Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

Baker’s Cyst (Popliteal Cyst)

Definition

• Baker’s cyst, also called a popliteal cyst, is a benign swelling located behind the knee.

• It may vary in size, from small to large.

• It is a fluid-filled cyst, not a solid tumor.

• The cyst should transilluminate.

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Anatomy and Pathophysiology

• The cyst lies posterior to the medial femoral condyle.

• It is connected to the knee joint through a valvular opening.

• The cyst is located between:

  • Semimembranosus muscle

  • Medial head of the gastrocnemius muscle

• It is usually located at or below the joint line.

Mechanism of Formation

• Knee effusion (excess intra-articular fluid) allows fluid to pass:

  • From the knee joint

  • Through the valve

  • Into the cyst

• The fluid movement is unidirectional, usually toward the back of the knee.

• Tears of the posterior horn of the meniscus extending to the capsule may create a defect or one-way valve between:

  • The knee joint

  • The bursa between gastrocnemius and semimembranosus

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Causes

The two most common causes are:

• Knee arthritis

• Meniscal tear (especially medial meniscus)

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Clinical Presentation

• Swelling behind the knee

• Pain

• Fullness

• Tenderness

• Baker’s cyst is more visible with the knee fully extended.

• Swelling behind the knee may alert the clinician to underlying intra-articular pathology.

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Diagnosis

Clinical Findings

• Posterior knee swelling

• Associated knee effusion

Imaging

• MRI

  • Confirms diagnosis

  • Identifies associated intra-articular pathology

• Ultrasound

  • Helpful diagnostic tool

  • Cyst located medial to the blood vessels

  • Especially useful if cyst is in an atypical location

Differential Diagnosis

• If located in an atypical position, consider tumor.

• Ruptured cyst may cause:

  • Calf pain

  • Swelling

  • Rule out:

    • Deep vein thrombosis

    • Thrombophlebitis

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Treatment in Adults

Conservative Treatment

• Ice

• Compression wrap

• Anti-inflammatory medication

• Strengthening exercises

Aspiration

• Can be performed:

  • Blindly

  • Under ultrasound guidance

• Fluid may be obtained during aspiration.

Surgical Treatment

• Excision of the cyst

• Recurrence is common if intra-articular pathology persists.

• Best treatment:

  • Arthroscopy

  • Debridement of intra-articular pathology

• Primary focus of treatment should be:

  • Meniscal tear

  • Arthritis

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Complications

• Cyst rupture

  • Causes calf pain and swelling

  • Must rule out deep vein thrombosis or thrombophlebitis

• Recurrence is common after removal if underlying pathology continues.

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Popliteal Cyst in Children

Characteristics

• Common soft tissue mass behind the knee

• Occurs more frequently in boys

• Usually asymptomatic

• Transilluminates

• Not a tumor

Pathology

• May not be intra-articular

• May not be connected to the knee joint

• Usually not associated with a meniscal tear

Management

• Surgery is rarely indicated.

• Usually managed with observation.

• Spontaneous resolution may occur within 10–20 months.

• In difficult cases:

  • Aspiration may be performed.

  • May respond to aspiration and steroid injection because it is not connected to the knee joint.