Masterclass: Surgical Management of Benign Bone Cysts – UBC and ABC

Introduction and Epidemiology

Benign bone cysts represent a common yet clinically challenging subset of orthopaedic pathology, primarily affecting the pediatric and adolescent populations. The two most prominent entities within this classification are the Unicameral Bone Cyst (UBC), also known as a simple bone cyst, and the Aneurysmal Bone Cyst (ABC). Although both present as cystic lesions within the skeletal system, their pathogenesis, natural history, and requisite management strategies diverge significantly.

Unicameral Bone Cysts

A UBC is a benign, fluid-filled, solitary cystic lesion that predominantly involves the metaphysis of long bones. Epidemiological data indicates that 40% to 80% of these lesions occur in the proximal humerus and proximal femur. The vast majority of patients (approximately 90%) present in the first two decades of life. UBCs are classified as either "active" or "latent." Active cysts are located directly adjacent to the physis and maintain the potential to impair growth or cause structural compromise. Latent cysts have migrated away from the growth plate into the diaphysis as longitudinal bone growth progresses, generally indicating a cessation of active cystic expansion.

The exact etiology of UBCs remains a subject of academic debate. Historical theories proposed a reactive or developmental process secondary to localized venous obstruction, leading to increased intraosseous pressure and subsequent focal bone resorption. However, recent cytogenetic analyses have identified clonal chromosomal abnormalities, specifically the translocation t(16;20)(p11.2;q13) and TP53 mutations in recurrent UBCs, suggesting a potential neoplastic origin. Macroscopically, UBCs consist of a cavity filled with yellowish or serosanguineous fluid. The cyst is lined by a thin, translucent fibrous membrane lacking a distinct endothelial or epithelial cellular lining.

Aneurysmal Bone Cysts

Conversely, an ABC is a benign, highly vascular, expansile, and locally aggressive osteolytic lesion. It is characteristically eccentric in location and most frequently arises in the metaphysis of long bones and the posterior elements of the spine. Similar to UBCs, ABCs predominantly affect patients in the first two decades of life.

The pathogenesis of primary ABCs has been definitively linked to a neoplastic process. Cytogenetic studies have demonstrated a recurrent chromosomal translocation, t(16;17)(q22;p13), which places the ubiquitin-specific peptidase 6 (USP6) gene under the regulatory control of the highly active osteoblast cadherin 11 (CDH11) promoter. This upregulation leads to the activation of matrix metalloproteinases and subsequent cystic bone destruction. Histologically, ABCs are characterized by cavernous, blood-filled spaces separated by fibrous septa. Crucially, these spaces lack a true vascular endothelial lining. The septa are heavily populated with multinucleated giant cells, spindle cells, and woven bone trabeculae.

It is critical to note that ABCs can also occur as secondary lesions in up to 30% of cases, arising in conjunction with other benign or malignant bone tumors such as giant cell tumors, osteoblastomas, chondroblastomas, and fibrous dysplasia.

Surgical Anatomy and Biomechanics

Understanding the regional anatomy and the biomechanical alterations induced by cystic lesions is paramount for preoperative planning and executing safe, effective surgical interventions.

Proximal Humerus and Femur Anatomy

Because the proximal humerus and proximal femur are the most frequent sites for benign bone cysts, surgical approaches must respect the local neurovascular structures and physeal anatomy.

In the proximal humerus, the deltopectoral interval serves as the primary internervous plane for extensive curettage and grafting. The cephalic vein must be identified and protected. The proximity of the axillary nerve, traversing the quadrangular space and wrapping around the surgical neck, necessitates meticulous soft tissue handling, particularly when addressing expansile ABCs that distort normal cortical landmarks. The proximal humeral physis contributes approximately 80% of the longitudinal growth of the humerus; therefore, iatrogenic injury during curettage or hardware placement in active cysts must be strictly avoided.

