(Radiographics. 1999;19:1585-1592.)
© RSNA, 1999
Musculoskeletal Infections: US Manifestations1
Nathalie J. Bureau, MD, FRCPC,
Rethy K. Chhem, MD, PhD, FRCPC and
Étienne Cardinal, MD, FRCPC
1 From the Department of Radiology, Hôpital Saint-Luc, Centre Hospitalier de l'Université de Montréal, 1058 rue Saint-Denis, Montreal, Quebec, Canada H2X 3J4 (N.J.B., E.C.); and the Department of Diagnostic Radiology, National University Hospital of Singapore (R.K.C.). Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received February 8, 1999; revision requested March 19; final revision received June 17; accepted June 21. Address reprint requests to N.J.B.
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Abstract
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One of the most important prognostic factors in patients with musculoskeletal infections is the delay in establishing therapy. Early diagnosis of septic arthritis requires analysis of joint fluid. Ultrasonography (US) is a rapid, portable, sensitive technique for confirming the presence of joint effusions. The study can be easily repeated for follow-up of lesions. US allows real-time guidance of fluid aspiration and can reduce the risk of contaminating other anatomic compartments, especially in the hands, wrists, and feet. Radiography provides complementary information and should be performed in conjunction with US. US is the imaging modality of choice for diagnosis of superficial abscesses. Dynamic compression with the US probe and color Doppler imaging can facilitate detection of superficial abscesses. US may help in the early diagnosis of osteomyelitis by demonstrating subperiosteal or juxtacortical fluid collections and by providing guidance for aspiration of these collections. Evaluation of osseous involvement requires additional imaging; a US examination with normal results does not allow exclusion of bone infection. US is not degraded by metallic artifact and may be useful in cases of osteomyelitis complicating metallic fixation in an extremity. After initial radiography, US can play an important role in the management of musculoskeletal infections.
Index Terms: Arthritis, septic, 40.20 • Bones, infection, 40.20 • Bones, US, 40.1298 • Muscles, infection, 40.20 • Muscles, US, 40.1298
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INTRODUCTION
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Musculoskeletal infections are common in clinical practice. The diagnosis is not always evident and is often part of a differential diagnosis along with a number of other diseases. Hence, a painful, swollen extremity may result from cellulitis, ruptured synovial cyst or tendon, thrombophlebitis, septic or nonseptic arthritis, subcutaneous abscess, muscle infarction, or other conditions.
Ultrasonography (US) can play an important role in differentiating these entities. US is a low-cost, widely available, nonionizing imaging technique that can be easily used in different clinical settings including the emergency department, radiology department, and intensive care unit. The study can be easily repeated for follow-up of lesions. US should be used in conjunction with radiography because the information obtained with each modality is complementary (1). Delay in establishing the diagnosis is an important prognostic factor in different musculoskeletal infections (2–5). US is very sensitive in the detection of fluid collections and allows real-time guidance for immediate needle aspiration. Hence, US may be useful in obtaining specimens for microbiologic examination to confirm the diagnosis of infection.
In this article, we review the appearances of musculoskeletal infections at US and describe potential applications of US in the management of musculoskeletal infections. The specific entities discussed are cellulitis, septic bursitis, infectious tenosynovitis, abscess, pyomyositis, septic arthritis, and osteomyelitis.
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CELLULITIS
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Cellulitis is an acute infection of the skin involving the subcutaneous tissues (6). The patient presents with a painful, swollen, erythematous extremity that may be associated with fever, and the diagnosis is usually established on clinical grounds. The microorganisms most frequently implicated are Staphylococcus aureus and Streptococcus pyogenes. US demonstrates diffuse thickening and increased echogenicity of the skin and subcutaneous tissues (Fig 1). A cobblestone appearance may be seen with anechoic strands randomly traversing the subcutaneous tissues (7). These strands are the result of an inflammatory exudative dissection in the tissue spaces (8). The US appearance of cellulitis is nonspecific and can be indistinguishable from that of other causes of soft-tissue edema. The role of US in diagnosis of cellulitis is limited because the diagnosis is usually established on clinical grounds. When unusual causative microorganisms are suspected because of a lack of response to antimicrobial therapy, US may provide guidance for aspiration of the inflammatory exudate dissecting the subcutaneous tissues. Complicating abscesses are often diagnosed clinically by means of their tendency to demonstrate fluctuation at palpation. US is useful in detection of clinically occult fluid collections and provides guidance for diagnostic aspiration and therapeutic drainage (7).
