Posterolateral Corner Injury

Posterolateral corner (PLC) injury is a disruption of the structures on the posterolateral side of the knee that resist varus force, external rotation and posterior translation. Its main structures are the lateral collateral ligament (LCL), the popliteus tendon and the popliteofibular ligament, with secondary contributions from the biceps femoris, the lateral capsule, the fabellofibular ligament and the iliotibial band.

It follows a varus force on the knee, a hyperextension injury, a posterolateral blow to the flexed tibia, or a knee dislocation, in road traffic collisions, sports (football, rugby, skiing) and falls. Isolated PLC injury is uncommon (under 30%); most are combined with posterior cruciate ligament (PCL) or anterior cruciate ligament (ACL) tears.

The Fanelli classification describes the instability: type A is an isolated rotational (posterolateral) instability, type B is combined rotational and varus instability, and type C is complete instability with varus and rotational laxity. Grading is by the lateral joint line gap on varus stress: grade I under 5 mm, grade II is 5 to 10 mm, and grade III is over 10 mm.

Complications are chronic posterolateral instability, failed cruciate reconstruction when the PLC is missed, common peroneal nerve palsy (15 to 29%), popliteal artery injury (with knee dislocation), arthritis, and varus thrust gait.

Pathophysiology.

The LCL resists varus stress at 30 degrees of flexion. The popliteus tendon and popliteofibular ligament resist external rotation of the tibia, particularly at 30 to 90 degrees of flexion. The PLC also acts with the PCL to resist posterior translation, so combined injuries cause greater instability and place heavy loads on any cruciate graft.

The common peroneal nerve runs behind the biceps femoris tendon and around the fibular neck, where it is stretched or torn by varus force.

Lateral and posterolateral knee pain, swelling and bruising, a sense of giving way when walking on an extended knee, and a varus thrust gait (a lateral thrust during stance). Acute injuries show bruising over the lateral knee, and tenderness over the fibular head, the LCL and the popliteus.

• Varus stress test at 0 and 30 degrees: laxity at 30 degrees alone indicates an isolated LCL or PLC injury, and laxity at 0 degrees indicates PLC with cruciate injury.
• Dial test at 30 and 90 degrees (prone or supine): increased external rotation of over 10 degrees compared with the other knee at 30 degrees alone means an isolated PLC injury; increased external rotation at both 30 and 90 degrees means combined PLC and PCL injury.
• Posterolateral drawer test, reverse pivot shift test, external rotation recurvatum test and the posterior drawer test.
• Examine the common peroneal nerve (foot drop, sensation over the dorsum of the foot and first web space, and great toe extension), the pulses and the popliteal fossa. Palpate the fibular head for avulsion.

• Radiographs: anteroposterior, lateral and skyline views of the knee. Look for a Segond fracture, an avulsion of the fibular head ("arcuate sign") or Gerdy tubercle, and a lateral tibial plateau fracture. Weight bearing long leg alignment views in chronic injuries to assess varus.
• Varus stress radiographs at 20 degrees of flexion: a lateral compartment gap of over 2.7 mm more than the other knee indicates an LCL injury, and over 4.0 mm suggests complete PLC injury.
• MRI (3 tesla) identifies the LCL, popliteus tendon, popliteofibular ligament and associated cruciate, meniscal and chondral injuries, with high accuracy in acute injury.
• Ankle brachial pressure index (below 0.9 needs angiography) when the knee was dislocated or pulses are reduced. Nerve conduction studies and electromyography at 3 to 4 weeks for peroneal nerve injury.

Differential diagnosis.

• Isolated LCL sprain and iliotibial band injury.
• Posterior cruciate ligament tear and anterior cruciate ligament tear.
• Lateral meniscal tear and lateral tibial plateau fracture.
• Knee dislocation and common peroneal neuropathy.

Initial management. Rest, ice, elevation and a hinged knee brace, with analgesia. Check pulses and the peroneal nerve repeatedly. Assess for knee dislocation (vascular injury) in every combined PLC injury.

Grade I and II (isolated, no instability). Hinged brace for 4 to 6 weeks, protected weight bearing, avoidance of varus and external rotation stress, and physiotherapy emphasising quadriceps and hamstring strength and neuromuscular control. Return to sport at 6 to 12 weeks when stable.

Grade III or combined injuries.

• Surgery within 2 to 3 weeks of injury (acute repair or reconstruction is better than chronic reconstruction, and delay beyond 3 weeks makes repair of tissue difficult).
• Anatomical repair of avulsion injuries (fibular head, popliteus, biceps femoris) with suture anchors, screws or a tension band. Reconstruct midsubstance tears with a graft (LaPrade technique: LCL, popliteus tendon and popliteofibular ligament with autograft or allograft).
• Treat the PCL and ACL injuries at the same time or in stages.
• Chronic PLC injury: reconstruction, with a high tibial osteotomy first when there is varus malalignment (over 5 degrees of varus), to protect the graft.
• Common peroneal nerve: explore and neurolyse with the repair if there is a deficit; late tendon transfer (tibialis posterior) for permanent foot drop. Use an ankle foot orthosis.
• Rehabilitation: non-weight bearing for 6 weeks, hinged brace, range of motion progressing from week 2 (limit flexion to 90 degrees initially), strengthening from 3 to 4 months, return to sport at 9 to 12 months.

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL); with severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Thromboprophylaxis. Enoxaparin 40 mg subcutaneously once daily, started within 12 to 24 hours after surgery and continued for 14 days or until the patient is fully weight bearing and mobile (20 mg once daily at creatinine clearance under 30 mL/min; adjust by anti-Xa level in obesity and renal impairment; check the platelet count on days 5 to 14 for heparin induced thrombocytopenia). Use mechanical prophylaxis until then.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• Oral morphine 5 to 10 mg every 4 hours as needed for severe pain for up to 3 days (start 2.5 to 5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2; avoid exceeding 60 mg in 24 hours without senior review); monitor sedation and respiration and prescribe a laxative.
• Neuropathic pain from peroneal nerve injury: gabapentin 300 mg orally at night, increased by 300 mg every 1 to 3 days to 300 to 600 mg three times daily (maximum 1800 mg daily; reduce at creatinine clearance under 60 mL/min); monitor sedation and dizziness.

Monitoring. Neurovascular status every 4 hours for 24 hours after injury or surgery. Peroneal nerve function at each visit, with electromyography at 3 to 4 weeks when a deficit persists. Varus and external rotation laxity, range of motion and quadriceps strength at 6 weeks, 3 months, 6 months and 12 months. Stress radiographs at 6 and 12 months. Late arthritis on weight bearing radiographs.

Referral and escalation. Refer every suspected PLC injury beyond grade I to orthopaedics or sports surgery within 1 week for acute repair. Emergency referral for a dislocated knee, absent pulses (vascular surgery), compartment syndrome, open injury, or a progressive nerve deficit.

Special groups.

• Athletes: combined injuries need staged reconstruction and a long rehabilitation; the return to sport takes 9 to 12 months.
• Children and adolescents: avulsion fractures of the fibular head and growth plate injuries are more common than ligament tears.
• Varus alignment: osteotomy before or with ligament reconstruction protects the graft.
• Kidney impairment: avoid ibuprofen, reduce morphine doses and reduce gabapentin.