Posterolateral Corner Injuries

Posterolateral corner (PLC) injuries make up about 16% of knee ligament injuries, and 60-70% occur with a cruciate injury. They are frequently missed at first presentation, and a missed PLC leads to cruciate graft failure, so every injured structure must be addressed. Keep a high index of suspicion in any PCL or ACL injury.

Who and how. The injury is usually the product of high-energy trauma. Motor vehicle collisions are a common cause, and in sport it is seen in football, soccer and skiing. The mechanisms:

• Varus force - a direct blow to the medial knee
• Hyperextension with a varus component
• External rotation on a planted foot
• Knee dislocation - the multiligament pattern
• Contact sports - a direct lateral blow

The structures. LaPrade's anatomic description centres on three primary static stabilisers. The fibular collateral ligament (FCL) runs from the lateral femoral epicondyle to the lateral fibular head. The popliteus tendon, the tendon of a muscle-tendon unit, has its femoral origin on the lateral femoral condyle and inserts on the tibia, and the popliteofibular ligament connects the popliteus to the fibular styloid. Tendon and ligament together form the popliteus complex.

The capsule. The posterolateral capsule, with the arcuate ligament, forms the arcuate complex, a further static stabiliser.

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What the corner resists. The PLC is the primary restraint to varus and to external rotation. The FCL is the primary varus restraint and the popliteus complex the primary external-rotation restraint. Secondarily the corner resists posterior translation at full extension and hyperextension (recurvatum). It tightens in extension, and as a secondary backup to the ACL it helps control anterior translation.

The PLC and the PCL. The two work together: the PCL resists posterior translation and the PLC resists rotation. Loss of one stresses the other.

The common peroneal nerve. It wraps around the fibular neck and is very vulnerable, injured in 20%+ of PLC injuries. Injury shows as foot drop with numbness over the lateral leg and the dorsum of the foot.

Grade. The grade is set by dial-test asymmetry compared with the contralateral side.

Injury pattern. The corner is torn alone or with other ligaments:

• Isolated PLC - rare (under 30%), and still usually needs reconstruction
• Combined PCL/PLC - the most common pattern (50%+)
• Combined ACL/PLC - also common
• Multiligament (knee dislocation) - check for vascular injury and examine every ligament

By structure. An FCL injury gives primary varus laxity at 30 degrees; a popliteus injury gives external-rotation laxity on the dial test. Usually both are involved. Avulsions may be femoral or fibular, and the arcuate sign marks a fibular one. MRI determines which structures are injured.

Fanelli classification. Fanelli grades the injury by external rotation plus the degree of varus laxity, and so maps it onto the structures torn.

The progression from A to C tracks increasing FCL involvement. A pure rotational Type A injury spares the FCL, whereas the varus laxity of Types B and C signals attenuation and then complete rupture of the primary varus restraint. The appearance of varus laxity is the clue that raises the injury from A to B or C and makes reconstruction, rather than rehabilitation, the answer.

PLC injuries are frequently part of a knee dislocation, and the dislocated knee needs both the Schenck (KD) classification and a vascular assessment.

A PLC injury with both cruciates is a KD-IIIL. The suffix C denotes an associated arterial injury and N a nerve injury.

The popliteal artery. Tethered proximally at the adductor hiatus and distally at the soleal arch, it is at high risk in any dislocation, and its assessment is the limb-threatening priority. Examine the pulses and measure the ankle-brachial index (ABI):

• ABI 0.9 or above - serial vascular observation is acceptable
• ABI under 0.9 - CT angiography to exclude an intimal or occlusive injury
• Hard signs (absent pulses, expanding haematoma, bruit, active bleeding) - straight to the operating theatre and vascular surgery; do not delay for imaging

History. The patient describes a varus blow, hyperextension or a motor vehicle accident, with lateral and posterolateral knee pain and varus instability or giving way. Ask about foot drop and numbness. A varus thrust gait belongs to the chronic injury.

