Causes of osteomyelitis

Osteomyelitis is an infection of the bone usually caused by bacteria.

Around 90% of cases are caused by bacteria called staphylococcus aureus, commonly found on the skin or in the nose.

Why the infection happens

Your bones are usually resistant to infection, but can become infected when:

• a pre-existing infection in the blood spreads to a bone
• there is an injury, such as a bone fracture
• bacteria enters a wound during or after surgery, such as a joint replacement operation
• there is a pre-existing health condition, such as diabetes, which means the bone does not get a steady blood supply, so infection-fighting white blood cells cannot reach the site of injury

Blood infections that spread to the bone are more common in children than adults. This may be because children’s bones are still developing, which makes them more vulnerable than adult bones. In addition, a child’s immune system (the body’s natural defence against infection and illness) is still developing, so it is less effective at fighting off infection.

Infection after injury, particularly to the foot or ankle, is the most common cause of osteomyelitis in adults.

When a bone becomes infected

When an infection develops inside a bone, the immune system will attempt to stop it with infection-fighting white blood cells.

If the infection is not treated and the immune system is unable to deal with the bacteria, a collection of dead white blood cells will build up inside the bone, forming a pocket of pus known as an abscess.

In cases of chronic osteomyelitis, abscesses can block the blood supply to the bone, which will eventually cause the bone to die. Dead bone with no blood supply must be removed if infection is to be cleared.

Increased risk

There are several things that can make people more vulnerable to developing osteomyelitis:

Weakened immune system

If your immune system is weakened, an infection in your body is more likely to spread to your bone. Your immune system may become weakened if you:

• are undergoing certain treatments, such as chemotherapy, radiotherapy or a long-term dose of steroid tablets
• have malnutrition, which is when your diet does not contain all the nutrients needed for good health
• have a health condition, such as HIV or AIDS (although this is an uncommon cause of osteomyelitis)

Poor circulation

People with health conditions that affect the blood flow are at greater risk of developing osteomyelitis. This is because their bones may not be getting a steady supply of infection-fighting white blood cells.

Conditions known to cause poor circulation include:

• type 1 diabetes and type 2 diabetes
• sickle cell anaemia, an inherited blood disorder where red blood cells do not function properly
• atherosclerosis, narrowing of the arteries, often caused by eating a high-fat diet and/or smoking
• peripheral arterial disease, where the main arteries in the legs get clogged by a build-up of fat

Diabetes and foot injury

People with diabetes are particularly vulnerable to osteomyelitis because they are at risk of developing foot injuries.

Increased levels of glucose in the blood can cause nerve damage, which means people with poorly controlled diabetes can lose sensation in their feet and small cuts to the feet go unnoticed. Due to poor circulation, a serious infection can quickly develop in the feet before spreading to the bone.

Read more about foot care and diabetes.

Injury and trauma

If you break a bone or have a serious puncture injury that exposes deep tissue to germs, there is a chance you will develop osteomyelitis. This risk is increased if you also have a weakened immune system and/or poor circulation. Any broken bone with a loss of skin cover needs emergency surgery to clean the wound, get rid of dead tissue and stabilise the fracture. 

Orthopaedic surgery

If you have orthopaedic surgery (surgery that involves the bones or joints) or you have had metalwork implanted, there is a small chance you may develop osteomyelitis. However, there is less than a 1% chance of this happening.

Intravenous drug misuse

People who regularly inject themselves with illegal drugs such as heroin or methamphetamine (crystal meth) have an increased risk of developing osteomyelitis. This is because many people who misuse drugs do not use properly sterilised needles, which significantly increases the risk of introducing bacteria into their bloodstream.