Why Is One Knee Swollen but Not the Other? – Story Of The Day!

Internal derangements are common structural causes.² A tear in the Anterior Cruciate Ligament (ACL) or the Medial Collateral Ligament (MCL) can cause the joint to fill with blood and fluid almost immediately, a condition known as hemarthrosis.³ Similarly, a meniscus tear—damage to the C-shaped cartilage that acts as a shock absorber—can cause the knee to swell gradually over several hours or even days.⁴ In older adults, even a simple “bone bruise” or a micro-fracture resulting from a minor fall can lead to significant localized swelling that lingers far longer than expected. If the swelling is accompanied by a sensation of the knee “locking” or “giving way,” the likelihood of a structural tear is high.+2

Beyond the ligaments and bones, the soft tissues surrounding the joint are often to blame. Bursitis is a prime example of a localized inflammatory response. The knee is equipped with several bursae—small, fluid-filled sacs that act as gliding surfaces to reduce friction between tissues.⁵ The prepatellar bursa, located right over the kneecap, is particularly vulnerable.⁶ Historically referred to as “housemaid’s knee,” prepatellar bursitis occurs when this sac becomes irritated, often due to prolonged kneeling or a direct blow.⁷ Unlike a deep joint injury, bursitis swelling often looks like a soft, fluid-filled “egg” sitting on top of the kneecap. While it can be painful and limit movement, it is technically outside the joint itself, though it creates a dramatic visual discrepancy between the two legs.+2

Another common but often overlooked cause of one-sided swelling is the presence of a Baker’s Cyst, or a popliteal cyst. This occurs when an underlying issue within the joint, such as osteoarthritis or a small cartilage tear, causes the knee to produce an excess of synovial fluid.⁸ This fluid has to go somewhere, and it often collects in a pocket at the back of the knee. While the front of the knee might look relatively normal, the back of the leg becomes tight and swollen, making it difficult to fully straighten or bend the limb. In some cases, these cysts can rupture, sending fluid down into the calf and mimicking the symptoms of a much more serious condition: a blood clot.⁹+1