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Popliteal Cyst in the Child

Popliteal cyst is a painless soft-tissue mass in the medial popliteal fossa behind the knee.
  • Most common soft-tissue lesion about the knee in children
  • Affects children 2-14 years old
Incidence decreases after 9 years of age.
Twice as common in males
Risk Factors
  • Most are isolated cases.
  • Juvenile rheumatoid arthritis
  • Other chronic inflammation of the knee
No Mendelian pattern is known.
  • Likely resulting from weakness in the posterior knee joint capsule between the semimembranosus muscle and the medial head of the gastrocnemius
  • Rarely related to intra-articular lesions
Signs and Symptoms
  • Protrusion between the medial gastrocnemius and semitendinosus muscles
  • Swelling of the medial side of the popliteal space just lateral to the semitendinosus muscle
  • Usually asymptomatic, but can cause discomfort and restrict ROM of knee if excessively enlarged
  • Usually waxes and wanes in size, depending on the child’s activity level
  • Typically present for some time before the child is brought to the physician
Physical Exam
  • Examine the affected lower limb for swelling of the medial side of the popliteal space just medial to the semimembranosus muscle.
  • Compress the cyst to check for pain.
    • Usually painless
    • The remainder of the knee examination usually is normal.
  • Examine the gait.
    • No limp should be evident.
  • Transilluminate the cyst in a darkened room with a point light source (e.g., strong penlight)
    • With the patient prone, place the light source on the skin next to the area of swelling.
    • If the mass illuminates more strongly and evenly than the surrounding fatty tissue, the fluid-filled nature of cyst is confirmed, and a diagnosis of solid tumor is excluded.
  • Aspiration is not commonly performed.
  • However, if the cyst is aspirated, the cyst fluid is clear and gelatinous.
    • If the cyst fluid is not clear and gelatinous, send the aspirate for the following tests to rule out septic arthritis or soft-tissue abscess:
      • Cell count
      • Gram stain
      • Culture
  • Plain-film radiography is optional to rule out bony disorder.
  • Duplex ultrasound and MRI (rarely indicated) characterize a questionable cyst further and rule out malignancy.
Pathological Findings
  • Synovial fluid-filled sac in the semimembranosus-gastrocnemius interval
  • Rarely related to intra-articular lesions
Differential Diagnosis
  • Malignant disease
  • Vascular anomaly
  • Soft-tissue abscess
General Measures
  • The patient’s activity may be restricted when the cyst is large.
  • Surgical excision may be necessary if the cyst is symptomatic (rare).
    • The recurrence rate after surgical excision is 20-40%.
  • No treatment is required if no intra-articular lesion is present.
    • Left untreated, 70% of cysts disappear spontaneously after months to years (they may wax and wane in size).
  • If it is desired to confirm the diagnosis and increase the chance of resolution, the cysts may be aspirated with a large-bore needle, followed by immobilization for immediate decompression.
    • However, the recurrence rate is high.
  • Excision of the cyst through a transverse incision in the posterior popliteal region:
    • May be done as an outpatient procedure
  • Immobilization for several weeks postoperatively
The rate of recurrence of the cyst after surgical treatment is ~20-40%.
Patient Monitoring
  • No routine follow-up is needed.
  • Instruct the parent to return if the lesion changes in symptoms or in character.
727.51 Popliteal cyst
Patient Teaching
  • Inform parents about the benign nature of the condition.
  • Explain the similarity of the pathologic process to that of the Baker cyst in adults.
  • Mention the lack of underlying knee disease and the absence of increased synovial fluid production.
Q: Is MRI indicated for a popliteal cyst in a child?
A: Not unless the cyst is atypical and does not transilluminate, or separate symptoms are referable to the knee.
Q: Is follow-up needed?
A: If it is a typical cyst, no follow-up is needed unless symptoms develop.

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