There are a number of normal fat pads around the knee, and all can be useful. On frontal view, these fat pads are seen adjacent to the distal femur (Fig. 10.1a) and can become obliterated with deep edema. On lateral view, similar fat pads are seen posterior to the distal femur (Fig. 10.1b). Other fat pads include the prefemoral fat pad just over the anterior surface of the distal femur, the infrapatellar or Hoffa’s fat pad inferior to the patella, and the suprapatellar fat pad which exists between the insertion of the quadriceps tendon and the neck of the suprapatellar joint bursa (Fig. 10.1b). The infrapatellar fat pad commonly is obliterated by the edema associated with Osgood–Schlatter and Sinding–Larsen–Johansson disease. With prepatellar cellulitis, the soft tissues anterior to the quadriceps tendon, patella, and infrapatellar tendon become thickened and trabeculated. In the normal knee, the infra- and suprapatellar tendons usually are clearly identified (Fig. 10.1b).

The knee joint does not widen significantly with accumulations of fluid, and thus one must depend on surrounding soft tissue and fat pad changes for the detection of fluid in the knee joint [1]. In this regard, it is the lateral view of the knee that is most useful. On this view, fluid almost always first visibly accumulates in the suprapatellar bursa, which lies behind the quadriceps tendon and in front of the prefemoral fat pad. If this occurs, the quadriceps tendon is displaced anteriorly and the fat pad posteriorly, and the fluid-filled suprapatellar bursa blends with the quadriceps tendon to make it look thicker than normal (Fig. 10.2).

Fluid in the knee also is detectable with ultrasound, CT, and MR imaging. Ultrasound, however, is noninvasive and easily demonstrates the presence of knee joint fluid. However, because plain films are so dependable, seldom is there need for any of these studies to be performed for the sole purpose of detecting fluid in the knee joint.

Cortical buckle (torus) fractures around the knee are uncommon in older children because the cortices of both the distal femur and proximal tibia are sturdy. However, in infants and young children, they are common both in the distal femur and proximal tibia (Fig. 10.3). These fractures can result from simple axial loading injuries but also commonly are seen with hyperextension injuries. In this regard, in the upper tibia, buckle fracturing of the anterior tibial cortex along with an associated posterior diastatic hairline fracture is common [2–5] and often due to a trampoline-induced injury [6]. In such cases, when one falls off the trampoline, the fall is braced with the hyperextended lower extremity and stresses are placed on the upper tibia (Fig. 10.4a, b). On AP view of the knee, one may see a transverse (Fig. 10.4c) or slightly oblique fracture which often is hairline. In addition, visualizing this fracture on the AP view probably occurs in less than 50 % of cases. In some cases, an associated cortical buckle fracture is seen (see Fig. 10.5c).

All of the foregoing notwithstanding, in most cases, it is the lateral view of the knee which is most informative with this fracture where the findings consist of (1) buckling of the anterior tibial cortex, (2) increased concavity of the notch for the tibial tubercle, (3) anterior tilting of the epiphyseal plate, and (4) posterior diastatic fracture (Fig. 10.4b). In many cases, these findings are very subtle and comparative views are invaluable (Fig. 10.5a, b). This fracture is very common in infants and young children, and we have arbitrarily named it the toddler II fracture and the spiral fracture described by Dunbar (see Fig. 10.28) as the toddler I fracture.

Upper tibial transverse fractures also can result from lateral forces applied to the knee, and both this fracture and the toddler II fracture can result in a valgus deformity of the tibia [7–10]. This deformity, often initially minimally present, can become more pronounced and persistent as time goes by (Fig. 10.6). For the most part, this is believed to result from the fact that there is overgrowth of the medial aspect of the proximal tibia. In this regard, it has been noted that normally there is a greater blood supply to the upper medial tibia and that, with the hyperemia associated with fractures, the growth accelerated and overgrowth occur [8, 9].

Epiphyseal–metaphyseal injuries are not that common about the knee but do occur. In gross form, they are not difficult to detect, but the more subtle, nondisplaced Salter–Harris type I and II injuries frequently elude initial observation. In these cases, one should be suspicious of the slightest degree of widening of the epiphyseal plate (Fig. 10.7a). Comparative views are very helpful and it must be emphasized that positioning must be accurate for the slightest malpositioning of the knee may erroneously suggest a wide epiphyseal plate.

A variation of the usually Salter–Harris fracture is the one which occurs when the proximal tibial epiphysis is totally or partially avulsed (Fig. 10.7b, c) and finally MR imaging is especially useful in detecting epiphyseal metaphyseal injuries which are occult or invisible on plain films (Figs. 10.8 and 10.9). MRI has been shown to change management in cases of epiphyseal–metaphyseal injuries by better defining the extent of the fracture and in some cases by diagnosing unsuspected fractures [11].

