Orthopaedic Surgery - Aneurysmal Bone Cyst

Basics

An aneurysmal bone cyst (ABC) is a benign, expansile, reactive cystic lesion of bone that occurs predominantly in children and adolescents. The lesion can enlarge rapidly and cause marked expansion of the affected bone.

An aneurysmal bone cyst may arise as a primary lesion, without another underlying bone abnormality, or it may develop as a secondary lesion associated with another tumor or bone disorder.

Secondary aneurysmal bone cyst formation may occur in association with lesions such as giant cell tumor, chondromyxoid fibroma, fibrous dysplasia, osteoblastoma, and osteosarcoma.

Although benign, ABCs can behave aggressively at the local level because progressive expansion may destroy surrounding cortical bone and adjacent structures.

Incidence

Aneurysmal bone cyst is a relatively common benign bone lesion.

It occurs most frequently in children and adolescents with open growth plates, reflecting its predominance in the skeletally immature population.

The lesion may involve a variety of bones but is commonly encountered in the metaphyseal regions of long bones.

Risk Factors

There are no clearly established specific risk factors for development of a primary aneurysmal bone cyst.

Secondary lesions occur in association with certain benign and malignant bone tumors, but the presence of these conditions does not necessarily predict development of an ABC.

Pathophysiology

Aneurysmal bone cyst is characterized by abnormal vascular spaces within a reactive bone lesion.

These vascular channels contribute to progressive expansion of the lesion and enlargement of the affected bone.

As the cyst enlarges, the inner or endosteal surface of the bone is progressively resorbed.

At the same time, the periosteum may produce a thin shell or rim of new bone around the expanding lesion, giving the characteristic appearance of an expansile lesion surrounded by a delicate bony margin.

Local Destructive Effects

Despite being histologically benign, an aneurysmal bone cyst may become locally destructive.

Progressive expansion can erode the cortex and weaken the structural integrity of the bone.

When the lesion is located near a joint or growth plate, it may damage the physis, articular cartilage, and surrounding ligamentous structures.

Large lesions can therefore produce significant deformity, functional impairment, or pathologic fracture.

Etiology

An aneurysmal bone cyst is considered a benign reactive vascular lesion.

The precise initiating cause is not completely understood.

Primary ABCs develop without another identifiable bone lesion, whereas secondary ABCs represent cystic and vascular changes occurring within another underlying bone tumor or disorder.

Associated Conditions

Primary aneurysmal bone cysts are not typically associated with systemic disease.

However, secondary aneurysmal bone cyst changes may occur within other bone lesions, including giant cell tumor, osteoblastoma, chondromyxoid fibroma, fibrous dysplasia, and osteosarcoma.

Identifying an underlying lesion is important because treatment and prognosis may differ substantially.

Diagnosis

Signs and Symptoms

The most common presenting symptom is localized pain over the affected bone.

As the lesion expands, a visible or palpable soft-tissue mass may develop.

Pain may gradually increase over a relatively short period because these lesions can enlarge rapidly.

When the lower extremity is involved, the patient may develop a limp or antalgic gait because weight-bearing produces discomfort.

Physical Examination

Examination commonly demonstrates localized tenderness over the affected area.

An expansile lesion may produce a palpable mass or visible swelling.

The overlying skin is usually normal unless the lesion has become very large.

Range of motion of a nearby joint should be assessed, particularly when the lesion is close to the articular surface.

The affected extremity should also be examined for deformity, weakness, and signs of impending or established pathologic fracture.

Imaging

Plain Radiographs

Radiographs frequently demonstrate an eccentric, expansile lesion, often involving the metaphysis of a long bone.

The lesion may progressively thin or destroy the cortex.

A thin shell of periosteal new bone may surround the lesion as the bone expands.

The overall appearance may be described as expansile or ballooned, with varying degrees of cortical thinning and destruction.

CT

Computed tomography provides detailed assessment of the cortical bone and internal architecture of the lesion.

An aneurysmal bone cyst typically appears as an expansile lesion without mineralized tumor matrix.

CT is particularly helpful for evaluating cortical destruction, defining the extent of the lesion, and planning surgery in anatomically complex areas.

MRI

MRI is useful for assessing both the internal characteristics and the full extent of an aneurysmal bone cyst.

A characteristic finding is the presence of multiple fluid-fluid levels, caused by layering of blood products of different densities within the cystic spaces.

MRI may also demonstrate edema in the surrounding cancellous bone, reactive changes in adjacent soft tissues, and periosteal reaction.

It is especially useful when determining involvement of the growth plate, joint surface, neurovascular structures, or surrounding soft tissues.

