An aneurysmal bone cyst (ABC) is a benign but locally aggressive bone lesion filled with blood that predominantly affects children and young adults. Despite the word “cyst” in its name, most cases are now recognized as true neoplasms driven by specific genetic rearrangements, not simple fluid-filled cavities. ABCs can expand rapidly, weaken bone, and cause pain or fractures, but they do not spread to distant parts of the body, and the long-term outlook after treatment is generally good.
What an Aneurysmal Bone Cyst Actually Is
Under a microscope, an ABC consists of blood-filled spaces separated by walls of fibrous tissue, with scattered bone-forming cells and large multi-nucleated cells. On imaging, the lesion looks like a balloon expanding outward from within the bone, often thinning the outer shell of bone (the cortex) dramatically. The blood inside these spaces is not flowing the way it does in a blood vessel; it pools and settles, creating the layered “fluid-fluid levels” that radiologists look for on MRI scans.
ABCs account for a small fraction of all bone tumors, but they are one of the more common benign bone lesions seen in pediatric orthopedic clinics. They cause local pain and swelling and can weaken bone enough to cause a fracture with minimal trauma.1PubMed Central. Current management of aneurysmal bone cysts Though the word “benign” is reassuring, the local destruction these lesions cause can be substantial, and they have a stubborn tendency to come back after treatment, especially in younger patients.
Primary Versus Secondary ABCs
One of the most important distinctions in understanding ABCs is the difference between primary and secondary forms. About 70% of cases are primary ABCs, which arise on their own as true tumors.2PubMed Central. Update on aneurysmal bone cyst: pathophysiology, histology, imaging and treatment The remaining 30% are secondary ABCs, meaning the blood-filled cystic change develops inside a pre-existing bone tumor.
The primary form has a clear genetic driver. Researchers have found that up to 69% of primary ABCs carry rearrangements involving a gene called USP6, which gets fused with a highly active gene promoter (most commonly from the CDH11 gene). This fusion essentially switches on USP6 in an uncontrolled way, driving the growth of the lesion.3PubMed Central. USP6 and CDH11 oncogenes identify the neoplastic cell in primary aneurysmal bone cysts and are absent in so-called secondary aneurysmal bone cysts This USP6 rearrangement has never been found in secondary ABCs or in giant cell tumors of bone, making it a useful diagnostic marker when pathologists need to tell the two forms apart.4PubMed. Anchored multiplex PCR for targeted next-generation sequencing reveals recurrent and novel USP6 fusions and upregulation of USP6 expression in aneurysmal bone cyst
Secondary ABCs can develop alongside a variety of other bone lesions. In one imaging study of 49 secondary ABCs, the most common underlying tumors were giant cell tumor (35%), chondroblastoma (22%), fibrous dysplasia (12%), osteoblastoma (8%), and osteosarcoma (8%).5PubMed. Secondary aneurysmal bone cysts and associated primary lesions: imaging features of 49 cases The presence of osteosarcoma in that list is worth noting: if a cystic bone lesion turns out to be a secondary ABC, the underlying tumor driving it could potentially be malignant. This is one reason pathologists take biopsy results seriously even when the initial imaging looks like a straightforward ABC.
The clinical difference matters for treatment planning too. A systematic review of spinal ABCs in adults found that secondary ABCs had significantly higher rates of recurrence (about 19%) and a small but real mortality rate, compared with primary spinal ABCs, which recurred less often and had no recorded deaths in that series.6PubMed. Aneurysmal bone cyst of the spine in adult patients: A systematic review and comparison of primary vs secondary lesions Distinguishing the two forms often comes down to a combination of the patient’s age, the tumor location, and the histological and genetic findings.7PubMed Central. Diagnosing and discriminating between primary and secondary aneurysmal bone cysts
Where ABCs Show Up
ABCs can appear in virtually any bone, but they have favorite locations. The long bones of the arms and legs are the most common site, with the vast majority of long-bone ABCs seated in the metaphysis, the flared region near the growth plate. The spine is another frequent location, particularly the posterior elements of the vertebrae. Craniofacial bones are less commonly affected but not rare.8Journal of Pathology and Translational Medicine. Aneurysmal bone cyst: a review
In a classic series of 95 cases, all but five of the long-bone ABCs were metaphyseal, with the few exceptions growing in the mid-shaft. When ABCs involved the spine, they typically announced themselves with pain, often radiating along a nerve root, along with muscle spasm and limited range of motion. Six patients in that series developed some degree of limb paralysis from spinal cord or nerve compression.9Mayo Clinic Proceedings. Aneurysmal Bone Cysts: An Analysis of 95 Cases Spinal lesions generally demand more urgent attention because of this risk to neural structures.
