Musculoskeletal Infection

Stephanie A. Bernard

Jeffrey J. Peterson

Thomas H. Berquist

▪ BASIC CONCEPTS

KEY FACTS

Depending on the organism and the patient’s immune status, musculoskeletal infections may present with rapidly progressive onset or in a more insidious fashion (Table 10-1). Source of infections may be hematogenous, from contiguous spread (adjacent soft tissues or joint), or from direct implantation (i.e., puncture wound, trauma, or surgery).
Insidious onset of infection often follows trauma or surgical intervention or infection with atypical organisms (Tuberculosis, Lyme).
Clinical presentation is dependent on patient age, virulence of organisms, patient condition, site of involvement, and circulation (Table 10-2).
Early detection and evaluation of the extent of involvement are essential for proper treatment and optimal prognosis.
Imaging of musculoskeletal infection usually requires a multimodality approach.

Radiography: Soft tissue swelling or joint effusion may be the only findings. Significant bone destruction must be present before radiographs are positive.

Table 10-1 INFECTIONS: TERMINOLOGY AND CATEGORIES

Term/Conditions	Clinical/Image Features
Osteomyelitis	Infections in bone and marrow. Bacterial most common
Infective osteitis	Cortical infection. Often associated with marrow or soft tissue infection
Infective periostitis	Periosteal infection often with marrow and cortex involved
Soft tissue infection	Involves skin, subcutaneous tissues, muscles, tendons, ligaments, fascia, or bursae
Sequestrum	Segment of necrotic bone separated from viable bone by granulation tissue
Involucrum	Living bone around sequestrum
Cloaca	Tract through viable bone
Sinus	Tract from infected region to skin
Fistula	Abnormal communication between two internal organs or internal organs and skin
Brodie abscess	Sharply defined focus of osteomyelitis
Garré sclerosing osteomyelitis	Sclerotic nonpurulent infection with intense periosteal reaction
Chronic recurrent multifocal osteomyelitis	Subacute or chronic infection common in children. May be associated with SAPHO
SAPHO	Palmoplantar pustulosis, articular, and hyperostosis and osteitis (SAPHO) periosteal inflammation Chronic course involving chest wall, spine, long bones, large and small joints
SAPHO, synovitis, acne, pustulosis, hyperostosis, osteitis.

Table 10-2 COMMON ORGANISMS IN MUSCULOSKELETAL INFECTIONS

Bacterial Infections		Fungal and Higher Bacterial Infections	Parasitic Infections
Gram-positive		Actinomycosis	Hookworms
	Staphylococcal	Nocardiosis	Cysticercosis
	Streptococcal	Cryptococcosis	Echinococcosis
	Meningococcal	Coccidioidomycosis
	Gonococcal	Histoplasmosis
Gram-negative bacilli		Sporotrichosis
	Coliform bacterial infections
	Proteus
	Pseudomonas
	Klebsiella
	Salmonella
	Haemophilus
	Brucella
Mycobacteria
	Tuberculosis
	Atypical mycobacteria

Molecular imaging: Bone scanning is sensitive and can detect abnormalities early, but there is less anatomic detail, and findings are not specific. Techniques include three-phase technetium-99m-methylene diphosphate, gallium-67-, and indium- or technetium-labeled white blood cell studies, technetium antigranulocyte antibody scans, and positron emission tomography (PET).
Computed tomography (CT): Less sensitive than magnetic resonance imaging (MRI) or molecular imaging for most infections. Highest sensitive for soft tissue gas. Contrast aides in identifying abscess. Will detect cortical changes along with sequestra and cloaca in chronic osteomyelitis.
Ultrasound: Soft tissue infection, abscess formation, foreign body localization, and joint effusions can be identified and can be used for aspiration.
MRI: MRI is particularly suited for evaluation of early bone or soft tissue infection. Contrast is superior to CT, and anatomic detail is superior to radionuclide scans. When patient is able to receive contrast, contrast-enhanced fat-suppressed T1-weighted images help with identifying extent of infection and tissue viability.
Aspiration/biopsy: Regardless of the imaging technique used, joint aspiration or synovial or bone biopsy under fluoroscopic or ultrasound guidance may be required to isolate the organism.

