Fig. 1
Three year-old boy with methicillin resistant staph aureus (MRSA) osteomyelitis. The patient presented with arm swelling, pain and fever. MRSA was found on blood culture prior to imaging. (a) Anteroposterior radiograph shows a subtle lucency within the proximal humeral metaphysis (arrow). At most, there is subtle abrnomality of diaphyseal bone. Soft tissue swelling is most prominent medially, near the elbow (arrowheads). (b) Fat-saturated sagittal T1-weighted MR image after gadolinium-based contrast (TR 628 ms, TE 15 ms) shows diffuse abnormality of the marrow space extending from proximal metaphysis to distal metaphysis. Areas of increased enhancement and areas of absent enhancement (abscess, necrosis) are seen throughout. A small superiosteal abscess is seen at the anterior aspect of the proximal humeral metaphysis (arrow). Synovial enhancement is partially seen at the elbow (arrowheads). Enhancement is seen within adjacent soft tissues
Fig. 2
Thirteen year-old girl with acute osteomyelitis of the distal femur. The girl had a preceding viral illness two weeks prior and questionable trauma to the knee area one week prior. She presented with pain worse with movement, swelling and fever. White blood count, erythrocyte sedimentation rate and C-reactive protein were all markedly elevated. Axial fat-saturated T1-weighted MR image after gadolinium-based contrast (TR 600 ms, TE 18 ms) shows a subperiosteal abscess at the posterior aspect of the distal femoral metaphysis (arrowheads). Low signal extruded fat is seen anteriorly within the subperiosteal abscess delineated by a fluid/fluid level (arrow). Periosteal and adjacent soft tissue enhancement is noted. Ant anterior, Post posterior
Seventy percent of patients with Staphylococcus aureus osteomyelitis have a complication, which include subperiosteal abscess, pyomyositis, deep venous thrombosis/septic thrombophlebitis and septic arthritis [1, 3]. MRSA often manifests as aggressive disease with complications (Fig. 1) [5, 6]. Other long term complications may include chronic osteomyelitis, bone infarction, pathologic fracture, joint destruction, growth plate fusion, deformity and leg length difference.
Spinal osteomyelitis occurs due to seeding of the metaphyseal equivalent of vertebral body endplates. In distinction, disciitis is infection of the disc space and vertebral body endplate. Imaging findings overlap. MRI is performed with spinal osteomyelistis to define the extent of disease and to exclude epidural abscess.
Pelvic osteomyelitis occurs as metaphyseal equivalents—adjacent to the sacroiliac joints, triradiate cartilage, pubic symphysis, ischiopubic synchondrosis and iliac apophyses [2, 8]. MRI is used to assess the site and extent of infection—osteomyelitis verses septic arthritis verses pyomyositis. With osteomyelitis, MRI is necessary to assess for intrapelvic extension [8].
Neonatal osteomyelitis is often indolent or masked by other medical issues. Delayed diagnosis is common. In neonates, group B streptococcus, Escherichia coli and Enterobacter are more frequent with a lesser incidence of Staphylococcus aureus than in older children [4]. More epiphyseal and joint disease also portends a worse prognosis.
Patients with sickle cell disease have a higher incidence of infection with encapsulated organisms including Salmonella due to decreased splenic function with relative immunosuppression [9]. Infection with Staphylococcus aureus is also common. Differentiation of infection from bone marrow infarction is challenging but may be suggested by high signal on fat-saturated unenhanced T1-weighted MR images [9]. There is a higher incidence of diaphyseal disease with sickle cell disease, likely related to seeding of bone marrow infarcts.
Subacute osteomylelitis is due to a less virulent organism and a partial host response which walls off the infection. On imaging, a well-defined lucent lesion (Brodie abscess) is seen in a metaphysis or metaphyseal equivalent (Fig. 3) [1, 3]. A tract may be seen to the adjacent physis and, occasionally into epiphysis. Adjacent sclerosis and non-aggressive periosteal new bone may be seen. Rarely, a similar lesion occurs within an epiphysis. Children with subacute osteomyelitis present with less severe and insidious symptoms, less fever and marginally abnormal laboratory tests.
Fig. 3
Twelve year-old girl with subacute osteomyelitis (Brodie abscess). The patient presented with ankle pain and swelling for one month, but no fever. Mortise view of the left ankle shows a well-defined ovoid lucency with faintly sclerotic margins bridging the medial distal tibial growth plate (arrows)
Chronic osteomyelitis is variably defined as osteomyelitis persisting for longer than 4–6 weeks [1, 4]. It is due to inadequately treated acute or subacute osteomyelitis. Chronic osteomyelitis is characterized by sequestra (fragments of necrotic bone within the infection), involucrum (cloaking periosteal new bone) and cloaca (draining sinus) [1]. Chronic osteomyelitis may complicate open fractures.
