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Findings:
Simple Bone Cyst (SBC) or Unicameral Bone Cyst is a benign lesion of unknown etiology. They may be due to local disturbance of bone growth, lymphatic or venous obstruction or synovial in origin.
SBCs are most common in the first two decades of life and more common in males than females. Under the age of 17 the most common sites are the proximal humerus (55-65%), femur (25-30%), tibia, fibula, radius and ulna are rare. In older patients, males and females are equally affected and SBCs occur in more atypical locations such as the calcaneus, talus and ilium.
Patients due not present with pain, unless there is a fracture, which occurs in 66% of cases.
Pathologically the SBC is composed of clear, straw-colored fluid within a cyst lined by thin flat epithelial-like cells of mesothelial origin.
SBCs enlarge during skeletal growth (active cyst) and become inactive, or latent, after skeletal maturity (latent cyst). The active cyst arises adjacent to the physis and may grow to fill most of the metaphysis. The bone may be slightly expanded with a thin cortical shell. Around 12 years of age the active cyst becomes the latent cyst and the cyst no longer enlarges. The cyst now becomes separated from the physis and the bony wall thickens. The latent cyst is less likely to fracture than the active cyst.
The goal of treatment is to prevent pathologic fracture. After pathologic fracture approximately 10% of SBCs will resolve. Currettage and bone grafting will lead to recurrence in 50% of active cysts and 10% of latent cysts. Injection of 80 to 200 mg of methylprednisolone leads to healing in 70-95%. Failure rates after injection are higher in weight bearing bones. Other less common techniques include autologous bone marrow injection, multiple drill holes, cryotherapy, and intramedullary nailing. Larger lesions and lesions with septations have a poorer response to all treatments.
Typical MRI appearance is homogeneous low signal on T1 weighted images and very high signal on T2 weighted images. Proton density shows the cyst to be decreased or isointense to the surrounding bone. Margins of the lesion should be well defined. When complicated with a high protein content or blood SBCs will have a higher signal on T1 and proton density weighted images and may have a fluid-fluid level. Septations within a cyst are low intensity on all sequences.
References:
Murphy,Mark, MD. Radiologic Pathology. First Edition. P.571.
Greenspan, Adam, MD. Orthopedic Radiology. 3rd Edition. Pp.619-622.
Stark, David D. MD, Bradley, William G., Jr., MD, PhD. Magnetic Resonance Imaging. Second Edition. Pp. 2286-87.
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