Arthritis is a general term covering over 100 different conditions, but a handful account for the vast majority of cases. Among the 67 million U.S. adults diagnosed with some form of arthritis, osteoarthritis makes up roughly half (33 million people), rheumatoid arthritis accounts for about 16%, and the remaining cases span dozens of less common types. Understanding which kind you’re dealing with matters because the causes, progression, and treatment approaches vary widely.
Osteoarthritis: The Wear-and-Tear Type
Osteoarthritis is by far the most common form. It happens when the cartilage cushioning your joints breaks down over time. Healthy cartilage constantly repairs itself, but in osteoarthritis, the enzymes that break down cartilage outpace the body’s ability to rebuild it. The surface of the cartilage develops tiny cracks and roughened patches that gradually deepen, eventually wearing away enough that bone grinds against bone.
This process is driven partly by inflammatory signals inside the joint. One key molecule ramps up cartilage-destroying enzymes while simultaneously blocking the production of new cartilage material and triggering cell death in cartilage-producing cells. The result is a cycle of damage that accelerates over years.
Osteoarthritis most commonly affects the knees, hips, hands, and spine. It tends to develop after age 50, though joint injuries earlier in life can set it in motion decades sooner. Symptoms include stiffness (especially in the morning or after sitting), aching pain that worsens with activity, and a gradual loss of range of motion. Treatment focuses on weight management, physical therapy, anti-inflammatory pain relief, and in advanced cases, joint replacement surgery.
Rheumatoid Arthritis: The Immune System Turns on Joints
Rheumatoid arthritis (RA) is fundamentally different from osteoarthritis. Instead of cartilage wearing down mechanically, the immune system mistakenly attacks the lining of the joints, causing chronic inflammation that can erode bone and deform joints over time. It typically affects joints symmetrically, meaning if your left wrist is inflamed, your right wrist probably is too.
RA often starts in the small joints of the hands and feet, then spreads to larger joints. Beyond joint pain and swelling, it can cause fatigue, low-grade fevers, and inflammation in other organs including the eyes, lungs, and heart. Blood tests for specific antibodies help confirm a diagnosis, though not everyone with RA tests positive for them.
Early, aggressive treatment makes a significant difference in outcomes. Current guidelines recommend starting with a cornerstone medication (methotrexate) combined with a short course of anti-inflammatory steroids. If that doesn’t bring the disease under control within three to six months, doctors typically add a biologic medication that targets specific parts of the immune response. The goal is remission or near-remission, and many people achieve it with modern therapies.
Psoriatic Arthritis: Skin Disease Meets Joint Disease
Psoriatic arthritis develops in people who have psoriasis, the skin condition that causes red, scaly patches. Most people develop psoriasis first, sometimes years before joint symptoms appear. Having psoriasis is the single biggest risk factor. About 1 million people in the U.S. have been diagnosed with psoriatic arthritis.
It shares some features with rheumatoid arthritis (joint pain, stiffness, swelling) but has its own distinct characteristics. Psoriatic arthritis can affect joints on just one side of the body. It frequently causes dactylitis, where entire fingers or toes swell into a sausage-like shape. Fingernails and toenails often develop tiny pits, become brittle or crumbly, or start lifting away from the nail bed. Some people develop spondylitis, causing pain and stiffness in the lower back and pelvis. Eye inflammation and severe fatigue are also common.
Gout: Crystal Buildup in the Joint
Gout happens when uric acid, a waste product your body normally filters out through the kidneys, builds up in the blood and forms sharp, needle-like crystals inside a joint. Blood levels above 6.8 mg/dL define hyperuricemia, the condition that sets the stage for gout. This can result from the body producing too much uric acid, the kidneys not clearing enough of it, or both.
Gout attacks are notoriously sudden and painful. They often strike the base of the big toe, though ankles, knees, and wrists can also be affected. The joint becomes red, hot, swollen, and exquisitely tender, sometimes overnight. Attacks typically last a few days to two weeks and then resolve completely, but without treatment they tend to come back more frequently and can eventually cause permanent joint damage.
A related condition called pseudogout (calcium pyrophosphate deposition disease) works by a similar mechanism but involves a different type of crystal. Pseudogout tends to hit the knees and wrists more than the big toe and is more common in older adults.
