Abstract
Musculoskeletal infection in children often poses a diagnostic challenge, so much greater as the younger the patient is. Although joint aspiration remains indispensable for definitive diagnosis, imaging is a valuable tool in the workup. No matter the etiologic agent, early and accurate definition of the infectious origin of the arthritis is paramount in order to minimize structural damage and to avoid complications and long-term sequelae. This chapter will emphasize the articular component of musculoskeletal infections as well as related diseases that should be considered in the differential diagnosis, such as transient synovitis and chronic recurrent multifocal osteomyelitis. Spinal infection will be addressed in Chap. 11.
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Keywords
- Bone Marrow Edema
- Joint Effusion
- Chronic Recurrent Multifocal Osteomyelitis
- Joint Aspiration
- Musculoskeletal Infection
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5.1 Introduction
Musculoskeletal infection in children often poses a diagnostic challenge, so much greater as the younger the patient is. Although joint aspiration remains indispensable for definitive diagnosis, imaging is a valuable tool in the workup. No matter the etiologic agent, early and accurate definition of the infectious origin of the arthritis is paramount in order to minimize structural damage and to avoid complications and long-term sequelae. This chapter will emphasize the articular component of musculoskeletal infections as well as related diseases that should be considered in the differential diagnosis, such as transient synovitis and chronic recurrent multifocal osteomyelitis. Spinal infection will be addressed in Chap. 11.
5.2 Pyogenic Arthritis
The term pyogenic arthritis (PA) is used to describe bacterial joint infection, which accounts for approximately 6 % of all childhood arthritides. PA is most often a monoarthritis, more frequent in males with less than 3 years of age. The joints of the lower extremities are involved in approximately 75 % of the patients, mostly the hips and the knees; ankles, elbows, and shoulders are other important sites of disease. Even though PA is usually secondary to hematogenous spread, in children younger than 18 months, the process may begin in a metaphyseal focus that disseminates via transphyseal vessels. These vessels involute in older children, in whom the growth plate acts as a barrier for dissemination. Less common ways of contamination include direct inoculation (open wounds, joint punctures, surgery) or dissemination from a contiguous focus in the adjacent soft tissues.
Time is a critical prognostic factor in PA, as delayed institution of treatment leads to irreversible joint destruction and permanent sequelae. The acute inflammatory response related to bacterial infection causes quick cartilaginous destruction, and increase in the intracapsular pressure – due to synovial hypertrophy and purulent effusion – may lead to joint dislocation and epiphyseal ischemia, mostly in septic hips. Definitive diagnosis relies on joint aspiration and analysis of the synovial fluid, which must be performed as soon as PA is suspected. Imaging plays a secondary role, given that none of the available methods is able to distinguish infectious from noninfectious arthritides and a normal imaging study does not rule out articular infection.
Radiographs must be obtained in all patients, even though this imaging modality is quite insensitive: irreversible joint damage is usually present by the time that radiographic abnormalities become evident (Fig. 5.1). Findings related to the early stages of articular infection are fairly nonspecific, including joint effusion, widening of the joint space, and soft-tissue swelling (Fig. 5.2). Narrowing of the joint space appears quickly, as well as epiphyseal osteopenia (less pronounced than that found in tuberculous arthritis) and peripheral erosions (Figs. 5.3 and 5.4). Despite the fact that the knee is the most affected joint, PA is usually more severe in the hips (Fig. 5.5). Polyarticular PA occurs in less than 10 % of all cases (Fig. 5.6).
Ultrasonography (US) is very useful for early detection of joint effusion and synovitis, being also very appropriate for image-guided joint aspiration. Common findings include joint effusion (often with suspended debris) and hyperemia of the inflamed synovium on Doppler studies, but they are not specific (Fig. 5.7). US is also able to demonstrate erosions (especially large ones) and dissemination of the infection to adjacent bursae and tendon sheaths.
