Presentation
Presented to the emergency department 3 weeks following a fall onto an outstretched hand. Examination revealed pain and restricted range of movement at the left wrist.
Patient Data
AP and lateral radiographs of the left wrist display an ovoid lucency at the distal radial metaphysis with adjacent sclerosis. There is cortical irregularity of the lateral radial metaphysis in keeping with a greenstick fracture. Two faint discrete areas of sclerosis at the metadiaphysis of the distal ulna are in keeping with undisplaced fractures.
At the lateral aspect of the metaphysis of the left distal radius there is a healing greenstick fracture. Within the new subperiosteal bone, an ovoid lesion is present which returns a homogenous high T1 signal, with complete signal loss on the fat-suppressed sequence, confirming intralesional fat. Bone marrow edema is present.
Radiograph taken 3 months later displays gradual reduction in size of the cyst and adjacent bone sclerosis, in keeping with healing.
Case Discussion
Post-traumatic osseous cysts are an uncommon finding and tend to affect children 1. They occur following non-displaced or minimally displaced fractures, particularly those at the radius 2, as in our case, although they may occur at other locations.
Their development is perceived to relate to disruption of the cortex following minor trauma, such as a buckle or greenstick fracture. Following such cortical damage, the overlying periosteum remains intact, which creates a potential space into which medullary fat may seep and become sequestered; this is demonstrated well at axial imaging with MRI. In addition to accurately locating the position of the lesion, MRI demonstrates the internal composition of the cyst. High T1 signal and complete fat suppression confirm a fatty lesion. The location of the cyst tends to be proximal to the fracture line 3.
The cyst does not impede fracture healing 4 and over time will decrease in size without risk of further pathological fracture.
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