Lateral knee bulge may be caused by fat accumulation, meniscus injury, chronic inflammation stimulation and so on. 1. Fat accumulation: mostly seen in obese people, mainly because of the body’s excessive fat tissue, accumulation in the local, thus forming the lump, but there is no other uncomfortable symptoms. 2. Meniscus injury: mostly caused by trauma or sports, which can lead to localized cystic fluid or fluid accumulation on the lateral side of the knee joint causing a limited bulge, accompanied by obvious pain and activity limitation. 3. Inflammatory stimulation: mostly seen in old age osteoarthritis, rheumatoid arthritis, etc., the acute phase of inflammation due to the joint cavity fluid may lead to pain and swelling of the knee joint, the phenomenon of lateral bulge of the knee joint, which may be accompanied by the limitation of joint activities. Lateral knee bulge may also be caused by other reasons, if accompanied by obvious pain, the patient should promptly go to the hospital to identify the cause of targeted treatment.