Medically known as a Baker's or popliteal cyst, a cyst in the knee occurs when the knee produces excess synovial fluid that accumulates in the back of the knee, says Mayo Clinic. This tends to happen when there is a knee injury or joint inflammation caused by arthritis. A blood clot can also cause swelling behind the knee, so medical examination is necessary to determine whether the swelling is a clot or a cyst, says Healthline.
In many cases, the cyst produces no symptoms, says WebMD. However, when the symptoms appear, they include slight pain, swelling and tightness behind the knee. In some situations, the sac containing the fluid tears, draining the fluid into the tissues of the lower leg. This leads to redness and swelling in the lower leg.
The cyst may improve on its own without treatment, says WebMD. If the cause is arthritis or another health condition, the doctor must treat the underlying cause for the cyst to go away. The doctor may consider draining the cyst if it fails to clear up or is causing pain. A patient may also use steroid drugs to ease swelling. Some home care measures include resting the knee, taking over-the-counter pain relievers and using a walker, crutch or cane to walk.