Aneurysmal Bone Cyst: Symptoms, Types, Causes and Treatment

Aneurysmal bone cysts (ABCs) are rare, benign, but potentially aggressive bone lesions that can affect people of all ages. Though non-cancerous, their rapid growth and destructive potential can lead to significant health concerns, especially in children and young adults. Understanding the symptoms, types, causes, and treatment options is crucial for early diagnosis and effective management. This comprehensive guide synthesizes the latest evidence to help patients, families, and healthcare professionals navigate the complexities of ABCs.

Symptoms of Aneurysmal Bone Cyst

Aneurysmal bone cysts often present with symptoms that can be alarming, but they are not always specific, making early identification challenging. Commonly, people notice pain and swelling at the site, but symptoms can vary depending on the location and size of the cyst. In some cases, ABCs are discovered incidentally during imaging for unrelated reasons.

Symptom	Description	Frequency/Location	Source(s)
Pain	Localized, persistent pain	Most common in long bones, spine	2 3 4 10 17
Swelling	Visible or palpable swelling/mass	Affected bone region	1 2 3 4 10
Pathologic fracture	Fracture with minimal trauma	Long bones, especially in children	2 4 7 10
Neurological deficits	Numbness, weakness, or paralysis	Spinal column involvement	2 3 17
Limb deformity	Limb length discrepancy, deformity	Growing children, near growth plates	2
Headache	Generalized or localized	Craniofacial/skull ABCs	1 3
Hearing loss, tinnitus	Sensory symptoms	Temporal bone/cranial ABCs	3

Table 1: Key Symptoms

Pain and Swelling: The Most Common Presentations

Pain is the hallmark symptom of ABCs, often described as dull and persistent. Swelling may accompany pain, especially as the lesion grows and becomes more prominent under the skin or in deeper tissues. These symptoms typically prompt patients to seek medical attention.

Effects Based on Location

• Long Bones: ABCs in the arms or legs (e.g., femur, tibia, humerus) often cause localized pain, swelling, and sometimes visible deformity. In children, growth plate involvement can lead to limb length discrepancies or angular deformities as the cyst interferes with normal bone development 2 4.
• Spine: When ABCs occur in the vertebrae, they may compress the spinal cord or nerve roots, leading to neurological symptoms such as numbness, weakness, or even paralysis. Back pain is common 2 17.
• Craniofacial Bones: ABCs in the skull or jaw can present as a non-tender or tender mass, headaches, hearing loss, or other site-specific symptoms such as tinnitus 1 3.

Pathologic Fracture

Because ABCs weaken the bone structure, affected bones are more susceptible to fractures, sometimes with minimal trauma. This is particularly concerning in weight-bearing bones and in children who are more active 2 4 7.

Other Site-Specific Symptoms

• Pelvic Lesions: May cause vague pain or swelling, sometimes discovered incidentally.
• Ear Symptoms: ABCs in the temporal bone may mimic chronic ear infections, as in cases with otorrhea (ear discharge), hearing loss, or tinnitus 3.

Types of Aneurysmal Bone Cyst

Not all ABCs are the same. They can be classified by location, their relationship to other lesions, and even by their appearance under the microscope. Understanding the types helps guide diagnosis and treatment.

Type	Description	Prevalence / Notes	Source(s)
Primary ABC	Arises de novo, no prior bone lesion	~65% of cases	2 6 7 17
Secondary ABC	Develops atop a pre-existing bone lesion (tumor, trauma, etc.)	~30–35% of cases; often with GCT, chondroblastoma	2 6 10 11 13
Intramedullary	Located within bone marrow (metaphysis of long bones)	Most common (80%)	2 4 7 10
Surface (Juxtacortical)	On bone surface; subperiosteal or cortical	Rarer	2 10
Capanna Classification	Types 1–5 based on location/expansion pattern	Detailed morphologic classification	2
Solid Variant	Densely fibrous with fewer cystic spaces	Rare	4 8

Table 2: Types of Aneurysmal Bone Cyst

Primary vs. Secondary ABC

• Primary ABC: These develop spontaneously in previously healthy bone, likely due to genetic mutations or vascular malformations. They make up the majority of cases 2 6 7 17.
• Secondary ABC: Occur on the background of another bone lesion, such as a giant cell tumor (GCT), chondroblastoma, or following trauma. The pre-existing lesion may be benign or, rarely, malignant 2 6 10 11 13.

Location-Based Types

• Intramedullary (Central): Most ABCs are found within the marrow cavity, particularly in the metaphyseal regions of long bones 2 4 7 10.
• Surface (Juxtacortical): These are less common and develop on the surface of the bone, either subperiosteally or cortically. They may erode bone from the outside in 2 10.

Capanna Classification

A widely used morphological system, the Capanna classification divides ABCs into five types based on their relationship to bone structure:

• Type 1: Central, contained within the bone
• Type 2: Expansile, thinning the cortex
• Type 3: Eccentric, involving one cortex
• Type 4: Subperiosteal, superficial erosion
• Type 5: Periosteal origin, expanding towards and into bone 2

Solid Variant

A rare form, the "solid" ABC, features dense fibrous tissue and fewer cystic spaces. It can be challenging to differentiate from other giant cell lesions 4 8.

Causes of Aneurysmal Bone Cyst

The exact cause of ABCs has been debated for decades. Recent research points to both genetic and reactive origins, depending on the type.

