Semimembranosus Bursa

bursa musculi semimembranosi

The semimembranosus bursa lies between the semimembranosus tendon insertion and the medial head of the gastrocnemius in the posteromedial knee, just medial to the medial gastrocnemius bursa. These two bursae are closely related and often communicate. When the semimembranosus bursa enlarges and communicates with the knee joint through the medial gastrocnemius bursa fenestration, it forms the classic Baker popliteal cyst.

Region: Knee

Clinical Relevance

Clinical Notes

The semimembranosus bursa can enlarge independently without joint communication in direct trauma or semimembranosus tendinopathy. Primary semimembranosus bursitis presents as a posteromedial knee swelling distinguishable from Baker cyst by ultrasound (no communication with joint, direct bursal enlargement). Semimembranosus tendinopathy from overuse produces pain at the posteromedial knee that is reproduced by resisted knee flexion and direct palpation of the tendon just above its insertion on the tibial plateau.

Pathology

Common Injuries & Conditions

Primary Semimembranosus Bursitis

Direct trauma or semimembranosus tendinopathy produces isolated semimembranosus bursal enlargement without knee joint communication, presenting as a posteromedial knee mass and pain; ultrasound confirms an isolated collection with no joint communication, managed by aspiration and corticosteroid injection rather than by treating underlying joint pathology.