Orthopaedic Surgery - Popliteal Cyst in the Child

Basics

A popliteal cyst in a child is a benign, fluid-filled soft-tissue mass located in the:

Posteromedial popliteal fossa behind the knee.

It is usually:

Painless

and is most often an isolated condition rather than a manifestation of significant intra-articular knee disease.

Epidemiology

Popliteal cysts are among the:

Most common soft-tissue masses around the knee in children.

They are typically seen between approximately:

2 and 14 years of age.

Incidence

The frequency tends to:

Decrease after approximately 9 years of age.

Sex

Historical series suggest that childhood popliteal cysts are approximately:

Twice as common in boys as in girls.

Risk Factors

Most cases occur:

In isolation.

Less commonly, a cyst may be associated with chronic inflammatory conditions such as:

Juvenile idiopathic arthritis

or another source of persistent knee synovitis.

Genetics

There is no recognized Mendelian inheritance pattern.

Etiology

Childhood popliteal cysts are thought to arise from a weak area in the:

Posteromedial knee capsule

near the interval between the:

Semimembranosus tendon

and

Medial head of the gastrocnemius.

Unlike adult Baker cysts, pediatric cysts are:

Rarely associated with intra-articular structural abnormalities.

Anatomy

The cyst typically occupies the:

Semimembranosus-gastrocnemius interval.

It contains:

Synovial-like fluid

and may communicate variably with adjacent bursae or the knee joint.

Diagnosis

Diagnosis is usually based on:

History

Physical examination

and, when necessary,

Ultrasound.

Signs and Symptoms

The typical finding is a:

Soft swelling in the posteromedial aspect of the knee.

Location

The mass is usually found between the:

Medial gastrocnemius

and

Semimembranosus/semitendinosus region.

It may appear as a swelling on the:

Medial side of the popliteal fossa.

Symptoms

Most children are:

Asymptomatic.

However, a very large cyst may occasionally cause:

Posterior knee discomfort

Tightness

or

Restriction of knee motion.

Change in Size

The cyst often:

Waxes and wanes in size.

Parents may notice that it becomes more prominent after:

Activity

and less noticeable after rest.

Duration

Many cysts are present for:

Weeks or months

before medical assessment because they are usually painless.

Physical Examination

Inspection

Inspect the affected knee for:

Posteromedial popliteal swelling

and compare with the opposite side.

Palpation

The cyst is generally:

Soft

Compressible

and

Painless.

Tenderness should prompt consideration of another diagnosis.

Knee Examination

The remainder of the knee examination is usually:

Normal.

Assess:

Range of motion

Effusion

Joint-line tenderness

Ligament stability

Gait

Observe the child’s gait.

A typical uncomplicated popliteal cyst should not produce:

A limp.

A persistent limp suggests the need to evaluate for another musculoskeletal problem.

Transillumination

Historically, transillumination has been used to support the diagnosis.

In a darkened room, a strong point light source is placed adjacent to the swelling while the child lies:

Prone.

A fluid-filled cyst may illuminate:

More evenly and brightly

than surrounding solid tissue.

This finding supports a cystic lesion but does not replace imaging when the diagnosis is uncertain.

Laboratory Tests

Routine laboratory studies are:

Not required.

Aspiration

Aspiration is not routinely performed.

If aspirated, typical cyst fluid is:

Clear

and

Gelatinous or viscous.

Abnormal Aspirate

If the fluid is:

Cloudy

Purulent

Bloody in an atypical setting

or otherwise unexpected, further evaluation may include:

Cell count

Gram stain

Culture

to exclude:

Infection

or

Abscess.

Imaging

Plain Radiographs

Plain radiographs are usually:

Optional.

They may be obtained if there is concern for:

Underlying bony abnormality

or another source of knee symptoms.

Ultrasound

Ultrasound is the preferred imaging modality when the diagnosis is uncertain.

