Simple bone cysts (SBC) are common benign non-neoplastic lucent bone lesions seen mainly in childhood and typically remain asymptomatic.
SBC accounts for the 'S' in the popular mnemonic for lucent bone lesions FEGNOMASHIC.
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Terminology
The term unicameral bone cysts (UBC) is no longer recommended in the World Health Organization Classification of Tumors (5th ed.) since the lesion can rarely be truly multilocular 10.
Epidemiology
Simple bone cysts are usually found in children in their 1st and 2nd decades (65% in teenagers) with the mean age at diagnosis being 9 years 8. They are more common in males (M:F ~ 2-3:1) 2,6. Active simple bone cysts occur most frequently between the ages of 1 and 10 years.
Diagnosis
Diagnostic criteria are 10:
essential: simple cyst lacking a true lining with typical imaging features
desirable: fibrin-like deposits with or without mineralization forming cementum-like structures
Clinical presentation
These lesions are usually asymptomatic and found incidentally, although pain, swelling and stiffness of the adjacent joint also occur. The most frequent presentation is due to pathological fracture 1,2,6.
Pathology
The etiology and pathogenesis are unknown 8,10. When uncomplicated by fracture the cysts contain clear serosanguineous fluid surrounded by a thin fibrous membranous lining.
During the active phase, the cyst remains adjacent to the growth plate. 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.
Location
They are typically intramedullary and active cysts are found in the metaphysis of long bones, abutting the growth plate 1. Locations include 1,2,5:
proximal humerus: most common 50-60%
proximal femur: 30%
other long bones
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occurrence elsewhere is relatively uncommon, and usually occurs in adults
As bone growth progresses the cyst loses its connection to the physis migrating into the diaphysis and subsequently healing. Simple bone cysts can be rarely seen in adults in unusual locations such as in the talus, calcaneus, or the iliac wing.
Radiographic features
Plain radiographs are the first-line imaging modality. 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 radiograph
Simple 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. They sometimes expand the bone with thinning of the endosteum without any breach of the cortex unless there is a pathologic fracture. Prominent ridges of bone can appear as pseudotrabeculation on x-ray but in fact, SBC is usually unilocular. Rarely, they are truly multiloculated, which can occur after repeated fractures 3,10.
If there is a fracture through this lesion a dependent bony fragment may be seen, and this is known as the fallen fragment sign. The rising bubble sign is considered pathognomonic and occurs when a gas bubble is seen at the most non-dependent part of the SBC 8,10.
CT
Features on CT are similar to plain radiographs but CT has the advantage of characterizing extent, detecting radiograph-occult fractures, and assessing internal density (usually between 10-15 HU) 8.
MRI
MR signal characteristics for an uncomplicated lesion include 8,10:
T1: low signal
T2: high signal
T1C+: peripheral enhancement
Fluid-fluid levels can be seen in the setting of fibrous septations, which can enhance 8. Internal signal heterogeneity, periosteal reaction and soft tissue edema can be seen in the setting of fracture 8.
Nuclear medicine
Scintigraphy
Simple bone cyst on bone scintigraphy tends to appear as foci of photopenia (cold spot). This is referred to as the doughnut sign which results in increased uptake peripherally and a photopenic center. However, a pathological fracture would cause an increased radioisotope activity.
Treatment and prognosis
Intervention is usually not required for an asymptomatic lesion. If large and threatening to fracture, or causing deformity then an intralesional steroid injection can be performed 3-5. If fractured the bone usually heals normally 5. In some instances, surgery with curettage and bone grafting is required. Local recurrence rates are ~15% (range 10-20%) 10.
History and etymology
Simple bone cysts were initially described by the German pathologist Rudolf Virchow in 1891 8,9.
Differential diagnosis
General imaging differential considerations include 8,10:
giant cell tumor of bone: usually older, extending to the articular surface
non-ossifying fibroma: eccentric, cortical base
haemophilic pseudotumor (intraosseous)
aneurysmal bone cyst (ABC): usually eccentric
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