(See Overview of Dislocations.)
Most anterior knee dislocations result from hyperextension; most posterior knee dislocations result from a posteriorly directed force to the proximal tibia while the knee is slightly flexed. Most knee dislocations result from severe trauma (eg, in high-speed motor vehicle crashes), but seemingly slight trauma, such as stepping in a hole and twisting the knee, can sometimes dislocate the knee, with vascular and neurologic complications, particularly in morbidly obese patients (1).
Knee dislocations always damage
Joint instability due to extensive ligament injury is a common long-term complication of knee injury.
Other structures that are commonly injured include the
Popliteal artery injury may initially affect only the intima and thus does not cause distal limb ischemia until the artery later becomes occluded. Undiagnosed arterial injury has a high risk of ischemic complications, which may lead to amputation.
General reference
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1. Vaidya R, Roth M, Nanavati D, Prince M, et al: Low-velocity knee dislocations in obese and morbidly obese patients. Orthop J Sports Med 3 (4):2325967115575719, 2015. doi: 10.1177/2325967115575719.
Symptoms and Signs
Diagnosis
A knee dislocation should be suspected if an injured knee is grossly unstable. Anteroposterior and lateral x-rays are diagnostic for dislocations that have not spontaneously reduced.
Vascular and neurologic evaluations are particularly important.
Popliteal artery injury should be suspected regardless of whether ischemia is evident. Clinical evaluation of the distal pulses cannot completely rule out a popliteal artery injury, even if the pulses are normal over a period of time.
The ankle-brachial BP index (ABI) should be determined to check for vascular injury; values ≤ 0.9 are very sensitive for vascular injury. However, CT angiography is the gold standard for vascular evaluation after knee dislocation. Some experts also recommend CT angiography even if the ABI is > 0.9 and no physical examination findings suggest ischemia.
If the ABI is ≤ 0.9 or if any findings suggest ischemia, immediate vascular surgical consultation is required. Clinicians should aggressively check for vascular injury because duration of ischemia greatly affects outcome. If surgery to repair the vascular injury is not done within 8 hours, amputation rates are higher.
Treatment
Treatment of knee dislocations is immediate closed reduction to 15° of flexion.
Vascular injuries are repaired immediately; a vascular surgeon should be consulted about repairing them. If tissue ischemia is present, fasciotomy may be necessary.
For gross instability, an external fixator is sometimes applied. Anteroposterior and lateral x-rays are usually taken to confirm reduction.
Knee ligaments can be reconstructed later, after the swelling resolves.
Key Points
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Many knee dislocations are accompanied by popliteal artery or nerve injuries.
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Knee dislocations always damage structures that support the knee joint, causing joint instability.
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Most knee dislocations are clinically obvious, but they may spontaneously reduce before they are evaluated; so suspect dislocation if an injured knee is grossly unstable.
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Always measure the ankle-brachial index and do CT angiography because the popliteal artery is commonly injured by knee dislocation.
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Immediately reduce the dislocated knee, and consult a vascular surgeon about repairing any vascular injuries.