Differential Diagnosis

The major differential diagnoses include giant cell tumor and telangiectatic osteosarcoma.

Telangiectatic osteosarcoma is particularly important because it can resemble an aneurysmal bone cyst clinically and radiologically, including the presence of fluid-fluid levels on MRI.

Careful imaging assessment and histological evaluation are therefore required when the diagnosis is uncertain.

Treatment

General Measures

Prompt evaluation and treatment are important because aneurysmal bone cysts can enlarge very rapidly.

Some lesions may increase markedly in size over only a few weeks.

Rapid enlargement can cause progressive cortical destruction, weakening of the bone, involvement of the growth plate, and damage to nearby articular or ligamentous structures.

Management should therefore focus both on controlling the lesion and preventing pathologic fracture.

Activity Modification

The affected extremity should be protected while definitive treatment is being planned.

For lesions involving the lower extremity, patients may be advised to use two crutches and reduce or avoid weight-bearing when there is significant structural weakening.

For upper-extremity lesions, activities that place excessive force through the involved bone should be avoided.

These precautions are intended to decrease the risk of a pathologic fracture through the weakened area.

Physical Therapy

Physical therapy is mainly used to educate the patient regarding safe mobilization and protection of the affected extremity.

Before surgery, the therapist may teach appropriate use of crutches or other assistive devices.

After treatment, rehabilitation can help restore joint motion, muscle strength, gait, and normal function while protecting the healing bone.

The rehabilitation program depends on the location and size of the lesion and the type of surgical reconstruction performed.

Surgical Management

Surgery is commonly required for aneurysmal bone cysts.

The traditional surgical approach involves intralesional curettage, in which the cystic and abnormal tissue is removed from within the affected bone.

Because the lesion is benign, a wide oncologic resection is not routinely required in most cases.

However, thorough removal is important because residual lesion tissue may contribute to local recurrence.

Curettage

During curettage, the cyst is opened and the abnormal tissue is carefully removed from the cavity.

The walls of the lesion may be further treated according to surgeon preference in an attempt to reduce recurrence.

The resulting bone defect can be substantial, particularly in large lesions.

The defect therefore often requires reconstruction to provide structural support and facilitate bone healing.

Bone Grafting and Bone Graft Substitutes

After curettage, the remaining cavity may be filled with bone graft or a bone graft substitute.

Various synthetic or processed materials are available.

The selected material should be biologically compatible and should not create an unnecessary immunologic response.

When graft substitutes are used, materials without risk of disease or viral transmission are preferred.

The choice of graft material depends on lesion size, location, patient age, and the amount of structural support required.

Follow-Up

Patients should be monitored closely after treatment because aneurysmal bone cysts can recur locally.

Follow-up usually includes periodic clinical assessment and radiographs.

When the patient remains asymptomatic and radiographs show satisfactory healing, plain radiographs alone are often sufficient for surveillance.

Evaluation of Recurrent Pain

The development of new or recurrent pain during follow-up should raise concern for local recurrence.

In this situation, repeat radiographs should be obtained.

MRI is also useful when symptoms recur because it can identify recurrent cystic changes before they become obvious on plain radiographs and can assess the surrounding bone and soft tissues.

Prognosis

The overall prognosis is excellent because an aneurysmal bone cyst is a benign lesion.

Most patients achieve good long-term function after successful treatment.

Outcome depends on adequate control of the lesion, restoration of bone strength, and preservation of nearby growth plates, joints, and neurovascular structures.

Local recurrence may occur, particularly in younger patients or after incomplete treatment, but recurrence can usually be managed with further therapy.

Complications

Despite its benign nature, an aneurysmal bone cyst can become very large and produce significant local destruction.

Potential complications include cortical destruction, pathologic fracture, deformity, and loss of mechanical strength of the affected bone.

Lesions adjacent to a growth plate may damage the physeal cartilage, potentially producing growth disturbance or angular deformity.

Joint and Ligament Damage

When an aneurysmal bone cyst extends toward a joint, it may damage the articular cartilage, potentially affecting long-term joint function.

Expansion may also disrupt nearby ligamentous structures.

These complications are particularly concerning in large or rapidly progressive lesions and provide an additional reason for timely treatment.

Patient Monitoring

Regular orthopedic follow-up is recommended after treatment.

Monitoring should include assessment of pain, swelling, limb function, joint movement, deformity, and evidence of pathologic fracture.

Serial radiographs are used to evaluate bone healing and detect local recurrence.

If new pain, swelling, or functional deterioration develops, MRI should be considered to assess for recurrent or residual disease.