The most common cause of osteoarthritis is a gradual, age-related breakdown of joint cartilage driven by a combination of mechanical wear and genetic vulnerability. Most cases fall into the category doctors call “primary” osteoarthritis, meaning no single injury or disease triggered it. Instead, decades of normal joint use, combined with inherited traits and body composition, slowly degrade the cushioning tissue between bones. Onset typically begins in the late 40s to mid-50s, and about 60% of people living with the condition are women.
Primary vs. Secondary Osteoarthritis
Osteoarthritis splits into two broad categories. Primary osteoarthritis develops without a clear triggering event. It’s the more common form, and it results from the cumulative effects of aging, genetics, body weight, and repetitive joint use over a lifetime. Secondary osteoarthritis, by contrast, has an identifiable cause: a specific injury, an infection, a congenital joint defect, or another underlying disease.
The distinction matters because secondary osteoarthritis can sometimes be prevented or slowed by addressing the root cause, while primary osteoarthritis is harder to pin on any single factor. In practice, most people diagnosed with osteoarthritis have the primary form, where multiple risk factors have compounded over years.
How Cartilage Breaks Down
Healthy cartilage is a firm, slippery tissue that lets bones glide smoothly against each other. It contains specialized cells that constantly maintain and repair the surrounding matrix of proteins and water. Over time, abnormal or excessive mechanical stress on a joint triggers a cascade of problems inside those cells: increased oxidative stress, chronic low-grade inflammation, and disrupted energy production within the cells’ mitochondria.
When these stresses outpace the cells’ ability to repair, cartilage thins and roughens. The underlying bone responds by forming small bony spurs, and the joint lining can become inflamed. Eventually, the cushioning between bones narrows enough that movement becomes painful and stiff. This process unfolds over years, which is why osteoarthritis is so strongly linked to age, even though aging alone doesn’t make it inevitable.
Genetics Play a Larger Role Than Most People Expect
Twin and family studies estimate that 40% to 65% of osteoarthritis risk is heritable, depending on which joint is affected. That means your genes may matter as much as, or more than, your lifestyle in determining whether you develop the condition. Inherited factors can include the shape of your joints, the quality of your cartilage, and how your body manages inflammation.
This doesn’t mean osteoarthritis is predetermined. It means some people start with joints that are more vulnerable to the same mechanical stresses everyone experiences. If you have a parent or sibling with osteoarthritis, your risk is meaningfully higher, but modifiable factors like weight and activity patterns still shift the odds.
Body Weight and Metabolic Stress
Carrying extra weight is one of the strongest modifiable risk factors for osteoarthritis, particularly in the knees. A large genetic analysis found that a higher BMI causally increases the risk of knee osteoarthritis by 91%, hip osteoarthritis by 52%, and even hand osteoarthritis by 21%. The knee and hip numbers make intuitive sense: more body weight means more force on weight-bearing joints with every step. But the hand finding reveals something important. Excess weight doesn’t just add mechanical load. It also creates a metabolic environment, with higher levels of circulating inflammatory molecules, that accelerates cartilage breakdown throughout the body.
This is why weight loss is consistently one of the most effective interventions for slowing osteoarthritis progression and reducing pain, even modest reductions of 5% to 10% of body weight.
Joint Injuries and Post-Traumatic Arthritis
A significant minority of osteoarthritis cases trace back to a specific injury. In a study of over 17,000 cases of knee osteoarthritis, roughly 36% were classified as post-traumatic, meaning the person had a documented knee injury before their diagnosis. ACL tears, meniscus damage, and fractures that extend into a joint surface are the most common culprits. These injuries alter the joint’s mechanics and can damage cartilage directly, setting the stage for accelerated degeneration years or even decades later.
Post-traumatic osteoarthritis tends to develop earlier than the primary form. Someone who tears their ACL at 25 may have symptomatic knee arthritis by their 40s, well ahead of the typical onset window.
Repetitive Occupational Stress
Certain jobs dramatically increase osteoarthritis risk in specific joints. The pattern is straightforward: joints that are loaded repeatedly, held in awkward positions, or subjected to sustained force for years are the ones that wear down.
- Knees: Occupations involving frequent bending, kneeling, or squatting show a strong positive association with knee osteoarthritis. Farmers are particularly affected. A study of Swedish dairy farmers found that those milking large herds or working more than five hours per day had dramatically elevated risk.
- Hips: Heavy lifting and prolonged standing contribute to hip degeneration. Agricultural work, construction, and jobs requiring repeated bending at the waist carry elevated risk.
- Hands and wrists: Forceful gripping, pinching, and repetitive hand motions increase risk. Farmers, forestry workers, and people in fisheries occupations had roughly 2.7 to 3.6 times the odds of developing hand and wrist arthritis compared to the general population.
Key ergonomic risk factors across all these occupations include repetitive motion, awkward posture, long duration without recovery time, forceful movement, vibration exposure, and cold or wet working conditions. Vineyard workers, apple harvesters, and nursery propagation workers face some of the highest combined exposures, with jobs requiring hours of repetitive gripping, heavy lifting, and sustained awkward positions.
Why It’s Usually Not One Cause
The most honest answer to “what causes osteoarthritis” is that it’s almost always a combination. A person might inherit joints that are slightly less resilient, gain weight in middle age, and work a job that loads their knees for 30 years. Each factor alone might not have been enough, but together they overwhelm the joint’s capacity to maintain itself.
Age ties all of these threads together. Cartilage cells become less efficient at repair over time, inflammatory signaling increases, and the cumulative effect of decades of mechanical use adds up. This is why osteoarthritis is rare before 40 and increasingly common after 50, even though the underlying processes may have started much earlier. The condition isn’t simply “wear and tear” in the way people often think of it. It’s an active biological process where the joint’s ability to heal falls behind the damage it accumulates, and genetics, weight, injuries, and occupational stress all determine how quickly that tipping point arrives.