Arthritis is inflammation of one or more joints, causing pain, swelling, stiffness, and reduced range of motion. It is not a single disease but a broad term covering more than 100 related conditions that affect the joints and surrounding tissues. As of 2024, about 21.3% of American adults have been diagnosed with some form of arthritis, making it one of the most common chronic health conditions in the country.
What Happens Inside an Arthritic Joint
A healthy joint is lined with a thin membrane called the synovium, which produces fluid that lubricates the joint and nourishes the cartilage. Cartilage is the smooth, rubbery tissue capping the ends of bones, allowing them to glide against each other without friction. In arthritis, this system breaks down in different ways depending on the type.
In osteoarthritis, the cartilage gradually wears away over time. As it thins, bones lose their protective cushion and begin to grind against each other, causing pain and stiffness. In rheumatoid arthritis, the immune system attacks the joint lining itself. The synovium becomes massively thickened, growing from its normal two or three cells deep to eight or ten cells thick. This overgrown tissue, called pannus, invades and erodes the cartilage and bone around it. Inflammatory signals also activate bone-destroying cells, which is why rheumatoid arthritis can cause permanent joint damage if left untreated.
In both cases, the joint’s cartilage cells can turn against themselves. Inflammatory signals push these cells to release enzymes that dissolve their own cartilage matrix, while simultaneously blocking the formation of new cartilage. This is why joint damage in arthritis tends to be progressive: the body is actively breaking tissue down faster than it can rebuild.
Inflammatory vs. Degenerative Arthritis
One of the most useful ways to understand arthritis is the distinction between inflammatory and non-inflammatory (degenerative) types. They feel different, behave differently throughout the day, and require different approaches to management.
Inflammatory arthritis, which includes rheumatoid arthritis and gout, tends to cause prominent fatigue and whole-body symptoms alongside joint pain. Morning stiffness typically lasts longer than an hour, and joints feel better with movement. Symptoms are often worst in the morning or after sitting still for a while. Degenerative arthritis (osteoarthritis) works in the opposite direction: stiffness usually fades within 30 minutes of getting up, and joints hurt more as the day goes on and you use them more. Rest helps, and systemic symptoms like fatigue are uncommon.
This pattern can be a helpful early clue. If your joints feel worst after rest and loosen up with activity, that points toward an inflammatory process. If activity makes things worse and rest brings relief, a degenerative condition is more likely.
The Most Common Types
Osteoarthritis
Osteoarthritis is the most common form of arthritis. It involves the gradual wearing away of cartilage, most often in weight-bearing joints like the knees, hips, and spine, as well as the hands. It develops slowly over years, and onset is typically gradual. Previous joint injuries significantly increase the risk: someone who tore their ACL, for example, is more likely to develop osteoarthritis in that knee later in life. Jobs requiring repetitive bending, squatting, or heavy physical work also raise the risk.
Rheumatoid Arthritis
Rheumatoid arthritis is an autoimmune disease in which the immune system mistakenly attacks healthy joint tissue, starting with the joint lining. It often begins in smaller joints like the fingers and toes and tends to affect matching joints on both sides of the body. Its onset is typically insidious, building gradually over weeks or months. Blood tests can detect markers of inflammation and specific antibodies (rheumatoid factor and anti-CCP antibodies) that help confirm the diagnosis, though not everyone with rheumatoid arthritis tests positive for these.
Other Notable Types
Gout is an inflammatory arthritis caused by the buildup of uric acid crystals in a joint. Unlike most other types, it strikes suddenly, often in the big toe, with intense pain that can peak within hours. Psoriatic arthritis affects the skin, joints, and the areas where tendons and ligaments attach to bone. It develops in some people who have the skin condition psoriasis. Ankylosing spondylitis primarily targets the spine, causing inflammation in the vertebral joints and ligaments that can eventually fuse sections of the spine together. Some infections, including Lyme disease and staph infections, can also trigger joint inflammation that mimics or leads to arthritis.
Who Is Most at Risk
Age is the single biggest risk factor. As you get older, cumulative wear on joints adds up, inflammation affects the body more readily, and muscle loss can leave joints less stable and more vulnerable to injury. But arthritis is not exclusively a condition of aging. It can develop at any age, including in children.
Sex plays a significant role. Women are more likely to develop osteoarthritis and rheumatoid arthritis, while men are more likely to develop gout. Family history matters too, particularly for rheumatoid arthritis, lupus-related arthritis, and ankylosing spondylitis, all of which have a genetic component.
Among modifiable risk factors, body weight is one of the most impactful. Carrying extra weight puts additional mechanical stress on the knees and hips, increasing the likelihood of osteoarthritis in those joints and worsening pain if it develops. Smoking raises the risk of rheumatoid arthritis specifically and makes it harder to stay physically active, which is one of the most effective ways to manage arthritis symptoms. Past joint injuries, whether from sports, accidents, or physically demanding work, set the stage for osteoarthritis years or even decades later.
Cardinal Symptoms to Recognize
The hallmark symptoms of arthritis are pain, swelling, redness, and warmth in one or more joints. Stiffness is nearly universal, though its pattern varies by type. Reduced range of motion often develops as the disease progresses, and some people notice a grinding or catching sensation when moving the affected joint.
The timing and behavior of stiffness is particularly telling. Morning stiffness lasting longer than an hour, especially if it improves with movement, suggests an inflammatory type like rheumatoid arthritis. Stiffness that clears within 30 minutes and worsens with use points more toward osteoarthritis. Joint pain that comes on explosively, reaching peak intensity within hours, is characteristic of gout or joint infection rather than the slower-building forms.
How Arthritis Is Diagnosed
Diagnosis typically starts with a physical exam and a detailed history of your symptoms: which joints are affected, when the pain is worst, how long stiffness lasts, and whether symptoms are symmetrical. Imaging, usually X-rays, can reveal cartilage loss, bone erosion, or joint space narrowing.
Blood tests help distinguish between types. Markers of general inflammation, such as the sed rate and C-reactive protein, tend to be elevated in inflammatory forms of arthritis. More specific antibody tests can point toward rheumatoid arthritis, though a negative result does not rule it out. For suspected gout, fluid drawn from the affected joint can be examined under a microscope for uric acid crystals. Osteoarthritis, by contrast, is largely diagnosed based on symptoms and imaging, since blood work is typically normal.