In the proximal femur, lesions frequently occupy the intertrochanteric or femoral neck regions. The lateral approach to the proximal femur is standard, utilizing the plane between the tensor fasciae latae and the gluteus medius, or splitting the vastus lateralis. The blood supply to the femoral head, derived primarily from the medial femoral circumflex artery, is at risk during aggressive curettage of the femoral neck. Furthermore, the proximal femoral physis and the greater trochanteric apophysis dictate the limits of safe intralesional excision.

Biomechanical Implications of Cystic Defects

Cystic lesions significantly alter the biomechanical properties of long bones. According to the principles of solid mechanics, the torsional strength of a tubular bone is proportional to the polar moment of inertia. A cystic defect effectively reduces the cortical thickness and increases the inner diameter of the medullary canal, drastically lowering the bone's resistance to torsional and bending moments.

When a cyst occupies more than 50% of the bone diameter, or when the cortical thickness is reduced to less than 2 millimeters, the risk of pathologic fracture increases exponentially. This is the underlying mechanism for the classic presentation of a UBC: a minor trauma resulting in a pathologic fracture. In UBCs, a fracture may result in the "fallen fragment sign," where a piece of cortical bone breaks off and falls into the fluid-filled dependent portion of the cyst, a pathognomonic radiographic finding indicating that the lesion is fluid-filled rather than solid.

Indications and Contraindications

The decision to proceed with operative intervention versus non-operative observation depends on the cyst type (UBC vs ABC), lesion location, biological activity (latent vs active), and the structural integrity of the host bone.

Operative vs Non Operative Management

For UBCs, asymptomatic latent cysts in the upper extremity are generally observed. Operative intervention is indicated for active cysts near the physis that are expanding, cysts in weight-bearing bones (proximal femur) with impending fracture risk, or cysts that have failed to resolve after a pathologic fracture.

For ABCs, due to their locally destructive and expansile nature, surgical intervention is almost universally indicated upon diagnosis to prevent progressive bone destruction, joint collapse, or severe deformity.

Pre Operative Planning and Patient Positioning

Thorough preoperative planning is essential to confirm the diagnosis, rule out malignant mimics (such as telangiectatic osteosarcoma), and determine the optimal surgical approach and fixation strategy.

Imaging Modalities

Plain radiography is the initial modality of choice. A UBC typically presents as a centrally located, well-circumscribed, purely lytic lesion in the metaphysis, with a narrow zone of transition and no periosteal reaction unless fractured. An ABC presents as an eccentric, expansile, osteolytic lesion with a "soap bubble" appearance and thinning of the overlying cortex.

Magnetic Resonance Imaging (MRI) is mandatory for ABCs and atypical UBCs. MRI provides critical information regarding the extent of the lesion, soft tissue involvement, and proximity to the physis. The hallmark MRI finding of an ABC is the presence of multiple fluid-fluid levels, representing sedimentation of blood products of varying ages within the cystic spaces. While fluid-fluid levels can occasionally be seen in UBCs complicated by fracture, their presence strongly suggests an ABC or a telangiectatic osteosarcoma.

Computed Tomography (CT) is highly valuable for assessing cortical integrity, particularly in areas of complex anatomy such as the spine, pelvis, or proximal femur, aiding in the decision for prophylactic internal fixation.

Biopsy Considerations

A definitive tissue diagnosis must be obtained prior to definitive treatment, especially when the radiographic picture is not classically benign. A percutaneous core needle biopsy is often sufficient. The biopsy tract must be meticulously planned so that it can be excised during the definitive surgical approach, adhering to oncologic principles. Intraoperative frozen section may be utilized to confirm the presence of benign giant cells and rule out high-grade malignancy before proceeding with curettage.

Patient Positioning

Patient positioning is dictated by the anatomic location of the lesion and the planned surgical intervention.
For proximal humeral lesions, the patient is typically placed in the beach chair position or supine on a radiolucent table with a bump under the ipsilateral scapula. The arm must be draped free to allow for full range of motion and intraoperative fluoroscopy.
For proximal femoral lesions, the patient is placed supine on a fracture table or a flat radiolucent table. The use of a flat table facilitates easier positioning for complex curettage and grafting, while a fracture table is preferred if closed intramedullary nailing or dynamic hip screw fixation is planned.

