Semimembranosus-Gastrocnemius Bursa

bursa musculi semimembranosi

The semimembranosus-gastrocnemius bursa is the anatomical basis of most popliteal (Baker) cysts in adults. In approximately 50 percent of adults the bursa communicates with the posterior knee joint through a valvular slit that allows fluid to flow from the joint into the bursa but not back. When the knee joint effuses from any cause (osteoarthritis, meniscal tear, inflammatory arthritis), fluid fills the bursa and produces the characteristic posterior knee swelling.

Region: Knee

Biomechanics

Function & Actions

Reduces friction between the semimembranosus and gastrocnemius during knee movement

Clinical Relevance

Clinical Notes

Baker cysts are the most common popliteal fossa mass and occur secondary to increased intra-articular pressure from joint effusion. They are not primarily treated by aspiration or excision of the bursa — the primary pathology (osteoarthritis, meniscal tear) must be treated to reduce joint pressure and allow the cyst to regress. Ruptured Baker cysts produce acute calf swelling and pain mimicking deep vein thrombosis.

Pathology

Common Injuries & Conditions

Baker Cyst

Popliteal bursa distension from communicating knee joint effusion producing a posterior knee mass, managed by treating the underlying joint pathology rather than the cyst itself.

Ruptured Baker Cyst

Acute calf pain and swelling from Baker cyst rupture mimicking DVT, confirmed by ultrasound showing cyst disruption and fluid tracking down the calf.