Unicameral bone cyst

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 children from the aged 5 to 15. 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.

Origins

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.

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.

Diagnosis

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.

Computerized tomography (CT) Scan

CT scans use x-rays and computer technology to produce accurate axial, coronal, and sagittal images. CT scans are more detailed than x-rays because they can create images that show fat, muscles, organs, and bones.

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)

Treatment

If there is a high probability of a fracture resulting from the unicameral bone cyst, then surgical treatment is necessary. Specific methods can be determined by the physician based upon the patient’s age, medical history, tolerance for certain medical procedures or medicine, health, and extremity of the disease. The treatment can involve or incorporate one or more of the following surgical methods, which are performed by a pediatric orthopedic surgeon:

• Curettage:

Surgeons create an incision or opening in the bone to drain out the fluids inside the cyst. Once the fluid is drained, curettage is then conducted and the lining tissue is scrape out the lesion, using a curette.

• Bone Grafting:

Bone grafting is proceeded with after curettage; the empty cavity is transplanted with donor bone tissue, bone chips taken from another bone, or artificial material.

• Steroid injection:

An injection of methylprednisolone acetate into the lesion helps reduce the levels of prostaglandin. Prostaglandin is a fatty acid that reduces cyst’s ability to be reabsorbed into the bone. To begin an operation using steroids, biopsy needles are placed into the cyst and the interstitial fluids are drained. The cyst is then filled with radiographic contrast to determine the volume and shape of the cyst. If the cyst can be filled, it will be injected with methylprednisolone acetate in several intervals for a time span of six to twelve months. Once the level of prostaglandin decreases, the cyst will be reabsorbed into the bone and disappear. Treatments using steroid injections are preferred over curettage, but there are few risks from the method, which are limited to infection, fracture, and reappearance of the cyst.

If a patient needs to be treated with surgery, a standard surgical procedure would be called for; the patient would be resting in Fowler’s position, a semi-sitting position, under general anesthesia. The exact size, shape, and distance between the acromion to the midpoint of the cyst are measured by a digital radiograph or MRI scan. A small, longitudinal skin incision, about 1 cm long, is made at the center of the cyst. Next, by using a trephine or drill bit, a small aperture is made inside the incision. Fluids contained in the cyst are drained and curved, metal impactors are used to break any septa, or membranes, within the cyst. Curettes are then used to remove the entire cyst from the diaphysis. After the removal of the cystic membrane, a 95% ethanol solution is injected into the cavity to produce a chemical cauterization to burn away any residual active membrane for 30 seconds and then aspirated. Saline solution is then immediately injected into the cavity to wash out any residual ethanol solution and to mitigate any damage to healthy tissue; this irrigation process of ethanol and saline solutions is repeated for another 2 to 3 times. A curved impactor is inserted into the cavity and used to penetrate the boundary between the cyst and bone marrow; the intentional penetrations will allow bone marrow cells to migrate into the cavity to produce a source of osteoinductive cells, cells that induce bony growth. Furthermore, the cavity is completely filled with bone graft substitute, such as calcium sulfate. Finally, one cannulated screw is placed into the aperture.