What could cause knee swelling without pain?

When your knee swells but doesn't hurt, it might be because a little sac of fluid called a bursa, which cushions the knee, becomes swollen. This is known as bursitis. Sometimes, bursitis might not cause a lot of pain if the swelling is mild or if the way the bursa is irritated is not severe.

Another possible reason is a gentle build‐up of extra fluid in the joint. In conditions like osteoarthritis, the knee can slowly develop more fluid inside or around it, and sometimes this extra fluid does not hurt much even though it makes the knee larger.

Other causes that usually hurt, such as monoarticular arthritis or spontaneous osteonecrosis of the knee, tend to cause pain along with swelling. If you see your knee swell and you don't have pain, it is important to get clarity on what might be causing your symptoms—you can start by taking a free symptom checker quiz to better understand your condition. A healthcare provider can then do simple tests and use imaging to understand the cause of the swelling and decide if you need treatment or just a little careful watch.