Revista Chilena de Radiología, año 2016. ARTÍCULO DE REVISIÓN Injuries and normal variants of the pediatric knee

Cristián Padilla C.a,* , Cristián Quezada J.a,b, Nelson Flores N.a, Yorky Melipillán A.b and Tamara Ramírez P.b a. Imaging Center, Hospital Clínico Universidad de Chile, Santiago, Chile. b. Radiology Service, Hospital de Niños Roberto del Río, Santiago, Chile.

Abstract: Knee pathology is a reason for consultation and a prevalent condition in children, which is why it is important to know both the normal variants as well as the most frequent pathologies. In this review a brief description is given of the main pathologies and normal variants that affect the knee in children, not only the main clinical characteristics but also the findings described in the different, most used imaging techniques (X-ray, ultrasound, computed tomography and magnetic resonance imaging [MRI]). Keywords: Knee; Paediatrics; Bone lesions.

Introduction posteromedial distal femoral metaphysis, near the Pediatric knee imaging studies are used to evaluate insertion site of the medial twin muscle or adductor different conditions, whether traumatic, inflammatory, magnus1. It is a common finding on radiography and developmental or neoplastic. magnetic resonance imaging (MRI), incidental, with At a younger age the normal evolution of the more frequency between ages 10-15 years, although images during the skeletal development of the distal it can be present at any age until the physeal closure, femur, proximal tibia and proximal fibula should be after which it resolves1. In frontal radiography, it ap- known to avoid diagnostic errors. Older children and pears as a radiolucent, well circumscribed, cortical- adolescents present a higher frequency of traumatic based lesion with no associated soft tissue mass, with and athletic injuries. During childhood and at school varying degrees of peripheral sclerosis, and on lateral age, atraumatic conditions such as infections or projection an excavation or proliferation with cortical inflammatory arthritis are observed, and not only irregularity can be seen1. In MRI it presents low signal neoplastic benign lesions but also malignant lesions on T1 and high on T2 and may present enhancement should be considered as relatively common in the with gadolinium1,2. It is surrounded by a hypointense pediatric knee. ring corresponding to the peripheral sclerosis seen on This review describes both normal variants and radiography1. Because of the typical location and the pathological findings related to the knee in the pe- radiographic findings, it does not require additional diatric age. imaging nor treatment (Figure 1)1,3,4.

Developmental and congenital variants without Normal femoral condylar ossification pathological significance Differentiating the normal ossification of the me- dial and lateral condyles of the epiphyseal pathology, Distal femoral cortical irregularity especially the osteochondritis dissecans, is a frequent This is a benign fibro-osseous lesion, along the challenge when analyzing pediatric knee radiographs.

Padilla C., Quezada C., Flores N., Melipillán Y. Ramírez T. Injuries and normal variants of the pediatric knee. Rev Chil Radiol 2016; 22(3): 121-132. doi: 10.1016/j.rchira.2016.08.003 Email: [email protected] (C. Padilla C.).

Received March 20, 2016; Accepted August 23, 2016. Available on the Internet September 29, 2016. © 2016. SOCHRADI. Published by Elsevier España, S.L.U. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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The irregular ossification in the posterior central the normal semilunar form, commonly affects the portion of the lateral femoral condyle is a variant of lateral meniscus and may be partial or complete1,2,6. normal development common in children younger Clinically it manifests in children and adolescents than 10 years (Figure 2). Osteochondritis dissecans with pain, click, popping or locking of the knee2. The presents predominantly in the medial femoral condyle, lateral discoid meniscus is more frequently associated in children older than 10 years, in the middle third of with meniscal injuries, possibly due to differences the condyle (and not posterior, as in normal condylar in both mechanical function and histological chan- ossification) and edema is frequent1. ges5. In coronal MRI slices the meniscus extends medially towards the tibial spine (greater than 13 Developmental and congenital variants that may mm of transverse diameter or 2 mm greater than have pathological significance the normal medial meniscus)2. In sagittal slices the continuity between the anterior and posterior horns Discoid meniscus is seen in 3 or more contiguous images in slices of This is a frequent congenital variant, multifacto- 4 to 5 mm thickness2,6. Most meniscal injuries in rial, with an estimated incidence of 0.4 to 17%1,5. The children under 10 years are secondary to discoid discoid meniscus is long, dysplastic, does not have meniscus (Figure 3)5.

