Q-angle abnormality is one of the clinical signs of patellar instability (unstable patella), a common cause of anterior knee pain, a common disorder of the patellofemoral joint, and an important cause of chondromalacia patella or osteoarthritis of the patellofemoral joint. The following describes the mechanism of occurrence of Q-angle abnormalities. 1. Static factors Mainly include patellar ligament, medial and lateral supporting ligaments, iliotibial bundle, medial and lateral femoral condyles, etc. The patellar ligament mainly restricts the superior displacement of the patella; the medial and lateral supporting ligaments restrict the lateral displacement of the patella; the iliotibial bundle also has the role of reinforcing the external superior patella. Therefore, the lateral patellar restriction mechanism is stronger than the medial one, and when the knee is in extension, the patella is slightly shifted outward when the quadriceps muscle is relaxed. The medial and lateral walls of the trochanteric groove have the role of limiting the lateral sliding of the patella, and when the angle of the groove increases, i.e., the groove becomes shallow or the femoral condyles are dysplastic, the patella loses this limiting effect and is prone to dislocation. In addition, the longitudinal length of the patella in normal people is almost equal to the length of the patellar tendon, and when the patellar tendon is longer than the patella, the patella is elevated, which is also a factor of patellar instability. The Q angle (quadriceps-angle) refers to the angle formed by the line from the anterior superior iliac spine to the center of the patella and the line from the center of the patella to the center of the tibial tuberosity. The normal Q angle is 5° to 10°. If the Q-angle is greater than 15°, the quadriceps muscle contraction generates a component force that moves the patella outward. As the Q angle increases, the lateral pulling force on the patella gradually increases, and the patella becomes less stable.