Acute pyogenic osteomyelitis is an inflammation of bone caused by an infecting organism. Staphylococcus aureus is the most common bacterium involved in the infection.
Acute hematogenous osteomyelitis has an incidence of 1 in 5000 children per year in the United States and is the most common musculoskeletal infection in children. About half of the cases of acute osteomyelitis occurs in children younger than 5 years.
The disease process involves 5 stages:
Inflammation: This stage represents initial inflammation with vascular congestion and increased intraosseous pressure; obstruction to blood flow occurs with intravascular thrombosis.
Suppuration: Pus within the bones forces its way through the haversian system and forms a subperiosteal abscess in 2-3 days
Sequestrum: Increased pressure, vascular obstruction, and infective thrombus compromise the periosteal and endosteal blood supply, causing bone necrosis and sequestrum formation in approximately 7 days
Involucrum: This is new bone formation from the stripped surface of periosteum
Resolution or progression to complications: With antibiotics and surgical treatment early in the course of disease, osteomyelitis resolves without any complications
Imaging plays an important role in the diagnosis of acute pyogenic osteomyelitis. It should always start with plain radiographs of the affected area.
Current imaging recommendations include plain radiography followed by 3-phase bone scanning and/or MRI, if available.
Although osseous changes become apparent on conventional radiographs 5-7 days into the disease process, plain radiographs are useful in ruling out other causes of bone pain, such as stress fractures. Plain radiography and radionuclide bone scanning greatly aid early diagnosis in cases of acute osteomyelitis. Plain radiography is useful for excluding other conditions; radionuclide scanning reveals evidence of inflammation at the site of bone pain.
Nuclear medicine bone scans are a highly sensitive (>90%) modality in the diagnosis of osteomyelitis. This procedure is performed in 3 stages. Technetium-99m (99mTc) is used to create images to determine areas of infection and bone remodeling dependent on local blood flow. The sensitivity of bone scans is often helpful when the exact site and extent of the infection is not known.
CT scanning allows for 3-dimensional (3D) examination of bone and surrounding soft tissue. CT scanning is an excellent modality for depicting periosteal new-bone formation and cortical bone destruction and for determining whether any sequestration or involucrum is present. Contrast-enhanced CT may show a ring-enhancing soft tissue abscess.
MRI, if available, is another useful modality for imaging acute osteomyelitis. Findings on MRI accurately demonstrate the extent and structure of the area involved in the pathologic process. The reported sensitivity is 88-100%; the specificity is 75-100%. Fat-suppression sequences allow for better detection of bone marrow edema; however, infection and inflammation cannot be differentiated. MRI may be the imaging modality of choice for infections involving the spine, pelvis, or limbs because of its ability to provide fine details of the osseous changes and soft tissue extension in these areas.
Additional imaging may be performed with indium-111–labeled leukocytes; gallium is used as needed. Gallium seems especially valuable in monitoring the efficacy of treatment.
Urso and associates evaluated 40 pediatric patients (aged 2-16 y) with osteomyelitis to assess the roles of various imaging modalities, including conventional radiology, bone scanning with 99mTc methylene diphosphonate (MDP), scintigraphy with 99mTc hexamethylenepropyleneamineoxime (HMPAO)–labeled leukocytes, CT, and MRI.
As for acute osteomyelitis (6 patients), conventional radiography showed a lytic lesion and periosteal new-bone formation and soft tissue swelling (4 of 6 patients). (No abnormalities were demonstrated in the other 2 patients.) Bone scanning, CT, and MRI depicted bone involvement. CT and MRI also showed involvement and the spread of an inflammatory lesion to surrounding soft tissue.99mTc-HMPAO scintigraphy was not performed to assess acute osteomyelitis, because of technical difficulties in performing the study promptly; thus, early analysis of the nature of the lesion was made with bone biopsy.
