Pain that occurs when the knee is straightened while bent is primarily considered to be an instability of the knee joint. Knee instability can be caused by a variety of factors, including laxity of the knee ligaments, laxity of the joint capsule, as well as damage to the meniscus and diseases of the patella and patellar ligament. When the knee joint is unstable, rotation or lateral displacement can occur, resulting in pain. When the knee joint is in a stable state, there is a tendency to flexion and extension, but because the surrounding ligaments, joint capsule and ligaments are strong enough to keep the knee joint from moving, there is no pain. Painful symptoms during movement tendencies often indicate instability of the knee joint, which can be caused by instability of the medial and lateral collateral ligaments, injury to the joint capsule, patellofemoral ligament disease, and meniscus injury. It is recommended that patients with this symptom should go to the hospital for an examination to find out which structure is the problem.