The pelvis is another noteworthy location. Pelvic ABCs can grow to a large size before symptoms prompt a visit to the doctor, simply because there is more room for the lesion to expand before it runs into something critical. Despite the dramatic local destruction they can cause, children treated surgically for pelvic ABCs tend to recover well functionally, with most reporting no lasting limitations in running, climbing stairs, or self-care.10PubMed Central. Functional outcomes and quality of life following surgical treatment of aneurysmal bone cysts of the pelvis in children
How ABCs Are Diagnosed
Imaging is the starting point. On plain X-rays, an ABC typically appears as a well-defined, expansile, purely lytic lesion, meaning the bone looks like it has been hollowed out and blown up like a balloon. The cortex may be paper-thin but often remains intact. MRI adds the most diagnostic detail: the hallmark finding is multiple fluid-fluid levels within the lesion, created by the layering of blood products at different stages of breakdown.11PubMed Central. Update on aneurysmal bone cyst: pathophysiology, histology, imaging and treatment These fluid levels were first described on MRI decades ago and remain one of the most recognizable features.12PubMed. Magnetic resonance imaging of fluid levels in an aneurysmal bone cyst and in anticoagulated human blood
However, fluid-fluid levels are not unique to ABCs. They also appear in telangiectatic osteosarcoma, a rare malignant tumor that can closely mimic an ABC on imaging. The stakes of confusing the two are high, since osteosarcoma requires aggressive treatment including chemotherapy. Several imaging features help distinguish them: ABCs tend to be smaller, have an intact (though thin) cortex, show no soft tissue mass extending outside the bone, and are often completely filled with fluid levels. In contrast, telangiectatic osteosarcoma is typically larger (averaging about 95 mm versus 46 mm for ABCs), more destructive to the cortex, and more likely to have a soft tissue component. When a lesion has a geographic pattern of bone destruction and is completely filled with fluid levels on MRI, the diagnosis of ABC can be made with confidence.13PubMed. The differentiation between aneurysmal bone cyst and telangiectatic osteosarcoma: a clinical, radiographic and MRI study
Biopsy can be surprisingly tricky. Because the interior of an ABC is mostly blood and thin, fragile tissue walls, a standard core needle biopsy often fails to capture enough solid tissue for a definitive diagnosis. In one series of patients with atypical features, core needle biopsy was diagnostic in only 2 out of 7 attempts, while the others returned misleading diagnoses including unicameral bone cyst, nonossifying fibroma, and giant cell tumor.14PubMed Central. Challenges in the Diagnosis and Treatment of Aneurysmal Bone Cyst in Patients with Unusual Features Some centers have experimented with specialized biopsy tools, such as endomyocardial biopsy forceps, to grab larger tissue fragments from the delicate cyst walls.15PubMed. Novel method to biopsy aneurysmal bone cysts in children When clinical and imaging features are characteristic enough, a biopsy may not be needed before proceeding with treatment, but in ambiguous cases, confirming the diagnosis is essential to rule out a malignant mimic.
Surgical Treatment
The traditional mainstay of ABC treatment is curettage: a surgeon opens a window in the bone, scrapes out the contents of the cyst, and then fills the cavity. Adjuvant techniques are usually added to reduce the chance of recurrence. High-speed burring of the cavity walls, application of phenol to destroy residual tissue, and packing the defect with bone graft (from the patient or a donor) are among the most common supplements.16PubMed Central. The treatment of aneurysmal bone cysts
Each adjuvant comes with trade-offs. Argon beam therapy and high-speed burring have been associated with a fracture rate around 20%, while phenol and cryosurgery carry a high recurrence rate. Curettage alone, without any adjuvant, performs worse than curettage with an adjuvant. The addition of bone graft after curettage helps fill the structural void, promotes healing, and appears to reduce both recurrence and fracture risk.17Journal of Orthopaedic Reports. Extended intralesional curettage with bone grafting for primary aneurysmal bone cyst of the talus In lesions where the cortex is still structurally adequate, or where thinned-out portions can be rebuilt with bone graft, curettage with burring and grafting remains the standard approach.18PubMed Central. Is curettage and high-speed burring sufficient treatment for aneurysmal bone cysts?