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Figure 1. Cellulitis of the right leg in an 80-year-old woman. Extended-field-of-view longitudinal US scan shows diffuse thickening of subcutaneous tissues with a predominantly echogenic texture (arrows) and hypoechoic strands (arrowhead). The presence of an abscess was excluded with US.
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SEPTIC BURSITIS
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Septic bursitis most frequently involves the olecranon or prepatellar bursa (9). The most common pathogen is S aureus. US demonstrates a fluid collection in the expected location of a bursa (Fig 2). The walls of the collection may or may not be thickened, and there may be debris or septa within the collection. Color Doppler US may show hyperemia in the walls of the collection. Like other imaging modalities, US does not allow reliable differentiation between an infected bursa and posttraumatic bursitis or bursitis characterized by inflammation alone. Aspiration is required for this distinction and can be easily performed with US guidance.
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Figure 2. Septic bursitis of the olecranon bursa. Extended-field-of-view longitudinal US scan of the posterior elbow shows a heterogeneous, hypoechoic fluid collection (arrows) adjacent to the olecranon (O) and triceps tendon (T). S aureus was cultured from fluid obtained with US-guided aspiration.
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INFECTIOUS TENOSYNOVITIS
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Acute suppurative tenosynovitis is caused predominantly by S aureus or S pyogenes and most frequently involves the tendon sheaths of the digital flexor muscles (9). It is usually the result of a penetrating injury (human or animal bite, puncture wound) and may be complicated by the presence of foreign bodies. Early diagnosis and treatment are crucial to prevent devastating complications such as tendon necrosis and contamination of other palmar spaces or adjacent joints (4). US shows accumulation of fluid (pus) within the tendon sheath (Fig 3). Tendons may appear enlarged when compared with the contralateral ones, and Doppler studies may reveal hyperemia. Noninfectious tenosynovitis may have a similar appearance at US, and the distinction usually requires fluid analysis. US may help exclude other diagnoses such as septic arthritis or digital cellulitis. US may also be useful in detecting foreign bodies and guiding their removal.
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Figure 3a. Acute bacterial tenosynovitis. C = capitate, L = lunate, M = third metacarpal, R = radius. (a) Longitudinal US scan at the level of the capitate-lunate joint shows a small effusion in the extensor tendon sheath (arrow). US-guided aspiration yielded 0.5 mL of fluid. (b) Longitudinal US scan shows injection of 2 mL of normal saline solution (arrow) for lavage, of which 1.5 mL was aspirated and sent for culture. S aureus was cultured from the aspirate.
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Figure 3b. Acute bacterial tenosynovitis. C = capitate, L = lunate, M = third metacarpal, R = radius. (a) Longitudinal US scan at the level of the capitate-lunate joint shows a small effusion in the extensor tendon sheath (arrow). US-guided aspiration yielded 0.5 mL of fluid. (b) Longitudinal US scan shows injection of 2 mL of normal saline solution (arrow) for lavage, of which 1.5 mL was aspirated and sent for culture. S aureus was cultured from the aspirate.
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Mycobacterial or fungal infections usually cause subacute or chronic proliferative tenosynovitis. US shows thickening of the synovial membrane as hypoechoic or isoechoic noncompressible tissue surrounding the tendon, and diffuse hyperemia is seen at Doppler US. US demonstrates fluid within the tendon sheath, which is usually scarce in cases of chronic tenosynovitis. Involved tendons may also appear enlarged when compared with the normal contralateral tendons (4). In cases of chronic infections, US may provide guidance for lavage of the tendon sheath or biopsy of the synovial membrane.
