The semimembranosus tendon has a complex distal anatomy with five insertional arms including the primary arm to the posterior medial tibial condyle, a reflected arm to the medial tibial flare, and expansions to the posterior capsule (forming the oblique popliteal ligament), the medial collateral ligament, and the popliteal fascia. This complex insertion provides dynamic posteromedial knee stability that complements the MCL. A semimembranosus bursa between the tendon and the medial head of the gastrocnemius is the origin of most popliteal (Baker) cysts.
Hip extension and knee flexion force transmission; posteromedial knee dynamic stabilisation
Semimembranosus tendinopathy at the posterior medial tibial condyle produces posteromedial knee pain reproduced by resisted knee flexion with internal tibial rotation, a pattern that is distinct from and often coexists with medial compartment osteoarthritis. The bursa between the semimembranosus tendon and the gastrocnemius medial head that communicates with the knee joint in many adults is the anatomical basis for Baker cysts, which enlarge proportionally to knee joint effusion.
Insertional degeneration at the posteromedial tibial condyle producing medial knee pain in distance runners, reproduced by resisted internal tibial rotation with knee flexion.
Semimembranosus-gastrocnemius bursa distension from communicating knee joint effusion producing a popliteal fossa mass that fluctuates with knee swelling.
