Infectious Arthropathies and Related Diseases

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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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).

Fig. 5.1

Radiographs of the index finger of the right hand of a 6-year-old female who sustained an open fracture of the middle phalanx 6 months before, presenting spontaneous pus drainage from a fistulous tract and marked soft-tissue swelling. There are signs of chronic osteomyelitis of the middle phalanx, with extensive osteolysis and bone sequestra in the medullary cavity and in the distal interphalangeal joint. Destruction of corresponding joint surfaces due to PA is also evident, with deviation of the distal phalanx

Fig. 5.2

A 9-year-old female with acute osteomyelitis of the right distal femur and secondary contamination of the joint cavity of the knee. There are no osseous abnormalities on radiographs, although swelling of periarticular soft tissues can be seen, mostly suprapatellar and more evident on the lateral view. There is also loss of definition of periarticular fat planes and subtle widening of the joint space. Nonetheless, these are nonspecific radiographic findings

Fig. 5.3

Anteroposterior view of the right hip of a child with PA whose onset occurred less than 1 month earlier. There is osteoporosis of the proximal femur and narrowing of the coxofemoral joint space, as well as irregularity of the acetabulum. Such findings represent advanced cartilaginous destruction

Fig. 5.4

Radiograph of the proximal portion of the right arm of a young child with bacterial osteomyelitis and secondary contamination of the shoulder joint. There are lytic lesions associated with cortical destruction in the proximal metaphysis, as well as marked swelling of periarticular soft tissues and widening of the proximal humeral physis

Fig. 5.5

Pyogenic arthritis of the left hip with poor evolution. There is marked bone destruction, with almost complete resorption of the left proximal femoral epiphysis and bone remodeling of the corresponding acetabulum; acetabular protrusion is also evident, with striking acetabular incongruity

Fig. 5.6

Radiographs of the left elbow (first image) and of the right shoulder (second image) of a child with multifocal bacterial osteomyelitis and secondary articular contamination. There is metaphyseal osteomyelitis in the left distal and in the right proximal humeri, with permeative osteolysis, periosteal reaction, soft-tissue swelling, and epiphyseal involvement, the latter more evident in the shoulder. Osteomyelitis of the right femur and pyogenic arthritis of the homolateral hip were also present (not shown)

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.

Fig. 5.7

US of the hips of two distinct children, both with pyogenic arthritis. The upper images show longitudinal scans of the left hip joint demonstrating joint effusion associated with irregular synovial thickening. In the lower row, there is a heterogeneous material filling the joint cavity of the right hip, representing synovial thickening and joint effusion with suspended debris. The proximal epiphysis of the right femur is irregular and of reduced size, notably if compared to the contralateral one (Courtesy of Dr. Telma Sakuno, Hospital Universitario da UFSC, Florianopolis, Brazil)

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).

Fig. 5.8

Coronal STIR image (upper-left image), transverse T1-WI (upper-right image), and post-contrast fat sat T1-WI in the coronal and sagittal planes (lower images) of the hips and thighs of a 9-year-old female with right-sided septic coxofemoral arthritis. MRI is very suitable to demonstrate joint effusion in the right hip and bone marrow edema pattern in the ipsilateral femur, which extends far beyond the subchondral bone, reaching the mid-diaphysis, indicative of associated osteomyelitis. There is post-gadolinium enhancement of the edematous bone and of the proximal quadriceps

Fig. 5.9

Coronal T1-WI and fat sat T2-WI (upper row) of the left hip of a 12-year-old adolescent with PA reveal joint effusion and cartilaginous thinning, as well as edematous changes of the soft tissues of the proximal thigh and of the subchondral bone marrow of the acetabulum. Transverse post-contrast fat sat T1-WI (lower row) disclose intense enhancement of the thickened synovium and extensive destruction of the anterosuperior acetabulum, with cortical discontinuity and infiltration of the adjacent soft tissues; contrast enhancement is also seen in the edematous tissues. This appearance may resemble those seen in aggressive bone tumors, like Ewing’s sarcoma

Fig. 5.10

Transverse post-gadolinium fat sat T1-WI of the left hip of a 13-year-old male with PA. Heterogeneous joint effusion, marked synovial thickening, and infected collections in the soft tissues of the proximal thigh can be seen, with post-contrast enhancement. The femoral head is deformed, and its appearance is suggestive of slippage of the proximal femoral epiphysis as a complication of septic arthritis