Ankylosing Spondylitis: Arthritis of the Spine
Ankylosing spondylitis primarily targets the spine and the joints where the spine meets the pelvis (the sacroiliac joints). Chronic inflammation in these areas triggers new bone formation, and over time, vertebrae can fuse together, reducing flexibility and creating a hunched posture in severe cases. Some people also develop inflammation in peripheral joints, the eyes (a condition called anterior uveitis), the skin, or the gut.
Genetics play a major role. Between 60% and 90% of people with this condition carry a specific genetic marker called HLA-B27. Those who carry it tend to develop symptoms at a younger age, have more severe spinal changes, and are more likely to have family members with the disease. People without HLA-B27 who develop ankylosing spondylitis tend to have a milder course and are more likely to also have psoriasis or inflammatory bowel disease.
Symptoms usually begin in the late teens or twenties with persistent low back pain and stiffness that improves with movement but worsens with rest. This pattern (worse in the morning, better with activity) is a hallmark that distinguishes it from most mechanical back pain.
Lupus-Related Arthritis
Systemic lupus erythematosus (lupus) is an autoimmune disease that can affect nearly every organ, and joint involvement is one of its most frequent symptoms. Lupus arthritis typically affects two or more peripheral joints with tenderness, swelling, or fluid buildup. It ranges widely in severity, from mild aching to disabling inflammation.
The key distinction from rheumatoid arthritis is that lupus arthritis has traditionally been described as non-erosive, meaning it doesn’t eat away at bone the way RA does. In practice, some people with lupus do develop erosive changes, but this is less common. Because lupus affects so many systems simultaneously (skin, kidneys, blood, brain), joint symptoms are usually managed as part of a broader treatment plan.
Septic Arthritis: A Joint Infection
Septic arthritis is a bacterial infection inside a joint, and it’s a medical emergency. Unlike every other type on this list, it can destroy a joint within 24 to 48 hours if antibiotics aren’t started promptly. Delayed treatment leads to cartilage destruction, bone loss, and permanent joint dysfunction.
The most common culprits are Staphylococcus aureus and Streptococcus species, though viruses, fungi, and other bacteria can also be responsible. It typically affects a single joint (often the knee) and causes rapid-onset pain, swelling, warmth, and fever in someone who hasn’t had a recent injury to that joint. Doctors diagnose it by drawing fluid from the joint and analyzing it for bacteria and white blood cells. Treatment involves intravenous antibiotics and often drainage of the infected joint fluid.
Juvenile Idiopathic Arthritis: Arthritis in Children
Arthritis isn’t limited to adults. Juvenile idiopathic arthritis (JIA) is diagnosed when a child develops joint inflammation lasting more than six weeks with no other identifiable cause. It’s classified into several subtypes based on how many joints are involved and what other symptoms are present.
- Oligoarticular JIA affects four or fewer joints in the first six months. It’s the most common subtype in children and sometimes stays limited, though it can spread to more joints later.
- Polyarticular JIA involves five or more joints from the start and resembles adult rheumatoid arthritis, particularly when blood tests for rheumatoid factor come back positive.
- Systemic JIA causes widespread inflammation beyond the joints, including spiking fevers, rashes, enlarged liver or spleen, and swollen lymph nodes.
- Enthesitis-related arthritis involves inflammation where tendons and ligaments attach to bone, often in the legs and feet, and is closely linked to the HLA-B27 gene.
- Psoriatic JIA combines joint inflammation with psoriasis or related features like swollen digits and nail changes.
- Undifferentiated JIA is diagnosed when a child’s symptoms don’t fit neatly into one category or overlap with more than one.
How These Types Compare
The most important distinction is between degenerative arthritis (osteoarthritis) and inflammatory or autoimmune types (rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, lupus-related arthritis). Osteoarthritis is a problem of cartilage breakdown and tends to worsen gradually over decades. The inflammatory types involve an overactive immune system, can progress quickly, and often affect other organs beyond the joints.
Gout and pseudogout sit in their own category as crystal-related diseases, triggered by metabolic imbalances rather than immune dysfunction or mechanical wear. Septic arthritis stands apart as an acute infection. Each type has a different trajectory and responds to different treatments, which is why getting the right diagnosis early shapes everything that follows.