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ABSCESS
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Abscesses may have different features at US (7). The lesion may appear as an anechoic or diffusely hypoechoic mass with increased through transmission or may be hyperechoic or isoechoic relative to surrounding tissues and lack mass effect (10). The margins may be well circumscribed or blend in with the surrounding tissues. Sometimes, an echogenic rim is seen. Septa may be present, as well as internal echoes (Fig 4), which represent debris or gas. Color Doppler imaging may be used to demonstrate hyperemia at the periphery of the mass and absence of flow in the center (Fig 5). Dynamic evaluation of the soft-tissue area by palpation or gentle compression with the US probe is useful to reveal motion of the liquefied purulent material in cases of isoechoic or hyperechoic abscesses. Although US plays a major role in the detection and management of superficial abscesses (7), deeper fluid collections, particularly in the lumbar and pelvic regions, usually require magnetic resonance (MR) imaging or computed tomography (CT) for diagnosis, determination of location and extent, and percutaneous management. MR imaging and CT also provide detailed information regarding osseous involvement, which would not be available with US.
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Figure 4a. Abscess of the right forearm in a 19-year-old intravenous drug abuser. (a) Longitudinal US scan shows a large subcutaneous fluid collection filled with echogenic material. (b) Longitudinal US scan shows fluid aspiration with a 16-gauge needle (arrow). The aspiration yielded 30 mL of purulent material and almost emptied the collection. (c) Longitudinal US scan obtained after repeat aspiration shows complete evacuation of the abscess. The patient was treated with intravenous antibiotics and did not require surgical drainage.
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Figure 4b. Abscess of the right forearm in a 19-year-old intravenous drug abuser. (a) Longitudinal US scan shows a large subcutaneous fluid collection filled with echogenic material. (b) Longitudinal US scan shows fluid aspiration with a 16-gauge needle (arrow). The aspiration yielded 30 mL of purulent material and almost emptied the collection. (c) Longitudinal US scan obtained after repeat aspiration shows complete evacuation of the abscess. The patient was treated with intravenous antibiotics and did not require surgical drainage.
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Figure 4c. Abscess of the right forearm in a 19-year-old intravenous drug abuser. (a) Longitudinal US scan shows a large subcutaneous fluid collection filled with echogenic material. (b) Longitudinal US scan shows fluid aspiration with a 16-gauge needle (arrow). The aspiration yielded 30 mL of purulent material and almost emptied the collection. (c) Longitudinal US scan obtained after repeat aspiration shows complete evacuation of the abscess. The patient was treated with intravenous antibiotics and did not require surgical drainage.
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Figure 5. Abscess of the right forearm in a 25-year-old intravenous drug abuser. Transverse color Doppler US scan shows diffuse hyperemia surrounding a multiloculated, hypoechoic fluid collection.
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PYOMYOSITIS
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Pyomyositis consists of a primary muscle abscess and is prevalent in tropical countries, in immunocompromised patients, and in intravenous drug abusers who traumatically contaminate their muscles with foreign material (1). The clinical presentation is often nonspecific with muscle aches and cramps and a deep induration and may at first suggest an intramuscular neoplasm. The causative agent is S aureus in over 90% of cases (11). Two US appearances have been reported and correspond to the two stages of the disease (12). The first stage consists of a phlegmon, which is characterized by localized muscle edema and appears as a nonspecific, hypoechoic, ill-defined area within one or more muscles. Later in the course of the disease, an intramuscular fluid collection corresponding to a formed abscess is seen (Fig 6), which usually requires drainage for complete resolution.
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Figure 6. Pyomyositis in a patient infected with human immunodeficiency virus. Longitudinal US scan shows a large, hypoechoic fluid collection (arrow) within the distal aspect of the gluteus medius muscle. S aureus was cultured from fluid obtained with US-guided aspiration.