Fig. 5.11

Female adolescent with chronic, low-grade infection in the right forefoot. In (a) and (b), PET-CT demonstrates intense enhancement adjacent to the third metatarsophalangeal joint, initially interpreted as an infection of the periarticular soft tissues with secondary involvement of the contiguous bone. MRI performed 3 months later (coronal STIR image c and sagittal post-gadolinium fat sat T1-WI d) demonstrates that the infection was primarily articular, with joint effusion, synovitis, and erosions in the head of the third metatarsal, as well as edematous changes of the bone and of the adjacent soft tissues

Fig. 5.12

MRI of the left ankle of a 12-year-old male with PA. T1-WI (upper-left image) and STIR image (upper-right image) evidence tibiotalar joint effusion, metaphyseal osteomyelitis of the distal tibia, and a small juxtaphyseal intraosseous abscess, anteriorly situated. Post-gadolinium fat sat T1-WI (lower images) demonstrate the intraosseous abscess with its hypointense content and peripheral enhancement draining to the joint cavity through a cortical break. Synovial enhancement is also present, distinguishing the thickened synovium from the joint effusion

Fig. 5.13

MRI of the same patient of Fig. 5.2, performed on the same day (upper row, coronal fat sat T2-WI and sagittal T2-WI; lower row, coronal and sagittal post-gadolinium fat sat T1-WI). There is diffuse infiltration of the bone marrow of the distal femur, with post-gadolinium enhancement, as well as subperiosteal abscesses, extensive edema of the periarticular soft tissues, and contamination of the joint cavity, with joint effusion and synovitis

Fig. 5.14

MRI of the left hip of a child with PA (upper-left image, coronal fat sat T2-WI; remaining images, transverse post-gadolinium fat sat T1-WI). There is a large and heterogeneous joint effusion on fat sat T2-WI, with extensive bone marrow edema pattern in the proximal femur and acetabulum and edematous changes in the adjacent soft tissues. Post-gadolinium images display a large, complex soft-tissue abscess surrounding the hip and extending to nearby bursae, with thick and irregular enhancement, as well as adjacent myositis

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).

Fig. 5.15

A 9-year-old male with PA of the right hip and septicemia. CT scan reveals joint effusion, marked swelling of periarticular soft tissues, densification of the subcutaneous fat, and juxta-articular abscesses. Gas bubbles can be seen in the collections, indicative of the bacterial nature of the infection (Courtesy of Dr. Arthemizio Rocha, Hospital Santa Marta, Taguatinga, Brazil)

Fig. 5.16

CT scan of the left knee of a 2-year-old child with PA. Transverse images with bone window (left images) and soft-tissue window (central images) and volume-rendered reconstructions (right images) reveal an extensive lytic lesion of the proximal metadiaphysis of the tibia, which extends across the growth plate to the proximal epiphysis. There is a wide area of discontinuity of the posterior cortex, with bone sequestra in the medullary cavity, joint effusion, and a complex abscess in the posterior soft tissues of the proximal leg

Fig. 5.17

Neonatal septic arthritis of the right shoulder. In the first image, radiograph of the affected joint reveals metaphyseal osteomyelitis of the proximal humerus, with a large intraosseous abscess and extensive periosteal reaction. In the following images, transverse CT sections corroborate the above-described findings, disclosing bone destruction and cortical rupture. There is contamination of the joint cavity, with a large joint effusion and abscesses in the adjacent soft tissues (Courtesy of Dr. Arthemizio Rocha, Hospital Santa Marta, Taguatinga, Brazil)

Fig. 5.18

A 17-year-old patient with PA of the right facet joint of L3-L4. In (a and b), transverse CT images evidence blurring and hypodensity of the soft tissues adjacent to the infected joint, without evidence of bone erosions. Transverse T2-WI (c) and post-gadolinium fat sat T1-WI (d) show fluid in the joint space extending deep to the homolateral ligamentum flavum, with discontinuity of the latter. Edema of the paravertebral soft tissues is also seen, as well as an epidural phlegmon at right, with contralateral displacement of the thecal sac. The inflamed tissues exhibit diffuse post-contrast enhancement

Fig. 5.19

Transverse CT images (a and b) and sagittal reformatted images (c) of the right ankle of a 7-year-old female. There are destructive lesions in the talus and in the navicular in the image with bone window settings (a), with sequestra in the affected bones and in the adjacent soft tissues. Diffuse edema of the periarticular soft tissues is evident in the images with soft-tissue window settings (b and c), as well as a large tibiotalar effusion. Post-contrast enhancement is seen in the thickened synovium and in the inflamed soft tissues

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.

