Good afternoon sir, Thanks for posting your query on HCM. I appreciate your concern for the painful swelling behind the knee. The features described in your case are suggestive of popliteal cyst (Baker s cyst) with subgastrocnemius bursitis (inflammation of bursa). You need to be examined by an orthopedic surgeon then the following investigations (Ultrasound & MRI) may be required to confirm the diagnosis. Most of the times, no treatment is required and a Baker s cyst will disappear on its own. If the cyst is very large and causes a lot of pain, following treatments are available: 1. Injection corticosteroid into your knee to reduce inflammation. This may relieve pain, but it doesn t always prevent recurrence of the cyst. 2. Fluid drainage or needle aspiration is performed under ultrasound guidance. 3. Physical therapy like icing, a compression wrap and crutches may help reduce pain and swelling. Gentle range-of-motion and strengthening exercises for the muscles around your knee also may help to reduce your symptoms and preserve knee function. Hope the suggestions given above could be useful in getting the treatment. Wishing you a good health.