Some people consider osteoarthritis (OA) a normal part of aging. The most common cause is the normal "wear and tear" of a joint. Mechanical stress on a joint over time may affect the enzymes (proteins that stimulate chemical reactions) in cartilage, but the exact relationship is unclear.

Some risk factors for OA have been identified. Risk factors make an individual more likely to develop a condition but do not mean the person will get the disease. In addition, people with no risk factors can still develop the disease. The most common risk factors associated with OA include:

• Age. OA usually develops after age 45 and is most common after age 65.
  
  
• Weight. Overweight or obese people are more likely to develop OA.
  
  
• Heredity. People with a family member with OA are more likely to develop the disease. Inherited bone abnormalities, such as a malformed joint or defective cartilage, may also increase the likelihood of developing OA. Even a minor difference in leg lengths, a common condition, may increase the risk of knee and hip OA, research has indicated.
  
  
• Sex. Although OA is less common before age 55, it occurs equally in both sexes in this age group. After age 55, it is more common in women than men.
  
  
• Lifestyle factors. People are more likely to develop OA when they have repetitive motion or stress on a joint from exercise or work conditions. Also, individuals who have long periods of immobilization are at risk for OA.

• Injury. Having a sports injury, such as a torn knee cartilage (meniscus injury) or a fracture near the joint, makes an individual more susceptible to OA. Knee surgeries, such as repair of a torn anterieror cruciate ligament, may increase risk of early OA. Weak muscles in the thighs may lead to the OA in the knees.

• Medical conditions. Diseases that cause a change in the structure or function of joints also increase the likelihood of OA. These include other forms of arthritis, such as rheumatoid arthritis and gout, and hemochromatosis, a condition in which excess iron can get deposited within cartilage causing destruction.
  
  
• Nerve disorders. Peripheral neuropathy, a condition that affects the nerves in limbs, may increase the likelihood of OA. Causes of neuropathy include diabetes and alcoholism.

Some research has contradicted the popular notion that cracking the knuckles or other joints contributes to osteoarthritis.