These sacs, called bursae, lie between tendons
or between tendons
and bones. Without these sacs, the tendons
would rub against each other. After an injury or infection, or because of overuse, the bursae sometimes becomes inflamed. Bursitis is very common in the shoulder and the knee. Treatment for mild cases includes resting that part of the body, heat, and painkillers. Injections of medicine, or surgery, are sometimes used in more serious cases.