A Popliteal/Baker's Cyst is neither a Ganglion Cyst nor a Skin and Subcutaneous Tissue abnormality, so neither 20612 nor 10160 would be correct. It is a deep, subfascial structure/lesion.
In adults, a Popliteal Cyst is an extension of the Knee Joint. The cyst is a swelling/fluid collection in a bursa between the Semitendinosus and Medial Gastrocnemius Tendons deep behind the knee. For what it is worth, all humans have the potential of developing a "cyst" from this bursa. There is a connection between the joint and the bursa, so the bursa and the Popliteal/Baker's cyst (a fluid filled bursa) are "extensions" of the joint. The fluid that fills the bursa to make it a cyst comes from the joint. Something is going on inside the joint that causes inflammation and subsequent excess joint fluid production, which fills and distends the joint (effusion). The fluid in the joint, particularly if the effusion is tense, can flow from the joint through the "connection/communication" to the bursa to fill the bursa resulting in the "cyst." The fluid in the cyst is joint fluid derived, but if there long enough can become syrupy or even thicker and gelatinous. The problem is that the fluid flow is usually only one direction, from the joint into the cyst, and usually does not flow backwards from the cyst into the joint. Consequently, the fluid is "trapped" in the cyst. Even if the joint inflammation resolves and excess fluid formation stops (i.e. the effusion resolves), the cyst may remain distended/full. If large and tight enough it can cause symptoms. There probably some patients that have "two way" fluid flow, but they are infrequent in my experience.
Evaluation and treatment are focused in the adult on identifying the cause of the joint inflammation, i.e. the intra-articular lesion/pathology, and treating that (MRIs and Arthroscopy or open surgery, etc). If that can be resolved, then the cyst may resolve on its own over time. If the joint is inflamed and has an effusion, and a cyst is also present, then aspiration and injection (steroid) of the joint may be sufficient for both. If that helps the joint but the cyst persists and is symptomatic, then aspiration (+/- steroid injection) of the cyst may be successful. However, this may be a temporary solution until the intra-articular disorder is ultimately resolved, i.e. the cyst may recur. Surgical excision of the cyst may be necessary if it is recurrent or significantly symptomatic to the patient, along or combined with the surgical treatment of the joint disorder (surgeon's decision).
So, the aspiration and injection (if done) of the cyst is in essence a treatment of the knee joint, and 20610 would be correct. In my years in practice, I saw and treated many patients with Popliteal Cysts. When I did cyst aspirations, +/- injection, I used this code. I was never informed by our "business office" that we were being denied payment for using this code. I would recommend you print off this reply and review it with your physician(s). If they agree, then it could probably be used to justify the use of this code.
I hope this is helpful to you.
Respectfully submitted, Alan Pechacek, M.D.