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Since there isn't a 'fema bone' in the human body (or any other body, for that matter), I'll base this answer on the theory that you're asking about the femur bone.

Directly from the Mayo Clinic website: A bone cyst is a noncancerous, fluid-filled cavity within the bone. Bone cysts occur most often in children in the long bones of the arms and legs. The cause isn't known. A bone cyst usually causes no symptoms. But sometimes a bone cyst can weaken the affected bone, which increases the risk of fracture. A bone cyst is often discovered only after a fracture occurs.

Treatment of a bone cyst, which is directed at preventing fractures, may include:

Corticosteroid medications
Injections of bone marrow aspirate
Surgical removal of the bone cyst

This answer was provided by Mayo Clinic physical medicine and rehabilitation specialist Edward Laskowski, M.D. Here's the link http://www.mayoclinic.com/health/bone-cyst/AN00437


Just for clarification, from Medline Plus Encyclopedia http://medlineplus.nlm.nih.gov/medlineplus/ency/article/002249.htm Long bones are hard, dense bones that provide strength, structure, and mobility. The femur (thigh bone) is a long bone. A long bone has a shaft and two ends. There are also bones in the fingers that are classified as "long bones," even though they are short in length. This is due to the shape of the bones, not the actual size. Long bones contain yellow bone marrow and red bone marrow, which produces blood cells.