The stability of the knee joint requires the integrity of the strong internal cruciate ligament. It is hidden deep inside the knee joint and cannot be seen or felt. Once a particular one is ruptured, knee instability can occur. Sometimes it is not very typical, and the pain is often overlooked as the joint swells down after a period of rest and the pain is not severe after the injury. However, the prominent manifestation is that the affected leg can’t eat the force when going up or down the stairs, and needs to hold the wall or walk with the body inclined. When running, the affected leg does not dare to make a sudden effort, and when running, the affected leg does not dare to stop sharply, and there is a fear of knee movement. Xiao Liu had this problem. On examination, the right knee posterior drawer test was positive, the knee joint was posteriorly unstable, the right knee upper thigh diameter was 37.5px shorter than the opposite side, and MRI showed posterior cruciate ligament injury in the right knee. After admission, knee arthroscopy was performed to confirm that the posterior cruciate ligament of the right knee was ruptured, with less than 1/4 fibers remaining and poor elasticity, and was unable to perform posterior cruciate ligament function, and minimally invasive posterior cruciate ligament reconstruction was performed. After the surgery, he was protected by a brace and resumed normal daily life after 3 months, and since then, he has not been afraid to go up and down the stairs. Mr. Li, a 35-year-old IT elite, had fallen down the stairs 5 years ago and suffered from left knee weakness, and sprained again this year, with swelling and pain in his left knee and limited movement. After admission to the hospital, arthroscopy revealed that the anterior cruciate ligament of the left knee had been absorbed and the tibial stop had completely disappeared. Magnetic resonance imaging (MRI) of the anterior cruciate ligament showed poorly, as shown by the arrow in the figure. He was treated with an arthroscopic minimally invasive ACL reconstruction and returned to normal work after 3 months, and the knee instability disappeared. Young patients with recurrent knee cruciate ligament injuries come to the doctor reminding me that I may need to educate myself about this. In young people, if you have chronic weakness after a knee sprain or fall, an inability to run fast, and a sense of fear in the knee going up and down stairs, be alert to possible internal knee ligament damage that needs prompt treatment to avoid muscle atrophy and further knee degeneration leading to a loss of quality of life. It is imperative that you seek professional medical help at this time.