Fig. 5.20

Late-stage sequelae of neonatal PA of the right hip. Radiographs show deformity of the proximal femur, delayed epiphyseal ossification, and remodeling of the acetabulum

Fig. 5.21

Radiographs of the right elbow of a 7-year-old male with late-stage sequelae of PA (a). Joint deformity, bone remodeling, and articular incongruity can be seen, with radial shortening due to premature physeal closure. In (b), pelvic radiograph of a patient with ­late-stage sequelae of PA of the right hip shows coxofemoral dislocation and superior migration of the homolateral femur. The femoral head is articulated with the iliac bone and the homolateral acetabulum is poorly developed, indicators of a disease of long evolution

Fig. 5.22

Radiographs of the left shoulder of a young child with PA. The image at left, taken during the active stage, shows metaphyseal lucencies and cortical irregularity due to osteomyelitis. The radiograph at right, taken after healing has occurred, reveals early closure of the proximal humeral physis and residual deformity

Fig. 5.23

Late-stage sequelae of PA of the right hip, probably complicated with avascular necrosis. Scanogram demonstrates shortening of the right lower limb due to deformity of the homolateral hip, characterized by a mushroom-shaped femoral head, a shortened and broadened femoral neck, flattening of the acetabulum, and early closure of the growth plates, with joint incongruity

Fig. 5.24

Sagittal post-gadolinium fat sat T1-WI of the right hip of a 3-year-old child with PA. In addition to the abnormalities usually found in septic joints, there is marked decrease in the enhancement of the proximal femoral epiphysis, as well as subtle loss of its sphericity, related to avascular necrosis

Fig. 5.25

Late-stage sequelae of PA of the right wrist. There is evidence of proximal row carpectomy at right, with fusion of the bones of the distal row and the adjacent metacarpals, as well as residual deformity and atrophy of the homolateral bones

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.

Fig. 5.26

Tuberculous arthritis of the left knee in a child. Phemister triad is present, with periarticular osteoporosis, predominantly peripheral erosions, and relative preservation of the joint space. Metaphyseal and epiphyseal lytic lesions are seen in the distal femur, representing active osteomyelitis (transphyseal spread)

Fig. 5.27

Increased size and osteoporosis of the epiphyses are seen in this anteroposterior view of the right knee of a child with tuberculous arthritis, with preservation of the joint space and swelling of the periarticular soft tissues

Fig. 5.28

Radiographs of the right knee disclosing posterolateral lytic lesions involving the metaphysis and the epiphysis of the distal femur, representing transphyseal dissemination of tuberculous osteomyelitis and concomitant tuberculous arthritis. There is discontinuity of the posterior cortex and diffuse swelling of the periarticular soft tissues

Fig. 5.29

Radiographs of the left knee of a 2-year-old child. There is a well-delimited lytic lesion adjacent to the anterolateral physis in the proximal tibial metaphysis. Tuberculous osteomyelitis

Fig. 5.30

Lytic lesion in the medial portion of the distal epiphysis of the right femur, ill-defined, associated with diffuse swelling of periarticular soft tissues, representing tuberculous osteomyelitis and concomitant arthritis

Fig. 5.31

Lateral views of both knees of a 5-year-old child with left-sided tuberculous arthritis. The left epiphyses are osteopenic and show increased size when compared to the contralateral ones; ipsilateral soft-tissue swelling is also evident

Fig. 5.32

Tuberculous arthritis of the left knee. There is increased size of the epiphyses and periarticular osteoporosis in the affected knee, with widening of the intercondylar notch, similar to those found in hemophiliac patients. Subtle narrowing of the joint space is already present

Fig. 5.33

Radiograph of the hips of a 15-year-old female with left-sided tuberculous arthritis. There is narrowing of the left joint space, with irregular joint surfaces and regional osteoporosis, as well as premature physeal closure in the ipsilateral femur

