The skeletal system forms the load-bearing framework of our body and, at first glance, conveys an impression of remarkable solidity. Yet beneath this solid exterior, lesions can develop within the bone itself that gradually cause serious structural damage. The aneurysmal bone cyst (ABC) stands out among these lesions for both the unusual character of its appearance and the complexity of its treatment — a benign yet aggressively behaving bone tumor. The word “benign” here refers solely to the absence of distant metastasis; its potential to destroy bone rapidly at the local level makes this a lesion that should never be underestimated.

What Is an Aneurysmal Bone Cyst?

An aneurysmal bone cyst is a benign lesion that expands within bone, consisting of blood-filled cavities separated from one another by thin septa. The word “aneurysmal” in its name derives from the way it balloons outward in a manner reminiscent of a vascular aneurysm.

On histological examination, the characteristic features include wide spaces filled with blood, thin fibrous septa dividing these spaces, giant cells (osteoclast-like cells) within the septa, fibroblasts, and islands of reactive bone. This complex architecture is one of the fundamental elements that makes both the diagnosis and the treatment of this lesion particularly challenging.

How Common Is It and Who Is at Risk?

Aneurysmal bone cysts account for approximately 1 to 2 percent of all primary bone tumors. Although they can occur at any age, there is a very pronounced age predilection: the large majority of cases — approximately 80 percent — are diagnosed in individuals younger than 20, particularly in the second decade of life (ages 10 to 20). The condition affects males and females with roughly similar frequency, with a slight female predominance reported in the literature.

Although any bone can be involved, the most common locations are:

• The metaphyses of long bones (femur, tibia, humerus): the most frequent site of involvement.
• The spine: the posterior elements of cervical, thoracic, and lumbar vertebrae (pedicles, laminae) are commonly affected.
• The pelvis
• The bones of the foot and hand

Spinal involvement is of particular importance, as a lesion in this location can compress the spinal cord and nerve roots, giving rise to a neurological picture.

How Does It Develop? Etiology

The precise cause of aneurysmal bone cysts was a subject of debate for decades. Today, two principal mechanisms have emerged at the forefront:

Primary ABC: This accounts for approximately 70 percent of cases. In this group, the lesion has been linked to chromosomal translocations involving the USP6 gene. The t(16;17) translocation in particular is the most frequently identified genetic alteration, and this finding is strong evidence supporting the view that ABC is a true neoplastic lesion.

Secondary ABC: In approximately 30 percent of cases, an ABC develops within or upon a pre-existing bone lesion. Giant cell tumor, chondromyxoid fibroma, osteoblastoma, fibrous dysplasia, and osteosarcoma are among the primary lesions that can give rise to secondary ABC. For this reason, when an ABC is diagnosed, it is critically important to search for an underlying lesion in the pathological examination.

What Are the Symptoms?

The clinical presentation of ABC depends largely on the location and size of the lesion. The most frequently encountered complaints, however, include the following:

• Pain: The most common and earliest symptom. Initially mild and intermittent, the pain intensifies as the lesion grows and can become constant.
• Swelling and tenderness: In locations where the lesion is close to the skin, a distinct bony swelling may be palpable over the affected area.
• Pathological fracture: Because ABC thins and weakens the bone cortex, a fracture can occur with minimal trauma — or sometimes spontaneously. This is a source of clinical urgency particularly in lesions of the lower extremity.
• Restriction of movement: Lesions near a joint may limit the range of motion of the affected joint.
• Neurological symptoms: In cases of spinal involvement, low back or neck pain, pain radiating into the arms or legs, numbness, weakness, and in advanced cases spinal cord compression may develop.

The Diagnostic Process

Plain Radiograph: This is the first imaging modality consulted in the diagnostic process. The classic appearance is an expansile, lytic lesion with a “blown-out balloon” appearance, thinning the bone cortex and surrounded by a thin periosteal shell, with well-defined margins.

Computed Tomography (CT): CT is superior for determining the true dimensions of the lesion within the bone, the integrity of the cortex, and its relationship to surrounding tissues. Characteristic “fluid-fluid levels” may be seen on CT — a finding that arises from the layering of blood-filled spaces of differing densities, and is a highly diagnostic sign of ABC.

