Active cysts usually develop in patients younger than 10 years of age. Typically, the cyst abuts the growth plate and occupies most of the metaphysis; it is expansile with a thin cortical shell, continues to enlarge during observation, and is commonly associated with fracture. The cyst is considered active as it arises and grows adjacent to the growth plate. With growth, it is left behind and becomes increasingly separated from the growth plate and at this point is considered latent.

Latent cysts are usually seen in patients older than 12 years of age. They have a thicker bony wall than active lesions. They remain static or diminish in size, show evidence of healing or ossification, and are less likely to result in fracture.

Diagnostic Studies. Radiographs show a central, well-marginated radiolucent defect in the metaphysis, with a symmetric appearance, usually without bony septations or loculations. The metaphysis is expanded, with marked cortical thinning that predisposes to fracture. After fracture, the eggshell-thin fragments are often displaced into the cystic cavity, thus creating the “falling leaf” sign (Plate 6-12).

The radiographic differential diagnosis is usually limited to fibrous dysplasia (see Plate 6-8) and aneurysmal bone cyst (see Plate 6-11). In some cases, the even radiolucency of the cyst may be difficult to distinguish from the ground-glass density of fibrous dysplasia. However, monostotic fibrous dysplasia is usually eccentric rather than central and diaphyseal rather than metaphyseal; in addition, the periosteal reaction is greater in fibrous dysplasia than in a simple bone cyst. An aneurysmal bone cyst (and even telangiectatic sarcoma) may also appear as a large radiolucent lesion, making it occasionally difficult to distinguish from a simple cyst.

Staging studies are indicated only when radiographs are difficult to interpret. Bone scans show an increased radioisotope uptake around the margin of the cyst, in contrast to the uniform uptake in fibrous dysplasia. The finding of straw-colored fluid on needle aspiration confirms the diagnosis of a simple cyst.

Histologic examination of an active cyst reveals a fibrous membrane lining a thin margin of reactive bone. The inner wall of the margin deep to the membrane is often covered with a network of osteoclasts. Between the membrane and the osteoclastic activity is a layer of areolar tissue containing fibroblastic and multinucleated giant cells. Latent cysts have a thicker membrane with little underlying osteoclastic activity, fewer giant cells, and more reactive bone.

Treatment/Prognosis. Simple cysts are treated with curettage and bone grafting; recurrence is high for active cysts (50%) and low for latent cysts (10%). Treatment with injection of corticosteroid is based on the theory that corticosteroids stabilize the mesothelial lining and induce healing of the cyst. Cyst fluid is aspirated with sterile percutaneous needle technique; then 80 to 200 mg of methylprednisolone acetate is infused into the cavity. In more than half of patients studied, the cyst healed after this technique, although multiple injections frequently were required. If the lesion fractures, there is a small chance that the fracture can heal and the lesion resolve.