SUGGESTED READING

Berquist TH, Broderick DF. Musculoskeletal infections. In: Berquist TH, ed. MRI of the Musculoskeletal System. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2006:916-947.

Palestro CJ, Love C, Miller TT. Infection and musculoskeletal conditions: imaging of musculoskeletal infections. Best Pract Res Clin Rheumatol. 2006; 20:1197-1218.

Resnick D. Osteomyelitis, septic arthritis, and soft tissue infection: mechanisms and situations. In: Resnick D. Diagnosis of Bone and Joint Disorders. 4th ed. Philadelphia: WB Saunders; 2002:2510-2624.

▪ OSTEOMYELITIS

KEY FACTS

The imaging appearance is dependent on the virulence of the organism and the stage of disease.
- Acute pyogenic osteomyelitis: permeative osteolytic destruction with periosteal
- Subacute osteomyelitis (Brodie abscess): well-defined lytic lesion with sclerotic border
- Chronic osteomyelitis: sclerosis with central sequestra of osteonecrotic bone, encasing involucrum, and draining cloaca
Early diagnosis and management are essential to avoid irreversible bone or articular damage.
Hematogenous osteomyelitis is more common in children than in adults and most frequently involves the lower extremities.
Metaphyseal regions, near the physis, are most commonly involved.
The open physis serves a protective function preventing infection from entering the epiphysis and, depending on joint anatomy, the joint from ages 1 to 16 years.
Imaging of osteomyelitis requires a multimodality approach (Table 10-3).
- Routine radiographs: may show soft tissue swelling. Bone changes may be inapparent early.
- Radionuclide scans: A positive technetium-99m scan is not specific, but a negative scan excludes the diagnosis. Combined indium-111 or technetium-labeled leukocytes and technetium-99m scans are more specific. PET may also be useful, especially for osteomyelitis in the spine.
- CT: cortical detail, sequestra, and cloacae
- MRI: T1- and T2-weighted magnetic resonance (MR) images are sensitive and can detect changes of infection early. T1-weighted signal abnormalities are most important. Gadolinium-enhanced T1-weighted images are routinely added in our practice. Anatomic detail is superior to radionuclide scans.

Table 10-3 IMAGING APPROACHES FOR OSTEOMYELITIS

Radiographic Features	Next Steps
Positive	Biopsy and treat
Negative; low suspicion	Technetium scan: Negative: stop Positive: MRI or combined radionuclide studies
Negative; high suspicion	MRI or combined radionuclide studies and MRI
MRI, magnetic resonance imaging.

FIGURE 10-1. The vascular supply to the metaphysis and epiphysis in infants (A), children (B), and adults (C). The physis serves a protective function for the epiphysis from ages 1 to 16 years (B).

FIGURE 10-2. Characteristic appearance of subacute osteomyelitis as a well-circumscribed metaphyseal lucency in the distal tibia. Radiographic osteolytic lesion with a sclerotic border (arrowheads).

FIGURE 10-3. Acute hematogenous osteomyelitis of the distal femur in a 15-year- old male. (A) Radiographically inapparent at time of magnetic resonance imaging (MRI). On MRI, the marrow demonstrates reticulated intermediate (B) T1-weighted infiltration of the marrow with increased signal on (C) Proton density fat-saturated imaging. Subperiosteal abscess (arrowheads) can be seen elevating the periosteum stopping at the level of the physis, where the periosteum is tightly adherent.

FIGURE 10-4. Osteomyelitis from direct extension from a diabetic foot ulcer in a 51-year-old female. (A) T1-weighted, (B) short-T1 inversion recovery (STIR), and (C) postgadolinium T1-weighted fat-saturated images of the hindfoot demonstrate an ulcer at the plantar aspect of the heel with replacement of the normal subcutaneous fat on T1 image to the bone surface and with contrast outlining sinus tract to skin (arrows). Underlying increased T2 signal and enhancement in the marrow with mild T1 marrow infiltration (arrowheads).