The rising bubble sign is considered pathognomonic and occurs when a gas bubble is seen at the most non-dependent part of the UBC 8,10. If there is a fracture through this lesion a dependent bony fragment may be seen, and this is known as the fallen fragment sign. Prominent ridges of bone can appear as pseudotrabeculation on x-ray but in fact, UBC is usually unilocular. Rarely, they are truly multiloculated, which can occur after repeated fractures 3,10. They sometimes expand the bone with thinning of the endosteum without any breach of the cortex unless there is a pathologic fracture. Unicameral bone cysts are well defined geographic lucent lesions with a narrow zone of transition, mostly seen in skeletally immature patients, which are centrally located and show a thin sclerotic margin in the majority of cases with no periosteal reaction or soft tissue component. CT and MRI add little to the diagnosis, however, can be helpful in eliminating other entities that can potentially mimic a simple bone cyst (see differential diagnosis below) 8. Plain radiographs are the first-line imaging modality. UBCs can be rarely seen in adults in unusual locations such as in the talus, calcaneus, or the iliac wing. Occurrence elsewhere is relatively uncommon, and usually occurs in adultsĪs bone growth progresses the cyst loses its connection to the physis migrating into the diaphysis and subsequently healing. They are typically intramedullary and active cysts are found in the metaphysis of long bones, abutting the growth plate 1. As the lesion becomes inactive it migrates away from the growth plate (normal bone is formed between it and the growth plate) and it gradually resolves 3,5. When uncomplicated by fracture the cysts contain clear serosanguineous fluid surrounded by a thin fibrous membranous lining.ĭuring the active phase, the cyst remains adjacent to the growth plate. The etiology and pathogenesis are unknown 8,10. The most frequent presentation is due to pathological fracture 1,2,6. These lesions are usually asymptomatic and found incidentally, although pain, swelling and stiffness of the adjacent joint also occur. DiagnosisĮssential: simple cyst lacking a true lining with typical imaging featuresĭesirable: fibrin-like deposits +/- mineralization forming cementum-like structures Active unicameral bone cysts occur most frequently between the ages of 1 and 10 years. They are more common in males (M:F ~ 2-3:1) 2,6. UBCs are usually found in children in the 1st and 2nd decades (65% in teenagers) with the mean age at diagnosis being 9 years 8.

Bone fragments classic