Left leg knee pain may be caused by trauma, osteoarthritis, degenerative lesions and other reasons. 1. Trauma: It may be caused by external force hitting the knee of the left leg, overextension, etc. It may cause blood vessel rupture and oozing out of tissue fluid, resulting in localized tissue swelling and painful sensation in the knee. 2. Osteoarthritis: it may be a chronic joint disease caused by degeneration, degeneration and injury of joint cartilage. At the beginning, patients may show mild or moderate intermittent hidden pain, and then joint stiffness, swelling and other symptoms. 3. Degenerative lesions: mostly due to inflammation of the knee joint or chronic strain, local injuries and other causes of degenerative changes in the knee joint surface, subchondral plate reactive hyperplasia, the formation of bone spurs, which can lead to joint swelling, deformation, pain and other symptoms. It is recommended that patients with left knee pain go to the hospital in time and follow the doctor’s instructions for treatment to avoid delaying the condition.