Aneurysmal Bone Cyst

6 Interesting Facts of Aneurysmal Bone Cyst 

  1. Aneurysmal bone cyst is a rare, locally destructive, benign skeletal tumor usually involving the metaphysis of long bones in children and young adults 
  2. Any bone may be involved; most common locations include proximal humerus, distal femur, proximal tibia, and spine 
  3. Presents with pain and tenderness at site of lesion, with or without soft tissue swelling or a palpable mass
  4. Classic radiographic appearance is a radiolucent cystic lesion within metaphyseal portion of bone 
  5. CT or MRI are used to delineate the characteristics and extent of the lesion and plan treatment; bone biopsy is required for definitive diagnosis before treatment
  6. Treatment is guided by orthopedic surgeon’s preference; curettage and bone grafting, with or without adjunctive therapy, is the most widely accepted approach 

Pitfalls

  • Failure to obtain biopsy may lead to a delayed diagnosis of an osteosarcoma or other cancerous lesion
  • Aneurysmal bone cyst is a rare, locally destructive, benign skeletal tumor usually involving the metaphysis of long bones in children and young adults° 
  • Any bone may be involved; most common locations include proximal humerus, distal femur, proximal tibia, and spine 

Classification

  • Primary: approximately 70% of cases 
  • Secondary to a different primary tumor (eg, giant cell tumor, unicameral bone cyst): 30% of cases 
  • Categorized according to stage and morphology
    • Enneking staging system for benign bone tumors
      • Inactive
        • Most benign form
        • Lesion is contained
        • Expansion is rare
        • Inflammation is minimal
      • Active
        • Lesion is associated with expansion and cortical thinning
        • Layer of reactive bone is visible on radiographs
        • Mild symptoms (eg, pain) are present
      • Aggressive
        • Rapidly expansive
        • Locally destructive
        • Highly symptomatic
    • Capanna morphologic types of aneurysmal bone cyst
      • Type 1 lesions
        • Centrally metaphyseal location and well contained
      • Type 2 lesions
        • Entire segment of bone is involved with cortical expansion and thinning
      • Type 3 lesions
        • Eccentric metaphyseal location with no, or minimal, cortical expansion
      • Type 4 lesions
        • Subperiosteal reaction develops with no, or minimal, cortical erosion
      • Type 5 lesions
        • Periosteum displaced toward soft tissues, penetrates cortex with extension into cancellous bone

Clinical Presentation

History

  • Long bones (eg, proximal humerus, distal femur, proximal tibia) and spine are most commonly involved 
  • Mild to moderate pain is the most common symptom; often present for weeks to months 
    • Sometimes associated with pain and numbness extending down affected limb 
  • Soft tissue swelling or palpable mass may be present 
  • Spinal lesions may present with back pain or stiffness, torticollis, painful scoliosis, or, rarely, neurologic deficits and radicular pain 
  • Rarely, associated with fracture 

Physical examination

  • Tenderness over lesion site 
  • Local swelling or palpable expansile mass 
  • Torticollis, scoliosis, or neurologic deficits may be observed in some spinal lesions

Causes and Risk Factors

Causes

  • Cause is unknown; may arise as a result of a reactive process to other pathology, or as a response to local hemorrhage within bone 

Risk factors and/or associations

Age
  • Most common in adolescents and adults younger than 20 years 
Sex
  • No definitive evidence of sex bias; some studies indicate slightly higher incidence in females, while other studies report slightly higher incidence in males 
Genetics
  • Genetic basis is implicated in the development of primary (not secondary) aneurysmal bone cyst; often features chromosome locus 17p13 rearrangements 
Other risk factors/associations
  • Bone lesions that predispose to secondary aneurysmal bone cyst include giant cell tumors, chondroblastoma, and fibrous dysplasia 

