Arthritis isn’t a single disease with a single cause. It’s an umbrella term covering more than 100 conditions that affect the joints, and each type has its own triggers. Roughly 67 million American adults have some form of diagnosed arthritis, and that number is projected to reach 78.4 million by 2040. Understanding what’s actually happening inside your joints depends on which type of arthritis you’re dealing with.
Osteoarthritis: Wear, Tear, and Cartilage Breakdown
Osteoarthritis is the most common form, and it starts with the cartilage that cushions the ends of your bones. Cartilage is maintained by specialized cells called chondrocytes. Over time, these cells begin dying off faster than they can be replaced. The process isn’t just simple “wearing down” like a brake pad. The cells actually self-destruct through several distinct pathways, including inflammation-driven rupture where the cell membrane breaks open and releases compounds that accelerate damage to surrounding cartilage. Iron imbalances within cells can also trigger a chain reaction of damage to cell membranes, and disruptions in how cells process copper lead to toxic protein buildup and energy failure inside the cell.
The net result is the same: cartilage thins, roughens, and eventually wears away entirely. Bone begins grinding against bone, causing pain, stiffness, and swelling. This process tends to affect weight-bearing joints like knees and hips, as well as the hands and spine.
How Body Weight Multiplies Joint Stress
Excess weight is one of the strongest modifiable risk factors for osteoarthritis, and the math is striking. Every single pound of body weight translates to roughly four pounds of force on your knees with each step. So losing just 10 pounds removes about 40 pounds of pressure per step during daily activities. Over thousands of steps a day, that adds up to an enormous reduction in cumulative stress on the joint.
This force-multiplying effect explains why even modest weight loss can significantly slow cartilage breakdown and reduce pain in people with knee osteoarthritis.
Rheumatoid Arthritis: The Immune System Attacks
Rheumatoid arthritis (RA) works through a completely different mechanism. Instead of cartilage wearing out, your immune system mistakenly targets the synovial membrane, the thin lining that surrounds and lubricates your joints. Three key inflammatory signaling molecules drive this attack: TNF-alpha, IL-6, and IL-1. Together, they recruit immune cells to the joint lining, activate bone-destroying cells, promote the growth of abnormal tissue called pannus, and trigger the release of enzymes that digest cartilage from the outside in.
TNF-alpha is particularly destructive. It ramps up immune cell activity, increases the stickiness of blood vessel walls so more inflammatory cells can migrate into the joint, and directly stimulates the release of cartilage-degrading enzymes. IL-6 activates the cells that break down bone and promotes the formation of new blood vessels that feed the growing pannus tissue. IL-1 amplifies the whole cycle by pushing joint lining cells to release even more inflammatory compounds. The result is a self-perpetuating loop of inflammation and tissue destruction that, left unchecked, can deform joints permanently.
Genetics and RA Risk
Not everyone develops rheumatoid arthritis, and genetics play a significant role in who does. The strongest known genetic link involves a gene called HLA-DRB1, which was first identified as a risk factor in the late 1970s and has been confirmed repeatedly in large studies since. People who carry certain variants of this gene are substantially more likely to develop RA. Since around 2005, researchers have identified over 100 additional genetic locations that each contribute a smaller amount of risk but together implicate the immune system pathways central to the disease.
Having these genetic markers doesn’t guarantee you’ll develop RA. It means your immune system may be primed to overreact given the right environmental trigger.
Gut Bacteria and the Autoimmune Connection
One of the most compelling findings in recent years links RA to changes in gut bacteria. People with new-onset RA frequently show an overgrowth of a bacterium called Prevotella copri, which promotes inflammation and pushes immune cells toward an aggressive, pro-inflammatory state. At the same time, beneficial bacteria that produce anti-inflammatory compounds like butyrate tend to be depleted. This imbalance disrupts the normal checks and balances between immune cells that attack and immune cells that calm things down.
Gum disease also appears to play a role. A bacterium called Porphyromonas gingivalis, common in periodontal infections, produces an enzyme that chemically modifies proteins in a way that can trigger the specific antibodies seen in RA. This link between oral health and joint disease is one reason periodontitis and RA frequently occur together.
Gout: Crystal Deposits in the Joint
Gout is caused by the buildup of uric acid crystals inside a joint, most famously the base of the big toe. Uric acid is a normal waste product from breaking down certain foods and your body’s own cells. Problems start when blood levels exceed about 6.8 mg/dL, the saturation point at which uric acid can no longer stay dissolved and begins forming needle-shaped crystals.
Several local conditions in the joint make crystallization more likely. Temperature matters: a drop of just 2°C (about 3.6°F) from normal body temperature lowers the solubility threshold from 6.8 to 6.0 mg/dL. This helps explain why gout so often strikes the big toe, which sits far from the body’s core, has relatively poor blood flow, and cools more than other joints. Lower pH in the joint fluid also promotes crystal formation, partly by increasing calcium concentrations that further reduce uric acid solubility.
Mechanical stress plays a role too. Joints experience repeated impacts during daily movement, and physical disturbance of a supersaturated solution can directly trigger crystal formation. Even components of cartilage itself, like chondroitin sulfate, have been shown to accelerate crystal growth. Once crystals form, the immune system treats them as foreign invaders, flooding the joint with inflammatory cells and causing the sudden, excruciating pain of a gout attack.
Interestingly, some people have normal uric acid levels during an acute flare, which means a single blood test during an attack doesn’t always tell the full story.
Infectious Arthritis: Bacteria in the Joint
Septic arthritis occurs when bacteria invade a joint directly, usually through the bloodstream. Staphylococcus aureus is the most common culprit in both children and adults. In young children, a gram-negative bacterium called Kingella kingae is a frequent cause. In sexually active young adults, gonorrhea is the most common trigger for sudden, non-traumatic joint infection. Other bacteria like Salmonella can cause septic arthritis in people with sickle cell disease, and Pseudomonas is associated with puncture wounds.
This form of arthritis is a medical emergency. Staph infections of the joint carry a mortality rate that can exceed 50%, while gonorrhea-related joint infections are rarely fatal but still require prompt treatment to prevent permanent joint damage.
Reactive Arthritis: Joint Pain After Infection
Reactive arthritis is different from septic arthritis because the bacteria aren’t actually inside the joint. Instead, an infection elsewhere in the body, typically in the gut or urinary tract, triggers an immune response that mistakenly inflames the joints weeks later. The most common triggers are Yersinia enterocolitica and Chlamydia trachomatis, followed by Salmonella, Shigella, and Campylobacter.
People who carry the HLA-B27 gene variant are especially susceptible. This genetic marker appears to make the immune system more likely to cross-react with joint tissue after encountering these specific microbes.
Post-Traumatic Arthritis: Injury as a Starting Point
A serious joint injury, such as a fracture, dislocation, or torn ligament, can set the stage for osteoarthritis years later even if the injury heals well. The timeline is sobering: within 10 years of tearing an ACL and having it surgically reconstructed, about 25% of people develop osteoarthritis symptoms in that knee. By 15 years, the number climbs to roughly 50%.
The initial trauma damages the cartilage surface and alters the joint’s mechanics in subtle ways. Even small changes in how forces distribute across the joint can accelerate cartilage breakdown over time. This is why post-traumatic arthritis accounts for a meaningful share of osteoarthritis cases, particularly in younger adults who wouldn’t otherwise be at risk for age-related joint disease.