Magnetic resonance imaging (MRI) is capable to identify the earliest changes related to PA, allowing for accurate assessment of disease extent and helping in preoperative planning. It is highly sensitive for joint effusion (which may be heterogeneous) and synovitis, mostly when intravenous contrast is administered, as there is enhancement of the synovium and of the inflamed tissues (Figs. 5.8, 5.9, 5.10, 5.11, 5.12, 5.13, and 5.14). Erosions and destructive changes of bone and cartilage are also clearly seen (Figs. 5.9, 5.10, 5.11, and 5.12), as well as dissemination of the infectious process to nearby bursae and tendon sheaths (Fig. 5.14). Subchondral bone marrow edema is common and not necessarily indicative of osteomyelitis, as it may be merely related to reactive (noninfectious) osteitis. Osteomyelitis is more likely if the edematous areas extend far beyond the subchondral bone, notably if there is prominent low signal intensity on T1-weighted images (T1-WI) (Figs. 5.8, 5.12, 5.13, and 5.14). Cortical discontinuity, periosteal reaction, intraosseous abscesses, and soft-tissue collections are typical of osteomyelitis (Figs. 5.9, 5.12, 5.13, and 5.14).
Computed tomography (CT) has restricted usefulness in the assessment of PA in pediatric patients (see Chap. 1). It is usually reserved for selected cases, most commonly used as an alternative (or an adjunct) to MRI for osseous assessment or to evaluate anatomically complex joints (Figs. 5.15, 5.16, 5.17, 5.18, and 5.19); the use of intravenous contrast is formally recommended. If there is associated osteomyelitis, CT is very helpful to demonstrate bone destruction and the presence of sequestra (Figs. 5.16, 5.17, and 5.19). Post-contrast enhancement is more evident in the thickened synovium (Fig. 5.19). CT is the best imaging method to detect gas bubbles in the infected tissues, which are indicative of the bacterial nature of the infection (Fig. 5.15).
Despite its high sensitivity, bone scintigraphy also plays a limited role in the assessment of articular infection (see Chap. 1). Nevertheless, because of its ability to assess the whole body in a single study, bone scintigraphy may be useful to detect “occult” sites of infection in multifocal PA. Increased uptake is generally present in the affected joint, although decreased uptake may be seen in the epiphysis if avascular necrosis ensues (which is more frequently seen in the hip). The role of PET-CT is not yet established (Fig. 5.11).
Long-term sequelae are found in up to 40 % of children with PA. Abnormal joint alignment/joint deformities (Figs. 5.20 and 5.21), premature physeal closure (Figs. 5.22 and 5.23), limb-length discrepancy (Fig. 5.23), early-onset osteoarthritis, avascular necrosis (Figs. 5.23 and 5.24), and, in advanced cases, bony ankylosis (in opposition to the fibrous ankylosis seen in tuberculous arthritis – Fig. 5.25) are among the most important complications.
5.3 Tuberculous Arthritis
Tuberculosis (TB) is an infectious condition known since ancient times. Even though several strains of mycobacteria may cause TB, Mycobacterium tuberculosis is the most important of them all. There has been a global increase in the incidence of TB in recent times, related mainly to the HIV/AIDS pandemic. Osteoarticular disease may account for up to 35 % of all cases of extrapulmonary TB, and it is noteworthy that less than half of these patients have concomitant pulmonary disease. Musculoskeletal TB is more common in children than in adults, notably in Asia and Africa, occurring in children younger than 10 years of age in up to 50 % of the cases. Tuberculous arthritis is typically monoarticular, though multifocal involvement is found in approximately 10 % of patients. Affected individuals typically present a slowly progressive monoarthritis that affects weight-bearing joints, such as the knees and the hips, even though any joint can be involved. In extraspinal TB, osteoarticular involvement is more frequent than isolated osteomyelitis, and infection of the joint cavity is usually due to transphyseal dissemination of an active tuberculous focus in the metaphysis of a long bone. This is characteristic of osteoarticular TB and uncommon in pyogenic arthritis, highlighting the importance of being acquainted with tuberculous osteomyelitis to better understand the findings of tuberculous arthritis. Only the involvement of the peripheral joints will be discussed in the following paragraphs, as spinal involvement, the most common presentation of musculoskeletal TB, will be the studied in Chap. 9.
