A lump on the side of the knee could be a result of an injury, swollen or torn ligaments or a cartilage tear, according to WebMD. Knee injuries can cause fragments of the bone or cartilage to chip and get stuck in the joint, causing swelling and lumps.
Swelling from knee injuries and ailments produces lump-like appearances on both sides of the knee. For example, the bursa, a sac of fluid that protects and cushions the joints, can become irritated and enlarged, explains WebMD. This condition, known as bursitis, is often deemed housemaid's knee or preacher's knee because the injury is typically caused by kneeling. Swelling of the bursa can also be caused by arthritis.
Knee conditions such as iliotibial band syndrome and medial plica syndrome also cause the side of the knee to become enlarged due to swelling. The iliotibial band becomes swollen when the tissue that runs down the hip to the shin is overused or irritated, notes WebMD. The plica, which is a fold of tissue in the knee joint, can also swell from overuse, causing pain and a lump on the outer side of the knee. In severe cases, a partially dislocated kneecap causes the kneecap to slide out of position, which can form the appearance of a lump on the side of the knee due to swelling.