Arthritis is not a single disease. It’s a broad term covering more than 100 conditions that cause pain, swelling, or stiffness in one or more joints. Some types wear down the protective tissue inside a joint over years. Others involve an immune system malfunction that attacks healthy joint tissue. Still others are triggered by crystal deposits forming inside a joint. What they all share is that something has gone wrong in or around a joint, and it hurts.
In the United States, about 18.9% of adults have been diagnosed with some form of arthritis. The rate climbs steeply with age: 3.6% of adults aged 18 to 34 have it, compared with 53.9% of those 75 and older. Women are more likely to be affected than men, at 21.5% versus 16.1%.
What Happens Inside a Joint
A healthy joint is an engineering marvel. The ends of two bones meet, each capped with a smooth layer of cartilage that absorbs shock and lets the bones glide past each other with almost no friction. A thin membrane called the synovium lines the joint capsule and produces a small amount of slippery fluid that lubricates everything. Ligaments hold the bones together, and tendons anchor nearby muscles to bone.
Arthritis disrupts one or more of these components. In some types, the cartilage breaks down. In others, the synovial membrane becomes inflamed and thickened. In crystal-driven types, tiny sharp deposits lodge in the joint space and trigger intense inflammation. The result, regardless of the specific cause, is some combination of pain, swelling, stiffness, and reduced range of motion.
Osteoarthritis: Wear and Structural Change
Osteoarthritis is the most common form. It was once described simply as “wear and tear,” but the picture is more complex. The cartilage does thin over time, but the bone underneath changes too. Persistent mechanical stress causes microfractures in the bone just below the cartilage, which triggers a repair response that actually makes the bone denser and thicker. That remodeling stiffens the joint and disrupts the normal relationship between cartilage and bone.
Inside the cartilage itself, the cells responsible for maintaining it start behaving abnormally. In late-stage disease, they cluster together in some areas and die off in others. Blood vessels and tiny nerve fibers begin growing into tissue that is normally free of both, which is one reason osteoarthritis becomes more painful as it progresses. The calcified layer at the base of the cartilage expands upward into the softer cartilage above it, further thinning the cushion.
Osteoarthritis most commonly affects the knees, hips, hands, and spine. It tends to develop gradually, worsening over years. Stiffness is often worst after periods of inactivity, like sitting for a long time or first thing in the morning, but it typically loosens within about 30 minutes of moving.
Rheumatoid Arthritis: The Immune System Turns Inward
Rheumatoid arthritis is fundamentally different. It’s an autoimmune condition in which the immune system mistakenly attacks the synovial membrane lining the joints. White blood cells flood into the joint space, and the synovium becomes inflamed, swollen, and thickened. The inflamed tissue releases a cascade of inflammatory signaling molecules that break down cartilage and erode bone from the inside.
Both arms of the immune system are involved. Cells that normally fight infections, like macrophages and T cells, accumulate in the joint and sustain a chronic inflammatory loop. The resident cells of the synovium itself also change, becoming more aggressive and invading nearby cartilage and bone. This is why rheumatoid arthritis can cause permanent joint damage if it isn’t controlled early.
It typically affects joints symmetrically, meaning both wrists, both hands, or both knees at the same time. Morning stiffness in rheumatoid arthritis tends to last longer than in osteoarthritis, often persisting for an hour or more. Fatigue, low-grade fever, and a general feeling of being unwell can accompany the joint symptoms, because the inflammation isn’t confined to the joints alone.
Gout: Crystal Deposits That Trigger Flares
Gout is a form of arthritis caused by the buildup of uric acid in the blood. When uric acid levels exceed roughly 6.8 mg/dL, it can crystallize and deposit in joints as tiny, needle-shaped crystals. These crystals are directly toxic to cells in the joint lining. When cells die, they release alarm signals that recruit immune cells, which in turn release a burst of inflammatory molecules. The result is a sudden, severe flare of pain, redness, and swelling, often in the big toe, though it can strike ankles, knees, wrists, and other joints.