Cause/Mechanism	Description	Notes/Examples	Source(s)
Genetic mutation	Translocation of USP6 gene drives primary ABC	t(16;17)(q22;p13); USP6 overexpression	2 12
Pre-existing lesion	Secondary ABC develops atop another bone pathology	GCT, chondroblastoma, osteosarcoma, trauma	2 10 11 13
Vascular anomaly	Arteriovenous fistula or local hemodynamic changes	May cause cystic, blood-filled spaces	9 10
Trauma	Physical injury or fracture may trigger ABC formation	Often implicated in secondary ABCs	2 9 13
Reactive process	Local response to other bone lesions or insults	Non-neoplastic in some secondary cases	9 10

Table 3: Causes and Mechanisms of ABC

Genetic Drivers in Primary ABC

Recent advances have clarified that most primary ABCs are true bone tumors caused by specific genetic events. The fusion of the USP6 gene, typically via t(16;17)(q22;p13) chromosomal translocation, leads to overactivity of pathways that promote bone resorption and cyst formation 2 12. This discovery helps distinguish primary ABCs from secondary forms.

Secondary ABC: The Role of Other Lesions

Secondary ABCs are reactive lesions that arise in response to an underlying bone tumor or lesion. Up to a third of ABCs are secondary, with the most common precursor being giant cell tumor of bone (GCT), followed by chondroblastoma, osteoblastoma, and, more rarely, malignant tumors such as osteosarcoma 2 10 11 13. Trauma, including fractures, can also initiate secondary ABCs.

Vascular and Reactive Theories

Some ABCs are believed to develop due to abnormal blood vessel formations (arteriovenous fistulae) within the bone, leading to elevated pressure, local bone destruction, and cyst formation 9 10. This mechanism is more likely in secondary ABCs or in cases where trauma is implicated.

Treatment of Aneurysmal Bone Cyst

The management of ABCs has evolved dramatically, with a focus on effective cure and minimization of recurrence and complications. The choice of treatment depends on patient age, lesion location, size, and aggressiveness.

Treatment Modality	Description	Indications / Outcomes	Source(s)
Intralesional Curettage	Surgical removal of cyst contents	Standard of care; high recurrence if alone	4 15 16 17 18
Curettage + Adjuvant	Use of bone graft, cryosurgery, or sclerotherapy	Reduces recurrence; cryosurgery effective	9 14 15 18
En bloc resection	Complete surgical removal of affected bone segment	For aggressive/recurrent or inaccessible lesions	1 4 17
Minimally Invasive	Percutaneous injection (e.g., Ethibloc, bone marrow)	Selected cases, especially in pelvis/children	12 14 18
Embolization	Blocking blood supply to lesion	For spinal/pelvic or inoperable ABCs	17 18
Radiation Therapy	Rarely used due to risk of malignancy	Historically used; risk of sarcoma	1 15

Table 4: Main Treatment Options for ABC

Surgical Approaches

• Curettage: The mainstay of ABC treatment. The cyst contents are scraped out, and the cavity may be filled with bone graft or substitute. However, recurrence rates can be significant if curettage is used alone 4 15 16 17.
• Curettage with Adjuvant Therapy: Adding techniques like cryosurgery (liquid nitrogen application), high-speed burring, or sclerosing agents to destroy residual tumor cells can lower recurrence rates dramatically 9 14 15 18.
• En Bloc Resection: Complete removal of the affected bone segment may be necessary for large, aggressive, or recurrent ABCs, or when vital structures are at risk. This is more common in craniofacial or spinal lesions 1 4.

Minimally Invasive Techniques

• Percutaneous Sclerotherapy: Injection of agents like Ethibloc or alcohol into the cyst cavity causes it to scar and collapse. This is increasingly popular for select cases, especially in children or pelvic lesions, to avoid extensive surgery 12 14 18.
• Bone Marrow/Bone Matrix Injection: Introduction of demineralized bone and autologous bone marrow can stimulate healing and ossification without open surgery 14.

Embolization

Selective arterial embolization blocks the blood supply to the cyst, leading to shrinkage and healing. It is especially useful for ABCs in locations where surgery is hazardous, such as the pelvis or spine 17 18.

Radiation Therapy

Radiation is rarely used due to the risk of inducing secondary malignancy (sarcoma). It may be reserved for inoperable cases where other options have failed 1 15.

Monitoring and Follow-Up

Imaging (MRI, CT, or X-ray) is used to monitor for recurrence, which is highest within the first two years after treatment. Most recurrences can be managed successfully with repeat intervention 4 15.

Conclusion

Aneurysmal bone cysts, while benign, can have serious implications if not promptly and properly managed. Here are the key takeaways from our comprehensive review:

• Symptoms: Pain and swelling are the most common, but site-specific symptoms such as neurological deficits or hearing loss can occur, depending on location.
• Types: ABCs are classified as primary or secondary, with further subtypes based on location and morphology (e.g., Capanna classification, solid variant).
• Causes: Primary ABCs are now understood to be neoplastic, driven by USP6 gene translocations; secondary ABCs arise atop other bone pathologies or trauma.
• Treatment: The standard is surgical curettage, often with adjuvant therapy to prevent recurrence. Minimally invasive options and embolization are viable in select cases, while radiation is rarely indicated.
• Prognosis: Most ABCs can be cured, but recurrence is not uncommon—particularly after simple curettage. Long-term follow-up is essential.

By staying informed about the latest insights into aneurysmal bone cysts, patients and clinicians can work together for timely diagnosis, optimal treatment, and the best possible outcomes.