Acute Osteomyelitis

General characteristics

A single pathogenic organism usually is responsible for acute hematogenous osteomyelitis.

Single or multiple bones can be affected, with infection of multiple sites seemingly more common in children than in adults.

There are three main routes of contamination:

• Hematogenous infection (infection via bloodstream from sources such as pharyngitis, pyodermia, otitis) • Spread from an adjacent infection (e.g., sinusitis, or cutaneous or dental infection) • Direct implantation of infectious material (penetrating injury)

The fourth route is postoperative infection, which may occur by the above-mentioned three principal routes. Osteomyelitis often manifests as a severe clinical illness, with swelling and fever. Local pain and reduced range of motion of an extremity are suggestive of osteomyelitis. The metaphyses of the long bones are the most common site of infection. Since early treatment is required in order to avoid long-term sequelae, diagnosis should be made at an early stage. Conventional radiography may be negative for up to 14 days after onset of symptoms, whereas three-phase bone scintigraphy and leukocyte scintigraphy show early intense tracer uptake.

Imaging

In osteomyelitis, the amount of intramedullary fluid and inflammatory cells is increased. Therefore, MRI shows a signal intensity (SI) decrease in the bone marrow on T1-weighted images. In T2-weighted images, the inflammatory process results in high signal intensity, so that contrast between inflammation and normal bone marrow is reduced or absent. This is also true for contrast-enhanced T1-weighted images. Thus, short-tau inversion recovery (STIR) and proton density (PD) fat-suppressed sequences are highly sensitive for the detection of osteomyelitis; in these sequences the inflammatory process shows high signal contrasted against the low signal of normal bone marrow. Also, contrast-enhanced T1-weighted fat-suppressed images are excellent for the detection of osteomyelitis. Inflammatory tissue is depicted with strong contrast enhancement, and abscesses are clearly visualized with marked enhancement of the abscess membrane.

Any extraosseous spread of infection is best detected on STIR, PD fat-suppressed, and contrast-enhanced T1wfat- suppressed images; it might be missed in non-enhanced T1-weighted images. In contrast to the SI increase in tumors, the SI increase in paraosseous soft tissues is indistinct and poorly demarcated from the surrounding muscles and fat. If a soft tissue abscess is present, then demonstration of its fluid content and surrounding abscess membrane allow for clear diagnosis.

In order to select the appropriate therapy, it is essential to differentiate between isolated soft tissue infection, osteomyelitis with concomitant soft tissue infection, and osteomyelitis alone. MRI is superior to all other imaging methods in making this distinction. Especially in patients with diabetes, this differentiation is crucial because local operative procedures together with antibiotics for a short time are sufficient therapy for soft tissue infections, whereas more extensive surgery and long-term antibiotic therapy are required when bone also is involved.