A 12-year-old boy presents with lateral knee pain after a football injury.

Figure 1A is a frontal radiograph of the left knee that shows an avulsion fracture adjacent to the lateral femoral condyle. The lateral view revealed a suprapatellar joint effusion. Figures 1B and 1C are coronal and axial fat-suppressed fluid-sensitive images, respectively, of the same knee showing the avulsion. Attached to the avulsed fragment is the popliteus tendon. There is a moderate joint effusion with a fluid-fluid level.

This is a case of isolated popliteus tendon avulsion. The popliteus tendon, a component of the posterolateral corner of the knee, originates from a groove at the lateral femoral condyle, just inferior to the lateral collateral ligament attachment site. The tendon courses inferomedially and the muscle inserts into the posteromedial surface of the proximal tibial metaphysis. The muscle also has origins from the fibular head and the lateral meniscus [3]. The popliteus muscle opposes lateral meniscal displacement and limits posterior tibial translation and rotation [4, 5]. Although injury to the popliteus tendon is not rare, isolated avulsion fracture of the popliteus tendon is [3, 6].

In 1990, Gruel [7] first described this lesion in a 15-year-old girl who sustained a twisting injury to her knee. In the skeletally immature population, the increased incidence of such avulsion injuries is likely secondary to the relative weakness of bone at tendon insertion sites [8]. It is, therefore, not surprising that adults are more likely to experience intrasubstance tears of the popliteus tendon [5, 6]. Anterior translation of the femur with respect to the tibia accompanied by rotation during knee flexion is thought to be the mechanism of injury.

The popliteus tendon is a dynamic stabilizer, and reflex muscular contraction is likely to contribute to the injury and helps explain its isolated nature [3, 8, 9]. The ideal management of patients with isolated popliteal tendon avulsion injuries is controversial. Success has been reported with both nonsurgical and surgical treatments [7???11].

A 16-year-old boy presents with knee pain and swelling after sustaining an injury during soccer practice.

Sagittal T2 fat-suppressed (Fig. 2A) and proton density (Fig. 2B) images of the knee show a nondisplaced avulsion fracture (type I) of the anterior tibial spine. There is edema in the avulsed fragment and the underlying tibial epiphysis. The anterior cruciate ligament (ACL) is normal. Note the moderate joint effusion.

A more common bony avulsion injury of the skeletally immature knee is the tibial spine avulsion fracture. Before physial closure, anterior tibial spine avulsions are relatively common, whereas ACL tears become more prevalent after skeletal maturity [12]. The chondroosseous tibial spine is the weakest component of the ACL complex in the skeletally immature patient. Kocher et al. [13] have shown that the size of the intercondylar notch also influences the type of injury sustained. Anterior tibial spine avulsion injuries are more common in patients with a wide intercondylar notch, whereas those with a narrow intercondylar notch are more likely to suffer intrasubstance ACL tears. Tibial spine avulsions may be radiographically subtle or may be occult on conventional views, so oblique and tunnel projections may be helpful [14].

The multiplanar nature of MRI greatly aids in visualization of the avulsion injury. If acute, there is edema in the avulsed fragment and the underlying tibial epiphysis. There may also be high or intermediate signal within the substance of the ACL, indicating a partial-thickness ligament tear [15]. Based on the Meyers and McKeever [16] classification system, a tibial spine avulsion fracture is categorized as type I, II, or III. Type I is nondisplaced and does not inhibit knee extension. A type II fracture is displaced anteriorly and hinged posteriorly. With a type II fracture, knee extension is limited and the anterior horn of a meniscus may be trapped between fracture fragments. There is complete displacement in a type III fracture, and the patient typically holds the knee in a mildly flexed position.

Type I fractures are treated conservatively, whereas type II and type III injuries require reduction and fixation of the avulsed fragment [2]. This patient was treated nonsurgically.

A 16-year-old boy who is a long-distance runner presents with a 3-month history of anterior proximal tibial pain.

A coned-down frontal radiograph (Fig. 3A) of the left tibia shows sclerosis in the proximal tibia accompanied by surrounding periosteal reaction. A coronal STIR image (Fig. 3B) clearly shows the low-signal fracture line surrounded by marrow edema. There is also soft-tissue edema. These findings are consistent with a proximal tibial stress fracture.

Repeated stress on normal bone???particularly if the strain is concentrated on a focal area not meant to withstand the load???can lead to a stress fracture [17]. For the skeletally immature population, the approximate distribution of stress fractures is as follows: tibia, 50%; fibula, 20%; pars interarticularis of the lumbar spine, 15%; femur, 3%; metatarsals, 2%; and tarsal navicular, 2% [17]. Although radiography is typically the first imaging study performed when a stress fracture is suspected, the initial radiographs show positive findings for a stress fracture in only 10% of the cases, probably because radiographic findings require 2???10 weeks to manifest. Positive findings include periosteal reaction, endosteal cortical thickening, and fracture lucency [17].

