The posteriorlateralcorner (PLC) injuries are mostly caused by car accidents, fall injuries and sports injuries, and are often complicated by other ligament injuries such as posterior cruciate ligament (PCL) and anterior cruciate ligament (ACL), mainly accompanied by PCL injuries. The injury can cause internal knee valgus, tibial external rotation and knee retroflexion deformity, so the repair and reconstruction after PLC injury is crucial for knee stability. If treatment is neglected or delayed, it will lead to knee instability and cause failure of surgical reconstruction of other ligaments [1-3]. 1, Anatomy of PLC and its function The anatomy of PLC is complex, and the posterior external horn ligament structure is closely related.Seebacher et al [4] divided it into three layers, the first layer mainly has biceps femoris tendon, iliotibial bundle, the second layer mainly has lateral collateral ligament, N tendon, N fibular ligament, and the third layer has joint capsule, lateral head of gastrocnemius muscle. Its main function is to limit internal rotation of the knee joint, external rotation and to limit posterior external rotation of the tibia relative to the femur. Of these three layers, the most studied structure is the second layer, especially the recognition of the importance of the N muscle tendon and the N fibular ligament, which starts posteriorly from the proximal tibia and ends just below the lateral collateral ligament attachment point of the femoral epicondyle, with the N fibular ligament connecting it to the fibular head in its stroke and the tendonous portion forming a connection with the lateral meniscus via the intra-articular. The N muscle is currently considered to be an important dynamic structure in the lateral structures of the knee, and the N fibular ligament limits the external rotation of the tibial tibia relative to the femur [5].LaPrade et al [6] concluded from a cadaveric anatomical study that the main structures maintaining posterior lateral stability are the lateral collateral ligament, the N tendon, the N fibular ligament, and the lateral head of the peroneal muscle. The lateral collateral ligament mainly prevents internal rotation of the knee and also assists in preventing external rotation and posterior fall of the tibia, the N peroneal ligament plays an important role in preventing posterior fall of the tibia and internal rotation of the knee and also has a limiting effect on excessive external rotation of the tibia, and the tendonous portion of the N muscle mainly prevents external rotation of the tibia and also assists in preventing posterior fall of the tibia and internal rotation of the knee [7,8]. 2 Mechanism of PLC injury Postero-lateral violence acting on the anteromedial tibia in the extended knee position is a common type of PLC injury, and internal rotation violence acting on the externally rotated tibia and hyperextension violence acting on the internally rotated tibia are also common mechanisms of PLC injury.Simple PLC injury is rare, accounting for about 1.6% of knee ligament injuries and 43-80% of combined injuries to other ligaments [1,2,9]. Instability caused by posterior posterolateral structural injuries mainly includes straight posterolateral instability and posterior posterolateral rotational instability. In the diagnosis of posterior posterolateral structural injuries, it is important to first recognize that simple lateral collateral ligament injuries are very rare, and the vast majority are combined injuries of the lateral collateral ligament, N peroneal ligament and N tendon, and simple straight lateral instability is rare, and more often posterior lateral rotational instability. 3, PLC injury diagnosis 3.1, because PCL injury usually combined with other ligaments of the knee joint combined injury, the initial diagnosis is easy to miss. Therefore, swelling, bruising, pressure points and special tests of the knee joint after trauma should be carefully examined bilaterally for comparison.The degree of PLC injury can be compared with the contralateral knee under 300 internal rotation stress in the flexed knee according to Hughston’s scale, with lateral knee opening 0-5 mm as I0, 6-10 mm as II0 and more than 10 mm as III0 [10].Fanelli [11] It is further divided into three types: type A: increased external rotation at 30° of knee flexion without internal instability. Type B: increased external rotation at 30° of knee flexion with 1 to 2° of internal rotation instability. Type C: increased external rotation at 30° of knee flexion, 3° of internal rotation instability. This indicates a tear of the N peroneal ligament, N tendon, and lateral collateral ligament, possibly combined with a cruciate ligament injury. Main clinical manifestations ① Pain in the posterior posterolateral corner of the knee is obvious in the acute injury period.Delee et al [12] found diffuse pressure pain in the posterior posterolateral region of the knee in acute PLC injury, with significant pressure pain in the fibular head and joint space in those with avulsion fractures of the fibular head or lateral tibial plateau joint capsule attachment (Arcuate and Segond fractures). ② Symptoms of posterior lateral rotational instability. It is manifested by playing soft leg when going up and down stairs or walking up and down slopes, etc. (3) Abnormal force line and gait of the lower extremity. patients with PLC injury have inversion of the knee joint when standing or walking, showing a typical inversion-extrusion gait (Varusthrust), or an overextension-inversion-extrusion gait during the support period of walking. in order to relieve pain and joint instability, some patients walk with a protective knee flexion. ④ Symptoms of common peroneal nerve injury. Some PLC tears can cause damage to the common peroneal nerve, with an incidence of 13%-16%. ⑤ Signs of vascular injury. PLC tears caused by knee dislocation may be accompanied by injury to the N vessels. Physical examination of PLC injury is performed by the following methods [2]: ① External rotation and reverse flexion test: the patient is supine, the examiner lifts the patient’s two big toes, and the presence of knee hyperextension, internal rotation and external rotation on the affected side is positive. Most patients have combined ACL or PCL injuries. (ii) Tibial external rotation (Dial) test: the patient was examined in supine or prone position with the knee flexed at 30° and 90°, respectively. If the angle between the external rotation of the foot and the thigh increased by 10-150 compared to the opposite side when the knee was flexed at 30° in the prone position, and there was no external rotation when the knee was flexed at 90°, it indicated an isolated PLC injury, and if the external rotation increased by 10-150 at both 30° and 90°, it suggested that both PLC and PCL were injured. (iii) Posterior lateral external rotation test: This is a combination of posterior drawer and external rotation test that correlates with LCL injury. The posterior drawer test is performed while the tibia is externally rotated with the knee flexed at 30° and 90°, respectively, and the posterior lateral tibial angle is palpated for subluxation. Again, the occurrence of subluxation at 30° rather than 90° suggests PLC injury, while the occurrence of subluxation at both angles indicates combined PLC and PCL injury. Internal rotation stress test: 0-300 knee flexion under internal rotation stress, 0-5 mm lateral knee opening for I0, 6-10 mm for II0, and more than 10 mm for III0. 00 position positive indicates severe PLC injury, including LCL, lateral one-third portion of tibial capsular ligament, N tendon, and superficial iliotibial bundle. Examination at 300 in flexion for III0 inversion instability suggests complete LCL rupture. ⑤ Anti-axial shift test: gradual extension of the knee under the stress of simultaneous external rotation and valgus calf in flexion, and a sense of sliding of the tibial plateau that has been semi-dislocated posteriorly to the lateral side to produce a reset is considered positive. Suggests PLC and PCL injury. (vi) Posterior lateral drawer test: A posterior drawer test with the knee flexed at 80° and the lower leg externally rotated at 15° is used to determine the posterior displacement and external rotation of the tibial plateau to determine PLC injury. Recently, Jacobson suggested that this test should be performed at 30° and 90° of knee flexion to differentiate between a simple PLC injury or a combined PCL injury. Both angles of posterior displacement are increased in combined injuries, whereas isolated PLC injuries are positive for posterior displacement only at 30° of flexion.Veltri et al [13] found that the external tibial rotation (Dial) test and the internal rotation stress test are the most effective physical examinations to detect PLC injuries. Imaging: knee radiographs show widening of the lateral knee gap, avulsion fractures of the fibular head, and avulsion fractures of the joint capsule of the lateral tibial rim (Segond fractures), and only great violence can cause avulsion fractures of the joint rim due to the high mechanical strength of the middle and lateral joint capsule, although the presence of the lateral joint capsule sign usually indicates an ACL tear, it can also occur in patients with injuries of PLC [14]. MRI: LaPrade et al [15] invented a special scan sequence including the entire fibular tuberosity in a prospective study, a method that clearly shows the entire posterior lateral structure and helps to diagnose PLC injuries. The lateral “arch signal” (arcuatesign) of the knee is characteristic. Intramedullary edema of the head of the fibula and avulsion fractures of the head of the fibula can show bowed signals [16]. Arthroscopy: Arthroscopic laxity of the lateral joint space of the knee and a positive posterior lateral angle arthroscopic through sign (lateral space opening >1 cm on microscopy) suggest a PLC injury, while arthroscopic examination can avoid missing multiple ligament injuries. In a prospective study of 30 third-degree PLC injuries, Laprade found that only 5 were simple PLC injuries and 25 were composite PLC injuries. Therefore, arthroscopy in conjunction with PLC repair or reconstruction is valuable. However, in the acute phase of combined joint capsule injury, arthroscopy may cause fluid extravasation and cause calf fascial interval syndrome. 4, treatment of PLC injury Non-surgical treatment Suitable for Ⅰ and Ⅱ degree PLC injury, usually good prognosis, the affected knee is fixed in the straight position for 3-4 weeks and then perform extension and flexion functional exercise, during the fixation period the affected limb performs isometric muscle exercise, practice pushing the patella; when the pain can be tolerated, weight bearing can be started gradually. 