Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

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Popliteal Cyst in the Adult

Basics
Description
  • Popliteal cyst in an adult is a distended cyst in the posterior fossa of the knee that often is connected directly to the joint space.
  • Synonym: Baker cyst
Epidemiology
  • Bimodal distribution: 1 subset in children, 1 in adults >55 years old:
    • In adults, cysts usually are the result of intra-articular abnormalities.
    • In children, the cyst is the primary disorder.
  • Males and females are affected equally.
Risk Factors
Intra-articular knee disease (meniscus tear or arthritis)
 
Genetics
No known Mendelian pattern
 
Etiology
  • Herniation of the synovial membrane through a weak point of the medial posterior capsule between 2 expansions of the semimembranosus tendon
  • Formation of a 1-way valve between the joint and the bursa
Associated Conditions
  • Rheumatoid arthritis
  • Osteoarthritis
  • Chronic ACL tear
  • Medial or lateral meniscal tears
Diagnosis
Signs and Symptoms
  • Mass or fullness in the popliteal fossa of the knee
  • Knee effusion
  • Symptoms of ruptured cyst are warmth, tenderness, and erythema of the calf, or isolated calf swelling, mimicking DVT.
  • Large cysts can produce posterior tibial nerve compression neuropathy.
History
Acute or chronic history of pain and/or mass behind knee
 
Physical Exam
  • Fullness and tenderness posteriorly in the popliteal fossa
  • Calf tenderness and swelling in the case of ruptured cyst
  • Presence of other potential causes of knee swelling, such as meniscal injury (joint line tenderness) or chronic ligamentous tear
Tests
Lab
Rheumatoid factor
 
Imaging
  • Standing flexed posteroanterior and lateral radiographs
  • MRI to evaluate the size of the cyst, differentiate it from a soft-tissue tumor, and identify intra-articular abnormalities
  • Duplex ultrasound is the most cost-effective imaging study for diagnosis and can help rule out DVT as the cause of a ruptured cyst.
Pathological Findings
  • Swelling in the popliteal fossa bursae, usually secondary to intra-articular disease
  • Herniation of the synovial membrane through a weakened area in the posterior joint capsule
  • Differentiating from DVT:
    • Important because anticoagulation in the presence of a ruptured cyst could lead to hematoma or compartment syndrome
    • Physical examination can be unreliable.
    • MRA provides the best resolution, but duplex ultrasound is the best test because of its availability and low cost.
Differential Diagnosis
  • Semimembranosus bursa
  • Ruptured cyst
  • Soft-tissue tumor
  • Lipoma in the popliteal fossa
  • Synovial cell sarcoma
  • Pseudoaneurysm (usually pulsatile)
  • Primary bone malignancy (e.g., parosteal osteosarcoma)
  • DVT
Treatment
General Measures
  • Symptomatic: Needle aspiration with or without steroid injection:
    • Observation with symptomatic care is acceptable if pain is tolerable.
    • Address intra-articular pathology (e.g., arthroscopic meniscectomy for meniscus tear)
  • Asymptomatic cyst requires no treatment.
  • >90% of popliteal cysts have associated intra-articular abnormalities; these must be addressed to prevent recurrence of the cyst.
  • Cysts often resolve spontaneously after treatment of the intra-articular abnormality.
Activity
The patient may continue all activities, limited only by pain.
 
Special Therapy
Physical Therapy
  • General knee and core strengthening program
  • Postoperative therapy as dictated by the surgical procedure; early ROM and weightbearing usually allowed
Medication
Resolution of symptoms in most cysts with nonoperative care and reassurance
 
First Line
Analgesics and NSAIDs are treatment mainstays.
 
Second Line
Narcotic analgesics rarely needed
 
Surgery
  • Arthroscopic evaluation and treatment of any intra-articular abnormalities are indicated.
  • A cyst that persists after treatment of the joint disorder can be removed via a posteromedial incision, but it has a high rate of recurrence unless intra-articular abnormalities are addressed.
Follow-up
Prognosis
  • Most cysts resolve once the intra-articular disorder is treated.
  • Untreated cysts may increase in size, but they often reach a stable, constant size.
Complications
  • Recurrence
  • Rupture, causing a painful, swollen calf and lower extremity (simulating DVT)
  • Popliteal artery or vein occlusion, posterior tibial nerve entrapment, and compartment syndrome are rare.
Patient Monitoring
Patients are followed at 4-6-week intervals after surgery until ROM and function return.
 
Miscellaneous
Codes
ICD9-CM
727.51 Popliteal cyst
 
Patient Teaching
Reassure the patient that the cyst is benign and will not damage the knee.
 
FAQ
Q: How can a popliteal cyst be differentiated from a soft-tissue tumor such as a synovial cell sarcoma?
A: Popliteal cysts usually are asymptomatic with incidental findings on MRI evaluation for knee conditions. If the lesion is getting larger and is painful, particularly at night, synovial cell sarcoma should be considered as a diagnosis. MRI and needle aspiration are required to make the diagnosis.
 
Q: How does a ruptured popliteal cyst present?
A: Patients complain of acute pain and swelling in the calf. Symptoms mimic DVT, which must be ruled out with duplex sonography.

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