Popliteal Cyst in the Child

By admin Last updated Feb 23, 2024

Ovid: 5-Minute Orthopaedic Consult

Editors: Frassica, Frank J.; Sponseller, Paul D.; Wilckens, John H.

Title: 5-Minute Orthopaedic Consult, 2nd Edition

> Table of Contents > Popliteal Cyst in the Child

Popliteal Cyst in the Child

Paul D. Sponseller MD

Basics

Description

Popliteal cyst is a painless soft-tissue mass in the medial popliteal fossa behind the knee.

Epidemiology

Most common soft-tissue lesion about the knee in children
Affects children 2–14 years old

Incidence

Incidence decreases after 9 years of age (1,2).

Prevalence

Twice as common in males (2)

Risk Factors

Most are isolated cases.
Juvenile rheumatoid arthritis
Other chronic inflammation of the knee

Genetics

No Mendelian pattern is known.

Etiology

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

Fig. 1. A diagnosis of popliteal cyst in a child may be confirmed by transillumination.

Diagnosis

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) (Figs. 1 and 2).

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.

Fig. 2. Prone 7-year-old with popliteal cyst. A: External appearance of cyst. B: After transillumination. Note that the cyst picks up light remote from the source in comparison with surrounding tissues.

Tests

Lab

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

Imaging

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 (3).

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

P.337

Treatment

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% (2).
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) (3,4).
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.

Surgery

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

Follow-up

Complications

The rate of recurrence of the cyst after surgical treatment is ~20–40% (2).

Patient Monitoring

No routine follow-up is needed.
Instruct the parent to return if the lesion changes in symptoms or in character.

References

1. De Greef I, Molenaers G, Fabry G. Popliteal cysts in children: a retrospective study of 62 cases. Acta Orthop Belg 1998;64:180–183.

2. Willis
RB. Sports medicine in the growing child. Overuse injuries. In:
Morrissy RT, Weinstein SL, eds. Lovell and Winter’s Pediatric
Orthopaedics, 6th ed. Philadelphia: Lippincott Williams & Wilkins,
2006:1414–1421.

3. De
Maeseneer M, Debaere C, Desprechins B, et al. Popliteal cysts in
children: prevalence, appearance and associated findings at MR imaging.
Pediatr Radiol 1999;29:605–609.

4. Seil R, Rupp S, Jochum P, et al. Prevalence of popliteal cysts in children. A sonographic study and review of the literature. Arch Orthop Trauma Surg 1999;119:73–75.

Miscellaneous

Codes

ICD9-CM

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.

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.