It can confirm that the lesion is:

Cystic

and help define its:

Size

Location

Relationship to adjacent structures.

Duplex Ultrasound

Duplex imaging may be especially useful when a vascular lesion is part of the differential diagnosis.

MRI

MRI is rarely needed.

It may be obtained when the lesion is:

Atypical

Solid-appearing

Painful

Rapidly enlarging

or when there is concern for:

Soft-tissue tumor or other deep pathology.

Pathological Findings

The lesion consists of a:

Fluid-filled synovial or bursal sac

within the:

Semimembranosus-gastrocnemius interval.

In children, it is generally not caused by major intra-articular pathology.

Differential Diagnosis

Important alternatives include:

Soft-tissue sarcoma

Vascular malformation or vascular lesion

Soft-tissue abscess

Other cystic or solid popliteal masses

Soft-Tissue Tumor

An enlarging, firm, fixed, painful, or nontransilluminating lesion should raise concern for a:

Solid soft-tissue mass

and warrants appropriate imaging.

Vascular Lesion

A pulsatile lesion or one associated with vascular findings should prompt evaluation for:

Vascular anomaly.

Abscess

An abscess is more likely to be associated with:

Pain

Erythema

Warmth

Fever

or abnormal inflammatory findings.

Treatment

General Principles

Most childhood popliteal cysts require:

No treatment.

Observation is appropriate when the child has:

No pain

Normal knee motion

Normal function

and a secure diagnosis.

Natural History

Approximately:

80% of pediatric popliteal cysts

have historically been reported to resolve spontaneously over:

Months to years.

During this period, the cyst may repeatedly:

Increase and decrease in size.

Activity

Normal activity can usually continue.

If the cyst becomes very large or uncomfortable, temporary activity reduction may be reasonable.

Aspiration

Aspiration is generally:

Not first-line treatment

because recurrence is common.

It may occasionally be considered for a superficial symptomatic cyst after:

Appropriate imaging confirmation.

Post-Aspiration Care

When aspiration is performed, temporary:

Immobilization or compression

may be used after decompression.

However, recurrence remains:

Frequent.

Surgery

Surgical treatment is:

Rarely necessary.

It may be considered when the cyst causes:

Persistent pain

Significant limitation of knee motion

Repeated functional problems

or

Diagnostic uncertainty.

Surgical Excision

Excision is performed through a:

Posterior or posteromedial approach to the popliteal region.

The procedure can often be performed as:

Outpatient surgery.

Postoperative Immobilization

The knee may be immobilized for:

Several weeks

depending on the extent of the procedure and surgeon preference.

Recurrence After Surgery

Surgical excision has a substantial recurrence rate, historically approximately:

20–40%.

This is one reason observation is preferred for uncomplicated lesions.

Follow-Up

Routine long-term follow-up is usually unnecessary when:

Pain is absent

Knee motion is normal

The diagnosis is secure.

Parent Education

Parents should be advised that the cyst may:

Change in size

and often resolves without treatment.

They should return for reassessment if the lesion develops:

Pain

Rapid growth

Firmness

Skin changes

Loss of motion

Limp

or another change in character.

Prognosis

The prognosis is:

Excellent.

Most children remain fully functional and many cysts resolve spontaneously.

Complications

The principal complication of operative treatment is:

Recurrence.

Other surgical complications are uncommon but may include:

Scar problems

Stiffness

Neurovascular injury

Patient Monitoring

No routine surveillance is required in a child who has:

Normal function

No pain

and a stable, typical cyst.

Clinical reassessment is appropriate if symptoms or the characteristics of the mass change.

Key Principle

A popliteal cyst in a child is usually a benign, painless, fluid-filled mass in the posteromedial knee that is rarely associated with significant intra-articular disease.

Most cases should be managed with:

Observation and reassurance.

Because spontaneous resolution is common and surgical recurrence is relatively high, intervention is reserved for:

Persistent symptomatic or diagnostically uncertain lesions.