Parameniscal Cysts of the Knee

What they are

Cysts Next to the Meniscus — Cause and Mechanism

Parameniscal cysts are cystic formations located on the periphery of the meniscus — next to the joint capsule. They may be intrameniscal (within the meniscal tissue) or parameniscal (outside the meniscus but adjacent to it).

The most widely accepted pathogenic theory is the extrusion of synovial or myxoid fluid through a meniscal fissure — typically a horizontal tear — which acts as a one-way valve. The fluid accumulates at the periphery, forming the cyst. This association with a horizontal tear is present in up to 98% of cases.

Lateral cysts are more often symptomatic and detectable on physical examination — they present as a palpable mass on the lateral joint line. Medial cysts tend to be deeper and may grow into posterior areas, making them difficult to palpate but clearly visible on MRI.

Treating the parameniscal cyst without treating the underlying meniscal tear leads to recurrence. Arthroscopic surgery addresses the meniscus and decompresses the cyst at the same time.

Lateral vs. Medial Cyst — Clinical Differences

The location determines the symptoms, the clinical presentation and certain particularities of the treatment.

Lateral Parameniscal Cyst

More common and more often symptomatic
Palpable mass on the lateral joint line — typical clinical sign
Increases with extension, decreases with flexion (Pisani's sign)
Lateral knee pain, especially when weight-bearing
Almost always associated with a horizontal tear of the lateral meniscus
MRI and ultrasound readily confirm the diagnosis
Treatment: arthroscopy with cyst decompression + treating the meniscus

Medial Parameniscal Cyst

Less common — but not rare
Often not palpable — deeper and more posterior
Diffuse medial pain, difficult to localise
May grow into posterior areas (popliteal region)
Diagnosis often incidental on MRI
May mimic a Baker's cyst when posteriorly located
Treatment: arthroscopy with a posterior approach if necessary

Diagnosis and Treatment

MRI is the investigation of choice — it identifies the cyst, the associated meniscal tear and helps plan the surgery. Arthroscopic treatment addresses the meniscus and the cyst at the same time.

Diagnosis

MRI of the knee — gold standard. It identifies the cyst (location, size, extent), the associated meniscal tear and the communication between the two. Ultrasound is useful for superficial lateral cysts.

Meniscal Treatment

Arthroscopy: repair of the horizontal tear (when feasible and in good-quality tissue) or partial meniscectomy. Treating the cause — not just the cyst — is decisive in avoiding recurrence.

Cyst Decompression

Intrameniscal arthroscopic decompression (creating an intra-articular communication) or open excision for large cysts. Recurrence is rare when the meniscus is adequately treated.

Questions about Parameniscal Cysts

Not necessarily. Small, asymptomatic cysts can be monitored. When the cyst is symptomatic (pain, a painful mass, functional limitation) and the underlying meniscal tear is identified, arthroscopic surgery is the most effective treatment — addressing both the cyst and its cause at the same time.

Occasionally, small cysts may regress spontaneously. However, because they are almost always associated with an active meniscal tear that acts as a feeding valve for the cyst, the tendency is for them to persist or grow as long as the tear remains untreated.

A Baker's cyst forms in the popliteal fossa (behind the knee) through accumulation of synovial joint fluid. A parameniscal cyst forms next to the meniscus (lateral or medial) through the extrusion of fluid via a horizontal meniscal tear. Both have different causes and locations, although posterior medial parameniscal cysts may occasionally mimic a Baker's cyst clinically.

Recovery depends on the associated meniscal procedure. If only decompression with partial meniscectomy was performed, recovery is quick — immediate weight-bearing and a return to sedentary work within 1–2 weeks. If a meniscal repair was performed, recovery is more gradual — similar to the standard meniscal repair protocol (4–6 months for sport).