1a Answer C. Legg-Calvé-Perthes (LCP) disease is the result of idiopathic, avascular necrosis of the developing proximal femoral epiphysis that often presents between 4 and 8 years of age. There are four stages of pathogenesis with radiographic correlation: avascularity, revascularization, healing, and residual deformity.

The radiographic pattern of a dense, flattened femoral epiphysis (arrow) with normal acetabular morphology are the imaging hallmarks of LCP, thus excluding hip dysplasia as a diagnosis. Slipped capital femoral epiphysis typically occurs in older children (mean age of 13 years), and radiographs reveal an irregularity of the physis, versus the epiphysis, that is more pronounced on frog-leg lateral projections. Osteomyelitis of the hip may result in a dense irregular epiphysis. However, there is typically an aggressive, osteolytic appearance to the proximal femur with involvement of the entire hip joint and rapid destruction if not treated urgently.

References: Dillman JR, Hernandez RJ. MRI of Legg-Calvé-Perthes disease. AJR Am J Roentgenol 2009;193(5):1394-1407.

Resnick D. Osteochondroses in diagnosis of bone and joint disorders, 4th ed. New York, NY: WB Saunders and Company, 2002:3686-3741.

Salter RB. In: Textbook of disorders and injuries of the musculoskeletal system, 3rd ed. Baltimore, MD: Williams & Wilkins, 1999:339-350.

1b Answer C. This case represents a late healing stage of Perthes disease with dense bone replacing trabecular bone at the middle pillar of the epiphysis. The initial, stage 1 of avascularity is radiographically occult. A symptomatic child with normal radiographs may be further evaluated by MRI with a high sensitivity for detecting osteonecrosis, marrow edema, and hip effusion. The figure below represents the different stages of Perthes disease. The earliest radiographic finding of a relatively dense epiphysis represents stage 2, revascularization. At this time, children are often asymptomatic. The “crescent sign” that is sometimes seen at stage 2 results from a pathologic, subchondral fracture of the anterosuperior epiphysis (see arrow in A). Stage 4 reveals a chronic, residual deformity of the femoral head, often with collapse, fragmentation, and lateral subluxation. Additional findings during disease progression may include a cystic lucency of the subphyseal, femoral neck (see arrow in B) and coxa magna with a short and widened femoral metaphysis.

Stages of Legg-Calvé-Perthes (LCP) disease on radiograph. A: 2 months; B: 4 months; C: 7 months; D: 10 months.

References: Dillman JR, Hernandez RJ. MRI of Legg-Calvé-Perthes disease. AJR Am J Roentgenol 2009;193(5):1394-1407.

Resnick D. Osteochondroses in diagnosis of bone and joint disorders, 4th ed. New York, NY: WB Saunders and Company, 2002:3686-3741.

Salter RB. In: Textbook of disorders and injuries of the musculoskeletal system, 3rd ed. Baltimore, MD: Williams & Wilkins, 1999:339-350.

2 Answer B. The radiograph reveals a geographic, lytic lesion without sclerotic margins (arrows) involving the epiphysis of the knee, the differential diagnosis of which includes both chondroblastoma and osteomyelitis. However, most chondroblastomas demonstrate a thin sclerotic margin radiographically.

Osteoid osteomas classically appear as a central, lucent lesion or nidus that is often cortically based but can occur in subchondral, intra-articular locations. Nonetheless, absent any surrounding cortical hyperostosis, an osteoid osteoma is unlikely. An osteochondral defect (OCD) should have an irregular fracture lucency, often with sclerotic margins abutting a fragment of subchondral bone, all of which are not present in this case.

The MRI appearance of a fluid signal, solitary lytic lesion centered at the distal epiphysis and cartilage (arrow) with surrounding reactive marrow signal is highly suggestive of osteomyelitis. Rim enhancement of the lytic lesion (not shown) would then suggest an associated abscess. Chondroblastomas also have surrounding T2 hyperintense marrow signal. However, the lesion itself is characteristically lobular with internal matrix that is T2 isointense/hypointense, not present in this case.

Understanding the imaging findings of osteomyelitis is important as the classic clinical history of fever, elevated white blood cell count, and inflammatory markers are not always present.

References: Guillerman RP. Osteomyelitis and beyond. Pediatr Radiol 2013;43(1):S193-S203.

Lyer RS, Chapman T, Chew FS. Pediatric bone imaging: diagnostic imaging of osteoid osteoma. AJR Am J Roentgenol 2012;198:1039-1052.

Weatherall PT, Maale GE, Mendelsohn DB, et al. Chondroblastoma: classic and confusing appearance at MR imaging. Radiology 1994;190:467-474.

