Patellar dislocation, as the name implies, is the dislocation of the patella out of its normal position in the patellofemoral joint, usually to the lateral side. The incidence of this disease is high in the adolescent population (especially in adolescent females), and the diagnosis of patellar dislocation is generally considered first in adolescents with a solitary joint free body. According to the course of the disease, it can be divided into acute patellar dislocation and recurrent patellar dislocation (some people also call it habitual patellar dislocation, but there is some difference between the two, which will be covered later). The first occurrence of patellar dislocation, whether due to trauma or congenital ligament laxity, femoral talus dysplasia, patellar maltracking, or abnormal lower extremity force lines, can be called acute patellar dislocation. At the moment of patellar dislocation, the violent action tears the medial patellar support band and dislocates the patella to the lateral side, while the injury is caused by abnormal impact between the medial patella and the lateral femoral carriage, and sometimes osteochondral fragments can appear, which is the source of joint free bodies. Patients complain of a sprained knee with a sensation of bone dislocation. Usually the patella has reset itself by the time the patient comes to the clinic, but a few patients need to be manually reset by the physician. Physical examination may reveal significant swelling of the suprapatellar capsule (another type of knee sprain with significant swelling is anterior cruciate ligament rupture), with significant pressure pain on the medial patella and medial femoral condyle, and severe pain induced by gently pushing outward on the patella. Recurrent patellar dislocation refers to patellar dislocation that occurs several times, but there is no definite conclusion as to how many times is here, and I generally consider dislocation for ≥2 times as recurrent patellar dislocation. For patients with recurrent dislocation, symptoms of knee swelling and pain and restricted activity are generally not obvious, and physical examination can mainly reveal patellar instability and abnormal force lines in the lower extremity, and those with more frequent episodes will have patellar friction sensation and positive patellar grinding test due to patellofemoral joint cartilage damage, and positive push patellar fear test. If imaging is performed on such patients, degeneration of the patellofemoral joint, femoral talus dysplasia, patellar subluxation, joint free body and other manifestations can be seen on X-ray; while MRI can give us more hints to see the signal of osteochondral damage of the patellofemoral joint, rupture and laxity or absence of the medial patellofemoral ligament, and also measure the TT-TG distance (which represents the degree of rotation of the tibia relative to the femur) to provide more reference for surgery.