Arthritis isn’t a single disease. It’s an umbrella term for more than 100 conditions that cause joint pain, swelling, and stiffness, and each type has distinct causes. About 21.3% of U.S. adults have been diagnosed with some form of arthritis, making it one of the most common chronic conditions in the country. The causes range from gradual cartilage wear and immune system malfunction to crystal deposits, infections, and joint injuries.
Osteoarthritis: Wear, Tear, and Inflammation
Osteoarthritis is the most common form, and it develops when the protective cartilage covering the ends of your bones breaks down faster than your body can repair it. For decades, this was viewed as simple mechanical wear from years of use. The picture is more complex. When cartilage is damaged, it releases signaling molecules that activate enzymes called MMPs, particularly one that targets the main structural protein in cartilage (type II collagen). These enzymes essentially digest the cartilage from within, accelerating the breakdown that everyday movement started.
Aging plays a central role. As you get older, the cells responsible for maintaining cartilage (chondrocytes) gradually enter a state of permanent shutdown. These aging cells don’t just stop working. They actively release inflammatory chemicals and tissue-destroying enzymes into the joint. Researchers have found that cartilage cells from older adults secrete significantly higher levels of these destructive compounds, creating a feedback loop: inflammation damages cartilage, which triggers more inflammation. Oxidative stress and accumulated DNA damage over a lifetime drive this process.
Obesity raises osteoarthritis risk through two pathways. The obvious one is mechanical: excess weight puts more force on your knees, hips, and spine with every step. But obesity also causes arthritis in joints that bear no weight at all, like the fingers and wrists. That’s because fat tissue produces inflammatory signaling molecules that circulate through the bloodstream and affect joints throughout the body, regardless of how much load those joints carry.
Rheumatoid Arthritis: The Immune System Attacks
Rheumatoid arthritis is an autoimmune disease. Your immune system, which normally fights infections, mistakenly attacks the lining of your joints (the synovium), causing painful swelling that can eventually erode bone and deform joints. Unlike osteoarthritis, which tends to affect joints you’ve used heavily, rheumatoid arthritis typically strikes symmetrically, hitting both wrists, both hands, or both knees at once.
Genetics set the stage. Specific variations of a gene called HLA-DRB1, particularly the variants known as HLA-DRB1*04 and HLA-DRB1*01, are strongly linked to higher risk. These gene variants produce proteins containing a particular amino acid sequence called the shared epitope, which appears to make the immune system more likely to misfire against joint tissue. Having these genetic variants doesn’t guarantee you’ll develop the disease, but it significantly raises the odds, especially when combined with environmental triggers.
Smoking is the single most well-established environmental risk factor. It appears to trigger changes in the lungs that cause the immune system to produce antibodies against the body’s own proteins, and those antibodies eventually target joint tissue. Occupational exposure to silica dust also increases risk. A large Swedish registry study found that men exposed to silica dust for more than 10 years had roughly 33% higher odds of developing the antibody-positive form of rheumatoid arthritis compared to unexposed men. Hormones likely play a role too: rheumatoid arthritis is two to three times more common in women than men, and symptoms often fluctuate around pregnancy and menopause.
Gout: A Metabolic Problem
Gout stands apart from other types of arthritis because its cause is well understood and directly measurable. It happens when uric acid, a waste product from breaking down certain foods and your body’s own cells, builds up in the blood and forms needle-shaped crystals inside a joint. These crystals trigger an intense inflammatory response that causes sudden, severe pain, often in the big toe.
Crystal formation becomes likely when serum uric acid levels rise above about 6.8 mg/dL, though the thresholds used in clinical practice are typically above 6 mg/dL for women and 7 mg/dL for men. Cold, acidic environments encourage crystal formation, which is one reason gout favors the extremities: toes, ankles, and fingers run cooler than your core.
