Osteomyelitis is an inflammation of the bone marrow and surrounding bone due to an infection. When a bone gets infected, the bone marrow (the soft part inside the bone) swells and presses against the bone's blood vessels. The bone cells can't get enough blood and parts of the bone may die. The infection can spread to the surrounding muscles and other soft tissues, causing pus to collect in the area. This is known as an abscess.
A bone infection is usually caused by bacteria, but sometimes other organisms such as a fungus may be the cause. The most common bacterial cause of osteomyelitis is Staphylococcus aureus. Other bacterial causes include Streptococcus group A and group B, H. influenzae, coliforms, and Pseudomonas aeruginosa, all of which can be associated with longstanding open skin ulcers that penetrate deep into the bone or with a traumatic injury that penetrates down to the bone. Bones are usually well protected and don't become infected easily.
Bone infection can occur if:
- there is trauma to the bone, or a bone has been broken and is protruding through the skin
- there is an adjacent infection of surrounding soft tissue that spreads to the bone underneath
- it is carried to the bone via the bloodstream
- there is poor circulation (as seen in diabetes)
An infection can spread into the bone in many ways. It can enter during bone surgery or through a broken bone that protrudes through the skin. It can also spread from an infected artificial joint, such as a knee joint, into the surrounding bone. Any contaminated object that pierces the bone, such as a piece of metal from a car accident, can cause an infection.
Infection in soft tissue, such as muscles or organs, can develop in an area that has been injured or has poor blood circulation. Once the infection appears, it can then spread to the nearby bone.
Children may develop bone infections in their arms and legs from an infection carried from another part of the body through the blood. Adults tend to develop infections in the bones of their spine (spinal vertebrae) through this route.
If you've had orthopedic surgery, such as hip replacement or knee replacement, where a piece of metal was attached to a bone, an infection can develop. The infection may occur early after surgery if bacteria from the surface of the skin contaminated the artificial hip or knee. The infection may occur years later if bacteria enter the bloodstream and are carried to the artificial hip or knee.
People who take illicit drugs by injection and people on kidney dialysis are at higher risk for developing osteomyelitis due to the higher risk of bloodstream infections in these groups.
The bacteria responsible for tuberculosis can also infect the bones in a rare condition called Pott's disease.
Symptoms and Complications
When a child gets a bone infection that has spread from the bloodstream, symptoms include fever and some pain in the infected bone. It may also be difficult to move the affected area, since the limb will often be swollen and tender.
In adults, symptoms usually develop slowly, and the spinal vertebrae are commonly infected. Symptoms include soreness, swelling, redness, and pain that often can't be relieved with painkillers, heating pads, or rest. Unlike children, adults with osteomyelitis generally have no fever. They may also experience weight loss and fatigue.
If the osteomyelitis has developed as a result of an infection from surrounding soft tissue, or has entered through a crack in the bone, it will cause the area over the bone to become painful and swollen. Pus will build up in the area and form an abscess on the skin or nearby soft tissue. Again, these types of infections don't cause fever. Blood tests are often normal as well.
If a person has an artificial joint that becomes infected, it will cause constant pain. If the bone infection isn't treated appropriately, it can become chronic.
There will also be pain and recurring infections in soft tissue, such as muscle and skin surrounding the infected bone. Pus may also drain from the bone into and out of the skin through a passage it creates called a sinus.
Sometimes, osteomyelitis (e.g., when it occurs in the shin) can persist but not be obvious for a very long time, even decades. People are aware of a scar over the area and occasionally see pus, but it may close over again and they ignore it.
Making the Diagnosis
The doctor may suspect osteomyelitis based on symptoms and a physical exam. A normal X-ray may not show anything until three weeks after the first symptoms appear. However, a bone scan called a radionucleide scan can often detect a bone infection in its early stages. In this procedure, small particles of radioactive material are injected into a vein and circulate in the blood throughout the body. Then a picture or scan is taken of the radioactive areas. If there are any abnormal areas, they will show up on the scan. The doctor may also order computerized tomography (CT) or magnetic resonance imaging (MRI) scans.
These scans create three-dimensional visuals of the body, but they can't always differentiate a bone infection from other types of bone conditions.
Taking a sample of blood, pus, joint fluid, or the bone itself is the best way to make a diagnosis of osteomyelitis. If you have a bone infection in your spine, your doctor will check for signs of infection by taking a sample of fluid from the spinal vertebrae with a needle during a test or during bone surgery.
Treatment and Prevention
Treatment of osteomyelitis depends on how the infection has spread to the bone and how deeply it has penetrated. If the bone infection has come through the blood and is a recent infection, treatment with oral antibiotic medications in high doses usually works. You'll be prescribed oral antibiotics against Staphylococcus aureus (such as cloxacillin* or clindamycin) unless your doctor has identified another type of bacteria responsible for the infection.
Because bacteria are increasingly resistant to commonly used antibiotics, your doctor may take a sample from the infection to create a culture (letting the bacteria from the infection grow in the lab). This is essential in choosing the antibiotic most likely to work.
If a fungal infection is suspected as the cause for the osteomyelitis, then an antifungal medication may be prescribed.
If the osteomyelitis is very severe, you might need to take intravenous antibiotics at first and then switch to oral antibiotic pills later, once the infection is under control. People usually take them for 4 to 6 weeks, except for recurrent infections or infections of the spinal vertebrae, which need a 6- to 8-week treatment.
For serious infections, the buildup of pus may need to be drained by surgery. If the infection has spread from surrounding soft tissue, the dead tissue and bone is removed by surgery, and the space is filled with healthy bone, muscle, or skin by grafting before antibiotics are given.
If an artificial joint is infected, it has to be surgically removed and replaced. Antibiotics are usually given before and after surgery. In rare cases, the infection may not be cured and the infected limb may need to be amputated or the joint fused with surgery.
Sometimes foot ulcers (infections caused by poor circulation) due to diabetes can spread to the bones of the feet. These infections are often difficult to cure with antibiotics alone, and sometimes the infected bone must be surgically removed. This is one of the reasons people with diabetes should learn to take care of their feet and follow their diet and treatment plan to get their blood sugar within normal ranges. Ulcers and osteomyelitis may not heal if diabetes is not controlled, which can lead to amputations.