The most common cause of osteoarthritis of the knee is age. Almost everyone will eventually have some degree of osteoarthritis. But several things increase the risk of having significant arthritis at an earlier age.

• Age. The ability of cartilage to heal decreases as a person gets older.
• Weight. Weight increases pressure on all the joints, especially the knees. Every pound of weight you gain adds 3 to 4 pounds of extra weight on your knees.
• Heredity. This includes genetic mutations that might make a person more likely to have osteoarthritis of the knee. It may also be due to inherited abnormalities in the shape of the bones that surround the knee joint.
• Gender. Women ages 55 and older are more likely than men to get osteoarthritis of the knee.
• Repetitive stress injuries. These are usually a result of the type of job a person has. People with certain occupations that include a lot of activity that can stress the joint, such as kneeling, squatting, or lifting heavy weights (55 pounds or more), are more likely to get osteoarthritis of the knee because of the constant pressure on the joint.
• Athletics. Athletes involved in soccer, tennis, or long-distance running may be at higher risk for osteoarthritis of the knee. That means athletes should take care to avoid injury. But it's important to note that regular moderate exercise strengthens joints and can decrease the risk of osteoarthritis. In fact, weak muscles around the knee can lead to osteoarthritis.
• Other illnesses. People with rheumatoid arthritis, the second most common type of arthritis, are also more likely to get osteoarthritis. People with certain metabolic disorders, such as iron overload or excess growth hormone, also run a higher risk of osteoarthritis. 

Osteoarthritis symptoms often develop slowly and worsen over time. Signs and symptoms of osteoarthritis include:

• Pain. Affected joints might hurt during or after movement.
• Stiffness. Joint stiffness might be most noticeable upon awakening or after being inactive.
• Tenderness. Your joint might feel tender when you apply light pressure to or near it.
• Loss of flexibility. You might not be able to move your joint through its full range of motion.
• Grating sensation. You might feel a grating sensation when you use the joint, and you might hear popping or crackling.
• Bone spurs. These extra bits of bone, which feel like hard lumps, can form around the affected joint.
• Swelling. This might be caused by soft tissue inflammation around the joint.