Several factors influence whether uric acid crystallizes in a given joint, including the pH of the joint fluid, electrolyte levels, and the concentration of proteins in the synovial fluid. This partly explains why some people with elevated uric acid never develop gout while others get frequent attacks. Between flares, the crystals can remain in the joint silently, doing low-level damage even when you feel fine.
Psoriatic Arthritis: Skin, Nails, and Joints
Psoriatic arthritis develops in some people who have the skin condition psoriasis. It targets not just the joint lining but also the entheses, the points where tendons and ligaments attach to bone. Inflammation at these sites, called enthesitis, causes tenderness and pain at places like the back of the heel, the bottom of the foot, or around the elbow.
A hallmark feature is dactylitis, sometimes called “sausage finger” or “sausage toe,” where an entire digit swells due to inflammation of the tendon sheath and surrounding soft tissue. Nail changes are also extremely common, affecting about 80% of people with psoriatic arthritis. The nails may pit, lift away from the nail bed, or become thickened and crumbly. This happens because the nail root is physically connected to the tendon that attaches to the bone at the fingertip, making the nail essentially part of the same structural system that’s under attack.
Psoriatic arthritis can affect joints asymmetrically (one knee but not the other) or symmetrically, and it varies widely in severity from person to person.
How Arthritis Is Diagnosed
There is no single test for arthritis. Diagnosis usually starts with your pattern of symptoms: which joints are affected, whether they’re stiff in the morning and for how long, whether the onset was sudden or gradual, and whether you have other symptoms like skin changes or fatigue.
Blood tests can help distinguish between types. Two proteins in particular, rheumatoid factor and anti-CCP antibodies, are markers for rheumatoid arthritis, though not everyone with the condition tests positive. Blood uric acid levels help evaluate gout. Markers of general inflammation in the blood can be elevated in several types but aren’t specific to any one.
Imaging fills in the rest of the picture. X-rays can show joint space narrowing, bone spurs, or erosion. They’re also used over time to track whether a condition is progressing. In some cases, ultrasound or MRI is used to catch earlier changes that X-rays miss, such as soft tissue inflammation or small bone marrow lesions.
How Arthritis Affects Daily Life
Nearly half of U.S. adults with arthritis, an estimated 24.8 million people, report that it limits their activities. That might mean difficulty climbing stairs, trouble gripping a jar, or being unable to walk comfortably for more than short distances. Among people who are unable to work or are disabled, the rate of activity limitation climbs to about 76%. Even among those who are employed, 38% report some level of limitation from their arthritis.
The impact goes beyond physical tasks. Chronic pain and reduced mobility affect sleep quality, mood, and social participation. Many people with arthritis gradually narrow their activities to avoid pain, which can lead to muscle weakening and further joint instability over time.
Managing Arthritis Across Types
Treatment varies by type, but a few principles apply broadly. Staying physically active is one of the most consistently supported strategies. Low-impact movement like walking, swimming, or cycling strengthens the muscles around joints, improves flexibility, and helps manage weight, which directly reduces stress on weight-bearing joints like the knees and hips.
For osteoarthritis, treatment focuses on pain management, maintaining function, and slowing progression. Physical therapy, weight management, and supportive devices like braces or shoe inserts are first-line approaches. When joint damage becomes severe enough to significantly impair quality of life, joint replacement surgery becomes an option.
For autoimmune forms like rheumatoid and psoriatic arthritis, the goal is to suppress the immune system’s attack on the joints. This typically involves medications that dampen the inflammatory response, and starting treatment early makes a meaningful difference in preventing permanent joint damage. For gout, management centers on lowering uric acid levels to prevent crystal formation and using anti-inflammatory treatment during flares.
Across all types, the practical reality is that arthritis is a long-term condition. It can be well controlled in many people, but it requires ongoing attention to symptoms, activity levels, and treatment adjustments as the condition changes over time.