI joint pain may be an initial symptom of AS or develop later as the disease progresses. The appearance of symptoms can vary from person to person.

However, SI joint pain does not necessarily mean someone has AS because many factors can cause SI joint pain.

In this article, we will look at how AS affects the SI joint, including the symptoms, diagnosis, and treatment options.

A woman with ankylosing spondylitis sitting with her lower back on a tennis ball to massage muscles.Share on Pinterest
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Yes, AS can affect the SI joint. Inflammation in the SI joint, which connects the pelvic bones to the bottom of the spine, is known as sacroiliitis. It is one of the hallmark features of AS.

Besides the SI joint, AS may also affect the:

  • shoulders
  • hips
  • ribs
  • hands
  • heels and feet

No — inflammation of the SI joint can occur for many reasons, so it does not always indicate someone has AS. Some other things that can cause SI joint pain include:

The first symptoms of AS usually start to show in people between the ages of 15 and 30. Most commonly, these initial symptoms include chronic back pain and stiffness, but a person may also have pain in the SI joint or hips as an initial symptom.

In the early stages of AS, common symptoms include:

  • early morning stiffness and pain lasting that eases throughout the day or with physical activity
  • pain that wakes a person during the night
  • pain in one or both buttocks and potentially also the backs of the thighs

Additionally, some people feel pain in the neck, shoulder, hip, or thigh.

Pain from AS tends to worsen when a person sits or lies still for extended periods, then improves when they begin moving around. Other symptoms someone may experience include:

  • a sore heel or arch on their foot
  • a painful or swollen finger or toe
  • tenderness at the base of the pelvis, making sitting on a hard surface uncomfortable
  • a pain or tightness in the chest, making it difficult to take deep breaths
  • inflammatory bowel disease
  • severe tiredness that does not improve with sleep or rest
  • uveitis or iritis, which causes painful and sometimes red eyes

If AS presents on an X-ray, it may show:

  • a widening of the space between the sacroiliac joints
  • whitening of the bones
  • lesions or damage to the bones

In later stages of the disease, the bones can also fuse, which will be visible on an X-ray. However, X-rays do not always show signs of AS in the early stages of the disease. It can take years for AS to show on an X-ray.

If a doctor cannot confirm AS with an X-ray, they may order an MRI scan. This can diagnose the condition earlier in the disease. On an MRI scan, doctors could see the same symptoms as above and swelling or damage to the soft tissues between joints.

Doctors will also use information about a person’s symptoms and a physical examination to inform their diagnosis. They may also test for the HLA-B27 gene, which has strong links to AS.

Doctors can grade inflammation of the SI joint using the modified New York criteria, which doctors use to diagnose AS. This includes clinical and radiological criteria, meaning it considers both the symptoms and findings from scans.

The radiological criteria are as follows:

  • Grade 0: typical
  • Grade 1: suspicious changes
  • Grade 2: minimal definite changes
  • Grade 3: distinctive changes
  • Grade 4: ankylosis, or bone fusion

For a doctor to diagnose AS, sacroiliitis must be at least grade 2 on both sides or at least grade 3 on one side. Additionally, a person must have at least one of the following clinical criteria:

  • lower back pain for at least 3 months that improves with exercise but not with rest
  • limited lower spine flexibility
  • limited lower chest expansion

Doctors treat SI joint pain due to AS by treating the underlying inflammation that drives the disease. This may include medicines, physical therapy, and in some cases, surgery.

Medicines to treat the pain and inflammation include:

Physical therapy can also help a person improve their muscle strength and maintain mobility in their spine and other joints.

Occasionally, an individual with AS may need surgery to replace a joint that has significant damage. These joints include the hip or knee.

Learn more about AS treatments.

Many people with AS remain independent and lead typical lives, but the severity and progression of the disease will vary from person to person.

If SI joint pain is an early symptom and the individual gets a prompt diagnosis, their treatment has a better likelihood of slowing or stopping permanent joint damage. However, if SI joint pain is a later symptom or someone has a delayed diagnosis, they may experience more effects.

The earlier a doctor diagnoses AS, the sooner treatment can begin to help slow or stop its progression. People with any concerns they could have AS should not hesitate to contact a healthcare professional.

AS itself does not usually affect life expectancy but can increase a person’s risk of developing conditions that do, such as heart attack and stroke. An individual can ask a doctor about ways to reduce the risks.

Ankylosing spondylitis (AS) is a chronic inflammatory condition of the spine. It often affects the sacroiliac (SI) joint at the bottom of the spine and can harm joints in the rest of the body. SI joint inflammation can cause pain in the lower back, buttocks, and legs.

AS joint pain tends to worsen with rest and improve with physical activity. Doctors treat the condition with physical therapy and medications to reduce inflammation and pain.