In all cases, a C-arm fluoroscopy unit must be positioned to provide unhindered orthogonal views (anteroposterior and lateral) of the entire bone segment.

Detailed Surgical Approach and Technique

The surgical management of benign bone cysts ranges from minimally invasive percutaneous techniques to extensive open intralesional excision with structural reconstruction. The chosen technique depends on the specific pathology.

Percutaneous Injection Techniques

For UBCs, percutaneous injection remains a first-line treatment, particularly for upper extremity lesions. The goal is to disrupt the cystic membrane, decompress the elevated intraosseous pressure, and introduce osteoinductive or osteogenic materials.

Under fluoroscopic guidance, a two-needle technique is employed. Large-bore trocars (e.g., Jamshidi needles) are introduced into the proximal and distal poles of the cyst. The fluid is aspirated and sent for cytological analysis to rule out malignancy. Radiopaque contrast is then injected to perform a cystogram, confirming that the cyst is unilocular and identifying any venous outflow obstruction.

Following contrast washout with normal saline, therapeutic agents are injected. Historically, methylprednisolone acetate was the standard agent, thought to decrease the production of cyst fluid by the fibrous lining. More recently, the injection of Bone Marrow Aspirate Concentrate (BMAC) combined with Demineralized Bone Matrix (DBM) has demonstrated superior healing rates by providing mesenchymal stem cells and osteoinductive growth factors directly into the defect. The needles are withdrawn, and the tracts are sealed to prevent backflow.

Curettage and Bone Grafting

For ABCs, and for UBCs refractory to injection or located in high-stress areas like the proximal femur, open intralesional curettage is the gold standard.

A longitudinal incision is made over the affected bone, utilizing standard internervous planes. The periosteum is incised and elevated. A large cortical window is created, ensuring it is oval or oblong to minimize stress risers and prevent iatrogenic fracture. The dimensions of the window must be adequate to allow complete visualization of the entire cystic cavity.

Once the cyst is accessed, aggressive intralesional curettage is performed. In ABCs, brisk bleeding is typically encountered upon breaching the cyst wall. Rapid, systematic removal of the fibrous septa and the soft tissue lining using variously sized curettes is essential. The use of a high-speed burr is mandatory to break down the bony ridges and extend the curettage into the reactive bone margin, effectively removing microscopic disease that could lead to recurrence.

Adjuvant Therapies for Aneurysmal Bone Cysts

Because ABCs have a high local recurrence rate (up to 20-30% with curettage alone), chemical or physical adjuvants are routinely employed following mechanical burring.
1. Argon Beam Coagulation: Delivers radiofrequency energy via a stream of argon gas, resulting in superficial thermal necrosis (1-2 mm depth) of the remaining cyst wall, effectively destroying residual neoplastic cells.
2. Phenol: A cytotoxic agent that denatures proteins. It is applied carefully with cotton applicators, followed by thorough neutralization with alcohol and copious saline irrigation.
3. Hydrogen Peroxide: Used for its hemostatic and mild cytotoxic properties.
4. Cryotherapy: Liquid nitrogen can be poured into the cavity to induce thermal necrosis, though this significantly increases the risk of postoperative pathologic fracture and is less commonly used today.

Following adequate curettage and adjuvant treatment, the resultant void must be managed. For smaller defects, autologous cancellous bone graft (e.g., from the iliac crest) or allograft chips can be tightly packed into the cavity. For larger defects, particularly in ABCs, the use of polymethylmethacrylate (PMMA) bone cement is an excellent option. PMMA provides immediate structural stability and generates an exothermic reaction during polymerization, acting as an additional thermal adjuvant.

Internal Fixation Strategies

Prophylactic internal fixation is highly recommended for lesions in weight-bearing bones or when the cortical integrity is severely compromised.

For proximal humerus UBCs, the use of Titanium Elastic Nails (TENs) has become increasingly popular. The nails are introduced retrogradely from just above the olecranon fossa and advanced across the cyst. The nails provide continuous mechanical decompression of the cyst, stimulate osteogenesis via the periosteal reaction, and provide internal splintage to prevent fracture displacement.

