Osteomyelitis: Keys To Diagnosis And Treatment
Comparing The Various Imaging Techniques
Radiography. Plain film radiographs remain necessary in the initial evaluation of bone infection. Early radiographic changes in the patient with osteomyelitis will often be subtle. Lytic osseous changes are often not visible for up to two to three weeks.18 By the time lysis is visible on radiographic images, extensive destruction of the bone has already occurred.
The photos at right demonstrate the rapid changes that can occur when one does not notice bone infection early. The top photo shows very subtle lytic and cortical changes. Within two weeks (see the bottom photo), the proximal phalanx is already showing signs of significant osteolysis, serpiginous changes and cortical dissolution. A study by Boutin and colleagues found radiographic evidence of osteomyelitis in as few as 3 to 5 percent of culture-positive osteomyelitis cases.19
Bone scintigraphy. Classically, osteomyelitis presents with focal hyperperfusion, focal hyperemia and demonstrates positive uptake on all three phases of bone scans. Bone scintigraphy is an early sensitive, but not specific, method of identifying bone infection.20 Nuclear imaging allows abnormalities of bone to be visible weeks or even months earlier than on plain film radiographs. One can cautiously confirm osteomyelitis after obtaining a positive 24-hour scan if no other differentials such as Charcot neuroarthropathy, bone tumor, stress injury or fracture exist. Negative results at three to six hours can effectively rule out osteomyelitis.21
In order to improve specificity, one can obtain indium oxide 111 labeled scans. Sensitivities between 70 and 90 percent occur with labeled leukocyte scanning.22,23 Drawbacks of the indium-111 scans include lower quality image expense, cost and exam complexity.20,24 Tc-HMPAO scans provide higher imaging quality in comparison to indium oxide 111. Leukocyte scans may demonstrate a comparative decrease in uptake as the resolution of osteomyelitis is occurring.25
Magnetic resonance imaging (MRI). One can detect bone edema very clearly on MRI and this is what a clinician should look for when evaluating for bone infection. Radiographs and bone scans will not reveal this information. Bone edema is visible as hypointense on T1-weighted imaging and hyperintense on T2-weighted imaging. Gadolinium can also enhance areas of cellulitis or abscess formation.26
One of the more difficult differential diagnoses when evaluating a patient with diabetes and potential osteomyelitis is Charcot neuroarthropathy. Diagnosis can be difficult and even impossible to delineate, based on plain film radiographs and general nuclear medicine imaging. One can often obtain valuable insight through MRI but the clear differentiation of infection and Charcot neuroarthropathy is still difficult based on MRI evaluation alone. Bone edema can be clearly visible with osteomyelitis and neuroarthropathy.27