When radiographs are negative, MRI is considered the examination of choice to evaluate for a stress fracture [18]. MRI is usually recommended rather than bone scintigraphy because MRI can more precisely discriminate between a stress fracture and other entities that may appear similar to a stress fracture on bone scintigraphy such as osteoid osteoma, osteomyelitis, a ligamentous injury with secondary osseous reaction, and neoplasm [17].

A clinically validated grading system for stress fractures has been proposed by Fredericson et al. [19]. In grade 1 injuries, high T2 signal is confined to the periosteum. If there is high T2 signal in the marrow without accompanying abnormal T1 marrow signal, then the injury is classified as grade 2. Abnormal low T1 signal within the marrow indicates a grade 3 injury. If a cortical signal abnormality (i.e., linear fracture line or a broader, less linear area) accompanies the marrow signal abnormalities, then the lesion is classified as grade 4 (Table 1). Grades 1, 2, and 3 are referred to as a ???stress response??? or ???stress reaction,??? whereas grade 4 represents a true stress fracture. As one would expect, each increasing grade corresponds to an increasing severity of injury and influences the amount and type of activity the patient can perform [20]. Most tibial stress fractures are focal and occur on the posteromedial side near the proximal metadiaphysis. Shin splints, on the other hand, involve a longer segment of the tibia and represent an enthesopathy. Signal abnormality (i.e., high T2) is confined to the periosteum and does not extend to the marrow [21].

A 4-year-old boy presents with chronic knee pain. Radiographic findings were negative.

Coronal STIR (Fig. 4A) and sagittal proton density (Fig. 4B) images show an abnormally large and thick lateral meniscus with internal intermediate-to-high signal; these findings indicate mucinous degeneration. The abnormal signal does not reach the articular surface. Note the normal shape and size of the medial meniscus.

This is a case of a lateral discoid meniscus. A discoid meniscus is an abnormally wide and thick meniscus that is prone to degeneration and tears. Most discoid menisci occur on the lateral side. The reported incidence ranges from 0.4% to 16.6% [22]. Several quantitative MR criteria for diagnosing discoid menisci have been described. The simplest and most popular method was described by Silverman et al. [23]: If three or more 5-mm-thick contiguous sagittal images show continuity of the anterior and posterior horns of the meniscus, the diagnosis of a discoid meniscus should be made. Araki et al. [24] claim the narrowest transverse width of the midsegment of the meniscus on a coronal slice should not exceed 14 mm. A discoid meniscus may be symptomatic even in the absence of internal signal abnormalities. However, diffuse increased internal signal intensity indicating mucinous degeneration is usually present and may extend to the articular surface. Other findings include meniscal tears, para- or intrameniscal cysts, and posterior horn extrusion. If a discoid meniscus is torn and displaced, a diagnosis of discoid meniscus may be difficult to make [2].

A 13-year-old girl who is on a volleyball team presents with chronic knee pain.

Coronal proton density (Fig. 5A) and STIR (Fig. 5B) images of the knee show an irregularity at the lateral aspect of the medial femoral condyle; this finding is an osteochondral defect (OCD). Note that the thin rim of hyperintensity on the STIR image is not as bright as the joint fluid. There is a small cyst associated with the OCD, but the cyst is singular and measured less than 5 mm. The overlying cartilage appears intact. This case would be characterized as stable juvenile OCD.

Most OCDs are caused by repetitive trauma, but other causes include familial dysplasia, avascular necrosis, and fat emboli [25???27]. Approximately 70% of the OCDs in the knee occur at the posterolateral aspect of the medial femoral condyle. The remaining cases occur in the central lateral femoral condyle (15???20%) and patella (5???10%) [28]. The incidence of OCDs of the knee in pediatric athletes is increasing, possibly secondary to increasing participation in competitive sports [28]. OCDs are segregated into one of two groups: juvenile or adult. A juvenile OCD occurs in skeletally immature patients and carries a more favorable prognosis than an adult OCD. A juvenile OCD usually heals completely with conservative management, but an adult OCD is more likely to require surgical intervention, lead to premature degenerative changes, become unstable, and worsen clinically [13, 20, 28???30].

