Arthritis isn’t a single disease. It’s a term covering more than 100 conditions that cause joint pain, swelling, or stiffness, and each type develops through a different process. Some result from gradual wear on your joints over decades. Others happen because your immune system attacks healthy tissue. Some are triggered by infections, injuries, or a buildup of crystals in your joint fluid. Understanding which type you’re dealing with changes everything about prevention and treatment.
Osteoarthritis: Wear and Breakdown
Osteoarthritis is the most common form, and it develops when the cartilage cushioning your joints breaks down faster than your body can repair it. Cartilage is a firm, slippery tissue that lets bones glide smoothly against each other. When it deteriorates, bone grinds on bone, causing pain, swelling, and stiffness that typically worsen over years.
The breakdown isn’t just “wear and tear” from use, though that’s part of it. Inside the joint, your body produces enzymes that actively dissolve cartilage components. One group of these enzymes chews through collagen, the tough protein that gives cartilage its structure. Another group breaks apart proteoglycans, the molecules that help cartilage absorb shock. In a healthy joint, your body balances destruction and repair. In osteoarthritis, that balance tips toward destruction, and once collagen is degraded, the damage is irreversible.
Several factors push that balance in the wrong direction. Mechanical stress from repetitive joint use, previous injuries, excess body weight, and aging all play a role. Genetics matter too. Some people inherit cartilage that’s less resilient or joints that are slightly misaligned, putting uneven pressure on the surface.
How Body Weight Affects Your Joints
Carrying extra weight is one of the strongest and most modifiable risk factors for osteoarthritis, particularly in the knees. A systematic review published in BMJ Open found that the risk of knee osteoarthritis increases by 35% for every 5-point rise in BMI. For context, that 5-point jump is roughly the difference between someone at the low end of “overweight” and someone at the low end of “obese.”
The effect isn’t purely mechanical. Fat tissue produces inflammatory molecules that circulate through your body and can accelerate cartilage breakdown even in joints that don’t bear weight, like your hands. So while the extra load on your knees and hips matters, the systemic inflammation from excess body fat compounds the problem.
Joint Injuries and Post-Traumatic Arthritis
A serious joint injury, like a torn ligament, a fracture that extends into the joint surface, or a meniscus tear, can set you on a path toward arthritis years later. About 12% of all osteoarthritis cases trace back to an initial joint trauma. In a study of military service members who sustained acute knee injuries, nearly 7% were diagnosed with post-traumatic osteoarthritis, with a median time from injury to diagnosis of just over two years.
The injury itself damages cartilage directly, but it also destabilizes the joint. A knee with a torn ACL, for instance, moves slightly differently even after surgical repair. That altered movement pattern concentrates force on areas of cartilage not designed to handle it, accelerating breakdown over time.
Rheumatoid Arthritis: Your Immune System Turns on Your Joints
Rheumatoid arthritis (RA) works through an entirely different mechanism. It’s an autoimmune disease in which your immune system mistakenly attacks the synovium, the thin membrane lining your joints. The synovium swells, thickens, and eventually invades the cartilage and bone underneath.
The attack is driven by immune signaling molecules. Two of the most important, TNF and IL-1, trigger a cascade that generates destructive enzymes and reactive oxygen species inside the joint. These signals also block the formation of new cartilage, so the joint loses its ability to heal while being actively destroyed. Activated immune cells in the synovium release still more inflammatory signals, including IL-6 and IL-8, which recruit additional immune cells and keep the cycle going.
What starts this process isn’t entirely clear, but genetics load the gun. A specific set of gene variants known as the “shared epitope” significantly raises RA risk. Many people with RA also produce autoantibodies, including antibodies against citrullinated proteins. Citrullination is a normal chemical modification that happens to proteins throughout your body, but in people genetically predisposed to RA, the immune system treats citrullinated proteins as foreign invaders.
The Gut Connection to Inflammatory Arthritis
One of the more surprising developments in arthritis research involves the bacteria living in your gut. People in the early stages of rheumatoid arthritis consistently show an overabundance of a gut bacterium called Prevotella copri compared to healthy people. When researchers colonized the guts of arthritis-prone mice with Prevotella-dominated bacteria from RA patients, the mice rapidly developed joint inflammation.
