Arthritis isn’t a single disease but a group of more than 100 conditions that damage joints, and each type has its own path of development. Some forms result from years of physical wear, others from immune system malfunction, and still others from metabolic problems or infections. Understanding what actually causes joints to break down can help you recognize your own risk factors and, in many cases, reduce them.
How Osteoarthritis Develops
Osteoarthritis is the most common form, and it starts with a shift in how your cartilage maintains itself. Healthy cartilage constantly breaks down and rebuilds in balance. In osteoarthritis, the enzymes responsible for breaking down cartilage become overactive while the body’s natural inhibitors get overwhelmed. The result is a net loss of the proteins that give cartilage its structure and cushioning ability. Inflammatory signaling molecules accelerate the process by suppressing cartilage repair, triggering cell death within the cartilage, and promoting further inflammation.
This isn’t simply “wear and tear” from aging. It’s a biochemical cascade that certain factors set in motion. Genetics play a large role: the heritability of osteoarthritis is estimated at about 51%, meaning roughly half of your risk comes from your genes. But the other half is shaped by things you can influence.
Weight and Joint Pressure
Every pound of body weight places four to six pounds of pressure on each knee joint. That means carrying an extra 20 pounds translates to 80 to 120 additional pounds of force hitting your knees with every step. Over years and decades, that mechanical stress accelerates cartilage breakdown. People with obesity are 20 times more likely to need a knee replacement than those at a healthy weight.
Excess weight also contributes through inflammation. Fat tissue produces signaling molecules that promote the same destructive enzyme activity seen in osteoarthritic cartilage, which is why obesity increases arthritis risk even in non-weight-bearing joints like the hands.
Joint Injuries and Post-Traumatic Arthritis
A serious joint injury can set you on a path toward arthritis years or even decades later. Between 20% and 50% of people who experience significant joint trauma eventually develop osteoarthritis in that joint, and these cases account for roughly 12% of all osteoarthritis diagnoses. The timeline varies widely. Some people develop symptoms within a year of the injury, while others remain pain-free for 10 to 20 years before arthritis appears.
ACL tears, meniscus injuries, and fractures that extend into a joint surface are particularly high-risk. The initial damage disrupts the smooth cartilage surface and alters the joint’s mechanics, creating uneven stress distribution that compounds over time.
Occupational and Repetitive Stress
Your job can meaningfully raise your risk of knee osteoarthritis. A large meta-analysis combining 71 studies found that people in physically demanding occupations had about 52% higher odds of developing knee osteoarthritis compared to those in sedentary jobs. The highest-risk occupations included agriculture workers, construction workers, floor layers, carpenters, metal workers, cleaners, and service workers.
The specific movements that drive this risk are lifting heavy loads, kneeling, climbing, squatting, and prolonged standing. If your work involves these activities daily for years, the cumulative stress on your joints mimics the effect of carrying extra body weight, just applied through repetition rather than constant load.
How Rheumatoid Arthritis Starts
Rheumatoid arthritis is fundamentally different from osteoarthritis. It’s an autoimmune disease where the immune system attacks the lining of your joints. Variations in more than 150 genes have been linked to the condition, with the most significant involving genes that help the immune system distinguish your own proteins from foreign invaders. When these genes carry certain variants, the immune system is more likely to misidentify joint tissue as a threat.
The heritability of rheumatoid arthritis is estimated at about 44%, so genetics alone don’t determine whether you’ll develop it. Environmental triggers appear to activate the disease in genetically susceptible people. Long-term smoking is the most well-established trigger and is also associated with more severe disease once it develops. Other suspected triggers include infections, hormonal changes (particularly in women), higher body weight, occupational exposure to certain types of dust or fibers, and gum disease. Periodontitis and gingivitis show a notable association with increased rheumatoid arthritis risk.
One striking feature of rheumatoid arthritis is that the immune markers used to diagnose it, specifically rheumatoid factor and anti-citrullinated protein antibodies, often appear in the blood years before any joint symptoms begin. The disease can be silently building long before you feel anything wrong.
Gout and Metabolic Causes
Gout develops when uric acid, a waste product from breaking down substances called purines, builds up in the blood and forms sharp crystals in a joint. The result is sudden, intense inflammation, most often in the big toe. Several metabolic and dietary factors raise uric acid levels to the point where crystals form:
- Diet high in purines: red meat, organ meats, and certain seafood are the primary dietary sources.
- Sugar-sweetened beverages: sodas and other drinks with added sugar increase uric acid production.
- Obesity and metabolic syndrome: excess body fat, high blood pressure, abnormal cholesterol, and high blood sugar all contribute to elevated uric acid.
- Chronic kidney disease: kidneys that can’t filter efficiently allow uric acid to accumulate.
- Conditions with rapid cell turnover: psoriasis and certain cancers break down more cells, releasing more purines into the bloodstream.
Unlike osteoarthritis, which develops gradually, gout often announces itself with a dramatic first attack. But the underlying uric acid buildup has usually been progressing for years before that first flare.
Infections That Trigger Joint Inflammation
Certain bacterial infections can cause reactive arthritis, a condition where joint inflammation develops days to weeks after an infection elsewhere in the body. The bacteria don’t actually infect the joint itself. Instead, the immune response to the infection misfires and targets joint tissue.
The most common bacterial triggers fall into two categories. Chlamydia trachomatis, a sexually transmitted infection, can trigger reactive arthritis starting from the urinary or reproductive tract. Gastrointestinal bacteria including Salmonella, Campylobacter, Shigella, E. coli, Yersinia, and C. difficile can trigger it following a bout of food poisoning or intestinal illness. Not everyone who gets these infections develops reactive arthritis. Genetic susceptibility, particularly carrying a specific immune system gene variant, plays a significant role in who reacts this way.
Age and Sex Differences
Arthritis risk climbs with age, but it’s not inevitable. Osteoarthritis rates are forecast to reach about 7% for men and 12% for women by 2040, reflecting both aging populations and rising obesity rates. Women are consistently affected at higher rates across both osteoarthritis and rheumatoid arthritis, likely due to hormonal factors, differences in joint anatomy, and changes in cartilage metabolism after menopause.
Rheumatoid arthritis most commonly appears between ages 30 and 60, while osteoarthritis typically becomes symptomatic after 50. Gout tends to strike men earlier, often in their 30s and 40s, while women’s risk rises after menopause when the protective effect of estrogen on uric acid clearance diminishes.
Factors You Can Control
While you can’t change your genetics, age, or sex, the modifiable risk factors are substantial. Maintaining a healthy weight removes the single largest controllable stress on your joints. Avoiding smoking eliminates a major trigger for rheumatoid arthritis. Treating gum disease may reduce autoimmune risk. Limiting high-purine foods and sugary drinks keeps uric acid in check. Strengthening the muscles around your joints helps absorb shock that would otherwise hit cartilage directly.
If your work involves heavy physical labor, using proper lifting mechanics, wearing knee pads when kneeling, and rotating between tasks can reduce cumulative joint stress. And if you’ve had a significant joint injury, staying active with low-impact exercise and maintaining a healthy weight becomes especially important, since your baseline risk of developing arthritis in that joint is already elevated.