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US is very sensitive in the detection of clinically occult fluid collections and may be used to guide aspiration. In most cases, aspiration is mandatory to differentiate an abscess or pyomyositis from other types of fluid collections, such as liquefied hematoma, seroma, and necrotic or cystic tumors.
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SEPTIC ARTHRITIS
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Bacterial arthritis is the most rapidly destructive joint disease and results in irreversible loss of joint function in 25%–50% of patients (2). It is usually the result of hematogenous seeding of a joint during bacteremia (13). S aureus is the primary cause of bacterial arthritis. Definitive diagnosis requires identification of the causative agent in the synovial fluid with a Gram stain or culture. US is very sensitive in detection of a joint effusion and may be particularly helpful in the hip, wrist, or shoulder (Fig 7), where physical examination is less reliable and radiographs are often noncontributory in the acute setting (1). A small joint effusion may be masked by excessive compression with the transducer (8). US does not allow differentiation of a noninfected joint effusion from septic arthritis; this distinction usually requires fluid analysis. US is an excellent imaging modality for guidance of arthrocentesis and may reduce the risk of contamination of other anatomic compartments, especially in the hand, wrist, or foot.
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Figure 7a. Acute septic arthritis in a patient infected with human immunodeficiency virus. C = capitate, L = lunate, M = third metacarpal, R = radius, T = extensor tendon. (a) Midsagittal US scan of the wrist shows a fluid collection at the capitate-lunate joint (arrow). (b) Midsagittal US scan shows needle aspiration (arrows).
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Figure 7b. Acute septic arthritis in a patient infected with human immunodeficiency virus. C = capitate, L = lunate, M = third metacarpal, R = radius, T = extensor tendon. (a) Midsagittal US scan of the wrist shows a fluid collection at the capitate-lunate joint (arrow). (b) Midsagittal US scan shows needle aspiration (arrows).
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The presence of a joint prosthesis is an important risk factor in septic arthritis (2). It is often impossible to differentiate mechanical loosening of a prosthesis from septic loosening solely on the basis of the clinical presentation and radiographic findings. Fluoroscopically guided aspiration of the prosthetic joint is often required to establish a definite diagnosis of periprosthetic infection. US allows evaluation of periarticular soft tissues and is very sensitive in detection of joint fluid, although it does not allow differentiation between infection and joint effusion resulting from mechanical failure of the prosthesis or other causes. In a painful prosthetic joint, demonstration of a large joint effusion (mean bone-to-capsule distance, 10.2 mm) associated with extraarticular fluid collections at US has been shown to be highly suggestive of an infection; in this situation, US should be followed by arthrocentesis (14).
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OSTEOMYELITIS
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Acute hematogenous osteomyelitis affects mostly young children and elderly patients. S aureus is the most frequently found offending organism in the former group, whereas gram-negative rod-shaped bacteria are frequently found in the elderly (5). MR imaging is the modality of choice in suspected cases of osteomyelitis because it can demonstrate osseous changes as well as the associated soft-tissue abnormalities. Radiography, CT, and nuclear medicine are also valuable in diagnosis of acute osteomyelitis. The earliest sign of osteomyelitis at US is nonspecific soft-tissue swelling adjacent to the affected bone (15). In the appropriate clinical settings, demonstration of a fluid collection immediately adjacent to the involved bone in adults (Fig 8) or a subperiosteal fluid collection in children and younger patients (Fig 9) at US is highly suggestive of osteomyelitis (16). Although a diagnosis of osteomyelitis cannot be established on the basis of demonstration of such fluid collections at US, the main role of US lies in the performance of immediate US-guided fluid aspiration to obtain specimens for microbiologic examination.
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Figure 8. Osteomyelitis complicating a fracture of the distal tibia in a 35-year-old patient. Transverse US scan of the anterior aspect of the distal tibia shows a hypoechoic fluid collection with internal debris (arrow) adjacent to a cortical disruption (arrowheads). With compression, fluid was seen moving in and out of the bone. US-guided needle aspiration of the abscess yielded a diagnosis of S aureus infection.
