A unicameral bone cyst, also known as a simple bone cyst, is a cavity filled with a yellow-colored fluid. It is considered to be benign since it does not spread beyond the bone. Unicameral bone cysts can be classified into two categories: active and latent. An active cyst is adjacent to the epiphyseal plate and tends to grow until it fills the entire diaphysis, the shaft, of the bone; depending on the invasiveness of the cyst, it can cause a pathological fracture or even destroy the epiphyseal plate leading to the permanent shortening of the bone.
A latent cyst is located away from the epiphyseal plate and is more likely to heal with treatment. It is typically diagnosed in under 20 year olds. Although unicameral bone cysts can form in any bone structure, it is predominantly found in the proximal humerus and proximal femur; additionally, it affects males twice as often as females.
Treatment options for unicameral bone cysts include invasive approaches such as injections, curettage and surgical fixation, and non-invasive procedures including observation to see if it does not get worse or resolves on its own, plaster casting or restricted activity.
Signs and symptoms
Most unicameral bone cysts do not cause any symptoms and are discovered as accidental findings on radiographs or CT scans made for other reasons. Large lesions can cause nearby areas of bone to thin, which may result in a fracture and cause pain.
Cause
There is not a specific theory behind the etiology of the unicameral bone cyst, however, according to many researchers and doctors, there is a commonly known theory hypothesized by Jonathan Cohen in 1970. Cohen studied interstitial fluid in six children undergoing treatment for unicameral bone cysts. He believed that the chemical composition of the fluid found in the bone cyst was similar to the chemical make-up in serum. Cohen theorized that the unicameral bone cyst occurs when interstitial fluids in cancellous bones quickly accumulate in one region from blockage.
One of the other theories is that the cysts result from a disorder of the growth plate. Another is that the cysts result from problems with circulation that are caused by a developmental anomaly in the veins of the affected bone. The role trauma plays in the development of these cysts is unknown. Some speculate that repeated trauma puts the bone at risk for developing a bone cyst. This, however, has not been proven.
Recently, some of these tumors have been found to contain cells that express the FUS-NFATC2 or EWSR1-NFATC2 fusion gene. (Fusion genes are formed from two previously independent genes that become united due to a chromosome translocation, deletion of some genetic material in a chromosome, or chromosomal inversion. These two fusion genes are described in FET gene family.) A recent study reported that the neoplastic cells in these cysts expressed a FUS-NFATC2 fusion gene in four and a EWSR1-NFATC2 fusion gene in two of nine test cases. It is suggested that the presence of these fusion genes indicates that SEC is a true neoplasm.
Diagnosis
X-rays
Unicameral bone cysts are found incidentally on X-rays. About 90 to 95% of the lesion is found in metaphysics of long bones. The cyst is centered, oblong in shape along the long axis of a long bone. Rarely, they are large and multicameral and are found in diaphysis. When fracture is present, there may be a small bone fragment migrated in the cystic fluid. This is called "fallen fragment sign" which is diagnostic of unicameral bone cyst. Besides, a bubble migrating upwards (known as "rising bubble sign") is another feature suggesting of unicameral bone cyst.
Computerized tomography (CT) Scan
CT scan shows a thin-walled lesion with pseudo septum (incomplete septum or a septum with perforations that allows communications between two chambers). CT scan is used to assess cyst wall thickness and risk of fracture.
Magnetic Resonance Imaging (MRI) Magnetic resonance imaging scans are used to identify the precise location of the cyst, to see how aggressive the disease is, and to determine the actual shape and size. The MRI uses a combination of magnets and radio-frequencies to produce various detailed, computerized images of the cyst and its surrounding body structures.
Bone scans
a nuclear imaging method to evaluate any degenerative and/or arthritic changes in the joints; to detect bone diseases and tumors; to determine the cause of bone pain or inflammation. This test is to rule out other cysts (which are quite unusual)