Early-stage radiographic findings are nonspecific and include widening of the joint space and soft-tissue swelling. The classic Phemister triad of tuberculous arthritis refers to a combination of periarticular osteoporosis, peripheral bone erosions, and relative preservation of the joint space (Figs. 5.26 and 5.27). Periosteal reaction, cortical irregularity, and lytic lesions may also be present (Figs. 5.26, 5.28, 5.29, and 5.30), with minimal bone sclerosis. Chronic granulomatous synovitis leads to joint effusion, synovial thickening, and pressure erosions; nevertheless, as tuberculous arthritis lacks the proteolytic enzymes found in pyogenic arthritis, progression of joint space narrowing is slower in the former. Hyperemia is the cause of marked juxta-articular osteoporosis, epiphyseal overgrowth (Figs. 5.7, 5.31, and 5.32), accelerated bone maturation with early/asymmetric physeal closure (Fig. 5.33), and widening of the intercondylar notch of the knees (Fig. 5.32). There is progressive destruction of the subchondral bone and extensive osteochondral damage (Figs. 5.33, 5.34, and 5.35), which may eventually lead to fibrous ankylosis. Subluxation of the femoral head is occasionally seen in tuberculous arthritis of the hip joint, just like in PA. The metaphyses of long bones (mainly the femora and the tibiae) are the most frequently affected sites in tuberculous osteomyelitis (Figs. 5.26, 5.28, and 5.29), but involvement of other sites (such as the ribs, patella, sternum, and skull) is not rare (Fig. 5.36). In children, there may be eccentric, well-delimited round, or ovoid lytic lesions, which are frequently multifocal and usually lack sclerotic borders, presenting metaphyseal expansion and periosteal reaction (Fig. 5.37); the term cystic TB (also known as osteitis cystica tuberculosa multiplex or multifocal tuberculous osteomyelitis) is often used to describe these multiple expansile metaphyseal lesions. As mentioned above, transphyseal dissemination is much more common in TB than in bacterial infections (Figs. 5.26 and 5.28); a cortical break may be present, disseminating the infection directly into the joint cavity, and sequestra are occasionally found. The term spina ventosa is used to describe a peculiar form of tuberculous osteomyelitis that is more common in children and usually affects the small bones of hands and feet (tuberculous dactylitis), characterized by osseous destruction, thickening of the overlying periosteum, and fusiform appearance of the bone. These lesions appear as cyst-like cavities on radiographs, leading to diaphyseal widening and swelling of the surrounding soft tissues.
MRI is the optimal imaging method for early demonstration of osteoarticular tuberculosis, revealing bone marrow edema adjacent to the affected joint, joint effusion (which is often heterogeneous), synovial thickening, cartilaginous damage, and bone erosions (Fig. 5.38). Administration of intravenous contrast is formally recommended, just like in pyogenic arthritis. Synovial proliferation may be hypointense or hyperintense on T2-weighted images (T2-WI), showing moderate to intense post-contrast enhancement when acutely inflamed (Fig. 5.38); nonetheless, chronically inflamed synovium may exhibit little enhancement or no enhancement at all. If compared to pyogenic arthritis, the erosive component is more prominent in tuberculous arthritis, with relatively less subchondral edema. When present, tuberculous abscesses usually display thin and smooth walls, while the walls of bacterial abscesses are most often thick and irregular; peripheral post-gadolinium enhancement is seen in both conditions. Intraosseous abscesses in TB are most often hypointense on T1-WI and hyperintense on T2-WI, also presenting post-gadolinium enhancement; the presence of hypointense areas on T2-WI usually indicates caseous transformation and is very suggestive of tuberculous osteomyelitis. CT is very useful to demonstrate bone sequestra (Fig. 5.39) and soft-tissue calcifications, the latter being typical of chronic osteoarticular TB; its advantages and drawbacks in the assessment of tuberculous arthritis are similar to those above described for pyogenic arthritis.
US is helpful in the assessment of tuberculous arthritis in children, being also able to guide joint aspirations. Just like in PA, common findings include joint effusion (which may be heterogeneous), synovial thickening/hyperemia, and soft-tissue swelling (Fig. 5.40); superficially located erosions may be seen. Soft-tissue abscesses appear as rounded/oval masses with variable echogenicity, depending on their content, presenting peripheral hyperemia on Doppler studies and posterior acoustic enhancement (Fig. 5.40).
5.4 Transient Synovitis of the Hip
Transient synovitis of the hip (TSH), also referred to as toxic synovitis, is an acute condition characterized by joint effusion and nonspecific synovial proliferation in the hip joint. It is one of the main differential diagnoses of the infectious arthritides and the most frequent cause of acute hip pain in children from 3 to 10 years of age, occurring bilaterally in up to 25 % of the patients. Its etiology is unknown and it is a diagnosis of exclusion. In most cases, prognosis is good and affected children will have complete recovery with conservative treatment. The main diagnostic challenge in TSH consists in distinguishing it from infectious arthritides, as the clinical picture and the imaging findings may be similar, but symptoms are usually more acute and severe in the latter. If left untreated, septic arthritis follows a relentless course, with abnormal laboratory tests; conversely, fever in children with TSH is usually low or absent, and there will be no clinical or laboratory evidence of systemic disease.