3 Answer A. The metaphyses of long bones are the most common sites of hematogenous osteomyelitis in children owing to the unique changes in metaphyseal and epiphyseal vascularity with age. In neonates, the epiphysis is at an increased risk of infection as nutrient vessels cross the physis. However, transphyseal extension of metaphyseal osteomyelitis may still occur in children older than 2 years of age despite the theoretic protection of an avascular physis.

References: Gilbertson-Dahdal D, Wright JE, Krupinski E, et al. Transphyseal involvement of pyogenic osteomyelitis is considerably more common than classically taught. AJR Am J Roentgenol 2014;203:190-195.

Guillerman RP. Osteomyelitis and beyond. Pediatr Radiol 2013;43(1):S193-S203.

4 Answer C. The gram-positive cocci (GPC), S. aureus followed by Streptococcus, are considered the most common pathogens for acute hematogenous osteomyelitis. However, the gram-negative bacillus, K. kingae, has an increased prevalence in children younger than 4 years of age.

MRI is highly sensitive for distinguishing K. kingae from GPC in young children when there is focal epiphyseal (and equivalent) cartilage involvement. There is also a diminished inflammatory response and bone marrow/soft tissue reaction with K. kingae compared to GPC. Pseudomonas and Escherichia coli are often associated with osteomyelitis from penetrating trauma, typically within the foot, but this is rare in the intra-articular epiphysis.

References: Gilbertson-Dahdal D, Wright JE, Krupinski E, et al. Transphyseal involvement of pyogenic osteomyelitis is considerably more common than classically taught. AJR Am J Roentgenol 2014;203:190-195.

Guillerman RP. Osteomyelitis and beyond. Pediatr Radiol 2013;43(1):S193-S203.

Kanavaki A, Ceroni D, Tchernin D, et al. Can early MRI distinguish between Kingella kingae and Gram-positive cocci in osteoarticular infections in young children? Pediatr Radiol 2012;42:57-62.

5a Answer A. The crescentic fracture fragment (arrow) arising from the lateral condyle is the typical location of a lateral condylar fracture. This often results from a varus injury to an extended supinated forearm. The more common supracondylar fracture is a horizontal fracture located at the distal humeral metaphysis, often involving the coronoid fossa without involvement of the physis.

Reference: Green NE. Fractures and dislocations about the elbow. In: Green NE, Swiontkowski MF. (eds). Skeletal trauma in children, 3rd ed. Philadelphia, PA: WB Saunders, 2003:257.

5b Answer C. Lateral condylar fractures most commonly occur between the ages of 5 and 10 years.

Reference: Green NE. Fractures and dislocations about the elbow. In: Green NE, Swiontkowski MF (eds). Skeletal trauma in children, 3rd ed. Philadelphia, PA: WB Saunders, 2003:257.

5c Answer C. Lateral condylar fractures of the elbow are most commonly Salter-Harris IV fractures.

The Milch classification groups lateral condylar fractures into types I and II based on fracture involvement lateral or medial to the capitello-trochlear groove, respectively. A Milch I fracture is lateral to the trochlea and extends through the capitellum. As the capitellum is usually ossified in this age group, it clearly represents a Salter-Harris IV fracture on elbow radiographs. However, the elbow remains stable as the humeroulnar joint is spared.

The Milch II fracture spares the capitellum and extends medial to the capitellotrochlear groove. There has been controversy classifying this fracture type as Salter-Harris IV when the involved trochlear epiphysis is not yet ossified and it appears radiographically as a Salter-Harris II fracture. However, the consensus is that the Milch II fracture pattern is also a Salter-Harris IV fracture as it involves the metaphysis, physis, and unossified trochlear epiphysis. Therefore, nearly all lateral condylar fractures are considered Salter-Harris IV fractures. However, the Milch classification is more important than the Salter-Harris classification for management because Milch II fractures are unstable and require surgical fixation.

This case highlights the more common Milch type II lateral condylar fracture as the metaphyseal fragment (arrow) extends medial to the capitellum. Radiographic evaluation is limited in this age group because of incomplete ossification of the trochlea. As a result, surgical management may rely on the degree of displacement of the metaphyseal fragment (>2 mm).

References: Bache E. Elbow injuries. In: Johnson KJ, Bache E (eds). Imaging in pediatric skeletal trauma. Berlin: Springer, 2008:257-270.

Green NE. Fractures and dislocations about the elbow. In: Green NE, Swiontkowski MF (eds). Skeletal trauma in children, 3rd ed. Philadelphia, PA: WB Saunders, 2003:257.

