What Is a Unicameral Bone Cyst?
A unicameral bone cyst is a non-neoplastic serous fluid-filled bone lesion that most commonly occurs in children and adolescents. It is also known as a simple or solitary bone cyst. Virchow initially reported it in 1891 as "cystic structures." The word unicameral refers to "one chamber," Therefore, a unicameral bone cyst describes a single fluid-filled chamber in the cyst. Unicameral bone cyst typically affects individuals in the 1 st and 2 nd decades of life. It most commonly affects males as compared to females.
Unicameral cyst mainly develops in the long bones, especially in the upper part of the humerus (upper arm bone) and femur (thigh bone). It can also be found in the distal tibia, distal radius, ilium, metacarpals, phalanges, and pelvis. With time unicameral bone cyst enlarges and results in thinning of long bones. The size of the unicameral bone cyst decreases as the patient approaches skeletal maturity, and it may heal once the bone growth is complete.
What Are the Types of Unicameral Bone Cysts?
The unicameral bone is classified into two types:
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Active Cysts: An active cyst is located near the epiphyseal plate and tends to grow over time. As the cyst enlarges, it causes the weakening of bones and results in a pathological fracture.
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Latent Cysts: Latent cysts are located away from the epiphyseal plate and are no longer growing. It usually heals after treatment.
What Are the Causes of Unicameral Bone Cysts?
The exact cause of a unicameral bone cyst is unknown; however, scientists have suggested many theories for the causation, but nothing has been proven. One of the theories is that a unicameral bone cyst results from a disorder of the epiphyseal plate. Another theory says that a bone defect or a blockage in venous drainage may cause fluid build-up and result in a unicameral bone cyst.
What Are the Symptoms of Unicameral Bone Cyst?
Most patients with unicameral bone cysts are asymptomatic. However, in some cases, the cyst may expand and weaken the bone and result in pathological bone fracture leading to the following symptoms:
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Bone pain.
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Swelling around the skin of the fractured bone.
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Bruising.
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Tenderness.
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Change in shape or alignment of the bone.
How Are Unicameral Bone Cysts Diagnosed?
The doctors may initially take the child's medical history and perform a complete physical examination to check for any symptoms. Then, since most patients are asymptomatic, the doctors suggest imaging tests and biopsy for a more accurate diagnosis.
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X-ray: Unicameral bone cysts appear as well-defined unilocular or multilocular radiolucent lesions in X-ray images. Mostly unicameral bone cysts in X-rays exhibit a thin sclerotic margin with no soft tissue component. However, in some cases, the cyst expands and causes pathological fracture leading to thinning of the endosteum, which can be seen in X-rays.
Unicameral bone cysts show two main radiographic signs in X-rays. It includes;
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Fallen-Fragment Sign: Due to pathologic fracture, sometimes a bony fragment may get displaced into the cyst. This radiographic feature is known as a fallen-fragment sign.
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Rising Bubble Sign: In an X-ray image of the unicameral bone cyst, a gas bubble is observed at the non-dependent margin of a cystic bone lesion. This characteristic radiographic appearance is known as the "rising bubble sign).
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MRI (Magnetic Resonance Imaging): MRI helps determine the extent of the unicameral bone cyst and how aggressive it is and also distinguishes it from other types of bone cysts.
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CT (Computed Tomography) Scan: CT scan provides cross-sectional images of unicameral bone cysts that help the doctor examine cysts in detail.
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Biopsy: Biopsy for unicameral bone cysts involves removing a small amount of tissue from the affected bone and sending it for pathological examination. The histological features of unicameral bone cysts could be determined by biopsy. The histologic features of the unicameral bone cyst help distinguish them from other cysts.
How Are Unicameral Bone Cysts Treated?
The treatment options for children with unicameral bone cysts depend on many factors, including:
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Child's age, overall health, and medical history.
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Size of the cyst.
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The extent of fracture and bone strength.
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Child's tolerance for specific treatment.
The doctors may not recommend any treatment and only monitor the cysts through routine X-rays in cases where the children are asymptomatic. For large cysts that cause symptoms or if the bone structure is weakened by fracture, the doctor suggests specific treatment such as:
1) Steroid Injection:
Injection of steroid methylprednisolone acetate can be effective in healing the cyst. Before a steroid injection, the healthcare provider numbs the injection site, places the spinal or bone marrow biopsy needle into the cyst, and aspirates the cystic fluid. Then the steroid methylprednisolone is injected into the cyst. The doctor will suggest steroid injections for six to twelve months until the cyst gets healed completely.
2) Curettage:
Curettage refers to the surgical scraping of the bone done with the help of a special instrument called a curette. In curettage, the surgeon makes an incision in the bone and creates an opening. The cystic fluid is then aspirated, and the tissue lining is curetted. After curettage, the remaining cavity is packed with an allogeneic bone graft (bone graft received from a donor) or autogenous bone graft (bone graft obtained from another part of the patient's body).
The treatment options for pathologic fracture in unicameral bone cysts include:
3) Immobilization:
The affected bone is immobilized by placing a splint, cast, or brace on the bone. In this procedure, initially, the surgeon surgically aligns the two broken ends of the bone, and then custom-made casts or splints, or braces are placed on the bone that helps rejoin the broken bone fragments through natural healing.
The doctor will suggest physical and occupational therapy after surgery to help the child gradually return to normal physical activities.
What Are the Differential Diagnosis of Unicameral Bone Cyst?
The differential diagnosis of unicameral bone cysts are:
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An aneurysmal bone cyst (a benign, non-neoplastic cystic bone lesion that most commonly develops in patients less than 20 years of age).
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Fibrous dysplasia (a rare bone disorder in which the affected bone is replaced by fibrous connective tissue).
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Enchondroma (a noncancerous bone tumor that develops in the cartilage found inside the bones).
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Chondroblastoma (a rare noncancerous bone tumor that mainly occurs in the epiphysis of long bones).
Conclusion:
Unicameral bone cysts usually recur after treatment in nearly half of the patients. The recurrence is more common in very young children than adolescents. The doctor will schedule follow-ups at regular intervals and suggest X-rays to monitor the cyst recurrence closely.
