Swelling along the backside of the knee could be caused by a few things. There could be some damage inside the joint causing effusion (excess fluid or swelling). There may be a torn ligament or damage to the meniscus (joint cartilage). Have you experienced trauma or some type of injury that could be causing this swelling?
Your general practitioner will conduct a history and physical exam (H&PE) to determine the cause of the problem. Questions about your symptoms and any events leading up to this swelling will help the physician identify the underlying pathology. That’s the history part. Some special tests might be done to rule out internal derangement (damage within the joint). That’s the physical exam portion of the diagnostic evaluation.
One of those tests is just palpating (pressing and feeling) the back of the knee. One of the most common causes of swelling in this area is a Baker’s cyst. This is just a fluid filled sac that forms when a tiny tear in the synovial lining allows synovial fluid to leak out. It’s considered benign or harmless, though it can cause pain when it is pressed or pinched. If enough fluid builds up, it can press on nerves in that area or restrict knee motion. Removal may be necessary.
Whatever is causing the swelling in your knee should be examined and diagnosed properly. Early recognition of problems and early intervention is usually the best way to avoid long-term or more serious complications later. See your general practitioner. He or she will refer you to a specialist if it’s warranted.