6-8 weeks to start closed chain exercise, 10 weeks to avoid N cord muscle exercise, 12-8 weeks to avoid N cord muscle exercise. Avoid N cord muscle exercises at 6-8 weeks, and free exercises at 12-14 weeks [17]. Surgical treatment: Grade III PLC injury requires surgical treatment, the earlier the surgery, the better, and the repair of acute injury should be performed within 2-3 weeks. Surgical methods include one-stage repair, enhancement of the posterior lateral structures, and reconstruction. Acute injury (within 3 weeks): surgical incisional anatomical repair of acute PLC grade III injury is better than the treatment of old injury. ross et al [18] concluded that anatomical repair of the posterior posterolateral horn is most important within 2 weeks after trauma, and the freshly traumatized posterior posterolateral structure is fixed with in situ strengthening sutures, the ligaments can heal and their stability can be completely restored, while direct suture repair after 2 weeks is poor, and reconstruction of the posterior posterolateral horn structure with ligament grafts is required Arthroscopic examination before repair can help to diagnose PCL injury, but it can also diagnose and repair combined meniscus and cruciate ligament injury at the same time. Combined cruciate ligament injuries should be reconstructed prior to PLC repair or reconstruction. A lateral straight or arcuate incision is used. The following major structures are exposed: (1) iliac path bundle, (2) biceps femoris, (3) common peroneal nerve, (4) lateral collateral ligament, (5) N muscle and N tendon, (6) N fibular ligament, and (7) posterior lateral joint capsule. The order of repair is appropriate from deep to superficial, and the repair can be done by direct sutures, tendon bone fixation through the bone hole and anchor nails, etc. Especially, repairing the joint capsule torn from the femur or tibia is very strong and convenient with sutures with threaded bone tracing nails.Stannard [19] compared the effect of posterior lateral complex repair and reconstruction and concluded that one-stage lateral collateral ligament reconstruction can effectively restore the lateral stability, while simple in situ repair of the lateral structure, the failure rate is as high as 37%, therefore, for patients with tears of the lateral collateral ligament and the parenchymal part of the N tendon, perform a lateral collateral ligament reconstruction with avulsion of the bony attachment point of the lateral collateral ligament and early in situ repair for fixation.Noyes [20] required simultaneous reconstruction of the anterior cruciate ligament while reconstructing the lateral collateral ligament, and if the anterior cruciate ligament is not repaired, the lateral structure will be secondary to laxity; it was also pointed out that the acute phase If the lateral collateral ligament injury is severe, reconstruction of the lateral collateral ligament with autologous or allogeneic patellar tendon can obtain better joint stability. Protracted injuries: The goal of treatment is to restore stability to the knee joint in internal rotation and external rotation, to maximize the return to pre-injury motion and to reduce the likelihood and severity of osteoarthritis. Management of old injuries is more complex than acute injuries due to extensive scar formation, secondary structural damage, and possible lower extremity force line abnormalities. In particular, abnormal lower extremity force lines with significant inversion deformity should first be treated with a high tibial valgus osteotomy to avoid reconstruction failure due to excessive stress and loading of the lateral joint capsule structures after reconstruction [13].Hughston [21] et al. applied a bone attachment point upshift of the arch ligament complex (lateral collateral ligament, lateral head of the gastrocnemius and N tendon) to treat 95 combined injuries with a follow-up of 2 -The objective evaluation rate was 85%, the subjective rate was 78%, and the functional rate was 80% at 13 years. This procedure not only does not damage the N-fibular ligament or the union of the N muscle with its tendon, but also displaces the lateral collateral ligament and the N tendon together upward, but the strength of this tension decreases over time.Jakob et al [22] reported that the stop of the N tendon on the femoral side can be deepened and tensioned to restore the N tendon tension when the PLC injury is mild, provided that the N muscle or N fibular ligament is intact.Noyes [23] used a method to restore postero-lateral knee stability by displacing and tightening the PLC structures on the femoral side in the direction of the lateral collateral ligament, requiring the parenchyma of these structures to be intact. Veltri et al [13] reported the repair of the injured N tendon using iliotibial bundle strengthening surgery or the reconstruction of the lateral collateral ligament and N fibular ligament by taking a long head of biceps femoris tendon and transferring the fibular attachment; Clancy [24] reported the free release of the biceps femoris tendon and Clancy [24] reported that the lateral collateral ligament was reconstructed by redirecting the transfer of the biceps femoris tendon to the lateral femoral condyle after free release and fixation, which achieved better treatment results. However, these methods are not anatomical reconstructions and secondary laxity can be up to 24% [25]. In recent years, anatomical reconstruction of the PLC has obtained better clinical results. Noyes [20] used autologous or allogeneic patellar tendon to reconstruct the lateral collateral ligament in 13 cases of PLC injury, including 7 cases of initial reconstruction and 6 cases of revision, and 93% recovered joint stability at a follow-up of 2-13.7 years after surgery. The indications for this reconstruction method are: (i) those with severe PLC parenchymal injuries that cannot be repaired with direct sutures, and (ii) all patients with chronic PLC injuries. Summary: As the anatomy and biomechanics of the postero-lateral knee have been studied, the treatment methods have been improved. All first-degree and most second-degree PLC injuries can be treated non-operatively with residual partial laxity, especially in patients with second-degree injuries. Acute Grade III PLC compound injuries are best treated with early (within 3 weeks) direct repair, or repair strengthening, or reconstruction of all injured ligaments. For old injuries, either isolated or multiple ligament injuries, the best treatment is reconstruction of the PLC along with reconstruction of the cruciate ligament of the combined injury in order to maximize the restoration of knee stability.