Letts M, Davidson D. Fractures of the lateral condyle of the humerus in children. Orthopaedic Knowledge Online Journal 2002;1(6). http://orthoportal.aaos.org/oko/article.aspx?article=OKO PED007

6a Answer C. Blount disease (tibia vara) is secondary to pathologic stress upon the posteromedial physis of the proximal tibia that results in medial growth suppression and associated tibia vara. As the name tibia vara implies, the lower extremity bowing (varus) is centered at the proximal tibia. The metaphyseal-diaphyseal angle is >11 degrees (˜20 degrees in this case). This angle is drawn from the metaphyseal beak to a line at the physis that is perpendicular to the lateral cortex of the tibial diaphysis (see angle). Although bilateral in this case, Blount disease is often unilateral or asymmetric and has infantile, juvenile, and adolescent presentations. The Langenskiold classification describes six stages of progressive metaphyseal depression, beaking, and fragmentation.

Developmental (physiologic) genu varus (bowing) normally resolves within 6 months of walking or by the age two. Congenital bowing classically presents as convex posterior and medial bowing of the tibial diaphysis and may be due to intrauterine positioning or skeletal dysplasia. In both cases, there is a normal medial metaphysis of the tibia. As rickets represents deficient mineralization of the growing physis, radiographs should display symmetric widening, cupping, and fraying of the growth plates of the distal femur and proximal and distal tibia (not present in this case).

References: Biko DM, Miller AL, Ho-Fung V, et al. MRI of congenital and developmental abnormalities of the knee. Clin Radiol 2012;67:1198-1206.

Cheema FI, Grissom LE, Harcke T. Radiographic characteristics of lower-extremity bowing in children. Radiographics 2003;23:871-880.

6b Answer D. Imaging findings of Blount disease include radiographical findings of depression of the medial tibial metaphysis. Additional MRI can better evaluate the growth plate demonstrating bony bridging, delayed ossification of the medial tibial epiphysis, widening of the tibial growth plate, and hypertrophy of the medial meniscus. The hypertrophy of the medial meniscus is likely compensatory hypertrophy due to abnormal forces within the knee.

Reference: Biko DM, Miller AL, Ho-Fung V, et al. MRI of congenital and developmental abnormalities of the knee. Clin Radiol 2012;67:1198-1206.

7 Answer C. Idiopathic chondrolysis (ICH) is a disease of unknown etiology that results in progressive articular cartilage destruction. ICH is often unilateral and presents with spontaneous hip or knee pain with worsening joint stiffness. There are neither systemic symptoms nor abnormal inflammatory biomarkers present. Early radiographs are often normal and performed to exclude more common acute causes of hip pain such as slipped capital femoral epiphysis. Later radiographs 10 to 12 months from symptom onset often reveal degenerative changes of concentric joint space loss, protrusio acetabuli, subchondral cysts, and sclerosis. It is important to distinguish ICH from secondary causes of cartilage loss from JIA or infection. The MRI findings in this case reveal a geographic pattern of T1/T2 signal prolongation confined to the middle third of the subchondral femoral head to the physis (arrows). This pattern is characteristic of early MRI findings of ICH. Synovial enhancement is less commonly reported with ICH compared to JIA. However, this negative finding is not specific. Additional findings of muscle wasting and atrophy are reported with follow-up imaging usually with associated joint contractures. Synovial biopsy is often performed for pathologic confirmation and to exclude infection. The prognosis is variable from spontaneous resolution to significant joint contracture and ankylosis.

References: Johnson K, Haigh SF, Ehtisham S, et al. Childhood idiopathic chondrolysis of the hip: MRI features. Pediatr Radiol 2003;33:194-199.

Laor T, Crawford AH. Idiopathic chondrolysis of the hip in children: early MRI findings. AJR Am J Roentgenol 2009;192:526-531.

8a Answer D. The imaging findings are most suggestive of chronic recurrent multifocal osteomyelitis (CRMO). CRMO is an idiopathic inflammatory disorder most commonly seen in children and adolescents. This disorder is characterized by multiple inflammatory bone lesions that demonstrate a relapsing/remitting pattern. On imaging, these lesions are most often lytic on plain radiographs initially followed by sclerosis in the chronic course. On MR imaging, the lesions demonstrate bone marrow edema and periostitis. CRMO commonly occurs in the long tubular bone and clavicle but can occur anywhere throughout the skeleton.

Reference: Khanna G, Sato TS, Ferguson P. Imaging of chronic recurrent multifocal osteomyelitis. Radiographics 2009;29:1159-1177.

