Finding out you have a Baker's cyst often comes with a frustratingly short explanation: there's fluid behind your knee, your joint is irritated, and you should rest or consider draining it. That's not wrong — but it skips over the part that actually matters.

Pain at the back of the knee that lingers, swells, and returns no matter what you try is a particular kind of frustrating. Semimembranosus bursitis — inflammation of the small fluid-filled sac nestled between the semimembranosus tendon and the medial gastrocnemius — is often underdiagnosed or lumped together with Baker's cyst and generic posterior knee pain.

If you have been told you have a Baker's cyst — or a dissecting popliteal cyst, where fluid has spread into the back of the calf — you already know the frustration. It may have been drained, it may have come back, and the explanation you received was probably brief: excess joint fluid, caused by something irritating the knee.