Septic Arthritis
Septic arthritis may occur due to hematogenous seeding of the synovium, extension of adjacent osteomyelitis or soft tissue infection, or by direct inoculation (trauma, surgery, iatrogenic) [10]. As previously noted, in infants less than 18-months old there is an increased association between osteomyelitis and septic arthritis due to transphyseal extension of infection.
Septic arthritis is most commonly due to Staphylococcus aureus [11]. Other organisms include group B streptococcus and Escherichia coli (particularly in neonates), Pseudomonas aeruginosa, Streptococcus pneumoniae, Kingella kingae (3-months to 3-years old), and Neisseria gonorrhoeae (neonates, sexually active teen). Peak presentation is 2–3 years of age. Lower extremity joints are more frequently involved than upper extremity joints. The hip and knee are most common with ankle and elbow less frequent. Symptoms are fever, localized joint pain and overlying warmth, erythema, soft tissue swelling, limp and refusal to bear weight.
ESR and WBC are increased. Blood cultures may be positive. Criteria for diagnosis on arthrocentesis are positive culture, positive gram stain or joint fluid WBC > 50,000/μL [10]. In approximately 30% of cases, an organism is not identified.
Depending on the joint involved, radiographs may show evidence of a joint effusion or adjacent soft tissue swelling, but are often normal. At the elbow, radiographs suffice to evaluate for effusion. Ultrasound is used at the hips to assess for effusion [7]. Ultrasound cannot distinguish infected for uninfected effusion from; however, abundant debris, synovial thickening or hyperemia on Doppler evaluation raises concern for infection (Fig. 4) [7]. On MR, increased signal (edema) and enhancement is seen in adjacent soft tissues and mildly increased adjacent bone marrow increase signal and enhancement may be seen. Distinction of reactive edema/enhancement from osteomyelitis may be difficult.
Fig. 4
Four year-old boy with transient synovitis of the right hip. The patient presented with a complaint of intermittent hip pain for 1 month, worsening in the last 24 h. He had some difficulty bearing weight, but full range of motion on exam. He was afebrile, with normal white blood count and C-reactive protein. Erythrocyte sedimentation rate was minimally elevated. Sagittal ultrasound images show a normal left (LT NORM) hip without effusion and a moderate him effusion on the right (RT, asterisks). Note that the effusion is most prominent anterior to the femoral neck and tapers over the femoral head. Femoral head cartilage is marked by arrowheads on each side. Arrows indicate the proximal femoral growth plates. The patient was treated conservatively without joint aspiration
The differential diagnosis for septic arthritis includes Lyme disease, viral infection, tuberculosis, Legg-Perthes disease, juvenile idiopathic arthritis (JIA), rheumatic fever and transient synovitis [10]. JIA is discussed later in this article. Rheumatic fever occurs post group A streptococcal infection with polyarthritis, carditis, erythema and subcutaneous nodules [11]. It is diagnoses with a strept test. Lyme disease is due to Borrelia burgorferi via tick bite [11]. Lyme disease simulates oligoarthritic JIA or septic arthritis. Lyme disease usually affects the knee or less commonly the hip, ankle and elbows. Patients with Lyme disease may also have myositis and lymph node enlargement.
Transient synovitis is the chief differential diagnosis for septic arthritis of the hip in young children. Transient synovitis often occurs after an upper respiratory tract infection. Patients have joint pain, mild fever and mildly increased ESR. Transient synovitis may be unilateral or bilateral. Treatment is conservative. Symptoms may recur.
Clinical factors may be used to distinguish septic arthritis from transient synovitis and guide management [12, 13]. Criteria include temperature (>38.5°C), WBC (>12 × 109/L), ESR (≥40 mm/h), CRP (≥20 mg/L) and weight bearing status. If all are abnormal, septic arthritis is certain—operative incision and drainage is indicated. If all are normal, septic arthritis is very unlikely—management may be conservative without joint aspiration (Fig. 4). It is patients in whom some criteria are positive and others not or with marginal abnormality who benefit most from joint aspiration to evaluate for infection [12, 13]. Depending on the joint, aspiration can be performed without or with imaging guidance (ultrasound or fluoroscopy).
Soft Tissue Infections
Pyomyositis is defined as a bacterial infection within muscle [7, 14]. Frank abscesses may form. Pyomyositis is most commonly seen in the pelvis. Care must be taken to exclude underlying osteomyelitis with extension into muscle and soft tissues. Prominent soft tissue findings may draw attention away from subtler bone findings.
Cellulitis is defined as a non-necrotizing infection limited to subcutaneous tissues, hypodermis and superficial fascia without muscle or deep fascial involvement [7, 14]. Imaging, principally ultrasound, is used to assess for fluid collection (abscess) or deeper extension. Necrotizing fasciitis is a rapidly progressive infection of deep fascia and adjacent soft tissues with areas of necrosis (which lack enhancement) [7, 14]. Fortunately, necrotizing fasciitis is uncommon in children. Differentiation of necrotizing fasciitis from pyomyositis and cellulitis may be challenging. Expedient surgical debridement is indicated for necrotizing fasciitis [14].
