There are more than 100 types of arthritis, but most fall into a handful of major categories based on what’s actually going wrong inside the joint. Some types wear down cartilage over time, others are driven by an immune system that attacks healthy tissue, and still others are triggered by infection or crystal buildup. About 67 million adults in the United States have some form of diagnosed arthritis, and that number is projected to reach 78.4 million by 2040.
Understanding the broad categories helps, because the type you have determines what treatment looks like and how the condition progresses.
Osteoarthritis
Osteoarthritis is by far the most common type. It’s a degenerative condition where the tissues inside a joint gradually break down. This goes beyond just cartilage. As the disease progresses, it can damage tendons, ligaments, the joint lining, the bone itself, and (in the knee) the meniscus. Researchers still don’t fully understand what triggers this breakdown in the first place.
The typical progression starts with cartilage thinning, which leads to pain and swelling. That pain makes you less active, which weakens the surrounding muscles, which puts even more stress on the joint. Over time, the joint can lose its normal shape, and small bony growths called bone spurs can develop along its edges. Osteoarthritis most commonly affects the knees, hips, hands, and spine, and it becomes more prevalent with age.
Rheumatoid Arthritis
Rheumatoid arthritis (RA) is an autoimmune disease. Your immune system mistakenly attacks the synovium, a thin membrane that lines your joints and produces lubricating fluid. As this lining becomes inflamed and thickens, fluid builds up inside the joint, causing swelling, stiffness, and pain. Left unchecked, the inflammation can erode cartilage and bone.
One hallmark of RA is symmetry. It typically affects the same joints on both sides of the body. If the knuckles on your left hand are inflamed, the same knuckles on your right hand usually are too. RA often starts in smaller joints like the fingers and toes, and it can also cause fatigue and a general feeling of being unwell.
Psoriatic Arthritis
Psoriatic arthritis (PsA) develops in some people who have the skin condition psoriasis, though joint symptoms occasionally appear before any skin changes. It shares features with both RA and osteoarthritis but has its own distinct signs.
About 40 percent of people with PsA develop dactylitis, sometimes called “sausage digits,” where an entire finger or toe swells due to inflammation in the small joints and surrounding tendons. Roughly 50 percent experience enthesitis, which is pain and inflammation where tendons and ligaments attach to bone, commonly at the heel or the bottom of the foot. Between 7 and 32 percent develop inflammation in the spinal column. Unlike RA, PsA tends to be asymmetric, affecting different fingers or toes on each side of the body.
Ankylosing Spondylitis
Ankylosing spondylitis primarily targets the spine and the joints where the spine meets the pelvis. Chronic inflammation can eventually cause vertebrae to fuse together, reducing flexibility and creating a hunched posture in severe cases. It belongs to a broader family of conditions called spondyloarthropathies.
There’s a strong genetic link. Between 60 and 90 percent of people with axial spondyloarthritis carry a gene called HLA-B27. That said, carrying the gene doesn’t mean you’ll get the disease. The lifetime risk for someone who is HLA-B27 positive is only about 1 to 2 percent. The gene’s prevalence varies widely across populations: 10 to 16 percent of Scandinavian populations carry it, compared to 25 to 50 percent of Inuit and some Indigenous North American groups, while it’s extremely rare in people of unmixed Sub-Saharan African ancestry.
Gout
Gout is a type of inflammatory arthritis caused by crystal buildup in a joint. Your body produces a substance called urate when it breaks down purines, compounds found naturally in your tissues and in many foods, particularly those from animal sources. When the body makes too much urate or doesn’t remove enough of it, the excess can form sharp crystals that deposit in joints.
Gout flares are notoriously painful. They often strike suddenly at night, with intense pain bad enough to wake you from sleep. The affected joint becomes swollen, red, and warm. The big toe is the most common target, though gout can affect other joints. Flares can be triggered by certain foods, alcohol, physical trauma, or illness. Drinking sugar-sweetened beverages like soda also raises the risk, as do conditions that cause rapid cell turnover, such as psoriasis.
Septic Arthritis
Septic arthritis happens when a joint becomes infected, usually by bacteria. The most common culprit is Staphylococcus aureus, followed by Streptococcus species. In older adults, gram-negative bacteria cause roughly 23 to 30 percent of cases. In adolescents and young adults, gonorrhea is an important cause to consider. Less commonly, viruses or fungi can be responsible.
This type of arthritis is a medical emergency. It typically affects a single joint, which becomes rapidly swollen, hot, and extremely painful. Unlike other forms of arthritis, septic arthritis requires prompt treatment to prevent permanent joint damage. Diagnosis requires drawing fluid from the joint and testing it for bacteria.
Lupus-Related Arthritis
Systemic lupus erythematosus (lupus) is an autoimmune disease that can affect virtually any organ, and joint pain is one of its most common symptoms. The arthritis of lupus looks a lot like RA on the surface, with pain, swelling, and stiffness in the hands and wrists. The key difference is that lupus-related arthritis is typically nonerosive, meaning it generally doesn’t cause the permanent bone damage that RA does.
Juvenile Idiopathic Arthritis
Arthritis isn’t limited to adults. Juvenile idiopathic arthritis (JIA) is a group of conditions characterized by arthritis that begins before age 16. The current classification system recognizes six categories: oligoarticular (affecting four or fewer joints), polyarticular (five or more joints, either with or without a specific blood marker called rheumatoid factor), enthesitis-related, psoriatic, systemic, and undifferentiated. Symptoms, severity, and outlook vary significantly between these subtypes, which is why doctors classify them separately rather than treating childhood arthritis as a single disease.
Fibromyalgia and Centralized Pain
Fibromyalgia is often grouped with arthritis conditions even though it doesn’t involve joint inflammation or damage. It’s a centralized pain condition, meaning the central nervous system processes pain signals abnormally, amplifying them. The best analogy is that the volume control on pain is turned up involuntarily. Something that wouldn’t cause pain for most people may feel genuinely painful, a phenomenon called allodynia.
This type of amplified pain processing can also develop alongside inflammatory or degenerative arthritis. Some people with RA, PsA, or osteoarthritis continue to experience significant pain even after aggressive treatment controls their joint inflammation, likely because their nervous system has developed centralized sensitization. Fibromyalgia often co-occurs with poor sleep, depression, and anxiety.
How Doctors Tell Them Apart
Diagnosing the specific type of arthritis relies mainly on a thorough medical history and physical exam, supported by blood tests and imaging. No single test is definitive, but several help narrow things down.
Rheumatoid factor (RF) is a blood marker associated with RA, but it’s far from perfect. Healthy older adults and people with hepatitis C can test positive without having RA, and some people with confirmed RA test negative. A more accurate marker is anti-CCP, which has about 95 percent specificity, meaning most positive results are true positives. Doctors often order both tests together. For conditions in the spondyloarthritis family, the HLA-B27 gene test can be informative. Antinuclear antibody (ANA) testing helps evaluate for lupus and other autoimmune conditions.
Imaging plays a supporting role. X-rays can reveal joint erosion and bone spurs associated with osteoarthritis or advanced RA. MRI and ultrasound are useful for detecting early inflammation and soft tissue changes before they show up on X-rays. For suspected gout, imaging or joint fluid analysis can confirm the presence of urate crystals.