Figure 1. Distal femoral cortical irregularity. AP radiograph and sagittal fat suppressed T2 MRI. Radiolucent lesion, well defined, with sclerotic margins, cortical base, on the posteromedial distal femoral metaphysis.

Figure 2. Normal femoral condylar ossification. Coronal and sagittal CT bone window. Irregular ossification in the posterolateral portion of the lateral femoral condyle as a normal variant.

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Figure 3. Discoid lateral meniscus. Coronal and sagittal fat suppressed T2 MRI. Discoid lateral meniscus with signal alteration of horizontal disposition compatible with rupture.

Bipartite patella Blount’s disease The patella is normally formed of 2 or 3 ossifica- Abnormal endochondral ossification of the medial tion centers, which begin to develop between 4 and 7 proximal tibia leads to Blount’s or tibia vara disease. years of age2,7. In the bipartite patella the secondary Radiography is sufficient for the diagnosis in pediatric ossification center at the superolateral aspect does patients who present genu varo after 2 years of age. not merge, which can be uni or bilateral2. It is con- The proximal tibia presents an epiphyseal-physeal- sidered a normal variant, but can be painful in both metaphyseal distortion, with medial inclination and acute and chronic stress injuries that can interrupt the fragmentation of the epiphysis, widening and irregula- synchondrosis between the accessory ossification rity of the physis and rounded or pointed metaphysis, center and the patella2,4,6,7. In radiography a fragment resulting in an increase in the metaphyseal-diaphyseal not attached to the rest of the patella can be seen, angle. It presents long-term complications, such as well corticated and generally along the superolateral premature degenerative changes secondary to pro- pole (Figure 4). gressive deformity, altered biomechanics or leg length

Figure 4. Bipartite patella. Coronal CT bone window. Small accessory ossification nucleus at the level of the superolateral pole of the patella. 123 Dr. Cristián Padilla C, et al. Revista Chilena de Radiología, año 2016; 22(3): 121-132. discrepancy if there is no appropriate treatment. The static stabilizers of the patella, which can lead to osteo- early or infantile form and the late or juvenile form chondral avulsion fractures of the medial patellar pole differ if the symptomatology occurs before or after 4 and ligament or retinacular ruptures1. The impaction of years of age respectively2. the patellar articular cartilage on the lateral femur may result in osteochondral fracture of the patella, trochlea, Anomalies of patellar alignment or lateral femoral condyle1. In extension, the patella moves medially and impacts the anterolateral femoral Impingement of superolateral Hoffa’s fat pad condyle resulting in the classic pattern of edema in the lateral femoral condyle (Figure 6)1. This corresponds to the friction syndrome of the lateral femoral condyle with the patellar tendon in the context of patellofemoral misalignment (a combination of alterations affecting the anterior extensor mechanism which includes a high patella, patellofemoral dysplasia and lateralization of the tibial tuberosity)1. It is most commonly seen in adolescent girls aged between 15-16 years with chronic anterior knee pain, and the most common structural abnormality observed is the high patella (patellar / patellar tendon ratio greater than 1.3 in men and 1.5 in women in sagittal MR imaging or an Insall-Salvati index on lateral projection of knee x-ray greater than 1.2)1,8. In MRI images, an increased signal in T2 was observed in the Hoffa fat pad (Figure 5)1. Figure 6. Acute lateral patellar dislocation. PD FAT SAT axial and coronal sequence MRI showing edema foci in the Acute lateral patellar dislocation medial margin of the patella, lateral portion of the lateral This is a consequence of poor patellar alignment. femoral condyle and an area of depression on the lateral surface of the lateral femoral condyle compatible with fracture Acute patellar dislocation is one of the most common secondary to episodes of reduced lateral patellar dislocation. injuries affecting the pediatric knee4,9. Lateral luxation is typically transient, but the patella rarely remains 9 dislocated in the initial images . After reduction, the Trauma frontal and lateral radiographs sometimes show only 4,9 In adults, traumatic lesions most frequently produce increased volume of soft tissue and joint effusion . In ligament and meniscal injuries, unlike children and flexion, the patella moves laterally injuring the medial adolescents in whom it characteristically affects the physis and adjacent bones, which are the weakest sites. This is why the pediatric knee is more suscep- tible to multiple types of fractures, some exclusive to this age, together with chronic stress injuries2.

