Unicameral bone cyst

Last revised by Dr Henry Knipe on 12 May 2022

Unicameral bone cysts (UBC), also known as simple bone cysts, are common benign non-neoplastic lucent bony lesions that are seen mainly in childhood and typically remain asymptomatic. They account for the S (simple bone cyst) in FEGNOMASHIC, the commonly used mnemonic for lytic bone lesions. 

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.  They are more common in males (M:F ~ 2-3:1) 2,6. Active unicameral bone cysts occur most frequently between the ages of 1 and 10 years.

Diagnostic criteria are 10:

  • essential: simple cyst lacking a true lining with typical imaging features
  • desirable: fibrin-like deposits +/- mineralization forming cementum-like structures

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.

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.

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
  • 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. UBCs can be rarely seen in adults in unusual locations such as in the talus, calcaneus, or the iliac wing.

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.

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. 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, UBC 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 UBC 8,10.

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

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.

Unicameral 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.

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.

Unicameral bone cysts were initially described by the German pathologist Rudolf Virchow in 1891 8,9.

General imaging differential considerations include 8,10:

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Cases and figures

  • Figure 1: distribution of unicameral bone cysts
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  • Case 1: with fracture
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  • Case 2
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  • Case 3: with fracture
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  • Case 4
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  • Case 5: with fracture
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  • Case 6: probable UBC
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  • Case 7: with classic "fallen fragment" sign
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  • Case 10: calcaneal cyst
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  • Case 11: with fallen fragment
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  • Case 12: probable UBC
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  • Case 13: healed UBC following fracture
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  • Case 14: with pathological humeral shaft fracture
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  • Case 15: with pathological fracture
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  • Case 16: with fracture
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  • Case 17: in an adult
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  • Case 18: tibia
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  • Case 19
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  • Case 20
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  • Case 21
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  • Case 22: of big toe
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  • Case 23: with new bone formation
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