There are more than 100 types of arthritis and related conditions. They range from extremely common forms like osteoarthritis, which affects tens of millions of people, to rare varieties tied to gut infections or mineral imbalances. Nearly 19% of U.S. adults had a diagnosed form of arthritis in 2022. Despite the staggering number of individual types, most fall into a handful of broader categories that help make sense of how and why they develop.
The Four Main Categories
Every form of arthritis works through one of two basic mechanisms: it either breaks down the tissue in your joint (degeneration) or triggers swelling that damages the joint over time (inflammation). Some types do both. Within those two mechanisms, arthritis sorts into four major groups.
- Degenerative arthritis is joint wear and tear. Osteoarthritis is the primary example.
- Inflammatory arthritis happens when your immune system mistakenly attacks your joints. Rheumatoid arthritis, psoriatic arthritis, and ankylosing spondylitis all fall here.
- Metabolic arthritis results from chemical crystals forming inside a joint. Gout and pseudogout are the two main types.
- Infectious arthritis is caused by bacteria, viruses, or fungi that invade a joint directly.
Osteoarthritis: The Most Common Type
Osteoarthritis involves the gradual wearing away of cartilage, the smooth cushioning that caps the ends of your bones where they meet in a joint. A lifetime of using your joints eventually thins that cartilage, so the condition becomes more likely as you age. It most often affects the knees, hips, hands, and spine. Pain tends to worsen with activity and improve with rest, and stiffness after sitting still usually loosens within about 30 minutes.
Because osteoarthritis is driven by mechanical wear rather than immune system malfunction, it behaves differently from inflammatory types. It typically affects joints on one side of the body or specific joints that have carried heavy loads or sustained old injuries, rather than appearing symmetrically.
Rheumatoid Arthritis and Other Autoimmune Types
Rheumatoid arthritis is the most recognized inflammatory form. The immune system attacks the lining of the joints, causing pain, swelling, and eventual joint damage. Unlike osteoarthritis, it often affects the same joints on both sides of the body, particularly the small joints of the hands and feet, and it can strike at any age.
Diagnosing it relies on a combination of physical examination, imaging, and blood tests. Two key antibodies help confirm the diagnosis: rheumatoid factor (RF) and anti-CCP antibodies. Anti-CCP is especially useful because it can appear before symptoms develop, allowing earlier treatment. That said, some people with rheumatoid arthritis test negative for both, so normal blood work doesn’t always rule it out.
Psoriatic Arthritis
Psoriatic arthritis develops in some people who have the skin condition psoriasis. It can affect any joint and often involves the fingers and toes, sometimes causing them to swell into a sausage-like shape. Not everyone with psoriasis develops joint problems, and in some cases the arthritis appears before any skin symptoms do.
Ankylosing Spondylitis
Ankylosing spondylitis primarily targets the spine and the joints where the spine meets the pelvis. Many people experience episodes of back pain and stiffness that come and go, but in more severe cases the vertebrae can fuse together, reducing flexibility. The condition can also cause inflammation in the eyes, skin, and gut. A gene called HLA-B27 raises the risk significantly, though carrying the gene doesn’t guarantee you’ll develop the disease.
Gout and Pseudogout
These two metabolic types share a similar mechanism: crystals form inside a joint and trigger sudden, intense inflammation. The difference is the type of crystal.
Gout is caused by uric acid crystals. When your body produces too much uric acid or doesn’t clear it efficiently, the excess can crystallize in a joint, most famously the base of the big toe. Attacks often strike suddenly, sometimes overnight, with severe pain and swelling.
Pseudogout works the same way but involves calcium pyrophosphate crystals instead. It tends to affect the knees and wrists rather than the big toe. People with certain mineral imbalances, such as excess iron or low magnesium, face a higher risk. An underactive thyroid or overactive parathyroid gland can also contribute. Previous joint injuries or surgeries raise the odds of pseudogout developing in that specific joint.
Infectious Arthritis
When bacteria, viruses, or fungi enter a joint, the resulting infection causes rapid, severe pain and swelling. The most common culprit is Staphylococcus aureus, the same bacterium behind many staph skin infections. Septic arthritis typically comes on fast and requires urgent treatment to prevent permanent joint damage.
Viral arthritis is a milder, usually temporary form triggered by viral infections, including COVID-19. It tends to resolve as the underlying infection clears, though joint pain can linger for weeks or months in some cases.
Juvenile Idiopathic Arthritis
Arthritis isn’t limited to adults. Juvenile idiopathic arthritis (JIA) is a group of conditions defined by joint inflammation that begins before age 16 and lasts longer than six weeks with no other identifiable cause. It’s further divided into six subtypes:
- Oligoarticular JIA affects four or fewer joints and is the most common subtype in children.
- Polyarticular JIA involves five or more joints and can be either RF-negative or RF-positive.
- Enthesitis-related arthritis causes inflammation where tendons and ligaments attach to bone, often in the legs and spine.
- Psoriatic JIA combines joint inflammation with psoriasis or a family history of it.
- Systemic JIA affects the whole body, causing fevers and rashes alongside joint symptoms.
- Undifferentiated JIA is the designation when symptoms don’t fit neatly into any other category.
Less Common Forms
Several rarer types account for much of the reason the total count exceeds 100. Reactive arthritis develops after an infection elsewhere in the body, typically a gut or urinary tract infection. Symptoms usually appear a few weeks after the initial illness and can include joint pain, eye irritation, and urinary problems.
Enteropathic arthritis is specifically linked to inflammatory bowel disease (IBD), affecting roughly 1 in 5 people with Crohn’s disease or ulcerative colitis. It can also develop after intestinal infections caused by bacteria like Salmonella or Shigella. The joint symptoms often flare alongside gut symptoms, though they can follow an independent pattern.
Lupus-related arthritis occurs when systemic lupus erythematosus, an autoimmune disease that can affect nearly any organ, targets the joints. The joint pain tends to shift from one area to another rather than settling permanently in one spot.
Why the Number Matters Less Than the Category
With over 100 types on the list, the specific count is less important than understanding which category your symptoms point to. Degenerative arthritis is managed very differently from autoimmune or crystal-driven types. Identifying the right category shapes everything from which blood tests are useful to what treatments are likely to help. If you’re experiencing persistent joint pain, swelling, or stiffness, the first practical step is figuring out whether the problem is mechanical wear, immune-driven inflammation, crystal deposits, or infection, because each path leads to a different set of options.