What drives uric acid levels up? Red meat, organ meats, shellfish, and alcohol (especially beer) are high in purines, compounds that break down into uric acid. But diet alone rarely causes gout. Most people with gout have kidneys that are inefficient at excreting uric acid, often due to genetics. Obesity, high blood pressure, kidney disease, and certain medications (particularly diuretics) also raise uric acid levels. Gout is becoming more common as rates of obesity and metabolic syndrome climb.
Psoriatic Arthritis: Skin and Joints Connected
About 30% of people with the skin condition psoriasis develop psoriatic arthritis, an inflammatory joint disease driven by the same immune pathways that cause scaly skin plaques. The connection runs through a specific signaling chain in the immune system. Immune cells in the skin and joints release a chemical messenger that drives a particular class of immune cells to multiply. Those cells, in turn, produce inflammatory signals that activate both skin cells and joint tissue.
In the skin, this cascade causes the rapid overgrowth of skin cells that creates psoriatic plaques. In the joints, the same signals recruit inflammatory cells into the synovial lining and promote the formation of cells that break down bone. The inflammatory molecules involved also directly stimulate bone-destroying cells, which is why psoriatic arthritis can cause permanent joint damage if left unchecked. The disease can also inflame the places where tendons and ligaments attach to bone, a feature that distinguishes it from rheumatoid arthritis.
Genetics influence who develops psoriatic arthritis, but not everyone with psoriasis gets it. Physical trauma, infections, and stress are common triggers for first flares.
Post-Traumatic Arthritis: Joint Injuries That Linger
A serious joint injury can set you on a path toward arthritis years or even decades later. Knee injuries lead to post-traumatic osteoarthritis in 25 to 50% of patients. ACL tears, meniscus injuries, and fractures that extend into a joint surface are the highest-risk injuries. Even after successful surgical repair, the joint environment changes in ways that promote long-term cartilage loss.
The initial injury causes bleeding into the joint, and the blood itself is toxic to cartilage. The inflammatory response that follows alters the chemical environment inside the joint, activating the same cartilage-destroying enzymes seen in age-related osteoarthritis but at an accelerated pace. This is why athletes who tear an ACL in their twenties can develop significant knee arthritis by their forties, well ahead of the typical timeline for osteoarthritis.
Septic Arthritis: Infection in the Joint
Septic arthritis occurs when bacteria (or less commonly, a virus or fungus) infect a joint directly. The most common culprit is Staphylococcus aureus, followed by Streptococcus species. In most cases, the bacteria reach the joint through the bloodstream during an infection elsewhere in the body. Less often, bacteria enter through a skin wound near the joint, a surgical site, or an injection.
Certain conditions raise the risk substantially: diabetes, HIV, immunosuppressive medications, intravenous drug use, and having a prosthetic joint. Existing joint disease, including rheumatoid arthritis and osteoarthritis, also makes joints more vulnerable to infection. Unlike other forms of arthritis, septic arthritis is a medical emergency. The combination of bacteria and the immune response they provoke can destroy cartilage within days.
Why Some People Get Arthritis and Others Don’t
For nearly every type of arthritis, the answer comes down to a combination of factors rather than a single cause. Genetics load the gun: family history raises risk for osteoarthritis, rheumatoid arthritis, gout, and psoriatic arthritis. But environment and lifestyle pull the trigger. Carrying excess weight accelerates osteoarthritis through both mechanical stress and body-wide inflammation. Smoking nearly doubles the risk of rheumatoid arthritis in genetically susceptible people. Diet and kidney function determine whether elevated uric acid tips into gout.
Age is the one risk factor that cuts across almost every type. Osteoarthritis prevalence rises sharply after 50 as cartilage cells lose their ability to maintain and repair tissue. Autoimmune forms like rheumatoid arthritis most commonly appear between ages 30 and 60. Gout becomes more common in men after 40 and in women after menopause, when uric acid levels rise. The cellular aging process itself, through the accumulation of worn-out cells that pump out inflammatory signals, creates joint conditions favorable to multiple forms of the disease.