Arthritis isn’t a single disease. It’s a blanket term for more than 100 conditions that cause joint pain, swelling, and stiffness. Each type has different causes, from immune system misfires to infections to simple wear on cartilage over decades. In the US, about 19% of adults have been diagnosed with some form of arthritis, and that number climbs sharply with age, reaching nearly 54% in people 75 and older.
Osteoarthritis: Cartilage Breakdown
Osteoarthritis is the most common form, and its root cause is the gradual destruction of cartilage, the smooth tissue that cushions the ends of bones inside a joint. This isn’t just passive “wear and tear” from use. It’s an active biological process driven by enzymes that chew through the two main structural components of cartilage: a protein called aggrecan and the collagen fibers that give cartilage its strength.
The breakdown follows a specific sequence. Aggrecan loss comes first. When enough aggrecan is stripped away, the collagen underneath becomes exposed and vulnerable to its own set of destructive enzymes. This matters because aggrecan loss can be reversed by the body, but collagen destruction cannot. Once that collagen network is gone, the cartilage cannot repair itself. The joint progressively loses its cushion, bone rubs against bone, and pain and stiffness set in.
Several factors kick this process into motion. Mechanical stress from joint injury or carrying excess weight is a primary trigger. Age plays a major role: as you get older, the cells that maintain cartilage (chondrocytes) become less effective at repair and more prone to producing those destructive enzymes. Gender matters too, with women (21.5%) significantly more likely than men (16.1%) to develop arthritis. Genetics influences how resilient your cartilage is and how aggressively your body produces inflammatory signals in the joint.
How Obesity Drives Joint Damage
Obesity doubles the lifetime risk of symptomatic osteoarthritis compared to people with a BMI below 25. The obvious explanation is extra mechanical load on knees and hips, but that’s only part of the story. Obesity also increases arthritis risk in non-weight-bearing joints like the hands, which points to something beyond simple pressure.
The missing piece is fat tissue itself. Fat doesn’t just store energy. It actively secretes signaling molecules called adipokines, including leptin and adiponectin, that promote inflammation throughout the body. These molecules reach joint tissue through the bloodstream and ramp up the same destructive enzyme activity that breaks down cartilage. Leptin levels in the blood are closely linked to osteoarthritis severity, and adiponectin has been tied to synovial inflammation inside the joint. This is why losing weight helps joints that aren’t even bearing the extra load.
Rheumatoid Arthritis: The Immune System Turns on Joints
Rheumatoid arthritis is fundamentally different from osteoarthritis. It’s an autoimmune disease where the immune system mistakenly attacks the lining of your joints, causing painful swelling that can eventually erode bone. The causes are a combination of genetic vulnerability and environmental triggers that flip the switch.
The strongest genetic risk factor sits in a set of genes that control how the immune system identifies threats. Certain variants, collectively called the “shared epitope,” dramatically increase susceptibility. About 60-70% of rheumatoid arthritis risk is thought to be genetic, but genes alone rarely cause the disease. Something in the environment has to activate it.
Smoking is the single most important environmental trigger. Studies consistently find that smokers face more than double the odds of developing rheumatoid arthritis, and smoking accounts for an estimated 20-30% of the environmental risk. The leading theory is that tobacco smoke causes chemical changes to proteins in the lungs, making them look foreign to the immune system. In someone with the right genetic background, this triggers the production of antibodies that eventually target joint tissue.
There’s also growing evidence that the disease may begin at mucosal surfaces, the moist linings of the mouth, lungs, and gut, where bacteria interact with the immune system. Periodontitis (gum disease) has been linked to the preclinical period before rheumatoid arthritis symptoms appear. People who carry rheumatoid arthritis-related antibodies but don’t yet have joint symptoms often already show signs of airway inflammation or lung abnormalities on imaging. The joints, it seems, may not be where the disease starts.
Gout: A Buildup of Uric Acid Crystals
Gout causes sudden, intense attacks of joint pain, most famously in the big toe, and it has a very specific chemical cause. Your body produces uric acid when it breaks down purines, compounds found naturally in your cells and in certain foods. When uric acid levels in the blood stay elevated above roughly 6 mg/dL, needle-shaped crystals can form and deposit inside joints, triggering a fierce inflammatory response.
