Arthritis is inflammation in one or more joints that causes pain, swelling, and stiffness. It’s not a single disease. The term covers more than 100 related conditions, and roughly 58.5 million adults in the United States have some form of it. That number is projected to reach 78 million by 2040.
How Arthritis Works Inside the Joint
What happens in an arthritic joint depends on the type, but the end result is similar: the structures that allow smooth, painless movement break down or become inflamed.
In osteoarthritis, the most common form, the problem starts with cartilage. Cartilage is the slippery tissue that caps the ends of bones where they meet. It’s maintained by cells called chondrocytes, which normally balance building new cartilage with clearing away old material. When a joint is stressed by years of use, excess weight, or a past injury, that balance tips. The chondrocytes start producing more destructive enzymes than they can offset with new tissue. These enzymes break down collagen and other proteins that give cartilage its structure. Over time, the cartilage thins, roughens, and eventually wears away, leaving bone grinding against bone.
Rheumatoid arthritis works differently. It’s an autoimmune condition where the immune system mistakenly attacks the synovium, the thin membrane lining the inside of the joint. White blood cells flood the joint space, and the body releases inflammatory signaling molecules, particularly one called TNF-alpha. These signals don’t just cause swelling and pain. They also activate cells that break down bone, which is why untreated rheumatoid arthritis can permanently deform joints over months to years.
The Most Common Types
Osteoarthritis affects more people than any other form. It typically develops gradually in weight-bearing joints like the knees and hips, or in the hands. It’s often described as “wear and tear” arthritis, though the biology is more active than that phrase suggests.
Rheumatoid arthritis tends to appear in matching joints on both sides of the body, often starting in the small joints of the hands and feet. Because it’s driven by the immune system, it can also cause fatigue, low-grade fevers, and inflammation in other organs.
Gout strikes suddenly, often in the big toe. It happens when uric acid, a waste product from breaking down certain proteins called purines, builds up in the blood beyond a concentration of about 6.8 mg/dL. At that point, the uric acid can crystallize into needle-shaped deposits inside a joint. The immune system treats those crystals as invaders, triggering intense inflammation. A gout flare can go from nothing to excruciating within hours.
Psoriatic arthritis develops in some people who have the skin condition psoriasis. It affects joints, skin, and the points where tendons and ligaments attach to bone. Ankylosing spondylitis primarily targets the spine and the joints connecting the spine to the pelvis, and over time it can cause vertebrae to fuse together. Reactive arthritis follows an infection elsewhere in the body, and its symptoms often resolve on their own within weeks or months.
Common Symptoms
Joint pain and stiffness are the hallmarks across nearly all types. In osteoarthritis, stiffness is usually worst after sitting still for a while and tends to ease within about 30 minutes of moving. In rheumatoid arthritis, morning stiffness often lasts longer, sometimes well over an hour, and is a key early signal.
Swelling, warmth, and redness around a joint are signs of active inflammation. You may also notice reduced range of motion, meaning you can’t bend or straighten the joint as far as you used to. Some forms of arthritis cause symptoms beyond the joints. Rheumatoid arthritis and lupus can bring fatigue, muscle aches, and a general feeling of being unwell. Gout flares are distinctive: the affected joint becomes extremely tender, swollen, and hot, often overnight.
Who Is Most at Risk
Arthritis can develop at any age, including in children (juvenile idiopathic arthritis is the most common chronic arthritis in kids), but risk climbs as you get older. Women are more likely to develop osteoarthritis, rheumatoid arthritis, and fibromyalgia. Men are more likely to develop gout.
Family history plays a clear role in several types. Rheumatoid arthritis, lupus, and ankylosing spondylitis all run in families. If a close relative has one of these conditions, your own risk is higher.
Carrying extra weight is one of the strongest modifiable risk factors. Excess body weight puts more 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 can make existing RA harder to control. Past joint injuries, even ones that healed well, increase the chance of osteoarthritis in that joint later. People who tore an ACL, for instance, are significantly more likely to develop knee osteoarthritis years down the line. Jobs that require repetitive bending, squatting, or heavy lifting also contribute. Among U.S. military veterans, about 1 in 3 have arthritis, compared with roughly 1 in 5 adults overall.
How Arthritis Is Diagnosed
There’s no single test that catches every type. Diagnosis usually combines a physical exam, blood work, and imaging.
For suspected rheumatoid arthritis, two blood tests are especially useful. One checks for rheumatoid factor (RF), an antibody found in many people with RA, though not everyone who tests positive has the disease. The other looks for anti-CCP antibodies, which are more specific to RA and can appear before symptoms start, making early diagnosis possible. General markers of inflammation in the blood help gauge how active the disease is.
Osteoarthritis is usually diagnosed based on symptoms, a physical exam, and X-rays that show narrowed joint space or bone spurs. X-rays aren’t always helpful early on, since visible damage takes time to develop. MRI and ultrasound can catch changes sooner and are particularly useful for evaluating rheumatoid arthritis in its early stages, when treatment makes the biggest difference. Gout is often confirmed by examining fluid drawn from the swollen joint under a microscope, where the characteristic needle-shaped uric acid crystals are visible.
Treatment Options
Treatment depends on the type and severity, and finding the right approach often takes some trial and error.
For pain and inflammation, over-the-counter anti-inflammatory medications like ibuprofen and naproxen are a common starting point. They reduce swelling and relieve pain but don’t slow the underlying disease. For rheumatoid arthritis and other inflammatory types, a class of medications called DMARDs can slow disease progression and prevent permanent joint damage. Some of these are conventional oral medications, while newer biologic versions target specific parts of the immune response. DMARDs work best when started early, before significant damage occurs, but they do increase susceptibility to infections.
Physical therapy is valuable across all types. Targeted exercises strengthen the muscles around affected joints, improve flexibility, and help maintain range of motion. A physical therapist can also recommend assistive devices like braces, canes, or shoe inserts that reduce joint stress during everyday activities. Simple strategies like alternating heat and cold on sore joints can provide meaningful relief. Heat loosens stiffness, while cold reduces swelling during flares.
For gout, management focuses on lowering uric acid levels to prevent future flares. Dietary changes, like reducing alcohol and foods high in purines (organ meats, certain seafood), help, and medications that lower uric acid production or improve its excretion can keep levels below the crystallization threshold.
When joint damage becomes severe, particularly in osteoarthritis of the knee or hip, joint replacement surgery can restore mobility and dramatically reduce pain. Most people who undergo hip or knee replacement report significant improvement in their quality of life and are back to normal daily activities within a few months.
Lifestyle Changes That Help
Staying physically active is one of the most effective things you can do, even though it may seem counterintuitive when your joints hurt. Low-impact activities like walking, swimming, and cycling strengthen supporting muscles without pounding the joints. Regular movement also helps maintain a healthy weight, which directly reduces stress on the knees and hips.
Losing even a moderate amount of weight makes a measurable difference for people with knee osteoarthritis. Every pound of body weight translates to roughly three to four pounds of force on the knee during walking, so small changes add up. Quitting smoking matters too, particularly for people at risk of or already living with rheumatoid arthritis, since smoking both increases the chance of developing RA and makes it harder to treat.