Loading...
Osteomyelitis is an infection of bone, typically bacterial, that results in inflammation and bone destruction.[1,2] Although bone is normally resistant to bacterial colonization, disruption of bone integrity may provide a pathway for infection. Delay in the diagnosis of osteomyelitis can lead to significant morbidity if targeted therapy is not initiated promptly. The sagittal T1- and T2-weighted magnetic resonance imaging (MRI) scans of the knee shown here demonstrate the classic decreased signal on T1 (left), with increased signal on T2 (right) due to bony edema; a necrotic sequestrum is seen centrally.
Images courtesy of Radiopaedia.
The classic presentation for osteomyelitis is fever, chills, fatigue, lethargy, and irritability. Patients may complain of joint pain or swelling, decreased joint motion, and an inability to bear weight on or use the affected extremity.[1-4] It is important to determine whether the patient has a history of trauma (including penetrating wounds), immunocompromised state (including diabetes),[5,6] and antibiotic treatments. On exam, localized signs of point tenderness, swelling, erythema, cellulitis, or even an open wound may be found. Infections near joints can result in an effusion or loss of joint motion. The only indication of infection may be a joint effusion adjacent to the infected bone (tibia or femur), as seen on the left, although the bone may be directly inoculated by a penetrating wound or abscess, as seen in the tibia on the right.
Images courtesy of Wikimedia and Flickr.
Initial workup for osteomyelitis will typically reveal an elevated erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level. A mild leukocytosis with left shift may be present. However, blood cultures are positive in only 50% of cases. Bone biopsy is necessary for definitive pathogen diagnosis. The initial imaging study of choice is conventional radiography, used to look for signs of osteomyelitis and rule out other etiologies.[7] Common findings are periosteal elevation or thickening; cortical thickening, sclerosis, and irregularity; loss of trabecular architecture; osteolysis; and new bone formation. Radiographs of the distal tibia shown here demonstrate periosteal elevation (left, arrowhead) and osteolysis (right, arrowhead), findings consistent with osteomyelitis. However, bony changes detectable by radiographs may not occur during the first week of infection in children and the first 2 weeks of infection in adults, so a normal radiograph does not preclude a diagnosis of osteomyelitis.
Images courtesy of Medscape Drugs & Diseases.
MRI is the best imaging modality for early detection of osteomyelitis, as positive findings are present earlier than in radiographs. MRI has a higher sensitivity and specificity than plain radiography and computed tomography (CT) scanning in the detection of osteomyelitis.[7] It is particularly good for evaluating vertebral osteomyelitis. Typical findings are decreased signal on T1-weighted images and increased signal on T2-weighted and short-tau inversion recovery (STIR) images. Contrast dye can enhance abscesses, which can be a surgical indication. The STIR MRI shown demonstrates increased signal within the vertebral body (yellow arrow) due to bone marrow osteomyelitis and enhancing fluid within the vertebral body (red arrow), likely an abscess full of pus.
Image courtesy of Radiopaedia.
Nuclear imaging can add additional information when performing a workup for chronic osteomyelitis, multifocal osteomyelitis, or periprosthetic infections and may be useful in detecting infection foci.[7,8] Indium-labeled white blood cell (WBC) scans have been used in the setting of orthopedic implants as a means of avoiding distortion of MRI and CT scans by metal artifacts. All nuclear imaging methods are very sensitive, but unfortunately none are specific, and nuclear imaging alone is not typically used to diagnose osteomyelitis. The bone scans shown demonstrate intense activity within the small toe of the right foot (yellow arrow) due to osteomyelitis.
Images courtesy of Radiopaedia.
In chronic or subacute infections, it is important to find the bony nidus, as its removal may be the only way to eliminate the infection. A sequestrum is a collection of necrotic bone that is the nidus of the infection. Involucrum is the new bone that forms around the area of necrosis. A CT scan can be valuable in evaluating the sequestrum and involucrum, which can aid in biopsy and surgical planning. This CT scan demonstrates a chronic infection in the right tibia, with a clearly outlined sequestrum (red arrow) and new bone formation (yellow arrow).
Image courtesy of Radiopaedia.
The most common causes of osteomyelitis in adults are posttraumatic causes (47%), vascular insufficiency (34%), and hematogenous seeding (19%). Posttraumatic osteomyelitis is usually caused by direct inoculation of bacteria during an injury or subsequent intervention. Open fractures, especially those with significant soft tissue injury, degloving, periosteal stripping, full thickness tissue loss, and vascular injury, increase the likelihood of osteomyelitis. Therefore, staged, damage-control–type interventions (multiple washouts and application of an external fixator) with broad-spectrum antibiotics are often used prior to any definitive treatment. This open fracture of the tibia, with significant periosteal stripping, is at a higher risk of osteomyelitis than a closed injury would have been.
Image courtesy of Wikimedia.
