Table of Contents
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 for Popliteal Cyst in the Child
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.
Appointment
If you would like an appointment / review with our child’s popliteal cyst specialist in Singapore, the best way is to call +65 6664 8135 or click here to book an appointment at the clinic. If you would like to speak to one of our clinicians first about e.g. popliteal abscess, popliteal fossa mass, popliteal cyst in child, baker’s cyst in child’s knee, lateral popliteal cyst, popliteal fossa swelling, popliteal fossa cyst etc, then please contact feedback2@bone.com.sg or SMS/WhatsApp to +65 6664 8135.
Rest assured that the best possible care will be provided for you.