Acute trauma Fissure fractures The distal femoral physis is responsible for 70% of the growth of the femur and 35% of the growth of the entire lower extremity. It is the fastest growing physis, with an average of 10 mm / year10. Before ske- letal maturation, physeal fractures are more frequent than ligamentous lesions2,11. Those fractures which affect the proximal femoral or distal femoral physis have a high risk of causing alterations in growth, with the consequent development of discrepancy in the length of the lower extremities and / or angular deformities2,10. These can be subtle in radiography and require MRI for a better characterization2. Fis- Figure 5. Impingement of superolateral Hoffa fat pad. sure fractures may occur until the physis is close to 11 Sagittal fat suppressed T2 MRI which shows an increased closure . Salter-Harris II fractures of the distal femur signal in the Hoffa fat pad. are the most common fractures (Figure 7)11.

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Figure 7. Salter-Harris III fracture. Coronal CT bone window. Fracture feature in relation to the intercondylar groove, slightly oblique course, extending towards the epiphyseal region without significant displacement. Figure 8. Avulsion fracture of the tibial spine. Coronal CT bone window. Fracture feature that compromises the tibial spine.

Avulsion fractures of the tibial spine Anterior bursitis of the knee The most common avulsion fracture of the There are 12 bursae around the knee, of which immature skeleton of the knee is that of the tibial 4 are more susceptible to inflammation (prepatellar, spine4. This is due to the composition of the tibial superficial infrapatellar, deep infrapatellar and pes spine at this age, which is weaker than that of anserine (goose’s foot)). the anterior cruciate ligament4,11. Prior to the phy- Trauma, infection, overuse, and hemorrhage may seal closure, avulsion of the anterior tibial spine result in inflammation of a bursa (Figure 9)7. is relatively common, whereas anterior cruciate ligament rupture becomes more prevalent after Chronic trauma physeal closure11. The width of the intercondylar Chronic lesions of the extensor mechanism of the notch would also play a role in the type of injury, knee include Osgood-Schlatter disease and Sinding- where the avulsion is more frequent in patients with Larsen-Johansson disease. wider intercondylar notch11. They can be subtle on radiography and initially only diagnosed with Osgood-Schlatter Disease MRI (Fig.8)4,11. This is a traction apophysitis after repeated trauma in the patellar tendon insertion in the tibial tuberosity, Lesions of the anterior cruciate ligament at an early stage of tubular ossification2,9. The typical Ligament injuries are generally less frequent presentation is an adolescent male with pain on the in pediatric patients due to ligamentous laxity, anterior aspect of the knee and increased volume on coupled with the relative weakness of the physis, the tibial tubercle2. It occurs bilaterally in up to 50%. In which makes bone lesions much more common2,6. MRI, a signal increase is observed in the ossification Anterior cruciate ligament injuries are common in center of the tibial tubercle at an early stage of the adolescents and more frequent in girls due to the disease, which then extends to the adjacent physeal combination of hormonal influence, valgus alignment cartilage2,6. The patellar tendon appears edematous and increased joint laxity1,2,6,9,12. The rupture criteria and thickened in its insertion in the tuberosity, a fin- are the same as for adults, the loss of parallelism ding also visible on radiography2,6. In ultrasound the with the Blumensaat line being a more sensitive findings are: fragmentation of the ossification center finding than the discontinuity of the fibers in this of the tuberosity, thickening of the patellar tendon and age group2,6. reactive bursitis13. In the final stage, separate ossicles