For spinal ABCs, especially secondary ones, the surgery can be more extensive. Corpectomy (removing an entire vertebral body) and spinal stabilization with hardware are frequently needed when the structural integrity of the spine is compromised.19PubMed. Aneurysmal bone cyst of the spine in adult patients: A systematic review and comparison of primary vs secondary lesions
Sclerotherapy as an Alternative to Surgery
For patients where surgery carries significant risk, or when the ABC is in a location where open surgery would be highly morbid, percutaneous sclerotherapy offers a less invasive path. The most widely studied agent is polidocanol, a sclerosing chemical injected directly into the cyst cavity under imaging guidance. It works by destroying the lining of the blood-filled spaces, triggering a healing response that gradually replaces the cyst with solid bone.
The results have been consistently encouraging across multiple studies. In a series of 72 patients followed long-term, sclerotherapy with polidocanol achieved a mean size reduction of about 77%, with clinical improvement in roughly 85% of cases. Recurrence was rare, occurring in under 3% of patients, and both recurrences responded to additional injections.20PubMed. Treatment of aneurysmal bone cysts with percutaneous sclerotherapy using polidocanol. A review of 72 cases with long-term follow-up A smaller series of 38 patients reported a 97% success rate, with only one patient eventually needing surgery. No major complications like anaphylaxis or tissue death occurred, though a few patients had minor inflammatory reactions at the injection site.21PubMed Central. Sclerotherapy with polidocanol for treatment of aneurysmal bone cysts: Good results in 37 of 38 consecutive patients Another single-center study found complete ossification (meaning the cyst fully converted to solid bone) in over 90% of patients, with 70% being pain-free within three months.22PubMed Central. Sclerotherapy for Aneurysmal Bone Cyst: A Single-Center Experience
Multiple injections are sometimes needed, typically spaced weeks or months apart, and the healing process is gradual. Sclerotherapy is not ideal when the diagnosis is uncertain (because there is no surgical specimen to examine) or when the bone is so weakened that fracture is imminent. But for confirmed ABCs in challenging anatomical locations, it has become a genuine first-line option.
Embolization and Ablation Techniques
Selective arterial embolization is another minimally invasive strategy, particularly useful for ABCs that have a rich blood supply or are in hard-to-reach locations like the pelvis or spine. The procedure involves threading a catheter into the arteries feeding the cyst and injecting a material that blocks those vessels. In a series of 36 patients treated with a specific glue-like agent, the treatment was effective in 94% of cases. About 61% needed only a single session, while the rest required two or three. Complications were infrequent but included two cases of skin death and one episode of temporary nerve weakness.23PubMed. Selective arterial embolization of 36 aneurysmal bone cysts of the skeleton with N-2-butyl cyanoacrylate
Thermal ablation is newer territory. Cryoablation (freezing the lesion) has shown promise as a standalone treatment in children, with one study reporting a mean volume reduction of about 82%, along with pain relief and new bone formation.24Journal of Vascular and Interventional Radiology. Cryoablation as the Sole Treatment for Aneurysmal Bone Cysts in Children Radiofrequency ablation, which uses heat instead of cold, has also been tested. In one series, pain scores dropped from a mean of about 8 out of 10 before the procedure to essentially zero afterward, with no recurrences during follow-up.25PubMed. CT-guided radiofrequency ablation in patients with aneurysmal bone cysts These approaches are still less established than curettage or sclerotherapy, and the existing studies are small, but they represent a growing toolkit for cases where traditional surgery is undesirable.
Denosumab for Difficult Cases
ABCs contain large numbers of osteoclast-like giant cells, the same type of bone-resorbing cells targeted by denosumab, a drug originally developed for osteoporosis and giant cell tumor of bone. The logic is straightforward: if you shut down the cells that are chewing through bone, the lesion should stop expanding and start calcifying.
