Orthopaedic Surgery - Unicameral Bone Cyst

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Basics

A unicameral bone cyst is a:

Benign, fluid-filled, membrane-lined cavity within bone

that occurs primarily during:

Childhood and adolescence.

It is also known as a:

Simple bone cyst

or

Solitary bone cyst.

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Natural History

These lesions generally become progressively less active with:

Skeletal maturation

and may eventually fill with:

Normal bone.

They are uncommon in:

Adults.

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Location

Approximately:

80%

occur in the:

Proximal humerus

or

Proximal femur.

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Less Common Sites

Other possible locations include:

Proximal tibia

Distal tibia

Distal femur

Calcaneus

Distal humerus

Radius

Fibula

Ilium

Ulna

and

Rib.

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Relationship to the Physis

Unicameral bone cysts are usually:

Centrally located

and develop adjacent to the:

Growth plate

within the metaphysis.

As the child grows, the cyst may become located farther into the:

Metaphyseal-diaphyseal region.

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Epiphyseal Extension

Extension across the:

Physis

into the epiphysis is:

Rare.

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Classification

Unicameral bone cysts are often classified as:

Active

or

Inactive.

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Active Cyst

An active cyst lies immediately adjacent to the:

Physis.

It is most commonly seen in children younger than approximately:

10 years of age.

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Active Cyst Appearance

The lesion may occupy much of the:

Metaphysis

and typically has a:

Thin cortical shell.

This increases susceptibility to:

Pathologic fracture.

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Active Cyst Recurrence

Active lesions have a relatively high likelihood of:

Persistence or recurrence

after treatment.

Historical recurrence rates of approximately:

50% or greater

have been reported in some series.

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Inactive Cyst

An inactive cyst is separated from the:

Growth plate

by a zone of normal:

Cancellous bone.

It is more commonly encountered in children older than approximately:

10 years.

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Inactive Cyst Cortex

Inactive cysts typically have:

Thicker surrounding cortex

and therefore carry a lower risk of:

Fracture

and

Recurrence.

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Epidemiology

Almost all unicameral bone cysts are diagnosed before:

20 years of age.

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Sex

The condition occurs more commonly in:

Boys

than girls.

A historical male-to-female ratio of approximately:

2:1

has been reported.

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Genetics

No consistent hereditary or:

Genetic association

has been established.

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Etiology

The exact cause remains:

Unknown.

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Proposed Mechanisms

Suggested mechanisms include:

Intraosseous hematoma

Venous obstruction

Lymphatic obstruction

or persistence of an:

Intraosseous synovial rest.

None of these theories has been definitively:

Proven.

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Pathology

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Gross Findings

Gross examination shows a:

Cystic cavity

lined by a thin membrane of variable:

Thickness.

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Cyst Contents

The cavity usually contains:

Clear to yellowish fluid.

The fluid may become:

Blood-tinged

or frankly:

Hemorrhagic

after a recent fracture or prior:

Aspiration or injection.

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Septations

Following fracture, the cyst may develop:

Internal septations

and

Loculated fluid collections.

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Microscopic Findings

The cyst lining consists mainly of:

Fibrous connective tissue.

Other findings may include:

Small bone spicules

Osteoclasts

Chronic inflammatory cells

and

Multinucleated giant cells.

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Associated Conditions

No specific systemic condition is consistently associated with:

Unicameral bone cysts.

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Diagnosis

Most lesions are discovered after:

Pathologic fracture

or incidentally on:

Imaging.

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Signs and Symptoms

Most patients are:

Asymptomatic

until fracture occurs.

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Pathologic Fracture

The cortex may become sufficiently thin that fracture occurs after:

Minimal trauma.

Examples include:

Throwing a ball

Minor falls

or routine:

Sports activity.

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Incidental Discovery

Lesions without fracture may be discovered incidentally during imaging performed for an:

Unrelated reason.

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Pain

Pain generally occurs when there is:

Fracture

or, less commonly, substantial weakening of the:

Bone.

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Physical Examination

The examination is often:

Normal

in an uncomplicated lesion.

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After Fracture

When fracture is present, examination may demonstrate:

Pain

Tenderness

Swelling

and reduced:

Limb use.

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Recurrent Fracture

In patients with repeated fractures, assess for:

Angular deformity

and

Limb-length discrepancy.