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origin of the patellar tendon in the lower pole of the patella, which represents a chronic traction injury of the immature bone-bearing junction2. It is typically seen in athletes aged between 10-14 years and may present with pain in the lower pole of the patella as- sociated with increased soft tissue volume and lower patellar pain, often associated with running, climbing or kneeling2,4,7. The bone fragments adjacent to the inferior pole of the patella are seen on radiography4. MRI shows fragmentation of the inferior patellar pole associated with thickening and edema of the proximal patellar tendon and also high T2 signal of the Hoffa fat pad1,2,7. In ultrasound the findings are: thickening of the patellar tendon in its proximal aspect, fragmen- Figure 9. Anterior bursitis of the knee. Ultrasonography, 14 longitudinal slice at the suprapatellar level. Bursa with a tation of the inferior pole of the patella and bursitis . 7 somewhat heterogenous thickening and a small amount Treatment is symptomatic . The avulsion lesions of the of anechoic fluid in its interior. inferior patellar pole are known as Sinding-Larsen- Johansson’s disease before the physeal closure, and as jumper’s knee after closure (Figure 11)1.

Figure 11. Sinding-Larsen-Johansson disease. Lateral x-ray projection. Small bony fragment in the lower pole of the patella, associated with thickening of the patellar tendon.

Figure 10. Osgood-Schlatter’s disease. Lateral x-ray projection. Irregularity of the anterior tuberosity of the tibia Juvenile osteochondritis dissecans with increased thickness of the patellar tendon. It is thought to be an acquired lesion of the sub- chondral bone characterized by varying degrees of bone resorption, collapse and abduction formation are seen, which represent avulsed fragments of the unrelated to an acute osteochondral fracture5. It is 2 tuberosity that are surrounded by edema (Figure 10) . defined as juvenile if it occurs before the closure of the physis1. Injury and separation of an intraarticular Sinding-Larsen-Johansson disease fragment of subchondral bone occurs with or without This is a traction apophysitis that affects the disruption of the overlying articular cartilage. The

126 Revista Chilena de Radiología, año 2016. ARTÍCULO DE REVISIÓN most common site in children and adults is the graph (AP), lateral and tunnel projection, where lateral aspect of the medial femoral condyle1,2,4,5. in lateral and tunnel projection posterior femoral The presentation is usually nonspecific, with poorly condyles are assessed which are not clearly seen localized gonalgia that increases with exercise or in the AP projection15. MRI is sensitive to show the climbing hills or stairs4. In the physical examination, subarticular bone defect in the early stages of the an antalgic position and pain on palpation over process15. The presence of mechanical symptoms, the anteromedial aspect of the knee in flexion, joint effusion, crackles and pain with movement is observed. Up to 25% have bilateral pain15. The may suggest instability of the lesion. Quadriceps initial imaging method is the anteroposterior radio- atrophy is an indicator of chronicity (Figure 12)15.

Figure 12. Juvenile osteochondritis dissecans. AP and lateral radiography. Radiolucent osteochondral defect in the medial femoral condyle.

Inflammatory pathology edema, local heat over the joint and functional impo- In the context of pain, lameness and edema, chil- tence. Given the clinical suspicion of joint involvement, dren are frequently evaluated for possible infectious or ultrasound plays an important role in evaluating the inflammatory arthritis. The presence of joint effusion magnitude of the effusion, the characteristics and may have multiple underlying non-traumatic causes synovial vascularization by color Doppler. In MRI, including septic arthritis, juvenile inflammatory arthritis associated bone involvement (osteomyelitis and / and hemophilia as more frequent causes2. or subperiosteal abscess) can be identified as the presence of signal anomalies and enhancement in Septic arthritis the adjacent bone suggest associated osteomyelitis This is a rapidly progressive inflammatory arthritis (Figure 13)2. most commonly caused by Staphylococcus aureus. More frequent in small children, with a peak at 2-3 Osteomyelitis years. The joints of the lower extremities are the most The most frequent clinical history is fever associated affected. Patients present with abrupt onset of fever, with decreased use of the affected limb. Older children

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may be able to identify the specific site of infection. In the anatomical distribution of acute bacterial osteoar- ticular infections in children, the knee is affected in 25 to 40%. The origin may be hematogenous, from a source of contiguous infection, penetrating or puncture injuries or postoperative infections. Although conventional radiography is recom- mended in all cases (may rule out an underlying fracture, for example), its sensitivity in acute infection is extremely low, since the findings do not appear until 50% of the bone material has been lost, 10 days after the onset of the symptoms. The imaging findings are ill-defined radiolucent lesions resulting from the destruction of the metaphysis and sclerotic images secondary to periostitis. MRI is sensitive for identifying edema and cor- tical inflammation and the bone marrow. The major advantage is that it delivers high-quality images of the bone, the joint and the surrounding tissue, critical information for deciding whether surgery is necessary (Figure 14)16.