For proximal femur lesions (both UBC and ABC), rigid internal fixation is mandatory. Depending on the patient's age and physeal status, options include pediatric dynamic hip screws, cannulated screws, or locking proximal femoral plates. The fixation construct must span the entire lesion and achieve secure purchase in the femoral head and the healthy diaphysis distal to the cyst.

Complications and Management

The surgical management of benign bone cysts is associated with several specific complications. Anticipation and prompt management are critical to optimizing patient outcomes.

Intraoperative and Postoperative Complications

Recurrence remains the most frustrating complication. In ABCs, recurrence typically happens within the first two years postoperatively. Routine radiographic surveillance is mandatory. If an ABC recurs, the secondary procedure must be more aggressive, often requiring structural allografts or PMMA cementing to ensure complete eradication of the neoplastic tissue.

Physeal arrest is a devastating complication in the young child. When curetting an active cyst abutting the physis, the surgeon must exercise extreme caution. Hand curettes should be directed away from the growth plate, and the use of high-speed burrs or thermal adjuvants (like PMMA or Argon beam) directly adjacent to the physis is contraindicated.

Post Operative Rehabilitation Protocols

Rehabilitation protocols must be tailored to the anatomic location of the lesion, the extent of cortical resection, and the stability of any internal fixation utilized.

Weight Bearing Guidelines

For lower extremity lesions managed with curettage and bone grafting without internal fixation, patients are typically restricted to touch-down weight bearing (TDWB) or non-weight bearing (NWB) with crutches for 6 to 12 weeks. Progression to full weight bearing is strictly contingent upon radiographic evidence of graft incorporation and cortical reconstitution. If rigid internal fixation (e.g., a dynamic hip screw) is employed, early partial weight bearing may be permitted, relying on the load-sharing characteristics of the implant.

For upper extremity lesions, patients are placed in a sling for comfort. Pendulum exercises are initiated early to prevent adhesive capsulitis, especially for proximal humerus lesions. Lifting, pushing, and pulling are strictly prohibited until the cortical window has healed, typically taking 8 to 12 weeks.

Range of Motion and Strengthening

Once radiographic healing is evident, a progressive physical therapy program is initiated. The focus shifts to restoring full active and passive range of motion. Isometric strengthening is followed by isotonic and eventually isokinetic exercises. Return to contact sports or high-impact activities is generally delayed for 6 to 12 months and requires complete radiographic consolidation of the cystic defect and restoration of normal cortical thickness.

Summary of Key Literature and Guidelines

The management of benign bone cysts has evolved significantly, driven by advancements in cytogenetics and long-term clinical outcome studies.

Pathogenesis and Genetics

The seminal discovery of the t(16;17)(q22;p13) translocation in primary ABCs fundamentally shifted the paradigm, reclassifying ABCs from reactive lesions to true neoplasms. The upregulation of the USP6 gene is now the definitive diagnostic marker for primary ABCs, distinguishing them from secondary ABCs and other benign mimics. Furthermore, recent literature identifying TP53 mutations in recurrent UBCs suggests a more complex, potentially neoplastic etiology for simple cysts than previously understood, warranting further academic investigation.

Treatment Outcomes

Historical data compiled by Campanacci and Mankin highlighted the unacceptably high recurrence rates of ABCs treated with simple curettage alone. Modern guidelines universally recommend extended intralesional curettage with a high-speed burr and the application of adjuvants.

For UBCs, the literature supports a step-wise approach. While steroid injections were the historical gold standard, recent prospective comparative studies demonstrate superior healing rates and lower recurrence with BMAC and DBM injections. The use of continuous decompression via flexible intramedullary nailing has also gained strong support in recent pediatric orthopaedic literature for the management of high-risk humeral UBCs, offering a reliable method to achieve cyst consolidation while simultaneously stabilizing the bone against pathologic fracture.

Clinical & Radiographic Imaging