Unicameral Bone Cyst
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Direct answer
The unicameral bone cyst, also called simple bone cyst, is a fluid-filled, non-neoplastic cavity with a fibrous wall, found mainly in the proximal humerus and femur of children. Its classic radiographic sign is the fallen-leaf sign, in which a cortical fragment drops into the cavity after a pathological fracture. Some cysts heal once the fracture unites, and the active options are steroid injection, curettage with grafting or intramedullary decompression.
What you must remember
- Age: children, mostly under twenty, with the lesion often found after a fracture.
- Site: the metaphysis of the proximal humerus and proximal femur, central and away from the joint surface.
- Histology: a fibrous wall with no true epithelial or endothelial lining, filled with serous fluid.
- Fallen-leaf sign: a fragment of cortex falls into the cavity, a classic radiographic feature.
- Presentation: a pathological fracture after minor trauma is the usual story, rather than pain alone.
- Healing: some cysts heal once the fracture unites, so initial observation can be reasonable.
- Treatment: steroid injection as a minimally invasive option, with curettage and grafting for recurrence or large lesions.
Managing the cyst after a fracture
Most unicameral bone cysts come to light after a fracture, so the first decision is whether the fracture needs management and whether the cyst will heal on its own. The steps below put those decisions in order.
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Confirm the fracture pattern: A metaphyseal lytic lesion with a fracture and a fallen fragment is the typical picture. In a child, the diagnosis is radiographic and does not usually need biopsy.
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Protect the fracture: Splinting or fixation is chosen according to the bone and its loading, because the fracture must unite before the cyst is judged.
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Reassess after union: Serial radiographs show whether the cyst has healed or is shrinking, which guides further treatment.
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Inject steroid if persistent: A steroid injection into the cavity is a minimally invasive option for cysts that persist or threaten to fracture again.
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Operate on failure: Curettage with bone graft or cement is used when injection fails, when the lesion is large, or when it lies near the growth plate.
Where the exam tries to catch you
The exam almost always tests the cyst in a child with a fracture, so the traps sit on diagnosis and first-line treatment.
- Trap: biopsying a typical metaphyseal fracture cyst Correct: the childhood age, the fallen fragment and the classic site make the diagnosis radiographic, so biopsy is not needed.
- Trap: confusing a unicameral bone cyst with aneurysmal bone cyst Correct: a unicameral cyst is a single fluid-filled space with a fibrous wall and no septa, whereas aneurysmal bone cyst shows septa and fluid-fluid levels.
- Trap: operating at the first visit Correct: some cysts heal after the fracture, so observation and steroid injection come before curettage.
Frequently asked questions
What is the fallen-leaf sign?
The fallen-leaf sign is a radiographic feature of a unicameral bone cyst in which a cortical fragment falls into the cavity after a fracture. It is seen most often in the proximal humerus and proximal femur of children.
Where do unicameral bone cysts usually occur?
Unicameral bone cysts usually occur in the metaphysis of the proximal humerus and proximal femur in children. They are central lytic lesions that lie away from the joint surface.
How is a unicameral bone cyst treated?
A unicameral bone cyst is commonly treated with steroid injection, a minimally invasive option, or with curettage and bone grafting if injection fails. Some cysts heal after a fracture, so observation is also reasonable.
How does a unicameral bone cyst differ from aneurysmal bone cyst?
A unicameral bone cyst is a single fluid-filled space with a fibrous wall and no septa. Aneurysmal bone cyst is a blood-filled, expansile lesion with septa and fluid-fluid levels on MRI.