Osteoarthritis is the most common form of arthritis, affecting about three million Canadians (one in ten). It usually develops after the age of 45 and affects men and women in equal numbers. Osteoarthritis is caused by the breakdown of a material that cushions the ends of bones known as cartilage. With the passing of time, osteoarthritis causes the tough elastic cartilage to crack and get holes in it. Bits of cartilage may break off and enter into the joint spaces causing irritation, pain and swelling.