Semimembranosus-Gastrocnemius Bursa (Baker Cyst)

bursa musculi semimembranosi

The semimembranosus-gastrocnemius bursa is the anatomical basis of Baker cysts. In most adults it communicates with the knee joint via a valve-like hiatus in the posterior capsule, allowing joint fluid to decompress into the bursa when intra-articular pressure is elevated. Baker cysts themselves are usually secondary to intra-articular pathology (meniscal tears, OA, inflammatory arthritis).

Region: Knee

Biomechanics

Function & Actions

Reduces friction between the semimembranosus and medial gastrocnemius; decompresses the knee joint when effusion develops

Clinical Relevance

Clinical Notes

Baker cysts present as posterior knee swelling that fluctuates with activity and is uncomfortable in extreme knee flexion. MRI confirms the cyst and identifies the underlying joint pathology (most commonly medial meniscal tears or OA). Cyst aspiration alone recurs because the underlying pathology persists. Treating the intra-articular cause (meniscal repair, corticosteroid injection for OA) resolves most cysts without cyst-specific intervention.

Pathology

Common Injuries & Conditions

Baker Cyst

Posterior knee semimembranosus-gastrocnemius bursa distension from knee joint effusion managed by treating the underlying intra-articular pathology.