The Baker cyst is a protrusion of the synovial knee joint capsule into the popliteal fossa between the tendons of the semimembranosus and gastrocnemius mm. Baker's cyst is not a disease in itself, but always a symptom of an intra-articularly increased accumulation of fluid. The causes can be inflammatory rheumatic, degenerative and traumatic. The increased intra-articular pressure causes a protrusion of the synovial capsule into the hollow of the knee.
Further details see below Baker cyst (vascular medicine)