Hemophilia Figure 13. Septic arthritis. X-ray in lateral projection. Increased It is a recessive disease linked to the X chro- volume in the knee joint and joint effusion. mosome that manifests itself as a deficiency of coagulation factors. Hemophilia A is due to factor VIII deficiency and hemophilia B, due to factor IX deficiency. Spontaneous bleeding or minimal

Figure 14. Osteomyelitis. Coronal CT bone window and axial window of soft tissue. Increased density and irregularity of the epiphysis cortex and proximal metaphysis of the tibia with presence of a possible Brodie’s abscess. Dense foreign body in the subcutaneous cellular tissue in the anterior portion of the proximal tibial metaphysis.

128 Revista Chilena de Radiología, año 2016. ARTÍCULO DE REVISIÓN trauma occurs. 85% of bleeding episodes are gradient. Foci of T2-weighted signals within the hemarthrosis, and the knee is the most affected synovium represent inflammation2,17. The epiphyses joint followed by the elbow, the ankle, the hip and appear larger and the patella appears square as the shoulder2. Recurrent episodes of bleeding in juvenile rheumatoid arthritis. The intercondylar can cause synovial hypertrophy and inflammation, notch is also widened. Chronically, thinned car- together with bone and cartilage erosions which tilage, subchondral cysts and osteophytes can lead to early degenerative changes2,17. In MRI, the be seen. Ultrasonography has high sensitivity for synovium is thickened and with low signal on T1 evaluating synovial hypertrophy and a borderline and T2 for hemosiderin deposition, which gives sensitivity (70%) for detecting small amounts of the characteristic blooming of low signal in echo effusion compared to MRI (Figure 15)17.

Figure 15. Hemophilia. Sagittal T2 MRI. Arthropathy with synovial thickening and hyposignal foci representing deposits of hemosiderin in the joint space in the context of hemophilic arthropathy. Joint effusion.

2 Juvenile rheumatoid arthritis cess and the surrounding degenerative changes . Systemic disease affecting children under 16 years of age with symptoms older than 6 months. Neoplastic pathology The knee is the most affected joint; presents pain, local heat and volume increase. MRI is used to Fibrous cortical defect and non-ossifying fibroma determine activity of the disease, its extent and the These are the most frequent benign lesions of response to treatment. the pediatric skeleton and often incidental findings 3 The most common findings in MRI are: joint on radiography . They are common metaphyseal 18 effusion, synovial thickening and nodularity, pannus lesions around the knee with identical histology . It formation and inflammation of the infrapatellar fat. The is called a fibrous cortical defect if it measures less thickened synovial and pannus show enhancement than 2 cm and non-ossifying fibroma if it measures 2 with gadolinium. With the progression of the disease, more than 2 cm . The characteristic radiographic changes in cartilage and bone are observed as a appearance is a lytic lesion with well-defined scle- 3 decrease or loss of joint space and bone erosions. rotic margins, eccentric and cortical-based . As the In a chronic stage, the degree of synovial thickening injury matures, its sclerotic component increases. depends on both the underlying inflammatory pro- The MRI signal varies depending on the degree of

