High-resolution ultrasound in the evaluation of musculoskeletal infections

Abstract

Soft tissue and osseous musculoskeletal infections are common but can be difficult to diagnose clinically. Signs, symptoms, and physical examination findings may be nonspecific, and laboratory values can be inconclusive. The extent of disease may also be underestimated on physical examination. Soft tissue infections most commonly occur secondary to direct inoculation from broken skin and less frequently due to the seeding of the soft tissues from hematogenous spread, while osseous infections are more commonly due to hematogenous seeding. Infections may also be iatrogenic, following surgery or other procedural interventions. High-resolution ultrasound is an extremely useful imaging modality in the evaluation of musculoskeletal soft tissue and joint infections, and can occasionally be used to evaluate osseous infections as well. Ultrasound can aid in the early diagnosis of musculoskeletal infections, allowing for prompt treatment, decreased risk of complications, and treatment optimization. Ultrasound is sensitive and specific in evaluating soft tissue edema and hyperemia; soft tissue abscesses; joint, bursal and tendon sheath effusions/synovitis; and subperiosteal abscesses. This article describes the typical high-resolution grayscale as well as color and power Doppler ultrasound imaging findings of soft tissue infections including cellulitis, fasciitis, necrotizing deep soft tissue infection, pyomyositis, soft tissue abscess, infectious bursitis, and infectious tenosynovitis. Ultrasound findings of septic arthritis as well as osteomyelitis, such as subperiosteal spread of infection (subperiosteal abscess). are also reviewed. In addition, the use of ultrasound to guide fluid and tissue sampling is discussed.

Keywords: US; abscess; cellulitis; fasciitis; osteomyelitis.

Fig. 1.

Retained foreign body and cellulitis. 13-year-old female with right hand injury after falling onto a tree stump. A. Initial radiographs were normal, without radiopaque foreign body or acute abnormality identified. Subsequent focused ultrasound was performed 4 days later when patient returned with increased swelling, erythema, and pain. B. Grayscale ultrasound (US) at area of concern between the thumb and index finger demonstrates linear echogenic foreign body with posterior acoustic shadowing (arrows). C. Color Doppler imaging shows hyperemia in the soft tissues surrounding the foreign body, consistent with cellulitis. No fluid collection was present

Fig. 2.

Cellulitis, abscess, and superficial thrombophlebitis. 45-year-old male with history of intravenous drug use and hepatitis C, now with severe elbow pain and swelling, fever, and elevated white blood cell (WBC) count. A. Frontal and B. lateral radiographs show stranding (arrows) in the subcutaneous fat, consistent with edema. C. Transverse grayscale ultrasound (US) image in the region of the antecubital fossa shows a cobblestoned, echogenic appearance of the subcutaneous fat (arrow), consistent with edema, suggesting cellulitis. D. Color Doppler US image shows hyperemia in the region of cellulitis. E. Transverse and F. long-axis grayscale US images in this region show a heterogeneous, mixed echogenicity collection (dashed arrows), measured by calipers, consistent with an abscess. G. Color Doppler image shows a longitudinal filling defect in a vein (short arrows), with loss of flow, consistent with superficial thrombophlebitis

Fig. 3.

Cellulitis and pyomyositis. 23-year-old female with history of intravenous drug use, presenting with a swollen, painful arm and fever. A. Transverse and B. long-axis grayscale US images of the upper arm show a cobblestoned, echogenic appearance of the subcutaneous fat, consistent with edema, which can be seen with cellulitis. Within the adjacent biceps brachii muscle, there is an anechoic lobulated collection (arrows), measured by calipers, consistent with abscess. C. Color and D. power Doppler US images show hyperemia of the wall of the abscess

Fig. 4.

Cellulitis, fasciitis, and soft tissue gas in necrotizing deep soft tissue infection. 30-year-old female with leg pain. A. Axial contrast-enhanced computed tomography (CT) image in lung window shows multiple foci of gas (arrows) in the medial soft tissues of the thigh. B. Same image, in soft tissue window, shows thickening of the skin, edema in the subcutaneous fat, and edema along the adjacent fascial planes (dashed arrows). C–E. Transverse grayscale US images in this region show a cobblestoned, echogenic appearance of the subcutaneous fat, consistent with edema, suggesting cellulitis. Multiple echogenic foci of gas are present (arrows), with ring down artifact (dashed arrows)

Fig. 5.

Cellulitis, bursitis, and diagnostic aspiration. 56-year-old male with severe hip pain and fever. A. Coronal and B. axial T2-weighted fat-saturated magnetic resonance (MR) images of the hip show a large, complex fluid collection (arrows) in the trochanteric bursa. Bone marrow edema is present in the adjacent femur (dashed arrows). C. Transverse and D. long-axis grayscale US images show a cobblestoned, echogenic appearance of the subcutaneous fat, consistent with edema, suggesting cellulitis. Fluid collection (arrows) is seen adjacent to the greater trochanter femoral cortex. E. US-guided aspiration was performed to confirm septic bursitis. Note needle (short arrows)

Fig. 6.

