Unicameral bone cyst
 Discussion of Unicameral bone cyst
- AKA Simple bone cyst
- AKA Solitary bone cyst
- Unicameral bone cyst is a common, benign, fluid-containing lesion, usually occurring in the metaphysis of long bones.
- The pathogenesis of unicameral bone cysts is unknown.
- On gross examination, the cyst expands the cortex of the bone and an intact periosteum covers this thin cortical shell.
- The cyst usually contains clear serous fluid. Occasionally, blood products may be found within the fluid if a previous fracture has occurred.
- Fibrous septa may form after a fracture and create a multilocular appearance.
- Half of all simple bone cysts present as pathologic fractures.
- Unicameral bone cysts occur more frequently in boys than in girls (ratio is 2:1).
- Most cysts occur in the first and second decades of life, with most occurring in children aged 4-10 years.
- Location of lesion depends on patient age
- In patients younger than 20 years, unicameral bone cysts are found in the humerus in 55-65%, in the femur in 25-30%, and rarely in the tibia, fibula, radius, and ulna.
- In patients older than 20 years, unicameral bone cysts are found more commonly in flat bones such as the iliac bone and calcaneus.
- Within the long bones, most unicameral bone cysts are situated in the proximal metaphysis.
 Imaging Findings for Unicameral bone cyst
- Well-defined, geographic lesions with narrow transition zones.
- A thin sclerotic margin is a typical finding.
- Usually are situated in the intramedullary metaphyseal region immediately adjacent to the physis.
- May cause expansion of the bone with thinning of the overlying cortex.
- A pathologic fracture through a unicameral bone cyst is a common occurrence and may lead to the "fallen fragment" sign (migration of a fragment of bone to a dependent portion of the cyst).
- MRI can confirm the presence of fluid within a simple bone cyst.
- Uncomplicated unicameral bone cysts have low signal intensity on T1-weighted images and high signal intensity on T2-weighted images.
- Lesions that have a pathologic fracture have heterogeneous signal intensities on both T1- and T2-weighted images because of bleeding within the cyst.
Patient #1: Radiographs and MR images demonstrate a unicameral bone cyst
 See Also
 External Links
 References for Unicameral bone cyst
- E-medicine simple bone cyst article.