Stress fractures around the knee most commonly occur in the upper tibia in older children. A fracture line usually is not visible but with healing typical sclerosis and periosteal new bone deposition are seen posteriorly (Fig. 10.10a). In the early stages, these changes can be subtle, but later periosteal new bone deposition is profound, and often a more serious lesion such as a bone tumor is erroneously suggested. Less commonly stress fractures occur in the upper anterior tibia, and both of these fractures are the equivalent of the toddler II fracture seen in infants and young children. The findings in the latter fracture usually consist of focal sclerosis in the upper anterior tibia (Fig. 10.10b–d). All of these findings also can be demonstrated with CT and MR where the occult fracture line often becomes visible. MR is valuable in differentiating stress reaction from stress fracture which has therapeutic implications with stress fractures requiring longer activity modification and rest.

Although not true stress fractures, impaction fractures of the distal femurs and proximal tibia can be considered within the ball park of stress fractures. In this regard, it is the tibia more often than the femur which is involved. Most often an axial loading force is exerted on the knee, and in most cases, the plain films are basically are normal. However, MR usually demonstrates the area of impaction (Fig. 10.10e, f).

Patellar fractures and dislocations are common. Overt fractures usually occur from direct blows to the patella and occasionally from acute quadriceps tendon stress-induced injuries (Fig. 10.11). In these latter cases, the proximal half or third of the patella is avulsed from the remainder of the patella, and the findings are readily recognizable.

In terms of patellar dislocations, both acute and chronic repetitive dislocations of the patella are common in childhood, but in most cases, the patella relocates before radiographs are obtained. Patellar dislocations usually result from twisting of the knee wherein the medial ligamentous stabilizers (retinacula) rupture and the medial margin of the patella strikes against the lateral femoral condyle [12]. In some cases of patellar dislocation, an avulsed fracture fragment is seen along the medial aspect of the patella (Fig. 10.12a). Transient patellar dislocation often results in a large hemarthrosis making clinical examination difficult. Often an ACL rupture is erroneously suspected clinically, and an MRI is ordered which reveals the correct diagnosis. With MR imaging the retinacular injury, the avulsed bony fragment, and bone bruises involving the medial margin of the patella and lateral femoral condyle are readily demonstrable (Fig. 10.12b–d). Finally, it should be noted that the avulsed medial patellar fracture fragment is not mobile and usually stays in one position. In addition, it often is the only finding seen in cases of patellar dislocation with subsequent relocation.

Lateral condylar fractures resulting from patellar dislocation are less well appreciated on plain films, but in some cases, one may see the fracture donor site (Fig. 10.13a). In other cases, the avulsed bony fragment, as opposed to the medial patellar fragment, can be very mobile and can be seen anywhere within the knee joint (Fig. 10.13b, c). On subsequent MR imaging, one usually can see the fracture site and the associated bone bruise of the lateral condyle (Fig. 10.13d, e).

Patients with chronic recurrent patellar dislocation often have associated patella alta, a condition in which the patella is higher in position than normal. Abnormal tracking of the patella results, and the patellofemoral groove becomes shallow [13]. Patella alta is best seen on lateral view, but the shallow groove and associated flattening of the lateral condyle are best seen on skyline views (with 30 % flexion) of the patella. Most dislocations, whether acute or chronic, are lateral dislocations.

The role of MRI is not only to diagnose unsuspected transient patellar dislocations but also to evaluate for patellofemoral dysplasia, osteochondral fractures [14], and presence of osteochondral loose bodies. Patellar dislocations with loose bodies are usually treated surgically. Patients without loose bodies are treated non-operatively with bracing and rehabilitation.

Other avulsion fractures around the knee [15] included suprapatellar quadriceps tendon avulsions. These usually are fairly straightforward as the avulsed fracture fragment is clearly obvious. They are not nearly as common as the other two avulsion fractures/tendon injuries, that is, Sinding–Larsen–Johansson disease [16] and Osgood–Schlatter disease [17–19].

With Osgood–Schlatter disease, the infrapatellar tendon, as it inserts on to the tibial tubercle, can avulse part of the tubercle, or the tendon may simply undergo a tendon injury. Clinically, findings consist of pretubercular swelling and pain, and on imaging, swelling is present over the tibia tubercle. In addition the insertion of the infrapatellar tendon is edematous, and tendon itself appears swollen (Fig. 10.14a). In many cases but not all, an avulsed fracture fragment can be seen (Fig. 10.14b). With healing, one can see considerable hypertrophic bone formation (Fig. 10.15) and eventually a small acquired exocytosis. All of these configurations are virtually pathognomonic of Osgood–Schlatter disease but occasionally can be mimicked by the very rare periosteal chondroma of the tibial tubercle [20]. This lesion is more bulky, mostly lytic, and associated with destruction of the upper tibia under the tibial tubercle.