Diet plays a direct role. Foods high in purines, particularly organ meats, certain seafood, and red meat, increase uric acid production. Alcohol, especially beer, both raises uric acid levels and makes it harder for the kidneys to clear it. Even yeast-rich foods like certain breads and fermented drinks can gradually elevate uric acid by promoting gut colonization with yeast that boosts uric acid secretion.
But diet isn’t the whole picture. Genetics strongly influences how efficiently your kidneys excrete uric acid, which is the main reason some people develop gout while eating the same diet as someone who never does. Kidney disease, certain medications, and conditions that cause rapid cell turnover also raise uric acid levels. Men are far more likely to develop gout than premenopausal women, partly because estrogen helps the kidneys clear uric acid.
Psoriatic Arthritis: When Skin Disease Reaches the Joints
Between 7% and 26% of people with psoriasis eventually develop psoriatic arthritis, an inflammatory condition that causes joint pain, swelling, and stiffness alongside the characteristic skin plaques. Skin symptoms typically come first, appearing an average of 12 years before joint problems begin, though in a minority of cases joint symptoms arrive first or at the same time.
The cause involves the same overactive immune response that drives psoriasis. Immune cells that are attacking skin also begin targeting joint tissue, tendons, and the places where tendons attach to bone. Genetics plays a large role, and the condition tends to run in families. Stress, physical trauma to a joint, and infections that activate the immune system can all serve as triggers in someone who’s already predisposed.
Ankylosing Spondylitis: Genetics and the Spine
Ankylosing spondylitis primarily affects the spine and the joints connecting the spine to the pelvis, causing chronic pain and stiffness that can eventually fuse vertebrae together. It has one of the strongest genetic links of any form of arthritis. A gene variant called HLA-B27 is present in roughly 90% of patients with ankylosing spondylitis, compared to just 7% of the general North American white population.
Carrying HLA-B27 doesn’t guarantee the disease. Most people with the gene never develop it, which means additional triggers are involved, likely including gut bacteria, infections, and mechanical stress on the spine and pelvis. The disease tends to start in the late teens or twenties, much earlier than osteoarthritis, and is more common in men.
Septic Arthritis: Joint Infection
Septic arthritis happens when bacteria (or less commonly, fungi) directly infect a joint. It’s a medical emergency because the infection can destroy cartilage within days. Staphylococcus aureus is the most common culprit in adults. In young, sexually active people, the bacterium that causes gonorrhea is the leading cause of sudden single-joint infection.
Bacteria can reach a joint through the bloodstream from an infection elsewhere in the body, through a penetrating wound, or during surgery. Puncture wounds and intravenous drug use are specifically associated with Pseudomonas infections in joints. People with existing joint damage, diabetes, weakened immune systems, or joint replacements face the highest risk.
Post-Traumatic Arthritis: Injury as a Starting Point
A significant joint injury, such as a fracture, ligament tear, or dislocation, can set the stage for osteoarthritis years or even decades later. The timeline is well documented for knee injuries: within 10 years of tearing an ACL and having it surgically reconstructed, about 25% of people show symptoms of osteoarthritis. By 15 years, that number reaches 50%.
The injury itself changes the mechanics of the joint, creating abnormal stress patterns on cartilage that was designed for a different loading pattern. It also triggers an inflammatory cascade inside the joint at the time of injury that can damage cartilage cells directly. Even a perfectly repaired ligament doesn’t fully restore the original joint mechanics, which is why surgical repair, while helpful for stability, doesn’t eliminate the long-term arthritis risk.
How Age Changes Your Joints
Aging affects joints in several ways that compound over time. Cartilage cells become less responsive to growth signals and less capable of producing new matrix to replace what’s been lost. The synovial fluid that lubricates joints changes in composition, becoming smaller in volume, and the protective molecules within it gradually decline in concentration. Tendons and ligaments lose elasticity, shifting more stress onto cartilage. Years of accumulated micro-damage add up, especially in joints that have absorbed decades of repetitive motion. None of these changes guarantee arthritis, but they lower the threshold at which other risk factors, like excess weight, old injuries, or genetic predisposition, tip a joint from healthy to symptomatic.