8b Answer B. Chronic recurrent multifocal osteomyelitis (CRMO) most commonly involves the metaphysis and metaphyseal equivalents. This is similar to the distribution of hematogenous spread of osteomyelitis, but CRMO may involve the clavicle.

References: Khanna G, Sato TS, Ferguson P. Imaging of chronic recurrent multifocal osteomyelitis. Radiographics 2009;29:1159-1177.

Mandell GA, Contreras SJ, Conrad K, et al. Bone scintigraphy in the detection of chronic recurrent multifocal osteomyelitis. J Nucl Med 1998;39:1778-1783.

8c Answer B. Multiple conditions are associated with chronic recurrent multifocal osteomyelitis (CRMO). These include dermatologic conditions such as psoriasis and pyoderma gangrenosum, autoinflammatory disorders such as Takayasu arteritis and Wegener granulomatosis, gastrointestinal syndromes such as inflammatory bowel disease, and genetic syndromes such as Majeed syndrome. CRMO is also associated with SAPHO syndrome (synovitis, acne, pustulosis, hyperostosis, osteitis) and spondyloarthropathies.

Reference: Khanna G, Sato TS, Ferguson P. Imaging of chronic recurrent multifocal osteomyelitis. Radiographics 2009;29:1159-1177.

9 Answer B. Lipomatosis of a nerve with macrodactyly is referred to as macrodystrophia lipomatosa. In this disorder, the affected nerve is enlarged by fibrofatty tissue. This can occur without macrodactyly or with macrodactyly (macrodystrophia lipomatosa). Clinically, this disorder is characterized by a slowgrowing mass most frequently within the upper extremity. Most cases involve the median nerve in the upper extremity and the medial plantar nerve in the lower extremity. Radiographs of this disorder demonstrate both soft tissue and bony overgrowth in the distribution of a sclerotome (arrows). MRI demonstrates a diffusely enlarged thickened nerve surrounded by adipose tissue.

References: Murphey MD, Carroll JF, Flemming DJ, et al. From the archive of the AFIP: benign musculoskeletal lipomatous lesions. Radiographics 2004;24:1433-1466.

Tripathi SK, Nanda SN, Kumar S, et al. Macrodystrophia lipomatosa—a rare congenital anomaly: a case report and review of literature. Ann Int Med Dent Res 2016;2(5):1-3.

10a Answer C. Thanatophoric dysplasia is a short-limbed dwarfism caused by a mutation of fibroblast growth factor receptor 3 gene (FGFR3). It is the most common lethal neonatal skeletal dysplasia. In this disorder, the long bones are short and may have a curved “telephone receiver” appearance (arrowheads). The ribs are shortened. Additionally, the vertebral bodies are flattened and may appear H shaped (arrows). This feature can be used to differentiate thanatophoric dysplasia from the short rib polydactyly syndromes such as Jeune syndrome.

Reference: Miller E, Blaser S, Shannon P, et al. Brain and bone abnormalities of thanatophoric dwarfism. AJR Am J Roentgenol 2009;192:48-51.

10b Answer C. The most common central nervous system manifestations of thanatophoric dysplasia are the cloverleaf skull deformity and megalencephaly. Additional central nervous system abnormalities are deep fissures and abnormal sulcation of the temporal lobes, a dysplastic hippocampus, and polymicrogyria.

Reference: Miller E, Blaser S, Shannon P, et al. Brain and bone abnormalities of thanatophoric dwarfism. AJR Am J Roentgenol 2009;192:48-51.

11a Answer D. Osteochondritis dissecans (OCD) of the capitellum is a focal injury of the articular cartilage and subchondral bone within the humeral capitellum. It is most commonly seen in this location in throwing athletes and is typically seen in patients between 12 and 17 years old. The suggested etiology of this lesion is repetitive microtrauma. OCD is most commonly seen within the knee.

On plain radiographs, there is most often a subchondral lucent focus (arrow), but there may be fragmentation or sclerosis. MRI best depicts the OCD where the subchondral abnormality is readily visible (arrowhead).

References: Cruz AI, Shea KG, Ganley TJ. Pediatric knee osteochondritis dissecans lesions. Orthop Clin N Am 2016;47:763-775.

Itsubo T, Murakami N, Uemura K, et al. Magnetic resonance imaging staging to evaluate stability of capitella osteochondritis dissecans lesions. Am J Sports Med 2014;42:1972-1977.

Jarret DY, Walters MM, Kleinman PK. Prevalence of capitellar osteochondritis dissecans in children with chronic radial head subluxation and dislocation. AJR Am J Roentgenol 2016;206:1329-1334.