The bacterium appears to contribute in at least two ways. First, it triggers an inflammatory immune response in the gut that spills over into the rest of the body. Second, proteins on the surface of P. copri are structurally similar to proteins in human joints, so antibodies trained to fight the bacterium may also attack joint tissue, a phenomenon called molecular mimicry. Other bacterial species have been implicated too. Collinsella aerofaciens increases intestinal permeability in animal models, essentially making the gut “leaky” and allowing inflammatory molecules to enter the bloodstream. Certain Lactobacillus species have also been found in higher-than-normal levels in people with early RA.
This doesn’t mean gut bacteria directly cause RA, but they likely act as an environmental trigger in people who are already genetically susceptible.
Gout: A Crystal Problem
Gout develops when uric acid, a waste product your body makes when it breaks down certain foods and its own cells, builds up in your blood beyond the point where it can stay dissolved. Once it crosses that saturation threshold, it forms needle-shaped crystals of monosodium urate that deposit in and around joints.
The crystals themselves don’t always cause symptoms right away. But when your immune system detects them, it launches an intense inflammatory response. White blood cells flood the joint, and a signaling molecule called IL-1β amplifies the inflammation rapidly. The result is a gout flare: sudden, severe pain, redness, and swelling, most often in the big toe, though it can affect any joint. The joint lining becomes swollen and packed with immune cells, predominantly neutrophils.
What raises uric acid levels? A diet rich in red meat, organ meats, shellfish, and alcohol (especially beer) increases production. Kidney problems that reduce uric acid excretion are another major factor. Genetics play a significant role in how efficiently your kidneys clear uric acid. Obesity, high blood pressure, and certain medications like diuretics also raise levels.
Psoriatic Arthritis and Ankylosing Spondylitis
Psoriatic arthritis develops in some people who already have psoriasis, the skin condition that causes red, scaly patches. Not everyone with psoriasis gets joint involvement. Roughly 30% do, and the transition appears to require both genetic susceptibility and an environmental trigger. Injuries, viral infections, and bacterial infections have all been identified as potential triggers that push the immune system from attacking skin to also attacking joints.
Ankylosing spondylitis primarily affects the spine and the joints where the spine meets the pelvis. It has one of the strongest genetic links of any form of arthritis. A gene variant called HLA-B27 dramatically increases risk, though it’s far from a guarantee. About 75% of children who inherit HLA-B27 from a parent with ankylosing spondylitis never develop the condition. Other genes involved in immune regulation, along with unknown environmental factors, determine whether someone with the genetic risk actually gets sick.
Septic Arthritis: Infection in the Joint
Septic arthritis is the one form caused directly by an infection. Bacteria, most commonly Staphylococcus aureus (staph), enter a joint and trigger rapid, severe inflammation. The bacteria can arrive through the bloodstream from an infection elsewhere in your body, like a skin wound or urinary tract infection. Less commonly, they enter directly through a puncture wound, a surgical procedure, or a joint injection.
Septic arthritis is a medical emergency. Unlike other forms of arthritis that develop over months or years, it can destroy cartilage within days if untreated. It typically affects a single joint, most often the knee, and causes intense pain, swelling, warmth, and fever.
Risk Factors That Cut Across Types
Several factors increase your risk for multiple forms of arthritis. Age is the most universal: the cumulative effects of joint use, immune system changes, and declining repair capacity all increase with time. Women are more likely than men to develop rheumatoid arthritis and osteoarthritis, while men are more prone to gout and ankylosing spondylitis. Smoking is a well-established risk factor for rheumatoid arthritis and worsens outcomes in several other types. Family history raises risk for nearly every form, though the specific genes involved differ.
Physical inactivity contributes to joint stiffness and weakness in the muscles that support joints, while certain occupations involving repetitive kneeling, squatting, or heavy lifting increase osteoarthritis risk in specific joints. Previous joint injuries remain one of the most preventable risk factors, making proper treatment and rehabilitation after sprains, fractures, and ligament tears genuinely important for long-term joint health.