Chronic Recurrent Multifocal Osteomyelitis (CRMO)
CRMO is a disorder uncertain cause that has features of infections and inflammatory process and not infrequently mimics an osseous neoplastic process in presentation [15, 16]. The disease is related to sternoclavicular hyperostosis/SAPHO (synovitis, acne, pustulosis, hyperostosis and osteitis) syndrome and HLA-B27 related diseases. CRMO is characterized by recurrent episodes of swelling and pain, low grade fever, leukocytosis and elevated ESR. On biopsy, no infectious agent is found and no neoplastic cells are found; rather, there are non-specific, chronic appearing inflammatory changes highlighted by the presence of plasma cells. The disease is characterized by exacerbations and recurrences.
On imaging, the appearance of CMRO is protean [15, 16]. Disease is most common in metaphyses and metaphyseal equivalents with mixed, ill-defined lucent and sclerotic lesions abutting the physis (Fig. 5). Periosteal reaction, if present, is non-aggressive in appearance. Characteristic locations include lower extremity metaphyses, the clavicles and pelvic metaphyseal equivalents. Multifocality is characteristic. On MRI, lesions have ill-defined increased signal and enhancement (Fig. 5). Whole body MRI is useful for delineation of sites of disease [17].
Fig. 5
Fifteen year-old boy with chronic recurrent multifocal osteomyelitis. The boy complained of chronic intermittent bone pain in his lower extremities. Erythrocyte sedimentation rate and C-reactive protein were mildly elevated. Multiple sites of disease were found on imaging. (a) Anteroposterior view of the right ankle shows ill-defined sclerosis in the distal tibial metaphysis with ill-defined lucencies near the growth plate. Mild soft tissue swelling is noted. (b) Sagittal inversion recovery MR image (TR 418 ms, TE 60 ms, TI 140 ms) shows increased signal within the distal tibial metaphysis and epiphysis, increased signal within the posterior calcaneus and a focal lesion with the medial cuneiform (arrow)
Inflammatory Disease
Juvenile idiopathic arthritis (JIA) is not a single disease, but rather a heterogeneous spectrum of diseases characterized by inflammation of the synovium [11, 18–21]. On histology, JIA affected joints show inflammatory cells, fibrin, collagen. Similar pathology findings can be seen with other arthopathies including some infections (i.e. Mycobacterium tuberculosis, Coccidiomycosis). JIA is a diagnosis of exclusion. It is of unknown cause. JIA is defined as having onset before 16-years of age and symptoms greater than 6 months in duration [18]. JIA is classified by the International League of Associations for Rheumatology (ILAR) classification (Table 1) [18].
Disorder | Percentage of JIA patients (%) | Criteria |
|---|---|---|
Systemic-onset JIA | 4–17 | ≥1 joint; with or preceded by fever; with one or more of: rash, lymph node enlargement, hepatosplenomegaly, serositis |
Oligoarticular JIA | 27–56 | 1–4 joints during first 6 months Persistent: not >4 joints affected throughout course of disease Extended: >4 joints affected after first 6 months |
Polyarticular JIA—RF-positive | 2–7 | ≥5 joints during first 6 months RF-positive |
Polyarticular JIA—RF-negative | 11–28 | ≥5 joints during first 6 months RF-negative |
Enthesis related arthritis (ERA) | 3–11 | Arthritis + enthesitis or arthritis or enthesitis with at least two of: sacroiliac tenderness or lumbosacral pain, HLA-B27 antigen positive, new arthritis in male > 6-years-old, acute anterior uveitis, history of ankylosing spondylitis, ERA, IBD with sacroiliitis, Reiter syndrome or acute anterior uveitis in a first-degree relative |
Juvenile psoriatic arthritis | 2–11 | Arthritis + psoriasis; or arthritis with at least two of: dactylitis, nail pitting or onycholysis, psoriasis in first-degree relative |
Undifferentiated arthritis | 11–21 | Fulfills criteria for none of the above OR fulfills criteria in ≥2 of the above |
Systemic-onset JIA (Still disease) accounts for 5–10% of JIA [19] Incidence is near equal in boys and girls. Extra-articular manifestations may precede joint disease. The diagnosis is defined by having fever for greater than two weeks with one or more of: rash, hepatosplenomegaly, lymph node enlargement or serositis [18, 19, 21, 22]. Systemic symptoms may mask and/or precede joint symptoms. Early, there a little or no radiographic changes [11]. Joint disease tends to be late-onset, symmetric and polyarticular. One third to one half of patients proceed to a chronic destructive arthritis. Greater overall morbidity is seen with systemic disease. Macrophage activation syndrome (MAS) is a potentially fatal complication characterized by pancytopenia, disseminated intravascular coagulation (DIC), liver dysfunction, sustained fever, and, eventually, multi-organ failure [21].
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