Osteomyelitis And Heel Ulcers: What You Should Know
What Diagnostic Imaging Can Reveal About Osteomyelitis
Osteolysis of bone only occurs after the infectious hyperemic process has “washed out” 30 to 50 percent of osseous mineralization.5 This process takes anywhere from 10 to 14 days after the onset of symptoms. Sclerosis and new perisoteal bone formation called involucrum surrounds infected and necrotic bone and sequestrum. A channel, cloaca, forms in new bone as bacteria and exudates drain. Chronic osteomyelitis involves the presence of bacteria living and surviving in sequestrum surrounded by involucrum. Chronic osteomyelitis can survive dormant for many years before a flare-up can occur.5
Radionuclide bone scans are useful in suspected osteomyelitis and are usually positive within 72 hours of infection.6 Tc-99 and In-111 are the most common initial scan techniques but the most useful radionucleotide scan for osteomyelitis is the Ceretec scan. While Ceretec scans are sensitive, the level of specificity lags behind the specificity of magnetic resonance imaging (MRI).7
Computed tomography (CT) provides excellent multiplanar reconstructions of the axial images, allowing delineation of even the subtlest osseous changes. In chronic osteomyelitis, CT scans demonstrate abnormal thickening of the affected cortical bone with sclerotic changes, encroachment of the medullary cavity and a chronically draining sinus. Although CT scans may show these changes earlier than plain radiographs, CT scans are less desirable than MRI because of the decreased soft tissue contrast as well as exposure to ionizing radiation.5,8
The major role of this technique in osteomyelitis is the detection of sequestra in cases of chronic osteomyelitis as surrounding osseous abnormalities on conventional radiography can mask these pieces of necrotic bone. The presence of pieces of sequestered bone suggests that the activity of the infectious process and its detection is helpful to guide the therapeutic options. Computed tomography is superior to MRI for the detection of sequestra, cloacas, involucra or intraosseous gas.5
Ultrasound has multiple advantages. It is readily accessible and one can perform it quickly and with minimal discomfort to the patient. Ultrasound is useful in regions that are not easily visible due to the presence of operative orthopedic instrumentation and therefore might not be easy to visualize with MRI or CT. Similarly, ultrasound is useful in patients in whom MRI is contraindicated. Finally, ultrasound has a lower cost, does not use ionizing radiation and offers real-time imaging. For these reasons, ultrasound is a useful tool in the evaluation of musculoskeletal infections and is particularly helpful in differentiating acute or chronic infections from tumors or non-infective conditions.
Ultrasound is also able to localize the site and extent of infection, identify precipitating factors such as foreign bodies or fistulae, and provide guidance for diagnostic or therapeutic aspiration or biopsy.9 Ultrasonography can detect features of osteomyelitis several days earlier than conventional radiographs (predominately in children). Using ultrasound, one would recognize acute osteomyelitis via elevation of the periosteum by a hypoechoic layer of purulent material.10 In cases of chronic osteomyelitis, ultrasonography can also assess the involvement of the adjacent soft tissues. Soft tissue abscesses related to chronic osteomyelitis appear as hypoechoic or anechoic fluid collections, which may extend around the bony contours. Finally, cortical erosions can become apparent on ultrasonography.10