Simple bone cyst (unicameral bone cyst, UBC) is one of the most common benign bone lesions of childhood and adolescence. It typically occurs in the metaphysis of long bones, particularly the proximal humerus and proximal femur. It is a unilocular cystic cavity filled with serous or serosanguinous fluid. The lesion is expansile but non-aggressive, causing cortical thinning without periosteal reaction. Pathologic fracture is the most common presentation, and the 'fallen fragment' sign is considered pathognomonic. Active lesions are adjacent to the growth plate, while latent lesions have migrated toward the diaphysis. Spontaneous regression is rare, and treatment involves curettage with grafting or steroid injection.
Age Range
5-20
Peak Age
10
Gender
Male predominant
Prevalence
Common
Simple bone cyst is a benign fluid-filled cavity within bone whose exact etiology remains debated. The most accepted theory proposes that local venous drainage obstruction increases interstitial fluid accumulation, leading to bone resorption. Elevated prostaglandin (PGE2) and interleukin levels in the cyst fluid stimulate osteoclastic activity, perpetuating bone destruction. This fluid accumulation manifests radiologically as low CT density and fluid signal intensity on MRI — low on T1, high on T2-weighted images. The expansion causes cortical thinning while periosteal integrity is preserved, reflecting a non-aggressive growth pattern. When pathologic fracture occurs, a bone fragment falls into the cyst and settles at the dependent portion due to gravity, creating the 'fallen fragment' sign — this finding proves the cyst content is fluid rather than solid.
After pathologic fracture, a free bone fragment within the cyst cavity settles at the most dependent portion due to gravity. This finding proves the lesion content is fluid and is pathognomonic for simple bone cyst. Since a fragment cannot move freely in solid tumors, the presence of this finding excludes solid lesions.
Central, well-defined, oval or fusiform-shaped lytic lesion in the metaphysis of a long bone. Internal density is consistent with fluid, measuring 10-20 HU. Cortical thinning (balloon expansion) is present but cortical integrity is maintained. The sclerotic rim is thin and smooth. No matrix calcification or solid component is seen. The lesion fills the medullary cavity and may extend from metaphysis to diaphysis.
Report Sentence
A centrally located, well-defined lytic lesion with fluid density (approximately ___ HU) is seen in the proximal [bone] metaphysis; the cortex is thinned but intact, with no periosteal reaction identified.
After pathologic fracture, a free bone fragment within the cyst cavity has settled at the most dependent (gravity-dependent) portion of the cyst. This fragment is a piece detached from the cortex or fracture line. The fragment is seen in a horizontal position at the base of the cyst. This finding proves the lesion content is fluid — in a solid tumor, a fragment could not move freely in this manner.
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A free bone fragment in the dependent position within the cyst cavity is seen along with a pathologic fracture line, consistent with the 'fallen fragment' sign — pathognomonic for simple bone cyst.
On T1-weighted sequences, the cyst cavity demonstrates homogeneous low signal intensity, consistent with fluid content. Signal intensity is equal to or slightly lower than skeletal muscle. A thin, low-signal peripheral rim (fibrous wall) may be seen. The signal remains homogeneous unless complications such as hemorrhage or infection are present. No internal septation or solid component is identified. In the presence of pathologic fracture, signal changes may accompany the fracture line and surrounding bone marrow due to edema.
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On T1-weighted sequences, the lesion demonstrates homogeneous low signal intensity consistent with fluid content; no solid component or internal septation is identified.
On T2-weighted sequences, the cyst cavity demonstrates homogeneous high signal intensity, consistent with simple fluid. The signal is comparable to CSF (cerebrospinal fluid). The cyst wall is thin and regular, with low signal on T2 (fibrous tissue). Thin septations may be seen in 5-10% of cases, but thick septa or solid nodules are absent. Surrounding bone marrow edema is generally absent (unless fracture is present). Fluid-fluid levels are not typical in simple bone cyst — this finding should suggest aneurysmal bone cyst instead.
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On T2-weighted sequences, the lesion demonstrates homogeneous high signal intensity consistent with simple fluid; no fluid-fluid levels or solid component are seen.
On STIR, the cyst cavity demonstrates markedly high signal intensity. Since surrounding bone marrow fat signal is suppressed, cyst-marrow contrast is maximized. This sequence is superior to T2 for evaluating the true extent and dimensions of the cyst. The thin fibrous wall appears as low signal. In the presence of pathologic fracture, surrounding bone marrow edema appears as conspicuously high signal on STIR. The signal is homogeneous unless complications are present within the cyst.
