Unfortunately I see that many of my Danish colleagues still do an 'open' surgical Baker's cyst removal as opposed to a keyhole (arthroscopic) procedure.
Fluid retained via a 'valve' action
The reason a Baker's cyst can occur is a fold (or plica) of the lubricating joint lining at the rear of the joint cavity that produces fluid which would be normal except that sometimes the fold acts as a valve - so that the fluid circulating at the back of the knee cavity runs into the cyst, but it cannot escape. If the cyst does not resolve by the method of manually emptying it and injecting steroid, then you need to have the cyst removed. For this you need to have a special arthroscopic procedure with removal of the plica at the rear of the knee.
My preferred surgical technique
My own preferred method is to deal with the problem arthroscopically, passing the keyhole surgery instruments into the back of the knee, locating the valve-like entrance into the cyst from within the knee cavity, and then trimming away the valve edges to allow the pressure in the cyst to equalise with that inside the knee All the cyst fluid should then simply drain back into the knee joint cavity and the swelling collapses.
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4. Zhou XN, Li B, Wang JS, Bai LH. Surgical treatment of popliteal cyst: a systematic review and meta-analysis. J Orthop Surg Res. 2016 Feb 15;11:22. doi: 10.1186/s13018-016-0356-3. [ Free full text ]