Basics
Description
Popliteal cyst is a painless soft-tissue mass in the medial popliteal fossa behind the knee.
Epidemiology
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Most common soft-tissue lesion about the knee in children
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Affects children 2-14 years old
Incidence
Incidence decreases after 9 years of age.
Prevalence
Twice as common in males
Risk Factors
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Most are isolated cases.
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Juvenile rheumatoid arthritis
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Other chronic inflammation of the knee
Genetics
No Mendelian pattern is known.
Etiology
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Likely resulting from weakness in the posterior knee joint capsule between the semimembranosus muscle and the medial head of the gastrocnemius
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Rarely related to intra-articular lesions
Diagnosis
Signs and Symptoms
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Protrusion between the medial gastrocnemius and semitendinosus muscles
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Swelling of the medial side of the popliteal space just lateral to the semitendinosus muscle
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Usually asymptomatic, but can cause discomfort and restrict ROM of knee if excessively enlarged
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Usually waxes and wanes in size, depending on the child’s activity level
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Typically present for some time before the child is brought to the physician
Physical Exam
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Examine the affected lower limb for swelling of the medial side of the popliteal space just medial to the semimembranosus muscle.
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Compress the cyst to check for pain.
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Usually painless
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The remainder of the knee examination usually is normal.
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Examine the gait.
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No limp should be evident.
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Transilluminate the cyst in a darkened room with a point light source (e.g., strong penlight)
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With the patient prone, place the light source on the skin next to the area of swelling.
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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.
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Tests
Lab
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Aspiration is not commonly performed.
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However, if the cyst is aspirated, the cyst fluid is clear and gelatinous.
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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:
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Cell count
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Gram stain
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Culture
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Imaging
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Plain-film radiography is optional to rule out bony disorder.
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Duplex ultrasound and MRI (rarely indicated) characterize a questionable cyst further and rule out malignancy.
Pathological Findings
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Synovial fluid-filled sac in the semimembranosus-gastrocnemius interval
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Rarely related to intra-articular lesions
Differential Diagnosis
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Malignant disease
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Vascular anomaly
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Soft-tissue abscess
Treatment
General Measures
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The patient’s activity may be restricted when the cyst is large.
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Surgical excision may be necessary if the cyst is symptomatic (rare).
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The recurrence rate after surgical excision is 20-40%.
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No treatment is required if no intra-articular lesion is present.
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Left untreated, 70% of cysts disappear spontaneously after months to years (they may wax and wane in size).
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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.
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However, the recurrence rate is high.
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Surgery
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Excision of the cyst through a transverse incision in the posterior popliteal region:
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May be done as an outpatient procedure
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Immobilization for several weeks postoperatively
Follow-up
Complications
The rate of recurrence of the cyst after surgical treatment is ~20-40%.
Patient Monitoring
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No routine follow-up is needed.
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Instruct the parent to return if the lesion changes in symptoms or in character.
Miscellaneous
Codes
ICD9-CM
727.51 Popliteal cyst
Patient Teaching
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Inform parents about the benign nature of the condition.
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Explain the similarity of the pathologic process to that of the Baker cyst in adults.
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Mention the lack of underlying knee disease and the absence of increased synovial fluid production.
FAQ
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.