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 General reference Knee dislocations are commonly accompanied by arterial or nerve injuries. Knee dislocations threaten limb viability. These dislocations may spontaneously reduce before medical evaluation. Diagnosis... read more ).
Knee dislocations always damage
Structures that support the knee joint, causing joint instability
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 (particularly in anterior dislocations)
Peroneal and tibial nerves
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.
(See Overview of Dislocations Overview of Dislocations A dislocation is complete separation of the 2 bones that form a joint. Subluxation is partial separation. Often, a dislocated joint remains dislocated until reduced (realigned) by a clinician... read more .)
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 of Knee Dislocations
Knee dislocations cause deformity that is clinically obvious. However, some dislocations spontaneously reduce before medical evaluation; in such cases, the knee remains very swollen and grossly unstable.
Fullness in the popliteal fossa suggests hematoma or popliteal artery injury.
Diagnosis of Knee Dislocations
Vascular and neurologic evaluation
A knee dislocation (as well as an anterior cruciate ligament [ACL] and/or posterior cruciate ligament [PCL] tear) should be suspected if an injured knee is grossly unstable (see also Knee Sprains and Meniscal Injuries Knee Sprains and Meniscal Injuries Sprains of the external (medial and lateral collateral) or internal (anterior and posterior cruciate) ligaments or injuries of the menisci may result from knee trauma. Symptoms include pain... read more ). 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 Diagnosis (ABI) should be determined to check for vascular injury; values ≤ 0.9 are very sensitive for vascular injury. The blood pressure in the ankle below the injured knee can be compared to the blood pressure in the ankle of the contralateral side if ABI cannot be obtained. 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 of Knee Dislocations
For vascular injury, immediate vascular repair and fasciotomy
Later elective ligament reconstruction
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.
Many knee dislocations are accompanied by popliteal artery or nerve injuries.
Knee dislocations always damage structures that support the knee joint, causing joint instability.
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.
Always measure the ankle-brachial index and do CT angiography because the popliteal artery is commonly injured by knee dislocation.
Immediately reduce the dislocated knee, and consult a vascular surgeon about repairing any vascular injuries.