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maturation. They have high signal on T2 in the early Osteosarcoma stage, then heterogenous and finally low signal It is the most frequent malignant primary bone 2,3 when the lesion is mature . These characteristics neoplasm around the knee2,3. It is usually observed correspond to the initial radiolucent appearance in the second decade of life and typically presents as 2 on radiography and later with sclerosis . Large a painful mass3. The origin is usually metaphyseal non-ossifying fibromas can be complicated by and medullary. The most common site is found in the 3 pathological fractures (Figure 16) . long bones of the lower limb, especially around the knee, followed by the humerus3. Radiography shows Osteochondroma aggressive lytic bone lesions, blastic or mixed with These are frequent injuries around the knee indistinct margins, cortical destruction, aggressive that arise when the growth cartilage plate begins periosteal reaction and soft tissue mass3,19. It is to move towards the metaphyseal region. It is an characterized by production of osteoid matrix3,19. MRI extension of normal bone showing continuity with is crucial for the local staging and to determine the the periosteum, cortex and medulla of the underlying extent of bone marrow, transphyseal, intraarticular, 3 bone . The majority can be diagnosed only by radio- and soft tissue involvement (Figure 19)3. graphy because of the characteristic of cortical and 2 medullary continuity with the native bone . They may Ewing’s sarcoma cause local complications by effect of mass such It is the second most common malignant pri- as bursitis (developing bursae on these tumors at mary bone tumor of infancy and young adults after the sites of friction), compression of the neurovas- osteosarcoma3,20. It is a primitive neuroectodermal cular package and formation of pseudoaneurysms tumor that is observed in the axial and appendicular 2,18 (Figure 17) . skeleton with the same frequency, whereas the vast majority of the osteosarcomas occur in the appendi- Chondroblastoma cular skeleton3. Within the long bones it is generally It is a rare benign tumor of immature cartilage, observed in the diaphysis and metadiphysis3. On with a peak incidence between 10 and 20 years of radiography it has a variable but generally aggres- age. About 50% of these tumors are diagnosed in sive appearance. The imaging findings include a patients with an immature skeleton. It has a pre- permeative, mixed lytic-sclerotic appearance with dilection for the epiphyses of the long bones, with destruction of the cortical and aggressive perios- the bones around the knee being most frequently teal reaction (spiculate or onion skin)3,20. It does affected. They often traverse the growth plate not usually produce a mineralized osteoid matrix toward the adjacent metaphysis. On x-ray the lesion as seen in osteosarcoma. MRI is crucial for local is eccentric, lytic, with geographic borders. Some staging and evaluation of skip lesions3. It is hypo calcifications can be seen within the lesion reflecting or isointense to the muscle in sequences sensitive 3 chondroid matrix (Figure 18) . to liquid due to its dense cellularity. A lytic lesion in

Figure 16. Fibrous cortical defect. Radiography in AP and lateral projection. Sclerotic bordered radiolucent image, Figure 17. X-ray in AP and lateral projection. Bone sessile geographic, well defined, without periosteal reaction, in exostoses which represent osteochondromas in the context the posteromedial cortex of the distal femoral diaphysis. of multiple hereditary exostoses. 130 Revista Chilena de Radiología, año 2016. ARTÍCULO DE REVISIÓN

Figure 18. Chondroblastoma. X-ray in AP projection. Oval lytic lesion at the proximal end of the tibia, compromising part of the metaphysis and epiphysis in relation to the medial tibial plate.

