Arthritis isn’t a single disease. It’s a term covering more than 100 conditions that cause joint pain, swelling, and stiffness, and each type has different underlying causes. The most common forms are osteoarthritis (wear-and-tear damage), rheumatoid arthritis (an autoimmune attack), gout (crystal buildup from excess uric acid), and psoriatic arthritis (linked to the skin condition psoriasis). What triggers one type can be completely unrelated to what triggers another.
Osteoarthritis: Cartilage That Can’t Keep Up
Osteoarthritis is the most common form, and it starts with a breakdown in the balance between cartilage repair and cartilage destruction. Healthy cartilage constantly rebuilds itself. The cells responsible for maintaining it produce fresh structural proteins while enzymes clear away damaged material. Osteoarthritis develops when destruction outpaces repair.
The process often begins with minor mechanical damage, whether from everyday use, repetitive stress, or an old injury. That damage produces tiny fragments of cartilage debris. Immune cells in the joint lining try to clean up these fragments, but over time the debris overwhelms them. The fragments then trigger inflammation, which causes cartilage cells to release even more destructive enzymes. Those enzymes break down more cartilage, creating more debris, fueling a self-reinforcing cycle. Meanwhile, the inflamed joint lining itself starts producing some of the same destructive enzymes, accelerating the damage from both sides.
As this cycle progresses, the cartilage absorbs more water and loses the dense structural proteins that give it strength. The collagen network weakens. The cartilage thins, cracks, and eventually wears away, leaving bone grinding against bone.
Aging and Cellular Slowdown
Age is the single biggest risk factor for osteoarthritis, and the reason goes deeper than simple wear over time. The cartilage cells themselves age. As you get older, these cells divide more slowly, produce smaller and less uniform structural proteins, and become less responsive to the growth signals that normally stimulate repair. Research from the University of Iowa found strong correlations between age and markers of cellular aging in cartilage: as cells aged, their ability to divide declined and their repair capacity dropped in lockstep. The result is cartilage that is fundamentally less able to bounce back from the daily micro-damage that joints experience.
Body Weight and Joint Pressure
Carrying extra weight dramatically increases the mechanical load on your joints. Every additional pound of body weight puts roughly four extra pounds of force on your knees. Being just 10 pounds overweight translates to an extra 15 to 50 pounds of pressure on the knee with each step. But the effect isn’t purely mechanical. Fat tissue is metabolically active and releases inflammatory molecules that circulate through the body, contributing to cartilage breakdown even in non-weight-bearing joints like the hands. This is why obesity is a risk factor for arthritis beyond what joint loading alone would explain.
Rheumatoid Arthritis: The Immune System Turns Inward
Rheumatoid arthritis has an entirely different origin. The immune system mistakenly attacks the synovium, the thin tissue lining the inside of joints. This causes painful swelling that can eventually erode bone and deform joints. The disease is driven by a combination of genetic vulnerability and environmental triggers.
Genetics account for an estimated 20% to 60% of rheumatoid arthritis risk. A specific set of genes known as the HLA-DR shared epitope is the strongest genetic link. People who carry two copies of these genes and also smoke face a risk 21 times higher than nonsmokers who carry no copies, according to research highlighted by the Johns Hopkins Arthritis Center. Smoking appears to chemically alter proteins in the lungs through a process called citrullination, essentially reshaping normal proteins into forms the immune system no longer recognizes as “self.” The body then generates antibodies against these altered proteins. In people with the right genetic background, those antibodies can trigger the widespread joint inflammation characteristic of rheumatoid arthritis.
Other environmental factors linked to increased risk include certain infections, hormonal changes, and prolonged exposure to air pollution, though smoking remains the most well-documented modifiable trigger.
Gout: A Metabolic Problem
Gout is caused by the buildup of uric acid in the blood and the formation of needle-shaped crystals inside joints. Uric acid is a normal byproduct of breaking down purines, compounds found in your own cells and in foods like red meat, organ meats, and shellfish. Problems arise when uric acid levels climb above the saturation point of about 6.8 mg/dL, at which point crystals begin to form.
For most people with gout, the issue is that the kidneys aren’t removing uric acid efficiently enough. This can be driven by genetic variation, obesity, insulin resistance, kidney disease, or certain medications like diuretics. Less commonly, the body simply produces too much uric acid due to high dietary purine intake or conditions that cause rapid cell turnover. Obesity is the strongest modifiable risk factor for gout. It both reduces the kidneys’ ability to clear uric acid and increases its production. Alcohol consumption, particularly beer and spirits, compounds the problem by raising uric acid levels and impairing excretion. Gout frequently occurs alongside high blood pressure, heart disease, and diabetes because these conditions share overlapping risk factors.
Hormones and Sex Differences
Women develop osteoarthritis at significantly higher rates than men, and the risk spikes after menopause. Cartilage is an estrogen-sensitive tissue. Estrogen promotes the production of the structural proteins that keep cartilage strong and resilient. It also helps dampen the inflammatory molecules that drive cartilage breakdown. When estrogen levels drop during menopause, cartilage loses both its repair support and its anti-inflammatory protection simultaneously. Hormone replacement therapy has been shown to reduce the expression of pro-inflammatory molecules in joint tissue, which helps explain why some studies find lower arthritis rates among women who use it, though that decision involves weighing other health considerations.
Joint Injuries and Post-Traumatic Arthritis
A significant joint injury, such as a torn ligament, fractured bone near a joint, or dislocated joint, substantially raises the risk of developing arthritis in that joint later. Between 20% and 50% of people who experience a serious joint injury eventually develop osteoarthritis in that joint. Across all osteoarthritis cases, about 12% trace back to a prior injury.
The timeline is unpredictable. Some people develop symptoms within a year of the injury. Others remain symptom-free for 10 to 20 years before arthritis appears. Initial post-traumatic inflammation usually resolves within two to three months. If joint pain and swelling persist beyond six months after an injury, that’s generally considered a sign the joint has entered a more chronic inflammatory state.
Psoriatic Arthritis and Immune Signaling
Psoriatic arthritis develops in roughly 30% of people with psoriasis. It’s driven by an overactive immune pathway in which specific signaling molecules ramp up inflammation in both skin and joints. The immune system produces elevated levels of these inflammatory signals, which promote the growth of a type of immune cell that attacks joint tissue. Genetic variations in the receptors for these immune signals have been directly linked to psoriatic arthritis susceptibility, confirming that the disease runs in families and involves a built-in vulnerability in immune regulation. The inflammatory molecules found at elevated levels in psoriatic skin are also found in the joint fluid of people with psoriatic arthritis, which explains why skin flares and joint symptoms often worsen together.
Infections That Attack Joints
Septic arthritis occurs when bacteria, viruses, or fungi directly infect a joint. The most common culprit is Staphylococcus aureus, the same staph bacteria responsible for many skin infections. The infection typically produces severe, rapidly worsening pain in a single joint and requires urgent treatment to prevent permanent joint damage. People with existing joint damage, compromised immune systems, or recent joint surgery face the highest risk. Unlike other forms of arthritis, septic arthritis can develop in hours to days rather than months or years.
Genetics Across All Types
Heritability plays a role in virtually every type of arthritis, though the degree varies. For osteoarthritis, genetic factors account for roughly 50% of overall risk, rising to about 64% for hip osteoarthritis specifically. For rheumatoid arthritis, heritability estimates range from 20% to 60% depending on the population studied. This means that while genes create vulnerability, they rarely act alone. Environmental and lifestyle factors, from body weight to smoking to diet, determine whether that genetic potential becomes disease.