Diagnostic Procedures

Primary diagnostic tools

  • Suspect diagnosis on basis of history, physical examination, and imaging characteristics
  • Initially evaluate with plain radiographs; classic appearance is a radiolucent cystic lesion within metaphyseal portion of bone 
  • Obtain CT or MRI to further delineate the characteristics of the lesion and plan surgical management; ultrasonography may also have a role 
  • Confirm diagnosis with bone biopsy in all cases 
  • Failure to obtain biopsy may lead to a delayed diagnosis of an osteosarcoma or other cancerous lesion

Imaging

  • Plain radiograph
    • Typically appears as an expansile radiolucent cystic lesion with a thin sclerotic shell within the metaphyseal portion of the bone; often eccentrically located 
    • May expand into surrounding cortical bone or soft tissue, mimicking a malignant lesion 
  • MRI
    • Examination of choice to complement plain radiograph 
    • May show characteristic multiloculated cavities and fluid-fluid levels, which signify solid blood components within the cystic cavity 
    • Aids in differentiation from other bone pathology
  • CT scan
    • Further delineates characteristics of the lesion; defines cyst margins 
    • Shows presence of fluid-fluid levels, which signify solid blood components within the cystic cavity; allows differentiation from unicameral bone cysts 
  • Ultrasonography 
    • May be used in initial evaluation of a soft tissue mass; can detect underlying bone tumors when the bone cortex is thinned, expanded, or disrupted
    • May identify mobile fluid-fluid levels suggesting aneurysmal bone cyst if the cortex is sufficiently disrupted 
  • Bone scan
    • Can help localize painful lesions, particularly spinal lesions in children 
    • Aneurysmal bone cyst is characterized by diffuse or peripheral tracer uptake with central area of decreased uptake 

Procedures

  • Sampling of bone for histopathologic examination, performed by orthopedic surgeon with patient under regional or general anesthesia
  • Optimal biopsy specimens are obtained by surgical incision 
  • Specimens may also be obtained via needle aspiration with radiologic guidance; however, these are often insufficient for diagnosis and not recommended in most cases 
  • Suspected aneurysmal bone cyst based on imaging 
  • Uncorrected coagulopathy
  • Histopathologic analysis demonstrates hemorrhagic tissue with cavitary spaces separated by fibrous septa, which are composed of different cells (eg, spindle cells, inflammatory cells, giant cells) 

Differential Diagnosis

Most common

  • Telangiectatic osteosarcoma
    • Uncommon variant of osteosarcoma; most important to exclude
    • Tumor primarily consists of large blood-filled spaces separated by thin bone septations
    • Radiographically, lesions may appear similar to aneurysmal bone cyst; however, telangiectatic osteosarcoma may be associated with thick nodular enhancement of tissue surrounding cystic spaces on MRI or CT 
    • Definitive diagnosis is by histopathologic examination of tissue obtained at bone biopsy; however, this may also be difficult owing to histologic similarities 
  • Giant cell tumor
    • Relatively common, usually benign bone tumors arising from metaphysis of long bones
    • Radiographic appearance is of a cystic lytic lesion, similar to that of aneurysmal bone cyst
    • Typically extends into the epiphysis of the long bones; this may be the only feature differentiating from aneurysmal bone cyst 
    • Definitive diagnosis is by histopathologic examination of tissue obtained at bone biopsy
  • Unicameral bone cyst
    • Common, benign, fluid-filled cystic lesion of bone, seen mainly in childhood
    • Can be difficult to differentiate from aneurysmal bone cyst
    • Tends to involve the metaphysis of long bones and may manifest with pain or limping; however, often asymptomatic 
    • May appear radiographically similar to aneurysmal bone cyst
    • MRI imaging helps differentiate; presence of double-density fluid level and intralesional septations usually indicates aneurysmal bone cyst rather than unicameral bone cyst 
    • Definitive diagnosis is by histopathologic examination of tissue obtained at bone biopsy

Treatment Goals 

  • Eradicate lesion
  • Minimize risk of recurrence
  • Relieve pain and functional impairment

Admission criteria

  • Admit for surgical treatment if this is the selected therapeutic approach

Recommendations for specialist referral

  • Refer to orthopedic surgeon for evaluation and management
  • Refer to spinal surgeon if lesions involve vertebrae

