Fat is not just used by the body as bulk tissue. In addition to its role in storing energy and regulating hormone action, fat is used in some parts of the body for its mechanical properties. The anatomy of anterior knee fat is more complex than it appears at first sight and is capable of withstanding considerable compressive and shear stress. Specific lesions occur when such mechanical stress exceeds the physiological limits and are yet little known. Superficial fat can be the site of either acute injury by closed degloving called the Morel-Lavallée lesion or chronic injury, when subject to repeat excessive shear forces, due to more complex and less well-defined disruptions that result in pseudo-bursitis. There are three main anterior, intracapsular and extrasynovial fat pads in the knee joint, which are the infrapatellar fat pad (IFP) or Hoffa's fat pad, the quadriceps fat pad and the prefemoral fat pad. The IFP plays an important role as a mechanical shock absorber and guides the patella tendon and even the patella itself during flexion-extension movements. In response to repeated excessive stress, an inflammatory reaction and swelling of the IFP is first observed, followed by a fibrotic reaction with metaplastic transformation into fibrous, cartilaginous or bone tissue. More rarely, the two other deep fat pads (quadriceps and prefemoral) can, if subject to repeated stress, undergo similar restructuring inflammatory reactions with metaplasia resulting in tissue hardening, anterior pain and partial loss of function.

Congenital diaphragmatic hernia is a rare congenital malformation, as well as kidney ectopia. Among kidney ectopias, the intrathoracic one is the rarest. Those malformations concern more frequently boys, and affected more the left than the right side. Their association is poorly reported in the literature. We report the rare case of an early sonographic prenatal diagnosis of intrathoracic kidney at 22 weeks of gestation in a female fetus, completed on the follow-up by the diagnosis of an associated diaphragmatic hernia at 33 weeks of gestation. If chest mass is diagnosed prenatally or in neonate, ITK should be considered in a differential diagnosis, all the more if the ipsilateral renal fossa is empty. An associated DH should be searched if ITK is confirmed. Isolated ITK usually requires no specific treatment, in contrast with ITK associated with DH.

The knee is one of the most studied anatomical structures by magnetic resonance imaging (MRI). Bone abnormalities are very frequently detected, whether or not related to the symptoms for which imaging was indicated. The aim of this pictorial study is to review the most commonly observed bone abnormalities of the knee, bearing in mind that the interpretation of MR images should always take into consideration both clinical and laboratory data, as well as the results of conventional X-ray imaging.

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