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Growth Disturbance

Recurrent injury near the physis may rarely produce:

Growth arrest

resulting in:

Limb-length inequality

or

Angular deformity.

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Imaging

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Plain Radiographs

The classic radiographic appearance is a:

Centrally located

Well-marginated

Radiolucent

and mildly:

Expansile lesion.

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Typical Location

The lesion is usually located within the:

Metaphysis

of a long bone.

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Cortex

The surrounding cortex is typically:

Thinned

but remains relatively:

Smooth.

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Periosteal Reaction

In an uncomplicated cyst, aggressive:

Periosteal reaction

is generally absent.

A fracture may produce secondary:

Callus or periosteal response.

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Fallen Fragment Sign

The:

Fallen fragment sign

is highly characteristic of a unicameral bone cyst complicated by:

Fracture.

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Mechanism of Fallen Fragment Sign

A piece of cortical bone breaks free and falls into the:

Dependent portion of the fluid-filled cavity.

This suggests a true:

Cystic lesion

rather than a solid tumor.

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Movement Away From the Physis

As the lesion becomes less active and the child grows, the cyst appears to migrate away from the:

Epiphysis

because normal metaphyseal bone develops between the cyst and:

Growth plate.

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MRI

MRI is useful when it is unclear whether the lesion is:

Cystic

or

Solid.

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MRI Appearance

A simple cyst typically demonstrates homogeneous fluid signal, including:

High T2 signal

consistent with its:

High water content.

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Soft-Tissue Mass

A true uncomplicated unicameral bone cyst should not produce an:

Extraosseous soft-tissue mass.

The presence of a substantial soft-tissue mass should prompt reconsideration of the:

Diagnosis.

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Differential Diagnosis

Important alternatives include:

Aneurysmal bone cyst

Fibrous dysplasia

Enchondroma

Giant cell tumor

and

Eosinophilic granuloma.

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Aneurysmal Bone Cyst

An aneurysmal bone cyst is often more:

Eccentric

and

Expansile

and may show:

Fluid-fluid levels on MRI.

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Fibrous Dysplasia

Fibrous dysplasia often demonstrates a:

Ground-glass matrix

rather than a simple fluid-filled:

Cavity.

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Giant Cell Tumor

Giant cell tumor typically occurs after or near:

Skeletal maturity

and often extends to the:

Subarticular region.

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Treatment

Management depends on:

Location

Cyst size

Fracture risk

Symptoms

and whether a pathologic fracture is already:

Present.

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Observation

An incidentally discovered lesion with low risk of fracture may be managed with:

Observation.

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Pathologic Fracture

When a cyst presents with a fracture, the fracture may first be allowed to:

Heal.

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Spontaneous Cyst Healing After Fracture

A minority of cysts heal completely after:

Fracture union.

Historical studies have reported spontaneous cyst resolution in approximately:

15%

of such cases.

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Persistent Cyst

Most cysts remain at least partially present after:

Fracture healing

and may require further:

Observation or treatment.

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Percutaneous Treatment

Percutaneous treatment may include:

Aspiration

followed by injection of:

Corticosteroid

Bone marrow aspirate

Calcium phosphate

or other:

Bone-graft substitutes.

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Double-Needle Technique

Two needles may be inserted under:

Fluoroscopic guidance.

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Confirmation of Cystic Nature

Aspiration of fluid helps confirm that the lesion is:

Cystic.

Contrast may occasionally be injected to confirm:

Needle location

and cyst architecture.

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Failure to Aspirate

If no fluid can be aspirated or the lesion cannot be entered as expected, the possibility of a:

Solid lesion

should be reconsidered.

Further:

Biopsy

may then be necessary.

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Corticosteroid Injection

Historically, methylprednisolone has been injected into the cyst after:

Aspiration.

Older protocols used doses in the approximate range of:

40–200 mg

depending on cyst size and:

Technique.

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Repeat Injection

Percutaneous injection may need to be repeated.

Historical protocols repeated treatment at approximately:

2-month intervals

for up to:

Three sessions.

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Multiple Treatments

Approximately:

Half of patients

in some historical series required more than one:

Injection.

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Bone Marrow Injection

Injection of:

Autologous bone marrow

has also been used to stimulate:

Bone formation and cyst healing.

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Bone-Graft Substitutes

Injectable materials such as:

Calcium phosphate

or other osteoconductive substitutes may be used in selected:

Patients.