Treatment Options

Treatment is determined by preference of surgeon after orthopedic evaluation and biopsy for definitive diagnosis

Surgical curettage and bone grafting, with or without adjuvant therapy, is the primary method of treatment 

  • Associated with good local control rate, which is further improved by use of adjuvant therapies
  • Less invasive than complete surgical excision; however, does require open surgery

Several minimally invasive treatments that can be performed on an outpatient basis have emerged as alternatives to curettage, including sclerotherapy and selective arterial embolization 

  • Sclerotherapy with a fibrosing agent (eg, polidocanol) achieves local control rates comparable to excision with lower morbidity; however, multiple treatments are required 
  • Selective embolism of arteries feeding the cyst is more often performed in conjunction with another procedure, particularly with cysts involving the axial skeleton; technically demanding and associated with risk of complications such as postembolism syndrome, transient paresthesias, and skin and subcutaneous necrosis 
  • Medical therapy with denosumab has been used successfully but is not currently an established option 
  • Radionuclide ablation with CT-guided injection of chromic phosphate P 32 appears promising but is not currently considered a first line treatment option 

Complete surgical excision is reserved for lesions in locations such as fibula or clavicle, where it can be performed without compromising function or cosmesis 

  • Excellent local control rate; however, no longer preferred owing to significant risk of complications (eg, bleeding, growth disturbance) and need for local reconstruction 

Nondrug and supportive care

Procedures
Curettage and bone grafting

General explanation

  • Surgeon removes the active component of the cyst by curettage via open surgical incision, then implants allograft bone chips or polymethylmethacrylate bone cement 
  • Adjuvant therapy is often used to minimize risk of local recurrence
    • Commonly used methods include cryotherapy with liquid nitrogen, use of a high-speed burr for curettage, argon beam laser therapy, electrocautery, and instillation of phenol 
      • Currently, there are no available high-level controlled, comparative studies regarding adjuvant efficacy, and any strategy used is typically institution dependent 
    • Use of polymethylmethacrylate bone cement may be superior to bone grafting in preventing local recurrence

Indication

  • Standard treatment of most aneurysmal bone cysts 
Sclerotherapy

General explanation

  • Percutaneous instillation of a sclerosing agent (eg, polidocanol, alcohol, doxycycline) into bone cyst 

Indication

  • Minimally invasive alternative to curettage for initial treatment of aneurysmal bone cysts 
Selective arterial embolization

General explanation

  • Percutaneous image-guided instillation of an occlusive agent into feeder blood vessels

Indication

  • Option for sole treatment when surgical management is difficult or may be associated with high risk of complications 
  • May be used as preoperative treatment to minimize intraoperative bleeding in large bone cysts in axial skeleton and pelvis 

Contraindications

  • Proximity to blood vessels supplying vital structures (eg, spinal cord)
  • Absence of feeding vessels to catheterize
Surgical excision 

General explanation

  • Complete open surgical resection of bone cyst

Indication

  • Advocated as most appropriate treatment of aneurysmal bone cysts of spine
  • Second line surgical option in most cases owing to risk of complications and need for local reconstruction

Contraindications

  • Infeasible cyst location (will adversely affect function or cosmesis) or proximity to vital structures 

Monitoring

  • Long-term clinical and radiologic follow-up is needed to assess for recurrence and monitor function 

Complications

  • Pathologic fracture 
  • Recurrence 
  • Traumatic cyst rupture, leading to bleeding 
  • Malignant transformation (rare cases reported after treatment with radiation therapy) 

Prognosis

  • Overall cure rate is 90% to 95% 
  • Functional outcomes are usually good; however, disabilities may result from the cyst or its treatment 
  • Local recurrence rate is approximately 10% to 20%
    • Younger age, open growth plates, and a metaphyseal location have been associated with increased risk of recurrence 

References

Glazebrook KN et al: Ultrasound of primary aneurysmal bone cyst. Case Rep Radiol. 2014:101069, 2014

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