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 a different underlying cause. The most common forms are osteoarthritis (caused by joint wear and mechanical stress), rheumatoid arthritis (caused by the immune system attacking joint tissue), and gout (caused by crystal buildup from excess uric acid). Globally, osteoarthritis of the knee alone affects roughly 375 million people, and that number has been climbing steadily.
Osteoarthritis: Wear, Load, and Metabolism
Osteoarthritis is the most widespread form of arthritis, and its cause comes down to the breakdown of cartilage, the smooth tissue that cushions the ends of bones inside a joint. For a long time, people thought of this as simple “wear and tear” from aging, but the picture is more complex. Cartilage is a living tissue maintained by cells called chondrocytes, and osteoarthritis develops when those cells can no longer keep up with the damage being done to the tissue around them. The normal balance between cartilage repair and cartilage breakdown tips in the wrong direction.
Altered joint loading is one of the strongest drivers. That means anything that changes how force is distributed across a joint: excess body weight, a previous injury, joint misalignment (like knock-knees or bow-legs), or instability from a torn ligament. Excessive force, whether from high impact or sustained pressure over long periods, kills cartilage cells, disrupts the tissue structure, and causes swelling. On top of the mechanical damage, inflammatory signaling molecules interact with that stress to accelerate the breakdown. This is why osteoarthritis tends to hit weight-bearing joints like the knees, hips, and spine hardest.
How Obesity Adds Fuel
Carrying extra weight increases the mechanical load on your joints, but obesity also causes arthritis through a separate chemical pathway. Fat tissue is not inert storage. It actively releases inflammatory signaling molecules, particularly two called leptin and adiponectin, along with other compounds that promote cartilage cell deterioration. These molecules travel through the bloodstream and can affect joints throughout the body, which helps explain why obese individuals develop osteoarthritis in non-weight-bearing joints like the hands, not just in their knees.
This dual mechanism, mechanical overload plus chronic low-grade inflammation, makes obesity one of the single most important modifiable risk factors for osteoarthritis. Weight loss has consistently been shown to reduce both the load and the inflammatory signaling, slowing joint degeneration.
Rheumatoid Arthritis: The Immune System Turns Inward
Rheumatoid arthritis has a fundamentally different cause. It’s an autoimmune disease in which the immune system mistakenly attacks the synovium, the thin membrane lining the inside of joints. The synovium normally produces fluid that lubricates and nourishes the joint, but in rheumatoid arthritis it becomes inflamed, thickened, and invasive. Left untreated, the inflamed tissue erodes into the adjacent cartilage and bone, causing permanent damage.
The process often starts years before symptoms appear. Characteristic autoantibodies, proteins that target the body’s own tissues, are detectable in the blood long before the first swollen joint. These antibodies form immune complexes that deposit in the synovial lining, triggering a cascade of inflammation. Early biopsies of affected joints show immune deposits and widespread cell death in the lining layer even when deeper tissue still looks normal, suggesting the lining is the immune system’s primary target from the very beginning.
Genetics Play a Large Role
The heritability of rheumatoid arthritis is estimated at about 60%, meaning genetics account for roughly that much of a person’s overall risk. A specific set of immune-system genes in the HLA region contributes the most, accounting for up to 37% of the inherited risk in people who carry the relevant autoantibodies. But dozens of other genes are involved too, each adding a small increase in susceptibility. Having the genetic predisposition doesn’t guarantee you’ll develop the disease. It means your immune system is primed, and an environmental trigger could set the process in motion.
The Gut Connection
A growing body of research points to the gut as a surprising player in inflammatory arthritis. The concept, sometimes called the “gut-joint axis,” centers on disruptions in the balance of bacteria living in the intestines. When that microbial community shifts out of balance, the intestinal lining can become more permeable. Inflammatory compounds produced by certain bacteria, particularly a molecule called lipopolysaccharide found on the surface of many gut microbes, can leak into the bloodstream and trigger a systemic immune response that reaches the joints.
Studies have found distinct differences in gut bacteria composition between people with early rheumatoid arthritis and healthy individuals. Researchers have even detected bacterial DNA from gut-dwelling species in the joint fluid of people with the disease. Another mechanism, called molecular mimicry, may also be at work: some bacterial proteins look similar enough to the body’s own proteins that the immune system, once activated against the bacteria, accidentally targets joint tissue too. This process appears to have a stronger impact in people who already carry genetic susceptibility.
