Most people with ankylosing spondylitis (AS) develop pain and stiffness in their lower back, but it’s not uncommon to also develop symptoms in other joints.

Ankylosing spondylitis (AS) is a type of arthritis that usually develops under age 40. About 80% of people develop symptoms before they’re 30 years old.

AS develops when your immune system attacks your joints and causes inflammation. It’s not uncommon to experience symptoms in joints like your knees.

AS falls into a group of conditions called spondylarthritis. These conditions are characterized by inflammation of the spine and other joints.

Read on to learn more about why AS sometimes causes knee pain and how it’s treated.

AS is an autoimmune disease. Autoimmune diseases occur when your immune system attacks healthy cells in your body. Researchers don’t know why some people develop AS, but it’s thought that both your genes and environmental factors contribute.

People with AS experience symptom flare-ups that can last days to weeks. During these flare-ups, your immune system actively attacks healthy cells in your joints, which causes your body to release molecules that create inflammation.

Inflammation can cause symptoms like joint pain and stiffness. These symptoms usually get better between flare-ups when inflammation subsides.

Chronic inflammation in your joints over many years can lead to the formation of new bone. This, in turn, can lead to immobility and permanent disability.

Generally when a person experiences AS symptoms in their knees, only one knee is affected or one knee is affected worse than the other.

AS can cause inflammation inside your knee joint itself or cause a condition called enthesitis. This happens when inflammation occurs in the enthesis, the place where a tendon or ligament attaches to a bone. Anywhere from 30% to 70% of people with AS develop enthesitis.

Enthesitis can cause:

  • tenderness
  • stiffness
  • pain
  • redness and heat

Along with directly affecting your knees, AS can cause changes to your spine that alter your knee mechanics and contribute to knee pain.

It’s not uncommon to experience knee pain or stiffness if you have AS. However, it’s rare to have knee pain without more typical symptoms of AS like lower back pain.

Estimates on how many people with AS have knee symptoms vary among studies. Some studies estimate that about 20% of people with AS have knee involvement, whereas other studies estimate that the knees are affected in as many as 70% of people. More research is needed before any firm conclusions can be reached.

AS and HLA-B27 gene

Almost everybody with AS has the gene HLA-B27. However, only about 5% to 6% of people who have this gene actually develop AS, according to a 2019 research review.

More than 100 variations in this gene have been identified, as noted in the research review. Some variations are associated with AS more strongly than others, and some aren’t associated with AS at all.

AS does not have a cure, but a combination of lifestyle changes and medications can often help manage symptoms and improve quality of life.


People with AS usually have worse pain during periods of inactivity or rest. Regular exercise may help you avoid stiffness and improve the function of your joints.

In a 2019 review of studies, researchers found low- to moderate-quality evidence that, compared with no treatment, following an exercise program may slightly:

  • reduce pain
  • improve function
  • decrease disease activity

Doctors often recommend low impact exercises, such as:

It’s generally a good idea to avoid exercises that cause pain.

Learn more about exercise for AS.

Over-the-counter treatment

The first medications that doctors usually recommend for AS are nonsteroidal anti-inflammatory drugs (NSAIDs). These drugs can help reduce inflammation, stiffness, and pain.

Some medications in this class of drugs are available over-the-counter (OTC). They include:

  • ibuprofen (Advil and Motrin)
  • naproxen (Aleve)
  • diclofenac (Voltaren, which is available OTC as a topical treatment; the oral form is available only by prescription)

If you can’t take NSAIDs, your doctor may recommend alternative pain relievers like acetaminophen (Tylenol).

Learn more about medications for AS.

Medical treatment

If OTC and home remedies do not relieve your symptoms, your doctor may recommend prescription medications. These include:

The last resort treatment for knee pain caused by AS is a surgical procedure called a total knee replacement. This procedure replaces your entire knee joint with a prosthesis.

In a 2017 study, researchers found that the annual rates of total knee replacements in people with AS in the United States increased by 168% between 2002 and 2013.

If you haven’t previously received a diagnosis of AS, you may want to visit a primary doctor if you have unexplained knee pain. If a doctor suspects that you might have arthritis, they might refer you to another doctor who specializes in joints, called a rheumatologist.

If you’ve previously received an AS diagnosis, it’s a good idea to visit your doctor if you develop knee pain or any other new or concerning symptoms.

AS is a type of arthritis that usually causes lower back pain but can also cause stiffness and pain in other joints, such as your knees.

Most people with AS can manage their symptoms with home remedies or medications. In rare cases, a doctor may recommend a total knee replacement if more conservative treatments do not offer relief.