Type I: uniplanar instability (uniaxial displacement, single pathological rotation axis) 1. Medial instability: 0 degree position of extended knee, valgus stress, medial knee opening. Medial collateral ligament, medial joint capsule, anterior cruciate ligament, posterior oblique ligament, medial portion of anterior and posterior joint capsule injury, even with posterior cruciate ligament injury. Flexed knee in 30 degree position with valgus stress and mild medial knee tension. Limited injury to medial structures, excluding cruciate ligament injury. 2. Lateral instability: extended knee in 0 degree position, internal valgus stress, lateral knee opening. Lateral joint capsule ligament, lateral collateral ligament, biceps femoris tendon, iliotibial bundle . , arch complex, N fibular ligament, posterior cruciate ligament, and even anterior cruciate ligament injury. The knee is flexed in a 30 degree position with internal stress and mild lateral knee tension. Limited damage to the lateral structures, or normal, to be contrasted with the key side. 3. Posterior instability: posterior drawer experimental stress, tibial posterior displacement. Posterior cruciate ligament, arcuate complex, posterior oblique ligament injury (partial or complete). 4, Anterolateral instability: rotational neutral anterior drawer test stress, anterior tibial displacement. May include anterior cruciate ligament, medial and lateral capsular ligament injury; of which the presence of composite anterolateral rotational instability should be noted. Type II: Rotational instability (double axial displacement, double pathological rotation axis) 1. Anterior internal rotational instability: the medial tibial condyle is displaced forward relative to the femoral epicondyle (X), and the lateral joint space is opened (Z). Medial capsular ligament, medial collateral ligament, posterior oblique ligament, anterior cruciate ligament injury. 2, anterior external rotation instability: flexion 90 degrees position, tibial epicondyle relative to femoral epicondyle row anterior displacement (X) lateral joint space opening (Z), lateral joint capsule ligament, arch ligament compound, anterior cruciate ligament injury. In the subextension position, the tibial epicondyle is semi-dislocated forward relative to the femoral epicondyle (X) anterior cruciate ligament injury, possible lateral joint capsule injury (axial shift experiment, jerk,MacIntosh) 3. posterior external rotation instability: the tibial epicondyle is displaced posteriorly relative to the femoral epicondyle (X) lateral joint capsule opening (Z). n tendon, arch ligament complex, lateral joint capsule ligament, posterior cruciate ligament injury ( External rotation position knee antitension test, anti-axial shift test). 4. Posterior internal rotation instability: medial tibial condyle is displaced posteriorly relative to the medial femoral condyle (X) medial joint opening (Z). Medial capsular ligament, posterior oblique ligament, posterior cruciate ligament, medial posterior capsule, hallux valgus stop injury, possible anterior cruciate ligament injury. Type III: compound instability (biaxial displacement, multiple pathological rotational axes) 1. Anterior external + anterior internal rotational instability: both sides of the tibial plateau are simultaneously displaced forward relative to the femoral condyles, and both sides of the joint may have different degrees of opening. However, the forward displacement of the tibial plateau can disappear when the tibial plateau is externally or internally rotated. 2.Anterior external + posterior external rotation instability: The tibial epicondyle can be displaced both forward and backward relative to the femoral epicondyle, the lateral joint space is open, there is extensive lateral structural damage, anterior cruciate ligament damage, and occasionally posterior cruciate ligament damage. 3. Anterior internal + posterior internal rotational instability: the medial tibial condyle is displaced anteriorly and posteriorly relative to the medial femoral condyle, with medial opening. Medial structures, semimembranosus, anterior and posterior cruciate ligament injury.