Most cases of osteomyelitis are caused by germs in the bloodstream, specifically a germ called Staphylococcus aureus, which is commonly found on the skin or nose of even healthy individuals. There are a few ways in which these germs can get into the blood and infect the bone.
Sometimes, bacteria in other parts of the body, such as germs from pneumonia or a urinary tract infection, travel through the bloodstream to a weakened part of the bone—in children, this is most often in softer areas at the end of the long bones of the arms and legs called growth plates. The bacteria can also come from a nearby infection, for instance, from severe puncture wounds that carry germs deep inside the body. Direct contamination is also a culprit, particularly in cases in which a bone has been broken so severely that part of it is sticking through the skin, though it can also occur during surgeries to replace joints or repair fractures.
While bones are normally resistant to infection, there are a few risk factors that can make them more prone to infection, including: a recent injury or orthopedic surgery; circulation disorders such as sickle cell disease, peripheral arterial disease and poorly controlled diabetes that cause the body to have difficulty circulating infection-fighting cells; problems requiring catheters or IV lines, which can serve as a way for germs to enter the body; and immune system conditions or medications, such as chemotherapy drugs, diabetes, having had an organ transplant, or taking corticosteroids or tumor necrosis factor inhibitors. Individuals who use illicit drugs are also more likely to develop a bone infection if they use nonsterile needles and do not sterilize their skin before injections.