Imaging of Musculoskeletal Infection

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What is the role of imaging in the diagnosis of musculoskeletal infection?

Imaging examinations can help to diagnose the presence of and delineate the extent of infection involving the soft tissues or bone. However, radiologists rely on correlative clinical findings including history and physical examination findings and laboratory testing results to help establish the diagnosis of infection. The diagnosis of musculoskeletal infection is a collaborative process between radiologists and referring providers.

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What imaging modalities are used in diagnosing musculoskeletal infections?

Most imaging modalities have a role in the diagnosis of infection, and these roles are frequently complementary. These include radiography, computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (US), and nuclear medicine techniques.

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What is the role of US in the diagnosis of musculoskeletal infection?

In general, musculoskeletal US is an important adjunct to the other modalities. US excels in the evaluation of superficial soft tissues, and benefits from higher spatial resolution than MRI, higher soft tissue contrast than CT, and a lack of ionizing radiation. However, US suffers from poor penetration into deeper structures and is therefore best utilized to evaluate superficial tissues. Additionally, US is optimized for answering a specific clinical question, rather than for the survey of larger anatomic areas such as can be performed with radiography, CT, or MRI.

In the specific diagnosis of infection, musculoskeletal US can be used to identify fluid collections, particularly pockets of potentially drainable pus in the midst of an extensive cellulitis. US also excels in the identification of nonradiopaque foreign bodies that may act as a nidus for infection.

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What is the role of nuclear medicine techniques in the diagnosis of musculoskeletal infection?

Technetium-99m ( ^99m Tc) methylene diphosphonate (MDP) bone scintigraphy (“bone scan”), Indium-111 ( ¹¹¹ In) white blood cell (WBC) scintigraphy (“WBC scan”), and fluorine-18 ( ¹⁸ F)-fluorodeoxyglucose (FDG) positron emission tomography (PET) are available nuclear medicine techniques that can be used to diagnose musculoskeletal infection. Currently, their use has been largely eclipsed by MRI, although they remain useful for problem-solving applications.

For example, the diagnosis of periprosthetic infection (osteomyelitis adjacent to orthopedic hardware) is difficult by MRI because of field distortion effects around metal. Similarly, the diagnosis of infection at the site of a recent fracture (when bone marrow edema and soft tissue edema are expected) can be challenging with anatomic imaging techniques. In these and other challenging cases, nuclear medicine techniques can provide valuable diagnostic information.

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In general, what is the first-line imaging modality used to diagnose musculoskeletal infection?

Radiography. It is fast, inexpensive, and frequently the only imaging modality necessary to establish a diagnosis of infection.

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Define osteomyelitis, cellulitis, fasciitis, myositis, pyomyositis, phlegmon, and abscess.

• Osteomyelitis : Infection of bone.

• Cellulitis : Infection of skin and superficial soft tissues.

• Fasciitis : Infection of fascial tissues.

• Myositis : A general term indicating muscle inflammation.

• Pyomyositis : Infection of muscles.

• Phlegmon : A suppurative inflammatory process in the soft tissues.

• Abscess : An organized collection of pus.

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What is the clinical significance of an abscess?

Abscesses are difficult to treat with antimicrobial agents alone, particularly when large in size. They often require drainage for complete treatment. In contradistinction, a phlegmon cannot be drained (because it does not contain an organized pocket of fluid) and therefore is generally treated with antimicrobial agents alone.

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Define Brodie abscess, sequestrum, involucrum, cloaca, and sinus tract.

• Brodie abscess : An organized collection of pus within a bone (i.e., an intraosseous abscess), reflecting subacute osteomyelitis, most often due to Staphylococcus aureus infection.

• Sequestrum : A devascularized piece of bone that is seen within an intraosseous abscess.

• Involucrum : Thickened periosteal new bone formation surrounding a sequestrum, which represents the body's attempt to contain a region of osteomyelitis.

• Cloaca : An opening from a sequestrum through the involucrum.

• Sinus tract : A connection from a bone to the skin surface, lined with granulation tissue, also called a fistula. It can allow the drainage of pus.

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What is the first-line modality in the diagnosis of acute osteomyelitis?

Radiography.

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