A classic OCD located in the medial femoral condyle is best identified in the notch view of conventional radiographs. The lesion is typically oval with variable amounts of central lucency and surrounding sclerosis. MRI can be very helpful in determining the stability of an OCD. In adult OCD, MRI findings of instability include a rim of high T2 signal around the lesion, cysts underlying the lesion, multiple marginal discontinuities in the subchondral bone at the lesion, a second rim of low T2 signal surrounding the lesion, and displaced intraarticular fragments. Because patients with juvenile OCDs have a greater ability to heal, the criteria to suggest instability are slightly more guarded: The high-signal T2 rim must be as bright as fluid, not just relatively bright as in the adult cases; and the cysts underlying the lesion must be multiple in number or larger than 5 mm [20, 31].

The clinical outcome of OCDs in children is more favorable than that in adults. However, part of the reason for the better outcomes in children is likely from mistakes in diagnosis because some normal variants can mimic an OCD. For example, a normal secondary ossification at the posterior aspect of the lateral femoral condyle may be mistaken for an OCD (Figs. 6A and 6B). Careful evaluation will reveal normal overlying cartilage and no associated bone marrow edema [2, 32, 33].

A 15-year-old boy presents with knee pain after a skiing injury.

Axial T2 fat-saturated (Fig. 7A) and coronal STIR (Fig. 7B) images of the left knee show the sequelae of transient patellar dislocation. On the axial image, there is edema in the medial patella with a delamination injury to the patellar and trochlear cartilage. Soft-tissue edema and thickening medially indicate injury to the medial retinaculum. The coronal image shows edema in the lateral femoral condyle from patellar impaction before relocation.

Transient patellar dislocation is a frequent injury in adolescents and young adults and commonly occurs in patients who are between 14 and 20 years old [34]. The mechanism of injury involves the quadriceps applying lateral tension on the patella while a flexed knee is twisted [35]. Most dislocations relocate spontaneously.

Radiographic findings include a large joint effusion that is sometimes accompanied by a medial patellar avulsion fracture [36]. MR findings are classic for the mechanism of injury. Typically, there are impaction contusions of the lateral femoral condyle and the inferomedial patella. Edema in the medial soft tissue likely indicates injury to the medial retinaculum, the medial patellofemoral ligament, or both. Cartilage injury at the medial patellar facet or lateral trochlea is common and occurs in up to 72% of transient patellar dislocations [34]. MRI may also reveal predisposing factors for transient patellar dislocation, such as trochlear tilt, femoral trochlear dysplasia (shallow sulcus), or patella alta. However, care must be taken before diagnosing lateral subluxation or patella alta when interpreting images of an extended knee [34].

A 12-year-old girl who plays basketball presents with chronic anterior knee pain.

A lateral radiograph (Fig. 8A), sagittal proton density image (Fig. 8B), and sagittal T2 fat-suppressed image (Fig. 8C) of the left knee show some bony fragmentation at the inferior pole of the patella with associated bone marrow, mild patellar tendinopathy, and edema in Hoffa fat pad. The patellar cartilage is intact. These findings are consistent with Sinding-Larsen-Johansson disease.

In 1921 Sinding-Larsen described a condition in adolescents consisting of tenderness at the anterior knee with inferior patellar fragmentation on radiographs. A year later, Johansson independently described a similar condition that later became know as Sinding-Larsen-Johansson disease [37]. It is important to differentiate this entity from patellar sleeve avulsion and jumper???s knee because treatment varies depending on the condition. Jumper???s knee is a pain syndrome often occurring in adolescent athletes and is caused by chronic inflammation or irritation at the proximal or distal attachment sites of the patellar tendon. The tendon is often thickened but without frank tears or avulsions [14]. A patellar sleeve avulsion typically occurs from a more acute injury, and there is damage to the inferior patellar cartilage. Lateral radiographs may reveal an osseous fragment below the lower pole of the patella. MRI may be necessary to differentiate this entity from Sinding-Larsen-Johansson disease, which involves only the bony patella and has no cartilaginous involvement [38].

In all three conditions, the mechanism of injury is similar; it is believed to be a forceful contraction of the quadriceps against resistance, particularly in adolescent male athletes. The injury usually happens in isolation, and there is no known association with damage to other structures of the knee [39]. The more chronic conditions of jumper???s knee and Sinding-Larsen-Johansson disease are managed conservatively, whereas patellar sleeve avulsion often requires open reduction with internal fixation and extensor mechanism reconstruction [38].

A 12-year-old girl presents with a nontraumatic swollen knee.

An axial T1 gadolinium-enhanced image (Fig. 9) of the left knee at the level of the patellofemoral joint shows thick, nodular synovial enhancement with a moderate joint effusion.

This is a case of juvenile idiopathic arthritis. Juvenile idiopathic arthritis is a relatively uncommon condition (incidence: 5???18 cases per 100 persons; prevalence: 30???150 cases per 100,000 persons) characterized by persistent joint swelling, often of a single joint and most commonly of the knee [40]. Although radiographs have been used to establish a baseline and monitor disease progression in joints, incomplete ossification of the immature skeleton limits the role of conventional radiographs in children. The soft-tissue swelling and joint effusions seen on radiographs are nonspecific, and bony changes do not occur early enough in the disease to enable early diagnosis [41???43].

