There are more than 100 different types of arthritis. That number surprises most people, who tend to think of arthritis as a single disease. In reality, “arthritis” is an umbrella term for any condition that causes joint pain, swelling, or stiffness, and roughly 21% of American adults have been diagnosed with some form of it. These 100-plus types fall into a handful of major categories, each with a different cause and different approach to treatment.
The Major Categories
Most rheumatologists group arthritis into several broad families based on what’s actually happening inside the joint. The main ones are degenerative (wear-and-tear) arthritis, autoimmune and inflammatory arthritis, crystal arthritis, infectious arthritis, and spondyloarthritis (which primarily affects the spine). Some forms overlap categories, and a few rare types don’t fit neatly into any single group. Understanding which category your arthritis belongs to matters because the treatment strategy is completely different for each one.
Osteoarthritis: The Most Common Type
Osteoarthritis is by far the most prevalent form. It develops when the cartilage cushioning a joint breaks down over time. In a healthy joint, cartilage continuously repairs itself. In osteoarthritis, the enzymes that break down old cartilage outpace the body’s ability to rebuild it, leading to a net loss of cushioning. The process starts with small cracks in the cartilage surface and gradually works deeper, eventually exposing bone.
Age is the biggest risk factor, but osteoarthritis isn’t simply “getting old.” Joint injuries, repetitive stress from certain occupations or sports, obesity, and genetics all accelerate the process. It most commonly affects the knees, hips, hands, and spine. The hallmark symptom is pain that worsens with activity and improves with rest, along with stiffness that lasts less than 30 minutes after sitting still.
Rheumatoid Arthritis and Other Autoimmune Types
Rheumatoid arthritis is the most well-known autoimmune form. Instead of wear and tear, the immune system mistakenly attacks the lining of the joints, triggering chronic inflammation that can erode cartilage and bone. It typically starts in small joints like the fingers and wrists, and it tends to affect the same joints on both sides of the body.
Blood tests play a key role in identifying rheumatoid arthritis. One marker, called anti-citrullinated peptide antibodies, is found in about 70 to 75% of people with the disease and is highly specific to it (over 90% specificity). Another marker, rheumatoid factor, can appear in the blood years before symptoms start. Nearly 30% of people who eventually develop rheumatoid arthritis have detectable rheumatoid factor a median of 4.5 years before diagnosis.
Lupus is another autoimmune condition that frequently involves the joints. Up to 95% of people with lupus experience joint problems at some point. The difference is that lupus arthritis typically does not erode bone the way rheumatoid arthritis does, and it comes alongside other systemic symptoms like skin rashes, fatigue, and organ inflammation.
Psoriatic Arthritis
About 20% of people with psoriasis develop psoriatic arthritis, with that number climbing to 25% in those with moderate to severe skin disease. It can cause swollen, sausage-like fingers or toes (a sign called dactylitis), pitted or crumbling nails, and pain where tendons attach to bone. Some people develop skin symptoms years before joint problems appear, while for others the joint pain comes first.
Severity of psoriasis, the presence of nail changes, and a history of eye inflammation called uveitis are all predictors of who will go on to develop joint involvement. At its most severe, psoriatic arthritis can progress to a rare subtype called arthritis mutilans, which breaks down bone and tissue in the fingers and toes. That affects roughly 5% of psoriatic arthritis patients.
Gout and Crystal Arthritis
Gout develops when uric acid in the blood rises above its saturation point, causing sharp crystals to form inside a joint. The immune system reacts to those crystals with an intense inflammatory response, flooding the joint with white blood cells. A gout flare is famously painful, often striking the big toe, and can go from zero to excruciating within hours.
Between flares, people with gout can feel completely normal. But if uric acid levels stay elevated, flares become more frequent and crystals can accumulate into visible lumps called tophi. Over time, this creates a cycle of persistent, recurring inflammation that can permanently damage the joint.
A related condition, calcium pyrophosphate deposition disease (formerly called pseudogout), follows a similar pattern but involves a different type of crystal. It most commonly affects the knees and wrists and tends to appear in older adults. There is currently no way to dissolve the crystals once they form, so treatment focuses on managing inflammation during flares.
Spondyloarthritis: Arthritis of the Spine
Spondyloarthritis is a family of conditions that primarily affect the spine and the joints where the pelvis meets the lower back. Ankylosing spondylitis is the most recognized form. It causes inflammatory back pain that typically starts before age 45, feels worse after rest, and improves with movement. Over time, new bone can form along the spine, potentially fusing vertebrae together.
Genetics play a strong role. A gene called HLA-B27 is present in about 92% of white patients with ankylosing spondylitis. But carrying the gene doesn’t mean you’ll get the disease. Only 1 to 2% of all HLA-B27-positive people ever develop it. The same gene is linked to reactive arthritis (60 to 80% of cases) and arthritis associated with inflammatory bowel disease or psoriasis (about 60% each).
Infectious and Reactive Arthritis
Septic arthritis occurs when bacteria, viruses, or fungi directly infect a joint. Staphylococcus aureus and Streptococcus species are the most common culprits. This is a medical emergency: if antibiotics aren’t started within 24 to 48 hours of symptom onset, the infection can destroy cartilage and cause permanent joint damage. Symptoms come on rapidly and usually involve a single, hot, swollen joint along with fever.
Reactive arthritis is different. It develops after an infection elsewhere in the body, typically a gut or urinary tract infection, and the joint itself is not directly infected. Instead, the immune system’s response to the original infection triggers inflammation in the joints, usually in the knees, ankles, or feet. It often resolves on its own within several months, though some people develop chronic symptoms.
Certain viral infections can also trigger joint inflammation. COVID-19, for example, has been linked to viral arthritis in some patients.
Juvenile Idiopathic Arthritis
Arthritis isn’t limited to adults. Juvenile idiopathic arthritis (JIA) is the most common type of arthritis in children, and it includes several distinct subtypes. Oligoarticular JIA, the most common and mildest form in North America, affects four or fewer joints, usually the knees or ankles. Children with this form, particularly young girls who test positive for a marker called anti-nuclear antibody, need regular eye exams because they’re at risk for a type of eye inflammation called uveitis.
Systemic JIA affects the whole body, not just the joints. It typically starts with spiking fevers and a rash that come and go for at least two weeks. Joint inflammation may develop later, and in severe cases, inflammation can spread to the spleen, liver, lymph nodes, and the linings of the heart and lungs. Polyarticular JIA affects five or more joints and more closely resembles adult rheumatoid arthritis.
Rare and Unusual Forms
Beyond the well-known types, dozens of rarer forms contribute to the 100-plus count. Palindromic rheumatism causes episodes of joint pain and swelling that can last hours to days, then completely resolve, with joints appearing normal between attacks. It most often hits the fingers, wrists, and knees. Interestingly, it reportedly doesn’t cause lasting joint damage, though the recurring flares bring significant fatigue that can linger for days or weeks.
Other uncommon forms include arthritis triggered by Whipple’s disease (a rare bacterial infection of the gut), arthritis linked to sarcoidosis (an inflammatory condition that usually affects the lungs), and arthritis associated with hemochromatosis (iron overload). Each of these has a unique underlying cause, which is why rheumatologists spend so much time on diagnosis. The joint pain may feel similar across many of these conditions, but the right treatment depends entirely on figuring out which of the 100-plus types is responsible.