Knee Dislocation Management

Knee dislocation is rare, about 0.02% of knee injuries, and it is an orthopaedic emergency because the popliteal artery is stretched or torn as the tibia leaves the femur. The consequence of a missed arterial injury is the limb, which is why every dislocated knee, however it presents, goes down the vascular pathway on this page.

The knee that has already reduced. 20% of dislocations reduce spontaneously before arrival (estimates run as high as 50%), so the diagnosis is easy to miss unless suspicion stays high. A swollen, grossly unstable knee after a high-energy mechanism deserves that suspicion whatever the radiographs show.

Mechanism. Most are high-energy, but the low-energy dislocation in the obese patient is increasingly common:

• High-energy trauma (60%): motor vehicle collision, fall from height
• Low-energy in the obese (40%): simple fall, hyperextension
• Dashboard injuries and contact sports (football, rugby)
• Ultra-low velocity in the morbidly obese (BMI greater than 40)

What travels with it. Four associated injuries to look for:

• Popliteal artery injury in 30-40%, most often an intimal tear
• Common peroneal nerve palsy in 25-30%, lateral dislocations carrying the highest risk
• Compartment syndrome, especially after a vascular repair
• Meniscal tears in 50%, often a peripheral detachment

The 30-40% is the classic, often-repeated figure. The largest pooled series (Medina 2014, in the Evidence Base) found vascular injury in 18% and nerve injury in 25%; the classic numbers come from older, selected series, so know both.

The ligaments. The Schenck grade is defined by the number and combination of these four stabilisers that fail, so know their parts:

• ACL: anteromedial and posterolateral bundles
• PCL: anterolateral and posteromedial bundles
• MCL: superficial (tibial attachment) and deep (meniscal) layers
• LCL: fibular attachment, part of the posterolateral corner

The posterolateral corner. Four components:

• LCL: primary lateral stabiliser
• Popliteus tendon: dynamic posterolateral stabiliser
• Popliteofibular ligament: resists external rotation
• Lateral capsule: secondary restraint

Together they are critical for rotatory stability, and a missed posterolateral corner injury is a risk factor for persistent instability after reconstruction.

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The popliteal artery. The artery is tethered proximally at the adductor hiatus and distally at the soleus arch, so when the tibia displaces, as in an anterior dislocation, the vessel is stretched over its fixed points. The commonest lesion is an intimal tear without complete disruption: the pulses are normal at first and the vessel thromboses 6-24 hours later. That is the whole reason normal pulses do not exclude an arterial injury.

The common peroneal nerve. The nerve wraps around the fibular neck and is stretched there as a traction injury in lateral and posterolateral dislocations. Recovery after this injury is poor.

The cruciates. Both are intracapsular with minimal blood supply, and both are torn in 80% of dislocations. Both need reconstruction to restore stability.

The sequence. A high-energy force (a motor vehicle accident, a sports collision, a fall from height) tears the cruciates and capsule and the tibiofemoral joint displaces. The artery is stretched over its fixed points and the intima dissects, thromboses or the vessel transects outright; the nerve is dragged around the fibular head. The joint may then reduce itself, which masks the severity of what has happened.

Schenck's anatomic classification counts the ligaments torn and is the most widely used. The direction of dislocation is the other way of describing the injury, and it predicts which ligaments have failed and guides the surgical plan.

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History. The mechanism is high-energy trauma, a dashboard injury or hyperextension. Ask specifically whether the knee "popped out", with the spontaneously reduced dislocation in mind. Immediate haemarthrosis and inability to bear weight are expected; foot numbness suggests a nerve or vascular injury.

Look. Gross deformity if the knee is still dislocated, with swelling and ecchymosis.

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The dimple sign. A fixed pucker of the medial skin means the medial femoral condyle has buttonholed through the medial capsule. That knee is irreducible: closed reduction must not be forced, and repeated forceful attempts produce the medial skin necrosis that is the commonest complication of these injuries. The sign is not always present: in the pooled irreducible dislocations of Malik and colleagues (Evidence Base) it was seen in only 70%, so a dislocation that resists one careful attempt under adequate sedation is treated as irreducible whatever the skin looks like.

The neurovascular examination. Mandatory before and after reduction, and documented each time. It has four parts:

• Pedal pulses: normal is 2+ bilaterally; diminished or absent pulses mean urgent CTA, because pulses alone cannot be relied upon
• Capillary refill: normal is under 2 seconds; over 3 seconds is an indication for immediate reduction and reassessment afterwards
• Ankle-brachial index, described under Investigations
• Common peroneal nerve: foot dorsiflexion and eversion

What not to do. Do not stress-test the ligaments acutely; it risks re-dislocation. Do palpate the compartments for tightness, particularly after a vascular repair.

The spontaneously reduced dislocation. When the history suggests a transient dislocation, keep the suspicion high and look for the clues:

• High-energy mechanism
• Severe instability on examination
• Inability to bear weight despite normal radiographs

Obtain an MRI to define the multiligament injury.

