A knee dislocation, not to be confused with a patella dislocation, is a rare, but potentially limb-threatening injury. Most often caused by high-velocity trauma, knee dislocations account for less than 1.2% of orthopedic trauma. Knee dislocations often result in injury to multiple cruciate ligaments of the knee, but can also cause neurovascular damage. Many of these injuries may initially go unrecognized, however, as the knee may spontaneously reduce in 20-50% of cases. The significant morbidity associated with these vascular injuries make prompt recognition crucial.
The mechanism of injury may be low- or high-energy. Low-energy injuries are usually sports-related and result from hyperextension with varying degrees of varus or valgus stress, resulting in anterior dislocation. High-energy injuries are frequently seen in motor vehicle accidents and may result from hyperextension or dashboard injury, with the knee flexed 90º and force directed posteriorly.
The Kennedy classification is based on the direction of displacement of the tibia.
- The most common type of dislocation
- Follows hyperextension injury
- Usually involves tear of PCL
- Arterial injury – Intimal tear due to traction
- Second most common
- Occurs due to axial load to the flexed knee (dashboard injury) or fall on the flexed knee
- The highest rate of vascular injury [complete tear of popliteal artery]
- Due to varus or valgus force
- Involves tears of both anterior and posterior cruciate ligaments
- The highest rate of peroneal nerve injury
- Varus or valgus force
- Associated with a disrupted posterolateral corner or posterior cruciate ligament
- Posterolateral is most common rotational dislocation
- Usually irreducible due to buttonholing of femoral condyle through the capsule
Frequently damaged structures in knee dislocations include the ligamentous stabilizers of the knee and the popliteal artery and vein. The anterior and posterior cruciate ligaments originate from the lateral and medial condyles of the femur respectively and insert onto the tibia near the tibial spine in the intertubercular sulcus, providing stability in anterior and posterior translation of the tibia on the femur as well as some rotatory stability. The medial and lateral collateral ligaments originate from the femoral epicondyles and insert on the proximal tibia and proximal fibula respectively, providing resistance to varus and valgus stresses. Secondary stabilizers, such as the posterior-lateral capsular structures, may also be compromised.
The popliteal artery is particularly susceptible to injury in dislocations due to its tethering proximally at the adductor hiatus 10 cm proximal to the knee and distally at the soleus arch. Popliteal artery injury has been reported in 8-64% of dislocations and has been shown in cadaver studies to occur at 50º hyperextension.
Immediate reduction of the dislocation is necessary to relieve occlusion of arteries, tension on nerves, and pressure on the skin. This may usually be accomplished closed, although open reduction may occasionally be indicated. Evaluation of the injury should include a thorough neurovascular exam pre- and post-reduction (peroneal nerve injury in 14-35%, vascular injury in 8-64%), and plain radiographs to look for associated fractures. Arteriography, arterial Doppler ultrasound, and MR angiography may have a role to evaluate vascular injury (versus serial examinations in the presence of normal pulses).
AnkleBrachial Index (ABI) is a rapid, reliable, noninvasive tool for diagnosing vascular injury associated with knee dislocation.
In one study of 38 patients, 11 (29%) had an ABI lower than 0.90. All 11 had arterial injury requiring surgical treatment. The remaining 27 patients had an ABI of 0.90 or higher. None had vascular injury detectable by serial clinical examination or duplex ultrasonography. The sensitivity, specificity, and positive predictive value of an ABI lower than 0.90 were 100%. The negative predictive value of an ABI that reached 0.90 or higher was 100%.
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The value of the ankle-brachial index for diagnosing arterial injury after knee dislocation: a prospective study. J Trauma. 2004