Vascular insufficiency, which is commonly seen in patients with poorly controlled or long-standing diabetes, renal failure, or peripheral vascular disease, can lead to chronic, full thickness skin ulcers, with eventual direct inoculation of adjacent bone. Here, sagittal T1 (left) and T2 (right) MRI images show a chronic plantar heel ulcer with direct contact to and resultant infection of the calcaneus. Quite commonly, these infections are caused by multiple organisms and require broad-spectrum intravenous antibiotic treatment; they can ultimately result in amputation of the limb.
Images courtesy of Radiopaedia.
Hematogenous osteomyelitis is caused by bacterial seeding from the blood. An uncommon cause of osteomyelitis in healthy adults, it is more commonly seen in association with the following risk factors: indwelling vascular catheter, intravenous drug abuse, dialysis, immunocompromise, hemoglobinopathy, varicella infection, and rheumatoid arthritis. Hematogenous spread is more common in infants and children and is usually found in the long bone metaphysis. The sharp turns taken by metaphyseal capillaries, coupled with low pH and oxygen tension near the physis, make this area ripe for infections in children. Subperiosteal abscesses can be seen when the infection breaks through the metaphyseal cortex, as can septic arthritis in infections near the hip, shoulder, elbow, or ankle. Subperiosteal pus (green arrow) is seen on this STIR MRI scan of the tibia, as well as a Brodie abscess (intramedullary abscess) (yellow arrow).[9]
Image courtesy of Wikimedia.
Treatment for osteomyelitis begins with broad-spectrum, parenteral antibiotic coverage after blood and/or bone cultures have been obtained. Once the etiologic organisms have been identified, coverage can be narrowed. Intravenous antibiotic treatment for 4-6 weeks is the standard course. This often requires the insertion of a peripherally inserted central catheter (PICC) line, shown here, to allow for home intravenous antibiotic dosing.[10] Infectious disease and orthopedic comanagement is essential. Serial lab studies are obtained to monitor the effectiveness of treatment and any side effects of the medication. For patients with prostheses, rifampin should be included, because it acts on the bacterial biofilm and helps to prevent recurrence.
Image courtesy of Wikimedia.
Surgical indications in osteomyelitis include the following: subperiosteal or intramedullary abscesses, chronic osteomyelitis recalcitrant to medical treatment, pathologic fractures resulting in instability (as commonly seen in vertebral osteomyelitis), vascular insufficiency and failure of conservative treatment, and/or life threatening sepsis due to severe osteomyelitis, with resultant multi-organ failure.[5] In some cases, amputation may be necessary. A Brodie abscess (intramedullary abscess), shown here, is usually seen in pediatric chronic osteomyelitis and often necessitates surgical intervention to allow for drainage, removal of necrotic tissue, and resolution of the osteomyelitis.[9]
Image courtesy of Wikimedia.
Osteomyelitis of the vertebral bodies deserves special consideration because it can lead to permanent neurologic defects, spinal deformity, or death if untreated. Hematologic seeding is the presumed mechanism in a majority of cases. Disease onset is usually insidious, with progressive worsening of back pain that is unrelieved by analgesics. Physical examination findings include tenderness of the spinous process and paravertebral muscles. Neurologic deficits are late findings secondary to vertebral body collapse, epidural abscess, or paralysis. MRI is the best imaging study for osteomyelitis (yellow arrow) and may also reveal diskitis (blue arrow), abscess, and/or involvement of the cord (red arrow). Treatment involves some combination of antibiotic therapy, bracing, surgical drainage or excision, and reconstruction.
Patients with sickle cell disease are at an increased risk for bacterial infection, with osteomyelitis being the second most common infection seen in these patients. S aureus remains the most common microorganism responsible for infection, but Salmonella and Serratia species and Proteus mirabilis represent a disproportionate share compared with the general populace. The T1-weighted MRI shown demonstrates decreased signal within the metatarsal (arrow), due to osteomyelitis, in a patient with sickle cell disease.
Garré sclerosing osteomyelitis, or chronic nonsuppurative sclerosing osteomyelitis, is a form of chronic osteomyelitis. Mild inflammation and infection lead to subperiosteal bone deposition. The disease is frequently asymptomatic. The characteristic radiographic appearance is an area of periosteal proliferation surrounded by successive layers of condensed cortical bone (arrows), described as an onion skin appearance.
Image courtesy of Medscape Drugs & Diseases.
Imaging studies of primary bone tumors and osteomyelitis can look very similar. Always keep cancer in the differential when working up a suspected case of osteomyelitis. If infectious lab studies are normal and a destructive lesion is seen on a radiograph or an MRI scan, refer the patient quickly to an orthopedic oncologist. The T2-weighted MRI scan of the distal femoral metaphysis seen here shows increased signal in the intramedullary canal, fluid, bony destruction, and soft tissue reaction, features very similar to those seen in MRI scans of an infection. In this case, however, the disease is an osteosarcoma.
Image courtesy of Radiopaedia.
Previous
Next 
