A bulge next to the knee is commonly caused by degenerative knee disease, knee meniscal cyst or knee trauma, etc. The details are as follows: 1. Knee degenerative disease: such as osteoarthritis, synovitis, the patient will have significant chronic synovial hyperplasia, excessive synovial hyperplasia will protrude into the joint space and bulge next to the knee joint, which is soft when pressed, but has obvious squeezing pain. The pain is more pronounced when the patient walks with weight and sometimes causes a large amount of fluid in the joint, which affects joint movement. MRI can confirm inflammatory synovial hyperplasia, and arthroscopic surgery can remove the inflammatory synovial membrane and remove the mass. 2. Meniscal cysts in the knee: Severe degeneration of the meniscus can cause local cysts to form, which can expand and form a swelling next to the knee joint, with obvious clinical symptoms when pressed and local discomfort. This can be detected by MRI and also requires arthroscopic surgery to remove the cyst and suture the meniscus. 3. Knee trauma: A local hematoma can form, resulting in a bulge, often complicated by ligament damage, meniscal damage, bone contusions, etc. The patient should be seen in a hospital to clarify treatment options.