Unicameral (Simple) Bone Cyst Workup

Updated: May 12, 2023
  • Author: Charles T Mehlman, DO, MPH; Chief Editor: Harris Gellman, MD  more...
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Laboratory Studies

The diagnosis of a unicameral (simple) bone cyst (UBC) is strongly suspected on the basis of the lesion's typical radiographic appearance and is confirmed when an appropriate cyst fluid is demonstrated. Specific laboratory tests are not a routine part of the workup of a UBC. [20, 43, 44, 45]


Plain Radiography

Appropriate imaging studies for a UBC should always include plain radiographs. The appearance of the lesion on plain radiography is virtually diagnostic.

A particular radiographic sign (ie, the fallen-fragment sign) is, at times, very helpful in the radiographic diagnosis of a UBC. Reynolds is credited with describing this sign in 1969. [46]  Typically, the sign is identified when the patient with a UBC presents with a pathologic fracture. The interior of the bone cyst may contain complete or nearly complete thin bony septations within it.

At the time of pathologic fracture, a portion of one of these bony segments may actually break free and float to the bottom of the cyst. This is possible because the UBC is filled with fluid and is not a solid. The fallen-fragment sign is found in approximately 20% of patients who present with a pathologic fracture secondary to a UBC. [47, 48]

Some authors have altered the original description of this sign and refer to it as the fallen-leaf sign; they choose to imagine the broken fragment of bone gently wafting down from the top of the cyst to the bottom of the cyst as if it were a leaf slowly falling to earth from a tree.


Magnetic Resonance Imaging

If a UBC is in close proximity to a growth plate and growth impairment is a concern, magnetic resonance imaging (MRI) may prove quite helpful. [49, 50, 51]  MRI should not be a routine part of the workup of a UBC. Instead, it should be reserved for unusual or atypical situations. One such instance is a cyst in which growth-plate damage is a concern. Several authors have documented that such damage can occur about the proximal humeral growth plate.

Another situation in which preoperative MRI could be of value is in rare cases in which a more sinister diagnosis is suspected (eg, pseudocystic osteosarcoma or low-grade central osteosarcoma). In such instances, MRI is an appropriate part of preoperative staging of such a tumor.

A UBC can produce a wide variety of appearances on MRI, including rather heterogeneous fluid signals and even fluid-fluid levels (a sign much more commonly found in aneurysmal bone cysts [ABCs]).

Magnetic resonance angiography (MRA) may be useful for distinguishing between UBCs and ABCs. [52]