Knee arthritis develops from a combination of mechanical wear, biological changes, and systemic factors that break down the cartilage cushioning your joint. Osteoarthritis is by far the most common form, affecting roughly 375 million people worldwide as of 2021, with over 27 million cases in the United States alone. But “knee arthritis” isn’t one disease with one cause. It’s the end result of several overlapping processes, and understanding which ones apply to you helps explain why your knee hurts and what you can do about it.
How Cartilage Breaks Down
Healthy knee cartilage is a smooth, rubbery tissue that lets the bones in your joint glide against each other with almost no friction. The cells inside cartilage have low metabolic activity and very little ability to regenerate, which is why damage tends to accumulate over time rather than heal on its own.
In the earliest stages of arthritis, the cartilage surface still looks intact, but its internal structure is already changing. The molecular composition of the tissue shifts first. The cartilage cells detect the damage and attempt a repair response: they multiply, form clusters, and ramp up production of structural proteins. But this repair effort is disorganized and ultimately inadequate. The cells start behaving like bone-forming cells rather than cartilage-maintaining ones, producing enzymes that actually accelerate tissue breakdown. Over months and years, the cartilage thins, roughens, and eventually wears away in patches, leaving bone exposed to grind directly against bone.
Body Weight and Joint Load
Every step you take sends force through your knees, and that force is a multiple of your body weight. Research has quantified this precisely: each pound of body weight translates to roughly four pounds of force on the knee during normal walking. Losing just 10 pounds, then, removes about 40 pounds of pressure from your knees with every step. Over thousands of steps per day, that math adds up fast.
But weight affects knee arthritis through more than just mechanical pressure. Fat tissue is metabolically active. It releases signaling molecules called adipokines that travel through the bloodstream and directly promote inflammation in joint tissues. These molecules trigger cartilage cells to produce inflammatory compounds that break down the structural matrix of the joint. This is why obesity increases arthritis risk even in non-weight-bearing joints like the hands, and it’s why losing weight has benefits beyond simply reducing the load on your knees.
Injuries That Set the Stage
A major knee injury doesn’t just hurt in the moment. It rewires the long-term health of the joint. After an isolated ACL tear, about 13% of people develop knee arthritis. When a meniscus tear accompanies the ACL injury, that figure jumps to 21 to 48%. Even with surgical reconstruction, the numbers are sobering: 12% of patients are diagnosed with arthritis within five years of ACL reconstruction, and roughly half develop it within 12 to 14 years.
The reasons are layered. A torn ligament changes how forces distribute across the joint, creating uneven wear patterns. A damaged meniscus removes a key shock absorber. And the initial injury itself floods the joint with inflammatory molecules that can alter cartilage cell behavior for years afterward. This is why former athletes commonly develop knee arthritis decades after their playing days, even if their surgery was considered successful.
Alignment and Joint Mechanics
The angle of your leg bones determines where pressure concentrates inside your knee. In a bow-legged alignment, the body’s weight-bearing axis passes to the inside of the knee center, forcing the inner compartment to absorb disproportionate load. In a knock-kneed alignment, the opposite happens: the outer compartment takes the extra stress. Over years, that uneven loading wears down cartilage faster on one side than the other.
Alignment issues can be present from birth, develop during growth, or result from injuries. They’re one reason arthritis often affects one side of the knee more than the other, and they help explain why two people with the same activity level can have very different outcomes.
Occupational Wear and Tear
Certain jobs put knees at substantially higher risk. The strongest evidence points to work that combines heavy lifting with prolonged kneeling, which raises the odds of developing knee arthritis by roughly 2 to 8 times compared to less physically demanding jobs. Kneeling alone, heavy lifting alone, and frequent stair climbing each independently increase risk as well, with odds ratios ranging from about 1.5 to 7 depending on the specific demands and duration of exposure.
Construction workers, flooring installers, farmers, and warehouse workers are among the most affected groups. The risk reflects cumulative exposure, so it tends to surface after decades in the job rather than after a few years.
Genetics and Family History
Twin studies estimate that 45 to 60% of knee arthritis risk is heritable. That’s a surprisingly large genetic contribution for a condition most people think of as purely “wear and tear.” Researchers have identified over 100 independent genetic variants associated with knee arthritis, including one in the GDF5 gene that influences how cartilage and bone develop. However, all known genetic variants combined still explain only about 11% of the total heritable risk, meaning much of the genetic architecture remains unmapped.
In practical terms, if your parents or siblings developed knee arthritis, your own risk is meaningfully higher. You can’t change your genes, but knowing your family history gives you reason to manage the factors you can control, like weight, activity choices, and injury prevention.
Metabolic Conditions Beyond Obesity
Diabetes, high blood pressure, and metabolic syndrome each contribute to knee arthritis through distinct pathways, independent of body weight. Chronically elevated blood sugar damages cartilage in two ways. First, it overwhelms the cartilage cells’ ability to regulate glucose intake, generating harmful oxidative molecules that degrade tissue. Second, excess sugar reacts with proteins in the joint to form stiff, dysfunctional molecules that trigger inflammatory signaling and accelerate breakdown.
High blood pressure affects the knee more indirectly. It can constrict the tiny blood vessels in the bone just beneath the cartilage, reducing blood flow and creating a low-oxygen environment. Bone cells respond to this oxygen deprivation by shifting their chemical signaling in ways that promote cartilage degradation. Insulin resistance, a hallmark of type 2 diabetes and metabolic syndrome, compounds the problem by pushing immune cells toward a more inflammatory state, raising levels of tissue-damaging compounds throughout the body.
Why Women Are More Affected
Women develop knee arthritis more often than men, and the gap widens sharply after menopause. The pattern is striking: both the prevalence and severity of knee arthritis increase in women after menopause, with a greater tendency toward arthritis in multiple joints simultaneously.
Estrogen appears to play a protective role in cartilage health. The hormone helps regulate cartilage metabolism, and cartilage cells have receptors that respond to it. Animal studies consistently show that removing the ovaries (which eliminates estrogen production) accelerates cartilage deterioration. Large epidemiological studies in humans support the same conclusion, though the exact molecular pathways are still being untangled. The practical implication is that the hormonal shift of menopause represents a genuine biological turning point for joint health, not just a coincidence of aging.
Rheumatoid Arthritis: A Different Mechanism
Not all knee arthritis comes from wear and tear. Rheumatoid arthritis is an autoimmune condition where the immune system attacks the lining of the joint. The synovial membrane, a thin tissue that normally produces lubricating fluid, becomes inflamed and thickened. Specialized immune cells and inflammatory fibroblasts in this tissue work together to erode both cartilage and bone from the inside out.
The destruction happens through an imbalance in bone remodeling. Cells that break down bone become overactive while cells that build bone are suppressed. Inflammatory fibroblasts in the joint lining amplify this cycle by producing signals that recruit more bone-destroying cells. Unlike osteoarthritis, which tends to affect one or two joints asymmetrically, rheumatoid arthritis typically strikes matching joints on both sides of the body and can begin at any age. It requires a fundamentally different treatment approach focused on suppressing the immune response rather than managing mechanical stress.
How Multiple Causes Overlap
In most people, knee arthritis isn’t the result of a single factor. A person with a genetic predisposition who also carries extra weight and tore their meniscus playing weekend soccer at 35 faces compounding risks. Each factor on its own might not have been enough, but together they push the joint past its ability to maintain and repair itself. This is why two people with the same knee injury can have completely different outcomes 15 years later: the injury is just one variable in a much larger equation that includes body weight, alignment, metabolic health, genetics, occupational demands, and hormonal status.