Unicameral Bone Cyst

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Unicameral bone cysts [UBCs], also known as simple bone cysts, are common benign, fluid-filled lesions that develop within the bone in skeletally immature individuals. They typically begin in the metaphyseal regions of long bones, most frequently in the proximal humerus and proximal femur of growing children and adolescents. As the lesion ceases to be active, the growth plate will grow away from it, leaving it in the diaphysis. Although the exact cause is not fully understood, unicameral bone cysts are thought to result from disturbances in normal bone growth at the physis (growth plate). These cysts are most commonly diagnosed in children between the ages of 5 and 15 years, as they tend to occur during periods of active bone growth. While occasionally asymptomatic, they frequently weaken the bone structure and predispose it to mechanical pain and pathological fracture, even with minimal trauma.

Epidemiology

Unicameral bone cysts are relatively common, accounting for approximately 3% of all benign bone lesions. They almost exclusively develop within the first two decades of life, with the highest incidence between the ages of 5 and 15 years. There is a strong male predominance, with around 75% of cases occurring in males. The reasons for this gender disparity are not fully understood, but it may be related to differences in bone growth and remodeling between males and females during adolescence.

The most common anatomic locations for unicameral bone cysts are the proximal humerus (50-60% of cases), followed by the proximal femur (20-30%) and proximal tibia (10-15%). Cysts can rarely occur in other bones, but the long bones of the upper and lower extremities are by far the most frequently affected sites.

Interestingly, unicameral bone cysts are almost never observed after the closure of the growth plates, which typically occurs by the late teens to early 20s. This strong association with active bone growth and development suggests the underlying pathophysiology is fundamentally linked to disturbances in the normal processes of bone formation and remodeling.

Clinical Features

Unicameral bone cysts are occasionally discovered incidentally on imaging but are more commonly diagnosed when they present with pain or a pathologic fracture, particularly after minor trauma. Symptoms typically depend on the size and location of the cyst. Pain may be intermittent and related to activity, especially if the cyst is expanding and weakening the bone. The activity and aggressiveness of the lesion are influenced by the patient’s age and its proximity to the growth plate. In younger patients, and in lesions closer to the physis, UBCs are more likely to be active, symptomatic, and have an increased local recurrence rate.

Radiologic Features

On x-rays, a unicameral bone cyst appears as a full-width, central lesion that mildly expands the bone. Unlike the expansion seen in aneurysmal bone cysts, this expansion in unicameral bone cysts is never beyond the cortical boundaries and almost never damages the growth plate in the absence of a pathological fracture through the plate.

The expansion associated with a unicameral bone cyst is uniform and shows cortical thinning, distinguishing it from the scalloped appearance seen in chondrosarcoma (which are quite rare in children). A periosteal reaction is typically absent unless a pathologic fracture has occurred. The lesion is lytic and displays a geographic margin, a sclerotic rim (particularly in the latent stage), and lacks a bony matrix (except when filling in during healing). When a fracture is present, a “fallen leaf” or “fallen fragment” sign may be seen (Figure 1), indicating that a flake of fractured cortex has fallen into the cyst cavity and is floating within it. This sign suggests the cystic nature of the lesion, as a piece of fractured cortex would not fall through a solid tumor.

Figure 1: AP radiograph of the right proximal femur demonstrating a pathologic fracture through a unicameral bone cyst. The “fallen leaf” sign is seen by the small cortical fragment floating at the inferior aspect of the cyst.

The appearance of auto-sclerosis (spontaneous new bone formation) or bony loculations (internal septations) on imaging are considered early, positive signs that the unicameral bone cyst is entering a healing phase.

In cases where a unicameral bone cyst arises in the calcaneus, it is typically located just below Bohler’s angle and has an egg-like shape with its long axis pointing anteriorly (Figure 2).

Figure 2: Lateral radiograph of the foot demonstrating a unicameral bone cyst of the calcaneus. They are always seen in exactly this position in the mid-calcaneus.

Typically, neither MRI nor CT is routinely needed for diagnosing UBCs. Nevertheless, these imaging modalities may be used in cases where radiographic findings are atypical, or for surgical planning. An MRI scan with and without contrast will confirm the cystic nature of the lesion demonstrated by the presence of peripheral enhancement without central enhancement.

Pathology

Histologically, unicameral bone cysts consist of large, serous, fluid-filled cavities lined by a thin layer of fibrous tissue. This fibrous lining contains benign spindle cells, scattered reactive giant cells, hemosiderin pigment, and chronic inflammatory cells (Figure 3). Nuclear atypia is typically absent, consistent with the lesion’s benign nature.

Figure 3: H&E stained histology slide of UBC, demonstrating thin fibrous lining around a cystic cavity.

Differential Diagnosis

An active unicameral bone cyst adjacent to the growth plate in a young child can mimic an aneurysmal bone cyst. Although both demonstrate fluid-fluid levels, an active unicameral bone cyst is a full-width, central lesion with mild bone expansion while an aneurysmal bone cyst is an eccentric lesion with significant bone expansion. A latent unicameral bone cyst can be confused with fibrous dysplasia, which leaves behind a fluid-filled space when it degenerates within the bone marrow.

Disease Course: Treatment and Prognosis

Unicameral bone cysts are typically self-limiting lesions that tend to resolve spontaneously by the time the growth plate closes. However, they are frequently treated to alleviate associated pain or to treat impending or established pathologic fracture. The activity and behavior of unicameral bone cysts are closely tied to the patient’s age and the lesion’s proximity to the growth plate. Cysts are classified as “active” if they are adjacent to the physis and “latent” if separated from the physis by normal bone. Active cysts, which are more common in younger children, tend to be more aggressive and may require multiple treatment interventions to induce healing. In contrast, latent cysts in older teenagers often respond well to a single treatment.

Over the past few decades, the management of unicameral bone cysts has evolved. Before the 1980s, the standard approach was open curettage and bone grafting. In the early 1980s, intraosseous corticosteroid injections became more common. More recently, the most widely used technique in the United States has been percutaneous injection of calcium sulfate or calcium phosphate cement, a minimally invasive and highly effective option. Rare cases of local or distant embolization of cement material have been reported with calcium phosphate bone cement injections. Additionally, the use of cryotherapy has been explored in selected cases. Flexible intramedullary nailing or proximal femoral cannulated screw placement may be reasonable procedures should internal fixation be deemed necessary.

Regardless of the treatment method, ensuring that the intervention does not compromise the adjacent growth plate is essential, especially in younger patients, as disruption of the growth plate could lead to limb length discrepancies or angular deformities.

When a pathologic fracture occurs through a unicameral bone cyst, the fracture should be managed as if the cyst were not present, with the aim of allowing it to heal. Significant fractures may cause enough healing potential for the underlying cyst to resolve on its own. However, if the cyst persists, further intervention may be necessary (Figures 4 and 5).

Figure 4: AP radiograph of the right proximal humerus demonstrating a pathologic fracture through a unicameral bone cyst.

Figure 5: Following a short course of immobilization, the fracture shown in Figure 4 had healed and formed a lateral callus. However, the cyst persisted and required eventual percutaneous cement injection.

Key Test Topics

Unicameral bone cysts are benign, fluid-filled cavities in bone.
They are most common in children aged 5-15 years.
Clinical presentation: Occasionally discovered incidentally on x-rays but commonly present with pain or a pathological fracture.
“Fallen leaf” or “fallen fragment” sign on x-ray of pathologic fractures.
Pathologic fractures are treated with standard fracture management. If the cyst does not resolve with fracture healing, it can undergo further treatment as for cysts where fractures have not occurred.