The main causes of knee arthritis

Knee arthritis develops when the cartilage that cushions your knee joint wears down or becomes damaged. The three most common types — osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis — each start differently, but all end with the same result: bone rubbing against bone, pain, and stiffness.

Osteoarthritis is the wear-and-tear kind. It happens gradually over years as the protective cartilage in your knee thins from everyday use. Your risk goes up with age, extra weight, repetitive stress on the joint, and muscle weakness around the knee. It is the most common type in older adults.

Rheumatoid arthritis is different — it is an autoimmune condition where your immune system attacks the lining of your joints. It can start suddenly and affect both knees at once. Unlike osteoarthritis, it is not caused by wear and tear.

Post-traumatic arthritis develops after an injury: a torn meniscus, a fracture, or a ligament tear that was not treated or healed improperly. Even if the injury seemed to heal, damage to the cartilage can lead to arthritis years later.

Key Takeaways

  • Osteoarthritis, the most common type in older adults, develops when cartilage wears down gradually from age, weight, and repetitive use.
  • Rheumatoid arthritis is an autoimmune disease where your body attacks joint tissue, and it can affect both knees at the same time.
  • Post-traumatic arthritis can develop months or years after a knee injury, even if the injury seemed to heal completely.
  • Risk factors you cannot change include age and family history, but weight management and muscle strength can slow progression.

How age and weight affect your knee joints

Your knee cartilage does not repair itself the way other tissues do. Over decades, the constant impact of walking, climbing stairs, and carrying your body weight gradually wears it thinner. After age 50, this wear accelerates for many people, which is why osteoarthritis becomes more common in older adults.

Extra weight puts direct pressure on your knees. For every pound you gain, your knees absorb an extra three to six pounds of force when you walk. Someone who is 20 pounds overweight is putting 60 to 120 extra pounds of stress on their knees with each step. This speeds up cartilage breakdown and is one of the few risk factors you can influence.

Age and weight often work together. As you get older, muscle naturally weakens unless you stay active. Weaker muscles around your knee — especially your quadriceps — mean your joint has less support, and the cartilage bears more of the load.

Injury and damage that leads to arthritis later

A single knee injury can set the stage for arthritis even if you recover fully from the when ready damage. A torn meniscus, a ligament tear (like an ACL injury), or a fracture can disrupt the smooth surface of your cartilage or change how your knee moves. Over time, this abnormal movement wears the cartilage unevenly.

You might not notice the problem right away. Someone might tear their meniscus at 35, feel better after a few months, and then develop arthritis at 55. The injury itself healed, but the cartilage damage did not. This is why post-traumatic arthritis can appear years or even decades after the original injury.

Repeated minor injuries add up too. Athletes and people in jobs that stress the knee — construction workers, nurses who stand all day, dancers — have higher rates of arthritis because of the cumulative impact.

Muscle weakness and poor knee alignment

Your quadriceps (the muscle on the front of your thigh) is your knee's main shock absorber. When this muscle is weak, your knee joint takes more of the impact from walking and movement. Weak muscles also allow your knee to track slightly off-center, which wears cartilage unevenly and speeds up arthritis.

Poor alignment — knock-knees, bow-legs, or a difference in leg length — puts uneven pressure on your cartilage. Over time, one side of the joint wears faster than the other. You cannot change your basic alignment, but physical activity that strengthens your quadriceps and the muscles around your hip can help stabilize your knee and slow cartilage wear.

This is why people who stay active often have better knee health than sedentary people of the same age. Movement keeps muscles strong and cartilage nourished. The cartilage in your knee has no blood vessels, so it gets oxygen and nutrients from movement — when you move your joint, fluid flows in and out of the cartilage.

Family history and autoimmune conditions

If your parents or siblings have arthritis, your risk is higher. Genetics influence how your cartilage is built, how quickly it wears, and whether your immune system is prone to attacking joints. You cannot change your genes, but knowing your family history helps you understand your own risk.

Rheumatoid arthritis and other autoimmune conditions run in families more strongly than osteoarthritis does. If you have rheumatoid arthritis, your immune system is attacking your joints, and this can happen regardless of your age or activity level. Other autoimmune diseases — lupus, psoriasis — can also trigger joint arthritis.

Some people carry genes that make them more likely to develop arthritis if they also have other risk factors. For example, someone with a genetic predisposition might not develop arthritis unless they also gain weight or have a knee injury. The combination matters as much as any single factor.

Occupational stress and repetitive movement

Jobs that involve repetitive knee stress increase arthritis risk. Construction workers who kneel, farmers who walk uneven ground, retail workers who stand in one spot for hours, and athletes who run or jump all put extra wear on their knees. The risk is higher when the movement is repetitive and the person is not taking breaks to rest the joint.

Repetitive stress is different from varied activity. Someone who walks different distances at different paces and does different activities spreads the wear across the joint. Someone who does the same motion the same way every day wears one spot of cartilage faster. This is why cross-training — mixing different types of movement — is protective.

If your job stresses your knees, strengthening exercises and good footwear matter more. Proper shoes with cushioning and support reduce the impact your cartilage absorbs. Taking breaks to rest and move differently also helps.

Inflammation and how it speeds cartilage breakdown

Cartilage breakdown releases particles into your joint, and your body responds with inflammation. This inflammation produces enzymes that further damage the cartilage, creating a cycle: damage leads to inflammation, inflammation causes more damage. Over time, this cycle accelerates.

In rheumatoid arthritis, inflammation is the primary problem from the start. Your immune system triggers inflammation that attacks the cartilage and bone. In osteoarthritis, inflammation develops as a response to wear and tear, but it becomes a major driver of the disease as it progresses.

This is why managing inflammation matters. Staying active, maintaining a healthy weight, and avoiding prolonged stress on the joint all help reduce inflammation. Some people find that certain foods or supplements affect their inflammation levels, though the evidence varies from person to person.

Frequently Asked Questions

Can you get knee arthritis from running or exercise?

Regular running does not cause arthritis in healthy knees. People who run throughout their lives often have healthier knees than sedentary people. However, sudden increases in activity, poor form, inadequate recovery, or running on hard surfaces without proper shoes can injure the knee and lead to post-traumatic arthritis later. The key is gradual progression and listening to pain signals.

Does arthritis in one knee mean the other will develop it too?

Not necessarily. If you have osteoarthritis from wear and tear, both knees may eventually be affected because they both age and bear your weight. But if arthritis developed from an injury to one knee, the other may stay healthy. Rheumatoid arthritis typically affects both knees at the same time because it is an autoimmune condition affecting your whole body.

Can you prevent knee arthritis if your parents have it?

You cannot change your genes, but you can influence whether arthritis develops or how fast it progresses. Staying at a healthy weight, keeping your leg muscles strong, avoiding knee injuries, and staying active all reduce your risk or delay onset. Even if arthritis does develop, these habits can slow its progression and keep you moving longer.

Is arthritis pain always from cartilage wear?

Not always. Arthritis pain comes from inflammation, bone-on-bone contact, muscle weakness, and sometimes from the joint lining itself. Early arthritis may cause pain from inflammation even when cartilage wear is minimal. This is why pain level does not always match what shows up on an X-ray — two people with similar cartilage damage can have very different pain levels.

What is the difference between arthritis and arthralgia?

Arthralgia is joint pain without arthritis — your knee hurts but the cartilage and joint structure are not damaged. Arthritis means actual damage to the joint structure. You can have knee pain from muscle weakness, overuse, or inflammation without having arthritis. A doctor can tell the difference with imaging and examination.