Magnetic Resonance Imaging (MRI): MRI is the gold standard for assessing soft tissue involvement, the relationship of the lesion to adjacent structures, and particularly the effect on neural structures in spinal lesions. Fluid-fluid levels can also be clearly visualized on MRI.

Biopsy: A definitive diagnosis can only be established by histopathological examination. Investigation for an underlying secondary lesion in the biopsy material is always mandatory.

Angiography: May be used in cases where preoperative embolization is planned, to map the vascular supply of the lesion.

Treatment Options

The treatment of ABC is individualized according to the location of the lesion, its size, the patient’s age, and overall health. The fundamental goals of treatment are complete control of the lesion, restoration of the structural integrity of the bone, and prevention of recurrence.

Surgical Curettage and Grafting

This is the standard treatment option for accessible lesions. The surgeon thoroughly evacuates the contents of the lesion using a specialized instrument called a curette; the remaining cavity is then filled with bone graft (harvested from the patient’s own bone or from a bone bank) or bone cement. Although recurrence rates following curettage are quite variable — reported between 10 and 40 percent — adjuvant techniques such as phenolization, cryotherapy (application of liquid nitrogen), or bone cement may be employed to improve surgical success.

Selective Arterial Embolization

This is a minimally invasive technique preferred for lesions in difficult-to-access locations — particularly the spine and pelvis — or to reduce the risk of intraoperative bleeding before surgery. When the vessels supplying the lesion are occluded via catheter, the lesion gradually shrinks and may undergo fibrotic transformation. Multiple sessions may be required.

Denosumab Therapy

Increasingly supported by evidence in recent years, this approach employs a biological agent called denosumab, which targets the RANK-L pathway. Successful outcomes have been reported in lesions containing giant cells — particularly in cases where surgical access is difficult or where recurrences have been repeated. Radiological sclerosis and a reduction in lesion activity can be achieved.

Radiotherapy

Radiotherapy has an extremely limited role in the treatment of ABC today. It may be considered as a last resort in lesions that are surgically inaccessible, adjacent to vital structures, and unresponsive to other modalities. The risk of malignant transformation following radiotherapy is the primary reason for this limitation.

Special Approach in Spinal ABC

Cases with spinal involvement require special consideration both in terms of surgical planning and neurological risk. A multi-stage treatment strategy is generally followed in these patients.

Selective arterial embolization is first performed to reduce the blood supply to the lesion. The lesion is then surgically cleared; simultaneous or sequential instrumentation and fusion restore spinal stability. Denosumab has emerged as a valuable adjunct treatment option — particularly in spinal cases where complete resection is not feasible.

Prognosis and Follow-Up

Although ABC is a benign lesion, its recurrence rate is too significant to ignore. The great majority of recurrences occur within the first two years following surgery, making regular radiological follow-up mandatory.

The completeness of surgical curettage is the most critical prognostic factor. Long-term success rates are markedly higher in cases where the entire lesion contents have been cleared. In secondary ABC, prognosis is determined by the biological behavior of the underlying primary lesion.

Malignant transformation is exceedingly rare and is largely confined to cases in which radiotherapy has been administered.

Differential Diagnosis: What Else Should Be Considered?

ABC can be confused with a number of bone lesions owing to its imaging findings and histopathological features. The principal entities to be considered in the differential diagnosis include:

• Giant cell tumor (GCT)
• Simple bone cyst (unicameral bone cyst)
• Telangiectatic osteosarcoma — confusion with this malignant tumor is of particular importance, as it can lead to an entirely incorrect treatment plan
• Chondromyxoid fibroma
• Fibrous dysplasia

Histopathological examination by an experienced bone pathologist is indispensable for an accurate diagnosis.

Conclusion: An Aggressive Appearance, a Benign Identity

Despite carrying the label “benign,” an aneurysmal bone cyst is a lesion that can rapidly destroy bone, carries a significant tendency for recurrence, and can sometimes evolve into a neurological emergency. Early diagnosis, accurate assessment of the lesion’s location, and a comprehensive treatment strategy planned by an experienced orthopedic oncology team are the fundamental keys to preserving the patient’s long-term function and quality of life.

For this reason, unexplained bone pain, swelling, or signs suggestive of a pathological fracture should prompt timely consultation with a specialist without delay.

Prof. Dr. Mehmet Şenoğlu Neurosurgeon, İzmir

Note: This article is intended for informational purposes only. Please consult a qualified physician for diagnosis and treatment.