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We just got a call that our 6-year-old granddaughter fractured her upper arm. The X-ray showed a unicameral bone cysts as the cause of the break. We are frantically searching the Internet for any information we can find. What is it? How will they treat it? What will happen?

Cysts are cavities or spaces that form and usually fill up with some type of material -- either fluid, blood, or fibrous tissue. In the case of unicameral bone cysts, the cavities are fluid-filled. As the cavity gets larger and larger, the area of affected bone becomes thin and weak. That's why the bone fractures near the cyst. In the case of children, the two bones affected most often are the humerus (upper arm) and the femur (thigh). Boys are affected by fractures associated with unicameral bone cysts twice as often as girls. Although these lesions can occur earlier, the usual age-group affected are between 10 and 20 years old. No one really knows why unicameral bone cysts form. There are a couple of theories involving certain molecules and chemicals found in the cysts but no real answers. Treatment is also highly debated. Without any treatment, the cyst often just gets bigger and bigger, compromising the bone. Removing the cyst isn't always the end of the problem, either because they often come back even after treatment. It even appears that some cysts do better without treatment. But predicting which cyst is going to expand, which cyst will disappear on its own, and when to do surgery to remove it is all under investigation. In cases like your granddaughter, the fracture must be treated. Most likely the cyst will be removed at the same time. If the cyst is not surgically removed, the child will be watched very carefully to make sure the cyst doesn't continue to increase in size. A stable cyst (one that is not changing in size or at least not getting larger) may just be observed using periodic imaging studies. The surgeon may also choose to inject a steroid medication into the cyst. This treatment has been shown to help reduce the size of the cyst. It is minimally invasive but doesn't always do the trick the first time. Several more injections may be needed before the cyst is completely gone.

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