In TSH, radiographs are normal or show nonspecific findings, such as mild osteoporosis of the proximal femur, widening of the joint space, and obliteration of fat planes around the hip. Radiographs are especially recommended for children with less than 1 year of age and for those older than 8 years old because TSH is uncommon in these age groups, in which septic arthritis and child abuse (younger children) and slipped femoral capital epiphysis (older children) predominate. US is the first line of investigation for patients with TSH, as synovial thickening and even small joint effusions can be detected (Fig. 5.41) and image-guided joint aspiration may be performed in the same session. MRI is a second-line study in TSH, showing joint effusion and mild synovitis without significant bone marrow edema or erosive arthritis (Fig. 5.42). Other imaging studies are rarely used in patients with TSH.
5.5 Chronic Recurrent Multifocal Osteomyelitis
Chronic recurrent multifocal osteomyelitis (CRMO) is an idiopathic inflammatory disease of the bones that affects most often females from 9 to 14 years of age. CRMO is characterized by (1) multifocal and non-pyogenic bone lesions; (2) undulating course, with exacerbations and remissions; and (3) association with other inflammatory diseases. These patients present recurrent episodes of osteitis, with swelling and tenderness in the affected sites. Histological findings are compatible with chronic osteomyelitis, but there are no viable microorganisms in samples of the affected tissues, laboratory tests are nonspecific, and cultures are negative. CRMO is a diagnosis of exclusion, based on clinical data, histopathological findings, and imaging. Prognosis is usually good.
Radiographs are the first imaging study ordered for most patients. CRMO typically presents with one or more lytic, eccentrically located metaphyseal lesions surrounded by sclerosis, abutting the growth plate (Fig. 5.43). There is a distinct predilection for the long bones, and the distal metaphysis of the tibia is the most commonly involved site. The clavicles are also often affected, mainly in its medial portions, sparing the sternoclavicular joint. Pathologic fractures occur mainly in the thoracic vertebrae and may lead to scoliosis of acute onset. Recurrent episodes of osteitis lead to bone sclerosis, cortical thickening, and diaphyseal involvement (Fig. 5.44), with abnormal bone modeling and metaphyseal widening in long-standing disease. MRI may be helpful if radiographs are negative or inconclusive: active lesions are typically associated with bone marrow edema pattern, while inactive, chronic lesions exhibit predominance of sclerosis, with low signal intensity in all sequences (Fig. 5.45). Mild inflammation of the surrounding soft tissues may also be found, but abscesses are absent. Investigation of multifocal CRMO is usually performed with bone scintigraphy or whole-body MRI, aiming to detect “occult” sites of involvement.
Key Points
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PA is a rapidly evolving disease that leads to accelerated joint destruction. Radiographs are insensitive, and radiographic findings appear late in the course of the disease. Early-stage abnormalities include synovitis and joint effusion: MRI is the most sensitive imaging method, being also able to demonstrate bone marrow edema, while US is also fairly sensitive for soft-tissue abnormalities, but insensitive for bone assessment. CT and bone scintigraphy have limited usefulness.
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Radiographic findings typical of tuberculous arthritis include marked periarticular osteoporosis, indolent course, and relative preservation of the joint space. Findings similar to those found in other hyperemic arthropathies can be found, such as epiphyseal overgrowth and early closure of the growth plates. The role of the imaging methods is similar in PA and in tuberculous arthritis.
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TSH is a self-limited and nondestructive arthritis that affects the hip of children. Joint effusion and synovitis are common, while bone erosions and significant bone marrow edema are notably absent. Radiographs and US are the first line of investigation, while MRI is reserved for selected cases.
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CRMO presents lytic lesions eccentrically located in the metaphyses of tubular bones, which may be associated with abnormal bone modeling and sclerosis of the medullary bone in long-standing disease. The main role of MRI is to distinguish active from inactive lesions. Bone scintigraphy and whole-body MRI are useful to demonstrate “occult” sites of involvement, considering the multifocal nature of CRMO.
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Lopes Viana, S., Ribeiro, M.C.M., Beber Machado, B. (2013). Infectious Arthropathies and Related Diseases. In: Joint Imaging in Childhood and Adolescence. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-35876-0_5
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