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Curettage and Bone Grafting

Persistent, recurrent, or structurally threatening lesions may require:

Open curettage

and

Bone grafting.

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Graft Options

Possible graft materials include:

Autograft

Allograft

Bone marrow aspirate

and

Demineralized bone matrix.

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High-Stress Locations

Lesions in mechanically critical sites, especially the:

Proximal femur

require more aggressive consideration because fracture may lead to:

Displacement

Deformity

or

Avascular necrosis.

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Internal Fixation

Internal fixation may be appropriate when a lesion causes or threatens fracture in a:

High-stress region.

Examples include the:

Femoral neck

and

Proximal femur.

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Proximal Femur

Treatment may combine:

Curettage

Bone grafting

and

Plate or other internal fixation

to protect against:

Refracture.

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Follow-Up

Serial radiographs are used to assess:

Cyst size

Cortical thickness

Fracture healing

and evidence of:

Recurrence.

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Prognosis

The overall prognosis is:

Excellent.

Most lesions eventually become inactive and fill with:

Bone

as skeletal maturity approaches.

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Treatment Course

Resolution may require:

Repeated injections

Fracture healing

or occasionally:

Curettage and grafting.

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Recurrence

Recurrence depends on:

Age

Location

Cyst size

and proximity to the:

Physis.

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Proximal Humerus

Recurrence has historically been reported more often in the:

Proximal humerus

than in the:

Femur

or

Tibia.

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Flat Bones

When unicameral bone cysts occur in:

Flat bones

recurrence is relatively:

Uncommon.

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Lesion Size

Smaller lesions generally have a lower recurrence rate than:

Larger lesions.

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Age

Cysts occurring during the:

First decade of life

are more likely to:

Recur

because they are more often:

Active.

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Malignant Transformation

Unicameral bone cysts have no recognized tendency toward:

Malignant transformation.

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Complications

Potential complications include:

Pathologic fracture

Recurrent fracture

Growth arrest

Angular deformity

Limb-length discrepancy

and, in the proximal femur,

Avascular necrosis.

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Growth Arrest

Injury near the physis may rarely lead to:

Premature growth arrest.

This can produce:

Limb shortening

or

Angular malalignment.

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Malunion

Repeated fractures may heal with:

Angular deformity

if alignment is not maintained.

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Avascular Necrosis

A fracture through a proximal femoral cyst may compromise the:

Femoral head blood supply

and contribute to:

Avascular necrosis.

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Patient Monitoring

Patients should be monitored for:

Cyst progression

Fracture risk

Cortical thinning

Recurrent fracture

and any evidence of:

Growth disturbance.

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Clinical Summary

Typical patient: Child or adolescent with an incidental lesion or a pathologic fracture after minor trauma.

Most common locations: Proximal humerus and proximal femur, accounting for roughly 80% of cases.

Typical radiograph: Central, well-defined, radiolucent metaphyseal lesion with cortical thinning.

Classic sign: A fallen fragment sign after fracture strongly supports a fluid-filled unicameral bone cyst.

Active cyst: Abuts the physis, occurs in younger children, and has greater risk of fracture and recurrence.

Inactive cyst: Separated from the physis by normal bone and generally has a lower risk of recurrence.

MRI: Useful when distinction from a solid lesion is uncertain.

Treatment: Observation for low-risk lesions; fracture treatment when present; selected cases undergo aspiration and injection, curettage, grafting, or internal fixation.

High-risk location: Proximal femur, where fracture can lead to deformity or avascular necrosis.

Prognosis: Excellent; most cysts eventually become inactive and fill with bone, with no malignant potential.

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Key Principle

A unicameral bone cyst is a benign, fluid-filled lesion of childhood that usually arises centrally in the metaphysis of the proximal humerus or proximal femur.

Most patients are asymptomatic until a:

Pathologic fracture

occurs after relatively minor trauma.

The characteristic imaging appearance is a:

Central radiolucent lesion with thin cortex, sometimes demonstrating the:

Fallen fragment sign.

Management ranges from:

Observation

to

Percutaneous aspiration and injection

or, for persistent or mechanically dangerous lesions,

Curettage, grafting, and internal fixation.

The major clinical concerns are:

Recurrent fracture, growth disturbance, deformity, and proximal femoral complications, while malignant transformation is not expected.