Unicameral bone cysts: a review.



Unicameral bone cysts (UBC) are benign serous fluid-filled cavities lined by fibrous tissue. The M:F ratio is 3:1 and median age at diagnosis is 9 y.o. However, around 6-10% of UCB occur in the flat bones of adults.

It is important to differentiate a UBC from an aneurysmal bone cyst (ABC). ABCs tend to be more expansive then UBCs. ABCs may be wider than the metaphysis and have multiple blood-filled cavities.



UBCs are classified as active when they are adjacent to the physis and latent if there is a layer of normal bone separating them from the physis. When UBCs are adjacent to the physis they retain their growth potential but become latent as they move further away.

UBCs are more aggressive and are more likely to reoccur if they develop in the first decade. Pts are usually asymptomatic however, may present with symptoms of pathologic fracture, these include pain/swelling over the fracture, decreased ROM, and difficulty with ambulation.

The most common site of UBC is the proximal humerus (90%) followed by the proximal femur. Overtime, UBCs in the humerus grow distally and may be seen in the middle of the diaphysis, on the other hand, UBC in the proximal femur rarely grow past the proximal diaphysis. (1)

Plain radiograph is the imaging of choice and has high diagnostic accuracy. When a fracture has occurred a "fallen fragment" or "rising bubble" sign may be seen. CT and MRI are not usually necessary but can aid in eliminating other mimics of a unicameral bone cyst.



A variety of treatment options currently exist: immobilization, corticosteroid injection into the cyst, curettage and bone grafting with possible internal fixation.

Treatment is not required unless the bone cyst is large and leads to symptoms, or there is an impending fracture. (2) Unicameral bone cysts tend to spontaneously improve in the majority of patients, especially after skeletal maturity. (2)

The most common complications of UBC are recurrence and pathologic fracture. Fortunately, less than 10% of patients experience growth arrest of the affected bone.

Conclusion:
Unicameral bone cysts are non-neoplastic serous fluid-filled cysts that are typically identified incidentally however, patients may present with pain and swelling due to pathologic fracture. Plain radiograph is the imaging of choice.

References:
1: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6138978/
2: https://www.ncbi.nlm.nih.gov/books/NBK470587/

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