Teaching Case

A seven-year-old girl consulted the orthopaedic department after a fall onto her right wrist.

The initial radiograph showed no fracture. The plain radiograph was repeated after six weeks, thereby revealing a sharply defined lucent area in the distal radial diaphysis (Fig. 1a). This lucency is a new finding. The lesion is located at the posterior surface of the distal radius, best depicted on the lateral radiograph (Fig. 1b). There is an early subperiosteal reaction visible, covering the cortical defect.
A low dose CT scan through the cortical defect in the proximal radius is subsequently made, to evaluate this cyst-like lesion in greater detail. The lesion is sharply demarcated and has a fatty content with density around -130 HU (Hounsfield unit). This density is similar to the density measured in the medullar cavity (-141 HU) (Fig. 2a). The periosteal reaction is best seen in the sagittal plane (Fig. 2b).

Some other cases of a cyst-like cortical defect in children associated with trauma have been documented [1-3]. Cystic defects are rare compared with the frequency of fractures in children. They appear most often in the distal radius and a green stick fracture precedes the cystic lesion in almost all cases.

The pathogenesis of this type of lesions is believed to depend on the cortical breach with an intact, but detached periosteum. The periosteum is still tough in children, but can easily be detached. These lesions therefore appear most frequently (if not exclusively) in children. The most accepted theory regarding the origin of fat in the cystic lesion is proposed by Malghem et al [2]. Intracellular lipid, released from the damaged intramedullary lipocytes at the time of the fracture, migrates through the cortical defect, where it becomes trapped in the newly formed, traumatic subperiosteal haematoma. This theory is supported by CT imaging, where the density of the cystic lesion and the intramedullary fat are similar. These cyst-like lesions do not enlarge. There is often a time-lag of four weeks before they appear faintly, until the surrounding post-traumatic tissue becomes calcified [2].
These asymptomatic lesions occur adjacent to the fracture line in the undamaged part of the bone, but within the periosteal reaction zone [3]. Although our patient had no visible fracture on radiographs, it is to be assumed there would have been a minor transcortical defect.

The lipomatous content is a specific finding and excludes evolving entities. Because of these benign features, a further work-up is not performed. In case of doubt of the aetiology, an MRI can be helpful. The nature of this lipid-rich lesion is harmless and has no impact on fracture healing. Treatment is not necessary.