Arthritis isn’t a single disease with a single cause. It’s a group of over 100 conditions that all share joint pain and inflammation, and the reason you developed it depends on which type you have. Nearly 19% of U.S. adults have been diagnosed with some form of arthritis, and the likelihood climbs steeply with age, from about 4% of adults under 35 to 54% of those 75 and older. Women are also more likely to develop it than men (21.5% vs. 16.1%). Understanding what’s actually happening inside your joints can help you make sense of your diagnosis and what comes next.
Osteoarthritis: Cartilage Wearing Down Faster Than It Rebuilds
Osteoarthritis is by far the most common type, and it’s often described as “wear and tear,” though that’s an oversimplification. Your joints are lined with cartilage, a firm, slippery tissue that lets bones glide past each other. That cartilage is maintained by cells called chondrocytes, which constantly break down old material and build new material to keep the surface intact. In osteoarthritis, the balance tips toward destruction.
What actually happens is an enzyme-driven process. Your body produces enzymes that chew through the two main structural components of cartilage: collagen (the scaffolding) and proteoglycans (the shock-absorbing filler). At the same time, inflammatory signaling molecules suppress the chondrocytes’ ability to produce replacement material. So your cartilage is being broken down while repair is simultaneously shut off. Once collagen is degraded, the damage is irreversible, because cartilage can’t regrow that structural framework.
Several factors push this process along:
- Age. Decades of joint use accumulate minor cartilage damage, and chondrocytes become less efficient at repair over time.
- Excess body weight. Obesity increases mechanical stress on joints like the knees, but it also drives osteoarthritis in non-weight-bearing joints like the hands. That’s because fat tissue is metabolically active, releasing inflammatory molecules that circulate throughout the body and accelerate cartilage breakdown. Obese individuals show higher blood levels of the same inflammatory signaling molecules that directly degrade cartilage and suppress collagen production.
- Previous injury. A torn ACL, meniscus tear, or fracture that extends into the joint can set off a chain reaction. The initial damage releases a flood of inflammatory molecules into the joint fluid, triggering the same destructive enzyme pathways seen in osteoarthritis. This can progress through a long, symptom-free period and then show up as chronic arthritis 10 to 20 years after the original injury.
- Genetics. Some people inherit cartilage that’s structurally less resilient or joints that are slightly misaligned, concentrating force on certain spots.
Osteoarthritis tends to affect the knees, hips, spine, and the joints closest to your fingertips. Morning stiffness is common but usually fades within a few minutes of moving around. Pain typically worsens with activity and improves with rest.
Rheumatoid Arthritis: Your Immune System Attacks Your Joints
Rheumatoid arthritis is fundamentally different. Instead of cartilage wearing out, your immune system mistakenly identifies the tissue lining your joints as a threat and launches a sustained attack. This creates chronic inflammation that damages cartilage, bone, and the surrounding structures.
The “why” behind this misfiring involves both genetics and environment. A specific set of gene variants called HLA-DRB1 shared epitope alleles significantly raise your risk. But genes alone don’t cause it. Environmental triggers are needed to flip the switch, and smoking is the single biggest modifiable risk factor. In people who carry two copies of the high-risk gene variant, smoking accounts for 55% of cases that involve a specific type of antibody (anti-CCP antibodies). Overall, smoking is responsible for roughly 35% of antibody-positive rheumatoid arthritis cases, and the risk increases with the amount smoked.
Rheumatoid arthritis looks and feels different from osteoarthritis. It usually affects joints symmetrically, both wrists or both hands at once. Morning stiffness lasts an hour or more, rather than a few minutes. It commonly targets the hands, wrists, and feet but tends to spare the joints closest to the fingertips. It can also cause fatigue, low-grade fevers, and a general feeling of being unwell, because it’s a systemic disease affecting the whole body.
Gout: Crystals Building Up in Your Joints
Gout is a form of arthritis caused by uric acid crystals forming inside a joint, most famously the big toe. Your body produces a substance called urate when it breaks down purines, compounds found naturally in your tissues and in many foods. Normally, urate dissolves in your blood, passes through the kidneys, and leaves in your urine. When levels stay elevated for a long time, needle-shaped crystals can form and deposit in joints, triggering intense, sudden inflammation.
Several things push uric acid levels higher. Diets rich in purine-heavy foods (red meat, organ meats, certain seafood) and sugar-sweetened drinks like soda contribute. Chronic kidney disease reduces your body’s ability to clear uric acid. Conditions that cause rapid cell turnover, like psoriasis or certain cancers, also flood the bloodstream with purines. Alcohol, particularly beer, is another well-established trigger. You can have elevated uric acid for years without symptoms before your first flare, which is why gout often seems to come out of nowhere.
Psoriatic Arthritis: Skin and Joints Share a Trigger
If you have psoriasis, the same immune system malfunction that causes your skin to overproduce cells can also attack your joints. Psoriatic arthritis develops when the immune system mistakenly targets both healthy skin and healthy joint tissue. About 30% of people with psoriasis eventually develop joint involvement, sometimes years after the skin symptoms first appeared, and occasionally before any skin patches show up at all.
The joints affected vary widely. Some people have symptoms in just a few fingers or toes, while others develop widespread inflammation. Swollen, sausage-like fingers or toes are a hallmark. Unlike rheumatoid arthritis, psoriatic arthritis doesn’t always appear symmetrically.
Reactive Arthritis: Triggered by an Infection Elsewhere
Sometimes arthritis develops a few weeks after a bacterial infection in the gut or urinary tract. The bacteria most commonly responsible are Salmonella, Yersinia, Campylobacter, Shigella, and Chlamydia. Only a small number of people infected with these bacteria develop joint symptoms, and genetics likely determine who is susceptible.
The arthritis itself isn’t an active infection in the joint. Rather, your immune system, after fighting off the bacteria, continues reacting in a way that inflames joint tissue. Reactive arthritis typically affects the knees, ankles, and feet, and most cases resolve within a few months, though some people develop chronic symptoms.
Why You Specifically Developed Arthritis
In most cases, arthritis results from a combination of factors rather than a single cause. Your genetic makeup sets the stage, and then life experiences, whether that’s a knee injury in your twenties, decades of carrying extra weight, a smoking history, or simply aging, push things past the tipping point. For autoimmune types like rheumatoid and psoriatic arthritis, the trigger is often impossible to pinpoint with certainty, because the immune system misfires for reasons that aren’t fully traceable in any individual.
What you can take from this: some risk factors (age, genetics) are beyond your control, while others (weight, smoking, diet) offer real leverage. Losing even a modest amount of weight reduces both the mechanical load on your joints and the inflammatory molecules circulating in your blood. Quitting smoking lowers your risk of rheumatoid arthritis progression. Dietary changes can help manage gout flares. Knowing which type of arthritis you have is the starting point for understanding which of these levers matters most for you.