Differential Diagnosis

The spontaneously reduced dislocation and the acutely swollen, unstable knee can be confused with lesser injuries. The distinction matters because only the true (occult) dislocation mandates the full vascular pathway.

Radiographs. AP and lateral views. Before reduction they document the direction of dislocation and identify fractures (a KD V); after reduction they confirm a concentric reduction and look for occult fractures of the tibial plateau or femoral condyle. Stress views are not performed acutely.

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Ankle-brachial index. Mandatory in every case. An ABI below 0.9 has 95% sensitivity for arterial injury. Measure it before and after reduction and then serially, because a normal ABI does not exclude an intimal tear.

CT angiography. The gold standard once the ABI is abnormal: sensitivity 95%, specificity 99% for popliteal artery injury, showing intimal tears, pseudoaneurysms and complete disruption. Any abnormality is an urgent vascular surgery consult.

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MRI. Once the vascular status is secure, typically at 5-7 days, on a 3 Tesla scanner. T2-weighted sequences show all the ligament tears, the meniscal injuries and the chondral damage, and the result is what plans the staged reconstruction.

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Algorithm

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The first six hours. The goals are to restore vascular flow, prevent limb loss and document the injuries.

Recognise and document (0-15 minutes). Identify the dislocation, whether obvious or by a history of reduction, record the neurovascular status (ABI, pulses, peroneal nerve) and obtain AP and lateral radiographs if time permits. If the dislocation is obvious and there is neurovascular compromise, do not wait for radiographs or CT: reduce immediately in the emergency department under procedural sedation. Reduction improves vascular flow and reduces compartment pressure; image afterwards to confirm the alignment.

Reduce (15-30 minutes). Under procedural sedation (propofol or ketamine), apply longitudinal traction to the tibia while an assistant provides counter-traction at the thigh, and reverse the mechanism of injury. For an anterior dislocation, the most common, gently extend the knee while applying posterior pressure to the proximal tibia. For a posterior dislocation, flex the hip to 90 degrees, apply traction, then extend the knee while lifting the tibia anteriorly. Reassess the neurovascular status as soon as the knee is reduced. The exception is the knee with a medial dimple, which will not reduce closed (see Clinical Assessment).

After reduction (30-60 minutes). Repeat the ABI and pulses and document whether they have improved or deteriorated, confirm a concentric reduction on radiographs and splint the knee in 15-20 degrees of flexion, which prevents re-dislocation. An ABI below 0.9 is a vascular surgery consult.

Serial monitoring (0-24 hours). Neurovascular checks every 2 hours for 24 hours; the algorithm allows 24-48 hours, and Stannard's series (Evidence Base) argues for at least 48 hours in a KD-IV injury. Watch for compartment syndrome, especially after a vascular repair. Once the limb is safe, the MRI described above plans the reconstruction.

Vascular Injury

The finding on examination tells you the likely lesion and how fast you have to move. Of the knees presenting with diminished pulses and an ABI of 0.7-0.9, 20% progress to thrombosis.

Staged Ligament Reconstruction

Why staged. Early total repair, every ligament at once, carries a 50% arthrofibrosis rate. Reconstructing the cruciates early and the collaterals later brings stiffness below 20% while still achieving good stability. The cruciates go first because restoring anteroposterior stability unloads the healing collaterals.

The Spanning External Fixator

The missing middle step. Between the splint and definitive ligament surgery sits the knee-spanning (femur-to-tibia) external fixator, the intermediate option for the unstable or complex dislocation. Its indications:

• Gross instability where a splint or brace cannot hold the reduction, classically the large or obese limb after an ultra-low-velocity dislocation, where the soft tissue will not maintain alignment
• To protect a vascular repair: after popliteal artery repair or bypass, the fixator immobilises the joint so the fresh anastomosis is not stressed by knee motion
• Open dislocation, severe soft-tissue compromise, or polytrauma and damage control where definitive ligament surgery must be delayed
• KD-V fracture-dislocation, as part of staged skeletal stabilisation

Technique. Apply it with the knee in slight flexion (about 15 to 20 degrees), place the pins well away from planned ligament tunnel sites and any future surgical zones, and keep it on until the soft tissues and vascular status allow, commonly around 6 weeks, before converting to a hinged brace and proceeding to staged reconstruction.

The trade-off. Pin-site infection, and knee stiffness if it is left on too long. It is a temporising stabiliser, not a definitive treatment.

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Predictors of a poor outcome. Five things in the history and the injury pattern predict a worse result:

1. Vascular injury requiring repair (higher complication rate)
2. KD IV or KD V injuries (all ligaments torn)
3. Peroneal nerve palsy (10-20% permanent)
4. Delayed reconstruction (greater than 6 months)
5. High-energy mechanism (polytrauma, associated injuries)