Fig. 5.34

A 6-year-old male with chronic arthritis in the left knee and a skin fistula. Radiographs disclose advanced destructive arthritis, marked osteoporosis, altered shape of the epiphyses, large bone erosions (mainly posterior, along the distal femur and the proximal tibia), and swelling of the regional soft tissues. Osteoarticular tuberculosis

Fig. 5.35

Pelvic radiographs of two distinct children, both with advanced tuberculous arthritis of the left hip joints. Bone destruction with marked periarticular osteoporosis and irregularity of the joint surfaces can be seen, as well as hypoplasia of the left iliac bones and of the left proximal femora

Fig. 5.36

A 3-year-old child with severe malnutrition and palpable nodules in the frontal region. Anteroposterior skull radiograph shows two paramedian lytic lesions in the frontal bone. A small button sequestrum is seen in the left-sided lesion. Tuberculous osteomyelitis

Fig. 5.37

Multifocal tuberculous osteomyelitis affecting the right knee and the left elbow. There is lamellar periosteal reaction in the humerus and solid periosteal apposition in the tibia and in the bones of the ­forearm, with mild bone expansion. Periarticular osteoporosis is also present, with lucent lesions in the distal humerus and in the proximal radius and tibia, as well as nodules in the dorsal soft tissues of the forearm

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.

Fig. 5.38

Sagittal post-gadolinium T1-WI (a) and coronal STIR image (b) reveal cartilaginous loss (arrow in a), suprapatellar bursitis (open arrow in a), enlarged popliteal lymph nodes (arrowhead in a), and anatomical disorganization (arrow in b) in a 3-year-old boy with knee tuberculosis (Reprinted with permission from Prasad et al. (2012))

Fig. 5.39

Transverse CT images of the right knee of a child with tuberculous arthritis. Synovial thickening and a large joint effusion are present, as well as a lytic lesion containing a bone sequestrum in the distal femur

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).

Fig. 5.40

In (a), US of the left knee in the sagittal plane shows synovial hypertrophy (arrow) and joint effusion (open arrow) in a 10-year-old boy with tuberculous arthritis (Reprinted with permission from Prasad et al. (2012)). In (b), a longitudinal US scan of the left hip of a patient with tuberculous arthritis reveals a deeply seated, well-delimited ­heterogeneous abscess adjacent to the femoral cortex, with posterior acoustic enhancement

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.

Fig. 5.41

US of the right hip of a child with TSH. Joint effusion is quite evident as well as synovial thickening. Nevertheless, these are nonspecific findings and do not allow to infer the real nature of the arthritis. Laboratory analysis of the synovial fluid was negative for infection and arthritis subsided with conservative treatment

Fig. 5.42

T1-WI (a), STIR image (b), and post-gadolinium fat sat T1-WI (c) of a 4-year-old child with right-sided TSH. There is moderate joint effusion and synovial thickening presenting post-contrast enhancement. Erosions and bone marrow edema are notably absent. Complete recovery was achieved with conservative measures

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.

Fig. 5.43

Oblique view of the right ankle of a 12-year-old male with CRMO. Several well-delimited lytic lesions are seen in the distal metaphyses of the tibia and fibula, adjacent to the growth plates, surrounded by sclerosis and not associated with cortical rupture or periosteal reaction. Similar lytic lesions are also identified in the posterior portion of the calcaneus

Fig. 5.44

Anteroposterior view of the knees (a) and lateral views of the right femur and of the left tibia (b) of an adolescent with CRMO of long evolution. The affected bones are widened, showing cortical thickening and heterogeneous density, intertwining sclerotic and lucent zones, which are mostly metaphyseal and diaphyseal. The posterior cortex of the femur is markedly irregular. These findings are quite similar to those found in chronic infectious osteomyelitis

Fig. 5.45

Sagittal T1-WI (a) and fat sat T2-WI (b) and coronal fat sat T2-WI (c) of the right ankle of the same patient of Fig. 5.43. The lytic lesions seen on radiographs display low signal intensity on T1-WI and high signal intensity on T2-WI, with surrounding bone marrow edema and sclerotic borders. Edema of the periarticular soft tissues is also present

Key Points

• 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.

• 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.

• 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.

• 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.