Gout: A Crystal Problem
Gout is caused by the buildup of uric acid in the blood. Uric acid is a normal waste product created when the body breaks down purines, compounds found in many foods (especially red meat, organ meats, and shellfish). Normally, uric acid dissolves in the blood, passes through the kidneys, and leaves the body in urine. When the body either produces too much or excretes too little, uric acid accumulates and forms sharp, needle-shaped crystals inside joints. Those crystals trigger intense inflammation, causing the sudden, severe pain of a gout flare.
Several factors raise the risk of this happening. Diets high in purine-rich foods and sugar-sweetened beverages (especially fructose-containing sodas) increase uric acid production. Chronic kidney disease impairs the body’s ability to clear uric acid. Obesity, high blood pressure, and metabolic syndrome all contribute. Conditions that cause rapid cell turnover, like psoriasis or certain cancers, flood the bloodstream with purines as cells break down. Alcohol, physical trauma, and certain medications can trigger individual flares in someone whose uric acid is already elevated.
Psoriatic Arthritis: From Skin to Joints
Psoriatic arthritis develops in people with psoriasis, the autoimmune skin condition that causes red, scaly patches. In about 75 to 80% of cases, the joint symptoms appear after the skin symptoms, often years later. The typical gap between the onset of psoriasis and the development of joint disease is 7 to 12 years.
Not everyone with psoriasis develops arthritis, but certain features increase the risk significantly. More severe skin disease and nail involvement (pitting, thickening, or separation of the nail from the nail bed) are the strongest predictors. Nail changes caused by damage to the nail matrix are particularly telling, because the tendons that attach near the finger joints are closely connected to the structures that support the nail. The inflammation that disrupts nail growth may reflect a deeper process already affecting the joint.
Post-Traumatic Arthritis
A joint injury, whether from a sports accident, a car crash, or a fall, can set the stage for arthritis that develops months or years later. Unlike typical osteoarthritis, which progresses without a clear starting event, post-traumatic arthritis begins with a defined injury. The initial trauma damages cartilage cells directly and triggers a wave of inflammation in the joint lining that is visible within the first week. The severity of the original injury matters: fractures that break through the joint surface and exceed a certain threshold of damage are much more likely to lead to arthritis within two years of surgical repair.
This form of arthritis is especially common in younger, active people who suffer ligament tears, meniscus injuries, or fractures involving the ankle, knee, or hip. The injured joint may feel fully healed after surgery and rehabilitation, but the biological process of cartilage degradation can continue silently underneath.
Infectious Arthritis
Bacterial, viral, or fungal infections can cause arthritis by directly invading a joint. The most common culprit is Staphylococcus aureus, the same staph bacterium responsible for many skin infections. Bacteria typically reach the joint through the bloodstream from an infection elsewhere in the body, though they can also enter directly through a wound, surgery, or injection. This type of arthritis, called septic arthritis, develops rapidly and is a medical emergency because the infection can destroy cartilage within days if untreated.
Viral infections can also trigger joint inflammation. Some cases resolve on their own once the infection clears. In reactive arthritis, the joint inflammation appears after a bacterial infection in another part of the body, usually the gut or urinary tract, even though the bacteria themselves are not present in the joint.
Risk Factors That Cut Across Types
Several risk factors increase arthritis susceptibility regardless of the specific type:
- Age: The risk of osteoarthritis, rheumatoid arthritis, and gout all rise with age, as cartilage becomes less resilient, the immune system changes, and uric acid levels accumulate over time.
- Sex: Women are more likely to develop rheumatoid arthritis and osteoarthritis, while men are more likely to develop gout, especially before age 60.
- Smoking: Tobacco use is one of the strongest environmental risk factors for rheumatoid arthritis, particularly in people who carry the HLA genetic variants.
- Family history: With rheumatoid arthritis heritability around 60%, having a close relative with the disease substantially raises your risk. Osteoarthritis and gout also run in families, though to a lesser degree.
- Previous joint injury: Any significant joint trauma increases the long-term risk of osteoarthritis in that joint, even if the injury heals well.