Examination. The PLC examination:

• Dial test at 30 and 90 degrees - the key test
• Varus stress at 0 and 30 degrees
• External rotation recurvatum test - supine, lift the great toe; positive when the tibia drops into hyperextension and external rotation
• Reverse pivot shift - a PLC-specific test, positive as a clunk on extension
• Common peroneal nerve - motor and sensory

The dial test. With the patient prone and the knees flexed to 30 degrees, then 90, externally rotate both feet simultaneously and compare the thigh-foot angle side to side. An asymmetry of more than 10 degrees is positive. Positive at 30 degrees only is an isolated PLC injury; positive at 30 and 90 degrees is a combined PCL and PLC injury, because at 90 degrees an intact PCL prevents the extra external rotation. The distinction is critical for surgical planning.

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Varus thrust gait. A lateral thrust during stance phase indicates chronic PLC deficiency with significant laxity.

Plain radiographs. Scrutinise the fibular head and Gerdy tubercle for an avulsion that signals a PLC injury. Beyond the arcuate sign, look for a Segond fracture of the lateral tibial plateau, a widened lateral joint space on varus stress, and osteophytes in the chronic knee. Assess the alignment for varus malalignment.

Stress radiography. Varus stress views quantify lateral joint opening, and are only meaningful compared with the contralateral side. They are useful in chronic cases, for monitoring, and in research.

MRI. MRI is good for the PLC, but the structures can be subtle. Each is read on the images that show it best:

• FCL - coronal images
• Popliteus - sagittal and coronal
• Popliteofibular ligament - often difficult to see
• Arcuate complex - the posterolateral capsule

MRI also shows the associated injuries: peroneal nerve oedema, bone bruising and cruciate tears.

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The pathway. Grade and chronicity set the course once the acute knee has been assessed.

Repair or reconstruct. Repair has a high failure rate for isolated PLC injuries, and overall reconstruction is the more durable, with repair failing in around 37% against 9%. Reconstruction gives more reliable outcomes, addresses chronic attenuation and restores the anatomy. The exception is an acute avulsion with good bone, where the fragment can be fixed.

Combined injuries. Both cruciate combinations need the cruciate and the corner reconstructed. With a PCL tear this may be a single or staged operation, and acute surgery is preferred. Early surgery prevents PLC contracture and improves outcomes.

LaPrade anatomic reconstruction. The gold standard. It reconstructs the FCL (allograft), the popliteus tendon and the popliteofibular ligament, restoring both varus and rotational stability, with superior outcomes to isolated repairs. The graft is an Achilles allograft with a bone block, or split semitendinosus. The steps:

• Separate femoral tunnels at the FCL and popliteus origins
• A fibular tunnel at the FCL insertion
• Reconstruct the FCL from femur to fibula
• Reconstruct the popliteus and popliteofibular ligament, the graft crossing from the fibula to the tibial sulcus

The principles are anatomic tunnel placement, appropriate tensioning, and addressing every structure.

Alignment. Varus malalignment increases stress on the PLC, and without alignment correction a PLC reconstruction will fail. When significant varus is present, an opening-wedge HTO of the proximal tibia corrects it to neutral or slight valgus. It may be staged before the PLC reconstruction or performed with it.

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What predicts the result. Outcomes are good with anatomic reconstruction, early surgery, every structure addressed and alignment corrected. They are poor after isolated repair, delayed surgery, missed varus alignment or incomplete reconstruction, and with a peroneal nerve injury.

The long term. Addressed appropriately, the knee is stable, function is preserved, and the return to activity is good. Untreated or undertreated, it goes on to progressive varus deformity, lateral compartment osteoarthritis and cruciate graft failure.

Anatomic or fibular-based reconstruction. The LaPrade anatomic two-graft technique reconstructs the FCL, popliteus and popliteofibular ligament; fibular-based (Larson) techniques are simpler and reconstruct the FCL and popliteofibular ligament only. Anatomic methods better restore rotation biomechanically, but high-level comparative clinical data are limited, and many surgeons obtain good results with a hybrid approach.

Acute repair or reconstruction. Reconstruction is more durable overall, yet a good acute bony avulsion with healthy tissue can be primarily fixed. The threshold for choosing repair over reconstruction in the acute setting remains debated.

Staged or single-stage alignment. With genu varus some advocate osteotomy first, and a proportion then avoid ligament surgery; others perform combined realignment and reconstruction. Optimal sequencing, especially in multiligament knees, is unresolved.

Graft choice and nerve outcomes. Allograft versus autograft, the optimal tensioning angle, and whether early peroneal nerve exploration changes outcomes are all without consensus.