As for subacute osteomyelitis, 99mTc-HMPAO scintigraphy was performed in 8 of 23 patients; it proved to be highly sensitive, showing cell clusters and confirming the diagnosis of an inflammatory lesion. T1-weighted MRIs showed a focal area of intermediate to low signal intensity. These MRIs also showed small, focal, intralesional areas of low intensity; a low-signal perifocal rim; and diffusely low signal intensity of the surrounding bone marrow. T2-weighted images showed high signal intensity in both the abscess lesion and the bone marrow; the latter was probably the result of edema. For 5 patients, a paramagnetic contrast agent (gadopentetate dimeglumine) was administered during MRI and resulted in inhomogeneous enhancement of both the inflammatory lesion and the surrounding bone marrow.
Regarding chronic osteomyelitis (7 patients), MRI was performed in 5 patients. In 4 patients, the lesion appeared as a hypointense area on T1-weighted images; T2-weighted images showed an inhomogeneous hyperintense area. In the same patients, 99mTc-HMPAO scintigraphy was always positive. In the fifth patient, the lesion was represented by a hypointense area on both T1- and T2-weighted images; 99mTc-HMPAO scans were negative.
Therefore, in cases of chronic osteomyelitis, both MRI and 99mTc-HMPAO were useful in detecting spinal and peripheral bone involvement, which was asymptomatic at first observation in some cases. Conventional radiography, CT findings (3 of 4 patients), and MRI findings (4 of 4 patients) of extra-axial localizations were similar to those in the subacute-chronic forms.
(See the images below.)
This 47-year-old man was being treated for staphylococcal septicemia when he presented with pain in the left lower leg. Clinically, embolic osteomyelitis was suspected. Physical examination revealed no abnormality. Radiograph of the left tibia (the site of pain) showed no abnormality.
Technetium-99m diphosphonate bone scans obtained 2 days later in the same patient shown in the previous image shows intense activity in the left tibia; this was highly suggestive of osteomyelitis.
On the basis of the route of infection, acute osteomyelitis can be classified as hematogenous or exogenous (see the images below). Hematogenous osteomyelitis is predominantly seen in children and involves the highly vascular long bones, especially those of the lower limb. In adults, hematogenous spread is more common to the lumbar vertebral bodies than elsewhere. In neonatal osteomyelitis, isotopic bone scans are reportedly normal in most patients.
Chest radiograph in an 8-year-old girl who presented with staphylococcal pneumonia.
Streptococcal osteomyelitis in a 3-year-old patient presenting with periosteal new-bone formation of the tibia.
Before puberty, infection starts in the metaphyseal sinusoidal veins. Because bones are relatively rigid structures, focal edema accumulates under pressure and leads to local tissue necrosis, breakdown of the trabecular bone structure, and removal of bone matrix and calcium. Infection spreads along the haversian canals, through the marrow cavity, and beneath the periosteal layer of the bone. Subsequent vascular damage causes the ischemic death of osteocytes, leading to the formation of a sequestrum. Periosteal new-bone formation on top of the sequestrum is known as involucrum.
Osteomyelitis may be acute, subacute, or chronic. With acute osteomyelitis, the presenting complaint is usually local pain, swelling, and warmth. These often occur in association with fever and malaise.
Differentiating acute osteomyelitis from bone infarction in patients with sickle cell disease is a major challenge. The 2 conditions must be differentiated on the basis of clinical findings and imaging studies because both are common in patients with sickle cell disease. The 2 diseases are managed differently.
Fine-needle aspiration (FNA) or needle biopsy may be used under ultrasonographic, fluoroscopic, or CT guidance to obtain samples of pus, tissue, or both to establish a histologic diagnosis of acute osteomyelitis.
Limitations of techniques
Plain radiographs are often normal for at least 1 week following infection; the findings are nonspecific.
MRI is contraindicated in patients with certain implant devices and metallic clips, and it is not tolerated by all patients because of claustrophobia or morbid obesity. In addition, young children may require sedation. Use of MRI requires patient cooperation because patient motion may degrade the images.
CT is quick and inexpensive but exposes the patient to ionizing radiation. The risk of a reaction to radio-iodinated contrast material is low; the detection of bone destruction or a paraspinal mass does not require the use of contrast material.
Although radionuclide studies are sensitive, they can be time-consuming, and they have lower spatial resolution. The incidence of false-negative scans is low in neonates and in elderly patients with osteomyelitis.