Possible causes of pain in the knee when the left leg flashes include cruciate ligament strain, meniscus injury, patella fracture, etc. X-rays and other tests are needed to clarify the cause. If there is a clear history of leg injury accompanied by pain in the knee area, it is possible that during the course of the injury, damage to the various structures within the knee joint occurred due to overstretching, impingement, and twisting of the knee joint. Examples include cruciate ligament strains or tears, meniscus injuries, fractures of the patella, the patellar surface of the lower end of the femur, and the fibular head of the tibia. To clarify the cause of pain in the knee when the left leg flashes, a series of tests are required. For example, the over-flexion test, the grinding and pulling test, and the Mai’s sign test, as well as X-ray examination and MRI examination, etc., will be performed to clarify whether there is any damage to the cartilage, ligament, and other tissues in the knee joint such as swelling and cyst, and whether there is any fracture. If your left leg flashes and you have pain in the knee, it is recommended that you go to a regular hospital in time to find out the cause of the pain and get prompt treatment if necessary.