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On STIR, the lesion demonstrates markedly high signal intensity with clear delineation of cyst margins due to suppression of surrounding bone marrow fat signal.
On post-gadolinium T1-weighted sequences, no enhancement is seen within the cyst cavity — confirming the lesion content is avascular fluid. Thin peripheral rim enhancement may be seen (from capillaries in the fibrous wall). If thin septations are present, mild enhancement may occur. Post-fracture reactive changes may show surrounding tissue enhancement, but the cyst content remains non-enhancing. Internal nodular or thick septal enhancement is atypical for simple bone cyst and should raise concern for aneurysmal bone cyst or other lesions.
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On post-contrast sequences, no enhancement is identified within the cyst cavity, with thin peripheral rim enhancement noted; no internal solid component or nodular enhancement is present.
On diffusion-weighted imaging (DWI), simple bone cyst shows no restricted diffusion. Low signal is seen on high b-value images (b=800-1000) with high signal on ADC maps (no T2 shine-through). This finding confirms the cyst content consists of free water molecules without a cellular component. It is important in differential diagnosis from abscess (shows restricted diffusion) and solid tumors (variable diffusion restriction).
Report Sentence
On diffusion-weighted imaging, the lesion shows no restricted diffusion with high ADC values; this finding is consistent with simple fluid content.
Criteria
Adjacent to the growth plate (in contact with physis), usually under 10 years of age, higher recurrence rate, more symptomatic
Distinct Features
Direct adjacency to the growth plate, faster expansion, high recurrence after surgery (up to 50%), requires more aggressive treatment approach. Radiologically tends to be larger with more cortical thinning.
Criteria
Separated from the growth plate (migrated toward diaphysis), usually over 10 years of age, lower recurrence rate, often incidental finding
Distinct Features
Normal bone tissue between growth plate and lesion (>1cm distance), slower growth or stable size, low recurrence (10-20%), conservative treatment may be appropriate. Radiologically smaller with thicker cortical wall.
Criteria
After pathologic fracture or internal hemorrhage, may contain fluid-fluid levels, heterogeneous signal on MRI, high signal component on T1 (methemoglobin)
Distinct Features
High signal areas on T1 (methemoglobin), fluid-fluid levels (hemorrhagic sedimentation), high density areas on CT. Important in differential diagnosis from aneurysmal bone cyst — ABC shows more prominent multilocular structure and solid component. Fracture line and surrounding soft tissue edema may accompany.
Distinguishing Feature
Aneurysmal bone cyst (ABC) shows multilocular structure, prominent fluid-fluid levels, and solid components; simple bone cyst is typically unilocular with homogeneous fluid signal and no fluid-fluid levels. ABC demonstrates more eccentric and aggressive expansion.
Distinguishing Feature
Fibrous dysplasia demonstrates ground-glass matrix density and does not contain fluid density; simple bone cyst has fluid density (10-20 HU). Fibrous dysplasia maintains bone structure despite being expansile and cortical thinning is more irregular.
Distinguishing Feature
Langerhans cell histiocytosis (LCH) is an aggressive lytic lesion showing solid enhancement, with possible periosteal reaction and surrounding soft tissue component; simple bone cyst shows no enhancement and no periosteal reaction. LCH is more common in flat bones.
Distinguishing Feature
Non-ossifying fibroma (NOF) is an eccentric, cortically based lytic lesion with sclerotic rim typically adjacent to the posterior cortex in the metaphysis; simple bone cyst is centrally and medullary located. NOF's internal matrix density is not fluid and shows higher density on CT.
Urgency
lowManagement
Observation for latent cysts; curettage with bone grafting or steroid/bone marrow injection for active cysts; surgical stabilization if pathologic fractureBiopsy
Not NeededFollow-up
Serial radiographs every 6-12 months for active cysts; post-treatment follow-up for recurrence assessmentSimple bone cyst is a benign lesion not requiring urgent intervention. Observation may be sufficient for latent cysts. Active cysts are treated with repeated steroid injection (methylprednisolone), bone marrow aspirate injection, or curettage with grafting. If pathologic fracture is present, fracture healing is awaited first, then cyst treatment is planned. Biopsy is usually unnecessary — the radiologic appearance is diagnostic. Recurrence can reach up to 50% in active cysts. Growth plate damage and growth disturbance are rare but important complications.
Simple bone cyst is a benign lesion but clinically significant due to pathologic fracture risk. Treatment usually involves curettage + grafting or steroid injection. Lesions near the growth plate may cause growth disturbance. Spontaneous resolution is possible with skeletal maturation.