11b Answer A. The prevalence of capitellar osteochondritis dissecans (OCD) is increased in children with radial head subluxation. Capitellar OCD is seen in 32% to 33% of children with chronic radial head subluxation likely because of abnormal radiocapitellar mechanics.

Reference: Jarret DY, Walters MM, Kleinman PK. Prevalence of capitellar osteochondritis dissecans in children with chronic radial head subluxation and dislocation. AJR Am J Roentgenol 2016;206:1329-1334.

11c Answer C. The criteria for unstable osteochondritis dissecans (OCD) on MRI are the following:

Reference: Cruz AI, Shea KG, Ganley TJ. Pediatric knee osteochondritis dissecans lesions. Orthop Clin N Am 2016;47:763-775.

12a Answer B. The image demonstrates a small osseous fragment adjacent to the pelvis consistent with an avulsion fracture of the anterior superior iliac spine (arrow). The anterior superior iliac spine is the attachment point of the sartorius muscle and tensor muscle of the fascia lata. This injury occurs during forceful extension of the hip and is commonly seen in sprinters. Treatment is activity restriction.

Reference: Stevens MA, El-Khoury GY, Kathol MH, et al. Imaging features of avulsion injuries. Radiographics 1999;19:655-672.

12b Answer D. The table below lists the common sites for avulsion injuries of the pelvis and the associated muscular attachments.

Reference: Stevens MA, El-Khoury GY, Kathol MH, et al. Imaging features of avulsion injuries. Radiographics 1999;19:655-672.

13a Answer B. The image demonstrates a metaphyseal corner fracture of the proximal tibia (arrow). This is a disc-shaped fracture though the metaphysis where the fracture line is nearly parallel to the physis. The fracture has been described a corner fracture or bucket handle fracture depending on the orientation of the radiographic projection. This fracture has been called a classic metaphyseal lesion (CML) and is common in abused infants particularly <18 months of age. CMLs are considered highly specific for infant child abuse.

References: Kleinman PK. Diagnostic imaging in infant abuse. AJR Am J Roentgenol 1990;155:703-712.

Lonergan GJ, Baker AM, Morey MK, et al. Child abuse: radiologic-pathologic correlation. Radiographics 2003;23:811-845.

Thackeray JD, Wannemacher J, Adler BH, et al. The classic metaphyseal lesion and traumatic injury. Pediatr Radiol 2016;46:1128-1133.

13b Answer B. A metaphyseal corner fracture or classic metaphyseal lesion (CML) occurs because of shearing injury, which causes differential horizontal motion across the metaphysis. This shearing force is caused by to-and-fro movement such as seen in shaking an infant by holding the infant from the feet or hands or shaking an infant while holding the chest and whiplashing their extremities.

References: Lonergan GJ, Baker AM, Morey MK, et al. Child abuse: radiologic-pathologic correlation. Radiographics 2003;23:811-845.

Thackeray JD, Wannemacher J, Adler BH, et al. The classic metaphyseal lesion and traumatic injury. Pediatr Radiol 2016;46:1128-1133.

14 Answer D. Although skeletal injuries without an explanation may be concerning for abuse, skeletal imaging findings that are associated with infant nonaccidental injury are metaphyseal corner fractures, rib fractures, sternal fractures, vertebral spinous process fractures, and acromion fractures of the scapula. Metaphyseal corner fractures or classic metaphyseal lesions (CML) are due to shear injury that can be seen with shaking. Rib fractures in infants are rare injuries given the plasticity of the bones. Given this, rib fractures require substantial force but can be seen with squeezing of the chest. Given the rarity of sternal fractures, vertebral spinous fractures, and acromion fractures of the scapula in infants, these fractures are also concerning for abuse. Skull fracture patterns such as multiple fractures, fractures crossing sutures, and bilateral fractures have also been associated with abusive injury.

References: Kleinman PK. Diagnostic imaging in infant abuse. AJR Am J Roentgenol 1990;155:703-712.

Lonergan GJ, Baker AM, Morey MK, et al. Child abuse: radiologic-pathologic correlation. Radiographics 2003;23:811-845.

15 Answer A. All infants with suspected abusive injury should undergo a skeletal survey. Additionally, a repeat skeletal survey 10 to 14 days following may identify additional injuries that could not be seen on the initial skeletal survey. The recommended views for a skeletal survey of suspected infant abuse are the following:

References: Kleinman PK. Diagnostic imaging in infant abuse. AJR Am J Roentgenol 1990;155:703-712.

Lonergan GJ, Baker AM, Morey MK, et al. Child abuse: radiologic-pathologic correlation. Radiographics 2003;23:811-845.