Cellulitis and infectious tenosynovitis. 70-year-old male with pain and swelling of the dorsal hand following a cat bite. A. Lateral radiograph of the hand shows dorsal soft tissue edema (arrows), which can be seen with cellulitis. Metallic clips are present from a prior unrelated surgery. B. Long-axis and C. transverse grayscale US images in the region of the fourth extensor compartment show fluid (dashed arrows) along the extensor tendons (*), consistent with tenosynovitis. C. Long-axis and D. transverse color Doppler US images in this region show hyperemia

Fig. 7.

Joint effusion and synovitis. 75-year-old male with fever, elbow pain and swelling. A. Long-axis grayscale US image through the posterior elbow shows mixed echogenicity joint effusion (arrow), deep to the triceps tendon (*), adjacent to the olecranon (O). B. Microvascular flow imaging (MVFI) in the same areas shows mild hyperemia suggesting synovitis. C. Sagittal T2-weighted MR image shows increased synovial-fluid complex signal (dashed arrow) distending the joint. D. Contrast-enhanced sagittal T1-weighted MR image with fat saturation shows diffuse, heterogeneous enhancement of the joint contents (dashed arrow), consistent with effusion and synovitis

Fig. 8.

Cellulitis, joint effusion, synovitis, abscess, and septic arthritis. 58-year-old male with end-stage renal disease, left-sided chest pain, fever, and elevated WBC count. A. Transverse grayscale US image just superior to the sternoclavicular joint (SCJ) shows a lobulated, heterogeneously hypoechoic fluid collection in the anterior soft tissues (arrows, measured with calipers), consistent with a complex joint effusion. B. Color Doppler US image shows hyperemia in the surrounding soft tissues, consistent with cellulitis. C. Long-axis grayscale US image at the level of the SCJ shows a heterogeneous, complex joint effusion (dashed arrows). D. Color Doppler US image in the same region shows peripheral hyperemia, consistent with synovitis. E. Coronal short tau inversion recovery (STIR) MR image shows edema in the soft tissues around the SCJ, with superior high signal abscess (short arrow). F. Coronal reformatted CT image with algorithm shows widening of the SCJ with erosions (short arrows)

Fig. 9.

Osteomyelitis with subperiosteal abscess. Artist’s graphic of osteomyelitis and its associated findings. Osteomyelitis involving the cortex and medullary cavity of the long bone of a skeletally immature patient. Extracortical extension of infection into the subperiosteal space creating a subperiosteal abscess (green)

Fig. 10.

Subperiosteal abscess and osteomyelitis. 2-year-old male with 2-day history of ankle pain and inability to bear weight. A. Transverse grayscale US image at the distal fibula near the ankle joint demonstrates a heterogeneous complex subperiosteal fluid collection (arrow) adjacent to the distal fibular cortex at the level of the metaphysis. B. Coronal STIR, C. axial T1-weighted, and D. post-contrast axial T1-weighted fat-saturated MR images of the left leg demonstrate a heterogeneous hyperintense subperiosteal collection (arrow) with an internal locule of fat, which is hypointense on post-contrast images (dashed arrow) centered at the distal fibular metaphysis. Edema signal within the distal fibular metaphysis and epiphysis is noted, with corresponding T1 marrow replacement. Findings are consistent with distal fibula osteomyelitis. Note the enhancing edema in the adjacent subcutaneous fat along the adjacent superficial and deep fascial planes and involving the distal leg musculature, consistent with associated soft tissue infection

Fig. 11.

Chronic osteomyelitis. Artist’s graphic of chronic osteomyelitis and its associated findings. Chronic osteomyelitis with a defect in the cortex (cloaca, with escaping pus, green), with surrounding new bone formation (involucrum, striped light brown), and a fragment of necrotic bone (sequestrum, black fragment) within the medullary cavity

Fig. 12.

Chronic osteomyelitis. 30-year-old male previously treated for osteomyelitis involving ununited left femur fracture with multiple prior debridements and hardware removal. The patient did not complete full course of antibiotic therapy. He presented with new pain following trauma and was found to have acute on chronic osteomyelitis. A. and B. Long-axis grayscale US images show a heterogeneous hypoechoic fluid collection (arrows) adjacent to the irregular cortex (dashed arrows) from the ununited fracture. C. Transverse grayscale US image shows communication of the fluid collection to the skin surface (short arrow). D. and E. Axial contrast-enhanced CT images in soft tissue windows in the same region show chronic ununited fracture fragments (dashed arrows) as well as the peripherally enhancing fluid collection (arrows), extending from the skin surface into the fracture cavity. Note the gas in the abscess. F. Coronal reformatted CT image with bone algorithm shows sclerotic bone (dashed arrows) in the region of the fracture, consistent with chronic osteomyelitis. Note the tracts from prior hardware (short arrows)