MRI is regarded as the most sensitive technique for the investigation of juvenile idiopathic arthritis [44???47]. Johnson et al. [43] concluded that synovial hypertrophy, joint effusions, and popliteal lymph nodes were present on MRI of all patients with early juvenile idiopathic arthritis in their study. In addition, gadolinium administration improved the detection of synovial hyperplasia [43]. Thickened (> 3 mm) enhancing synovium is typically identified in the suprapatellar bursa. Hypointense rice bodies can sometimes be identified within the joint space. They represent detached synovial fragments. Other less common findings that occur later in the disease process include cartilage thinning, bony erosions, meniscal hypoplasia, and popliteal synovial cysts [2].

Although this patient was eventually diagnosed with juvenile idiopathic arthritis, septic arthritis can look similar. Other conditions that can cause similar synovial thickening and enhancement include pigmented villonodular synovitis and hemophilia. Gradient-echo sequences can be helpful in differentiating these causes from juvenile idiopathic arthritis. Pigmented villonodular synovitis shows nodular hemosiderin deposition, resulting in blooming related to the susceptibility artifact of paramagnetic hemosiderin. This nodular hemosiderin deposition is in contradistinction to hemophilic arthropathy where the widespread hemosiderin deposition throughout thickened synovium results in a more diffuse appearance of susceptibility artifact. In addition, the osseous deformities seen in chronic hemophilia are not seen as a classic feature of pigmented villonodular synovitis [48]. Finally, the synovial enhancement and joint effusion seen with septic arthritis can be differentiated from other causes primarily on the basis of clinical features and laboratory data, including the results of joint aspiration in suspicious cases.

An 8-year-old girl presents with right knee pain after falling.

A frontal radiograph (Fig. 10A) of the right knee shows a well-defined lucency in the medial femoral metadiaphysis. This lucency is confirmed to be within the posteromedial metadiaphysis on a lateral radiograph (Fig. 10B). An axial reconstruction of a 3D fluid-sensitive gradient sequence (dual-echo steady-state) (Fig. 10C) shows the lesion to be mildly hyperintense. There is no associated soft-tissue mass or adjacent bone marrow edema. This finding represents a distal femoral metaphysial irregularity, a benign finding.

When evaluating the pediatric knee, it is important to be aware of common benign entities that may mimic abnormalities. A distal femoral metaphysial irregularity is a benign entity typically located at the posterior cortex of the medial femoral condyle metaphysis and is intimately associated with the adductor magnus insertion site. The irregularity likely results from chronic stress at the insertion site [49]. Other benign entities are distal femoral cortical irregularity and cortical desmoid.

On radiography, the lesion may appear as a rounded or scalloped lucency. Nonossifying fibromas (> 2 cm) and fibrous cortical defects (??? 2 cm) are also common benign lesions typically occurring near the metaphysis of the distal femurs of children. On radiography, they appear as scalloped lesions with geographic margins and narrow zones of transition.

On MRI, the lesion is usually low signal on T1-weighted sequences and bright on T2-weighted sequences. There may be slight contrast enhancement within the lesion, but there should be no adjacent soft-tissue enhancement [50]. The MR appearance of this irregularity parallels the radiographic appearance by showing well-defined margins. The lesions are usually dark on T1-weighted images, and the T2-weighted appearance is dependent on their stage of maturation. Uniform brightness on a T2-weighted sequence implies a relatively new lesion. As the lesion matures, the T2 brightness becomes more heterogeneous and less conspicuous [51]. Mild contrast enhancement is not rare.

Goodin et al. [52] characterized the anatomic appearance and metabolic activity of nonossifying fibromas, fibrous cortical defects, and cortical desmoids on PET/CT images. The fibrous lesions showed variable metabolic activity from mild to moderate to intense ¹⁸F-FDG uptake. CT, conventional radiography, MRI, or clinical follow-up was used to confirm the diagnoses of these fibroosseous lesions [52].

A 13-year-old boy presents with chronic anterior knee pain.

Coronal STIR (Fig. 11A) and dual-echo steady-state (Fig. 11B) images of the right knee show a bipartite patella with hyperintensity within the synchondrosis.

Most commonly located superolaterally, a bipartite patella represents the failure of fusion of a secondary ossification center of the patella. It is usually asymptomatic but may cause anterior knee pain if chronic or direct trauma interrupts the synchondrosis between the ossification centers [53]. In symptomatic cases, MRI may show abnormal signal within the synchondrosis, edema in the bipartite fragment, and cartilage discontinuity [2].