Early case series support this approach for selected patients. In a series of nine patients treated with denosumab, imaging showed new bone formation in all patients within three to six months, and MRI showed decreased blood flow to the lesion in most.26PubMed Central. Denosumab in patients with aneurysmal bone cysts: A case series with preliminary results In a pediatric series focusing on spinal ABCs, three children treated with denosumab before surgery experienced calcification, size reduction, and decreased blood flow, making the subsequent surgical resection safer and more manageable. No new neurological problems developed during treatment.27Journal of Neurosurgery: Pediatrics. The role of neoadjuvant denosumab in the treatment of aneurysmal bone cysts: a case series and review of the literature
Denosumab is not a routine first-line therapy for ABCs. It is reserved for cases where the lesion is inoperable or where shrinking the tumor before surgery would substantially reduce surgical risk.28PubMed. Denosumab for treating aneurysmal bone cysts in children Open questions remain about how long treatment should continue, what happens when the drug is stopped (giant cell tumors of bone sometimes rebound after denosumab discontinuation), and whether it affects skeletal growth in children. But for a child with a large, surgically treacherous ABC threatening the spinal cord, it offers something that did not exist a decade ago.
Recurrence and Who Is Most at Risk
The single most frustrating aspect of ABCs is their tendency to come back. Published recurrence rates after surgery range widely, from about 10% to as high as 59%, depending on the series and the patient population.29PubMed. Surgical treatment and recurrence rate of aneurysmal bone cysts in children Most modern series report rates in the lower portion of that range, and one pediatric series found an overall 82% success rate after the first surgery, which is better than the older literature might suggest.
Age and location are the two most consistent predictors. Younger children, especially those under 12, appear more likely to experience recurrence. The risk is highest when the ABC sits right next to an open growth plate. In one study, roughly 42% of ABCs directly adjacent to an open growth plate recurred, compared with a much lower rate in other locations.30PubMed Central. Aneurysmal bone cysts recur at juxtaphyseal locations in skeletally immature patients The biology may relate to the rich blood supply and active cell turnover near the growth plate, which could provide a more fertile environment for residual neoplastic cells to regrow. Gender and tumor size have not been consistently linked to recurrence.
Recurrence is usually detected within the first two years after treatment, and a second procedure is often curative. Families should expect close follow-up with repeat imaging during this window. The possibility of recurrence should influence the initial treatment choice: in a young child with a growth-plate-adjacent lesion, a less invasive approach like sclerotherapy that can easily be repeated may be preferable to a large open surgery.
Long-Term Function After Treatment
Despite the local destruction ABCs cause and the sometimes extensive surgery needed to treat them, long-term functional outcomes tend to be good. In a study of children treated surgically for pelvic ABCs, the mean musculoskeletal tumor society functional score was about 93%, and roughly 85% of patients reported no limitations in common physical activities. Self-rated general health scores averaged about 87%.31PubMed Central. Functional outcomes and quality of life following surgical treatment of aneurysmal bone cysts of the pelvis in children Children and adolescents have impressive capacity to remodel bone and compensate for structural deficits, which works in their favor during recovery.
Outcomes depend heavily on site and on how much structural bone was lost. Long-bone ABCs treated with curettage and bone grafting typically heal well and regain full strength. Spinal ABCs carry more risk of lasting neurological effects, though timely treatment usually prevents permanent deficits. The worst functional outcomes tend to occur in cases complicated by pathologic fracture before treatment, or in locations where the joint surface is involved and cannot be fully reconstructed. Early diagnosis, before the lesion grows large enough to compromise structural integrity, remains the single biggest factor in ensuring a good functional result.
Why ABCs Are Easily Misunderstood
The name itself causes confusion. “Aneurysmal” suggests a vascular problem, and “cyst” implies a benign fluid sac. Neither label captures what is actually going on. Primary ABCs are neoplasms with a defined oncogene fusion. They are not cysts in the way a ganglion cyst or a kidney cyst is a cyst. The name dates to an era before the genetic basis was understood and persists out of convention.
Another common misconception is that ABCs are a pediatric disease exclusively. While they peak in the second decade of life, they can and do occur in adults. Adult spinal ABCs, for instance, present a distinct clinical challenge and have been the subject of dedicated research. And secondary ABCs, because they ride along with other tumors, can appear at any age that the underlying tumor occurs.
Perhaps the most consequential misunderstanding involves imaging. Because fluid-fluid levels are so strongly associated with ABCs, there is a temptation to make the diagnosis on MRI alone and skip a biopsy. In most straightforward cases, this works fine. But telangiectatic osteosarcoma, which also shows fluid-fluid levels, is a lethal cancer that requires chemotherapy. Missing that diagnosis because the imaging “looked like an ABC” is the kind of error that keeps bone tumor specialists up at night. When any imaging feature is atypical, whether the lesion is unusually large, the cortex is broken through, or there is a soft tissue mass, a tissue diagnosis before definitive treatment is not optional.