Figure 19. Osteosarcoma. X-Ray in AP and lateral projection (a) and FAT SAT T1 sequence MRI with coronal and sagittal gadolinium (b). Distal femoral metadiaphyseal focal bone lesion, mixed, predominantly sclerotic, ill defined, associated with areas of cortical destruction, poorly delimited margins and aggressive periosteal reaction. 131 Dr. Cristián Padilla C, et al. Revista Chilena de Radiología, año 2016; 22(3): 121-132. a child associated with a large mass of soft tissue on selected pediatric and adolescent knee conditions. should be suspected of being an Ewing’s sarcoma3. Pediatr Radiol. 2013; 43 Suppl. 1:S99-106. A curative treatment is possible in approximately 6. Pai D, Strouse PJ. MRI of the pediatric knee. AJR Am two-thirds of patients with localized Ewing’s sarcoma; J Roentgenol. 2011; 196: 1019-|027. 7. Samim M, Smitaman E, Lawrence D, Moukaddam however, the prognosis in patients with metastatic 20 H. MRI of anterior knee pain. Skeletal Radiol. 2014; disease is poor . 43: 875-893.Documento descargado de http://www. elsevier.es el 18-11-2016 Conclusion 8. Amis AA. Current concepts on anatomy and biome- The knowledge of the normal variants of the chanics of patellar stability. Sports Med Arthrosc. 2007; 15: 48-56. development and of the exclusive or more frequent 9. Merrow A, Reiter MP, Zbojniewicz AM, Laor T. Avulsion pathological conditions in the pediatric knee will fractures of the pediatric knee. Pediatr Radiol. 2014; allow a better interpretation of the imaging studies 44: 1436-1445. that are performed in children, as well as to diffe- 10. McKenna S, Hamilton SW, Barker SL. Salter Harris rentiate those pathologies that require only x-ray fractures of the distal femur: Learning points from two for its diagnosis and to focus the MRI for uncertain cases compared. J Investig Med High Impact Case findings or more precise indications about the extent Rep. 2013; 1: 1-4. of the evaluated lesion. 11. Strouse PJ. MRI of the knee: Key points in the pediatric population. Pediatr Radiol. 2010; 40: 447-452. 12. Wegmann H, Tschauner S, Singer G, Marterer R, Eberl Ethical Responsibilities R, Sorantin E. The pediatric knee: Diagnosis and ma- Protection of people and animals. The authors nagement of ligament injuries. Semin Musculoskelet declare that no experiments have been performed Radiol. 2014; 18: 489-497. on humans or animals for this research. 13. Suzue N, Matsuura T, Iwame T, Higashino K, Sakai Confidentiality of data. The authors declare that T, Hamada D, et al. State-of-the-art ultrasonographic findings in lower extremity sports injuries. J Med Invest. they have followed the protocols of their work center 2015; 62: 109-1013. on the publication of patient data. 14. Valentino M, Quiligotti C, Ruggirello M. Sinding-Larsen- Right to privacy and informed consent. The authors Johansson syndrome: A case report. J Ultrasound. 2012; 15: 127-129. declare that no patient data appears in this article. 15. Ghahremani S, Griggs R, Hall T, Motamedi K, Boechat MI. Osteochondral lesions in pediatric and adolescent Conflict of interests patients. Semin Musculoskelet Radiol. 2014; 18: 505- The authors declare no conflict of interest. 512. 16. Arnold J, Bradley JS. Osteoarticular infections in chil- Bibliography dren. Infect Dis Clin North Am. 2015; 29: 557-574. 1. Orth R. The pediatric knee. Pediatr Radiol. 2013; 43 17. Doria A, Keshava SN, Mohanta A, Jarrin J, Blanchette Suppl. 1: S90-8. V, Srivastava A, et al. Diagnostic accuracy of ultrasound 2. Gill K, Nemeth BA, Davis KW. Magnetic resonance for assessment of hemophilic arthropathy: MRI corre- imaging of the pediatric knee. Magn Reson Imaging lation. AJR Am J Roentgenol. 2015; 204: W336-47. Clin N Am. 2014; 22: 743-763. 18. Kransdorf MJ, Peterson JJ, Bancroft LW. MR imaging 3. Khanna G, Bennett DL. Pediatric bone lesions: Beyond of the knee: Incidental osseous lesions. Radiol Clin the plain radiographic evaluation. Semin Roentgenol. North Am. 2007; 45: 943-54. 2012; 47: 90-9. 19. Moore D, Luu HH. Osteosarcoma. Cancer Treat Res. 4. Thapa M, Chaturvedi A, Iyer RS, Darling SE, Khanna 2014; 162: 65-92. PC, Ishak G, et al. MRI of pediatric patients: Part 2, 20. 6McCarville M, Chen JY, Coleman JL, Li Y, Li X, normalvariants and abnormalities of the knee. AJR Am Adderson EE, et al. Distinguishing osteomyelitis from J Roentgenol 2012; 198: W456-65. Ewing sarcoma on radiography and MRI. AJR Am J 5. McKay S, Chen C, Rosenfeld S. Orthopedic perspective Roentgenol. 2015; 205: 640-51.

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