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Figure 9. Acute osteomyelitis in a neonate. Longitudinal US scan shows a hypoechoic subperiosteal abscess (arrow) of the proximal clavicle (C) adjacent to the sternoclavicular joint. S = sternum.
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Unlike MR imaging and CT, US is not degraded by metallic artifact and may be particularly useful in cases of osteomyelitis complicating metallic fixation in an extremity. US may demonstrate loosening of metallic hardware and fluid collections or sinus tracts in the soft tissues of the involved extremity (Fig 10). If a fluid collection is identified, it should be evacuated with US guidance and the specimens sent for microbiologic examination.
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Figure 10a. Osteomyelitis complicating metallic fixation of the right femur in a 35-year-old man. (a) Extended-field-of-view longitudinal US scan of the lateral aspect of the femur shows a hypoechoic fluid collection (straight solid arrow) adjacent to a metallic plate (curved arrow) and the underlying cortex of the distal femoral diaphysis (arrowhead). Loosened screws are also identified (open arrows). (b) Radiograph obtained within 5 days of the US examination shows metallic fixation of a comminuted fracture of the distal femoral diaphysis. Lucent areas surrounding some of the screws (arrows) are suggestive of loosening. S aureus was cultured from fluid obtained with US-guided aspiration of the hypoechoic collection, and osteomyelitis of the femur with loosening of the metallic hardware was confirmed at surgery.
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Figure 10b. Osteomyelitis complicating metallic fixation of the right femur in a 35-year-old man. (a) Extended-field-of-view longitudinal US scan of the lateral aspect of the femur shows a hypoechoic fluid collection (straight solid arrow) adjacent to a metallic plate (curved arrow) and the underlying cortex of the distal femoral diaphysis (arrowhead). Loosened screws are also identified (open arrows). (b) Radiograph obtained within 5 days of the US examination shows metallic fixation of a comminuted fracture of the distal femoral diaphysis. Lucent areas surrounding some of the screws (arrows) are suggestive of loosening. S aureus was cultured from fluid obtained with US-guided aspiration of the hypoechoic collection, and osteomyelitis of the femur with loosening of the metallic hardware was confirmed at surgery.
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A US examination with normal results does not allow exclusion of bone infection; correlation with clinical findings, laboratory data, and results of other imaging modalities (MR imaging, nuclear medicine, CT) is always indicated. In cases of well-established osteomyelitis or to determine the extent of intramedullary involvement and the presence of an intraosseous abscess, MR imaging is the imaging modality of choice. Although sequestra can be detected with MR imaging, they are probably best evaluated with CT, particularly when small. MR imaging is also indicated in cases of infection of the spine or pelvis (1).
Reactivation of chronic osteomyelitis is a common clinical problem that is often difficult to diagnose. In this situation, US may be useful in demonstrating soft-tissue abscesses and sinus tracts, which are characteristic of active osteomyelitis. For a more complete evaluation of the osseous changes, MR imaging is the modality of choice.
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CONCLUSIONS
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An important determinant of outcome in patients with musculoskeletal infections is the delay in establishing therapy. US is a rapid, nonionizing, portable, sensitive imaging modality for confirming the presence of soft-tissue fluid collections and joint effusions. The study can be easily repeated for follow-up of lesions. US allows precise, real-time guidance of fine-needle aspiration and can reduce the risk of contaminating other anatomic compartments, especially in the hands, wrists, and feet. US should be performed in conjunction with radiography because these modalities provide complementary information. Because US is not degraded by metallic artifact, it may be useful in cases of osteomyelitis complicating metallic fixation in an extremity. Along with other imaging techniques, US is a valuable modality for evaluation and earlier diagnosis of musculoskeletal infection.
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References
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Abstract
INTRODUCTION
