Unicameral Bone Cyst

Etiology

Unicameral bone cysts contain serosanguinous or serous fluid and are lined by a fibrous membrane. The precise etiology is unknown, but practitioners think they develop as a result of a bone growth defect, which results in the fluid filling this defect and ultimately thinning and expanding of the overlying bone[2]. Researchers also suggest that they develop as a result of a venous obstruction within the bone[3].

Epidemiology

Unicameral bone cysts are most commonly found in adolescents from birth to 20 years of age. They have a predilection for males (3:1, male:female). Unicameral bone cysts make up 3% of the primary bone lesions. The actual incidence is not known as many of these are never discovered[4].

Pathophysiology

When in the active phase, the cysts tend to arise near the physis (growth plate) and maintain their growth potential; however, as the cysts become more latent, they begin to migrate away from the physis and can remain as a static bone defect or may gradually resolve over time.

Histopathology

Grossly, these cysts show a thin membrane with multiple ridges within the inner surface of the cyst and contain serous fluid or serosanguinous fluid.

Microscopically, the unicameral bone cysts demonstrate connective tissue within the inner lining of the cyst and may also contain hemosiderin, giant cells, and reactive new bone. The lining may also contain cementum-like material. Fracture callus or blood products within the cyst might be seen if a fracture was present.

History and Physical

The age of the patient is an important consideration, as bone cysts can be more aggressive in the younger population with an increased recurrence rate of four-times more than those in the older population. Age is also helpful when narrowing the differential diagnosis. Other useful information of the patient’s history includes type and onset of pain and associated systemic symptoms.

The physical exam includes a general physical from head to toe with special attention to the musculoskeletal portion. The range of motion, neurologic function, and vascular function are important considerations as more aggressive bone tumors can cause neurovascular compromise.

Unicameral bone cysts are asymptomatic unless a pathologic fracture is present. They often present with tenderness at the fracture site with possible associated swelling and/or bruising. Other symptoms may include difficulty with ambulation or decreased range of motion of the affected extremity. These lesions do not typically get infected, and as such, no laboratory abnormalities are present.

Evaluation

Location usually characterizes unicameral bone cysts. They tend to be near or at the metaphysis and are more centrally located within the medullary cavity. However, as mentioned earlier, as the cysts become increasingly latent, they tend to migrate away from the physis and have been found within the diaphysis. They do not cross the growth plate. Most commonly, unicameral bone cysts are found in long bones (in decreasing order of frequency), the proximal humerus (90%), proximal femur, and proximal tibia in children. Other common sites include the fibula, ribs, radius, ulna, and phalanges. In adults, they may occur in the calcaneus, iliac wing, distal radius, or patella[5][6][7].

Generally, on radiographs, they appear as a lucent lesion with a narrow zone of transition and is sharply demarcated from normal bone. The rim is sclerotic, and the adjacent cortex can be thinned; however, the cortex typically remains intact without cortical destruction or soft tissue invasion. No adjacent periosteal reaction is seen unless a fracture is present. When a fracture is present, a bony fragment may be displaced and be termed a “fallen fragment.” This fragment settles within the dependent portion of the cyst and can change with patient positioning. Rarely, calcification or ossification occurs within the cyst.

On computed tomography (CT), they are centrally located with an intact cortex that is thinned. There is no enhancement when intravenous (IV) contrast is administered), and the internal contents are that of fluid attenuation. The “fallen fragment” will be in the dependent portion following a fracture, similar to radiographs. The “rising bubble” sign may be seen with CT, which demonstrates a bubble of gas within the nondependent portion of the bone cyst and indicates a pathologic fracture.

On magnetic resonance imaging (MRI), the cystic composition of the lesion is confirmed on fluid-sensitive sequences (high signal intensity) and are of low to intermediate signal intensity on anatomic sequences. The rim may enhance 80% of the time on postcontrast imaging. Less commonly seen is a central enhancement of the bone cyst (27%). The “rising bubble” sign may be seen on MRI, but the “floating fragment” sign is less commonly demonstrated[8].

CT and MRI do not add much additional information but can aid in eliminating other mimics of a unicameral bone cyst.

Treatment / Management

There is no consensus on best treatment, but treatment is generally unnecessary in asymptomatic individuals. There is a widely reported range of failure with various treatments with the most common failure being fracture or recurrence.

Unicameral bone cysts can be treated with a direct injection of corticosteroids (methylprednisolone) into the cyst itself, curettage, or bone grafting. Injection of corticosteroids may also require multiple treatments. Bone grafting and curettage tend to be reserved for larger bone cysts and those that compromise the integrity of the bone. Many factors, such as size, bone strength, age of the patient, and activity level play role when considering the most appropriate treatment pathway[9][10].

Differential Diagnosis

Differential diagnosis includes an aneurysmal bone cyst, fibrous dysplasia, enchondroma, or benign chondroblastoma. A giant cell tumor or chondromyxoid fibroma may be additional considerations.

Aneurysmal bone cysts occur near the metadiaphysis and are usually more eccentric in location than centrally located. Additionally, aneurysmal bone cysts will show a significant expansion of the cortex, unlike the mild concentric expansion was seen in UBCs.

Fibrous dysplasia occurs within the same age distribution and is also centrally located. They can occur within the metadiaphysis or diaphysis. The radiographic appearance of this lesion is usually more “ground-glass” density versus lucent as seen with the unicameral bone cysts.

An enchondroma has less sclerotic margins than the unicameral bone cyst and often contains a chondroid matrix. The most common location is the metaphysis of the proximal humerus, which is often seen with unicameral bone cysts.

Prognosis

Unicameral bone cysts tend to spontaneously improve in the majority of patients, especially after skeletal maturity. Less than 10% of patients experience growth arrest of the affected bone, which is usually seen after a pathologic fracture or recent curettage. Interestingly enough, both active and latent cysts tend to recur, even after resection.

Overall, the majority of lesions are clinically insignificant, and patients do well.