Knee dislocation is a relatively rare injury with potentially disastrous sequelae, including popliteal artery disruption with subsequent distal ischemia necessitating vascular repair in a significant percentage of cases.1 Many emergency physicians and consulting orthopedists perform routine angiography to localize suspected vascular injury and to identify occult vascular injury, facilitating prompt repair and improving outcomes, especially limb salvage.2–4 In this injury, clinicians must always perform an evaluation of the vascular structures around the knee.
Anatomy and Pathophysiology
Traumatic dislocation of the knee (tibiofemoral joint) is rare. The usual mechanism of injury occurs during a fall from a height or an automobile or motorcycle crash with very high impact forces, and during sports. Anterior dislocation due to hyperextension is most common followed by posterior dislocation (dashboard to flexed knee). Medial, lateral, and rotary dislocations are less common.
Invariably, there is significant associated soft tissue injury, with disruption of the associated ligamentous structures, disruption of the joint capsule, and trauma to surrounding muscles and tendons.
Neurovascular injury is relatively common in knee dislocation because of the anatomy of the neurovascular bundle in the popliteal fossa. The bundle (which contains the popliteal artery, popliteal vein, and common peroneal nerve) courses posteriorly behind the knee joint. The popliteal artery is firmly tethered, both proximally in the adductor magnus hiatus and distally in the fibrous arch of the soleus. During dislocation, this immobility leads to severe vascular injury. The popliteal vein and common peroneal nerve are susceptible to injury by a similar mechanism.
Vascular injury occurs in approximately 35 to 40 percent of complete knee dislocations,5 and is more common in anterior and posterior dislocations. Vascular injury is less common in lower energy dislocations, regardless of the direction of the dislocation. The popliteal artery and/or the lateral geniculate arteries of the knee may be stretched, contused, or lacerated. Other injury patterns include intimal flaps, arterial spasm, and thrombus formation. If the popliteal artery is damaged, meager collateral flow through the geniculates may provide blood flow. Remember, soft tissue trauma and edema may cause compression and ultimately compromise this collateral circulation.
History and physical exam, as always, should guide care. The diagnosis is straightforward when the knee is obviously deformed and remains dislocated on presentation to the ED. Making the diagnosis is more difficult when patients present with an injury that could have caused a subluxation or a dislocation that spontaneously reduced. When a knee injury produces severe ligamentous disruption, it should be considered a dislocation that has reduced spontaneously in the field and treated as aggressively as a dislocation that has not reduced.6
Any patient with an obviously dislocated knee should have a focused neurovascular evaluation, including distal sensation and movement and measurements of vascular status including dorsalis pedis and posterior tibial pulses, extremity temperature, and capillary refill. Reduction is achieved by longitudinal traction/countertraction, and should not be delayed by radiographs. During this maneuver, force (anterior or posterior as appropriate) may be applied to the femur to return it to its natural alignment. Reduction should be achieved in the neutral position. In particular, the knee joint should not be extended because this movement stretches the popliteal artery. Intravenous conscious sedation is usually sufficient, although some posterior reductions can require operative repair. The limb should be placed in a posterior splint with 15 to 20 degrees of flexion.
Post-reduction radiography to rule out fracture is necessary, as are assessment and treatment for concomitant injury. After reduction, pulses should be checked. Immediate orthopedic consultation is needed, with, at minimum, admission for observation. Vascular surgery consultation is necessary if there is any evidence of vascular compromise.
Most importantly, up to 40 percent of knee dislocations demonstrate arterial injury. In fact, the amputation rate for patients with vascular injury associated with knee dislocation is 85 percent if not repaired within eight hours. It is accepted practice that patients with obvious pulse deficits or cold extremity distal to knee dislocation need immediate surgery. It is generally agreed that arteriography need not be performed if it delays surgery because the site of the injury is almost always easily localized — the popliteal artery in the popliteal fossa. It also is generally agreed that in the patient with mildly diminished pulses or other soft signs of potential arterial injury, prompt angiography is indicated. Time spent doing angiography on a limb when blood flow is diminished but not gone is useful to localize the injury and to characterize the nature of the vascular injury.2,7
For many years, angiographic evaluation of the affected joint was axiomatic. Proponents of angiography point out that it is a useful post-reduction evaluation tool even when pulses are normal because it is a very sensitive indicator of even minor arterial injury, including small intimal flaps.1–2,7–8 In addition, physical exam is not sensitive in detection of arterial injury/vascular compromise, and delay in detection means delay in repair, which means bad outcomes. Jones, Gable, McCoy, and others cite multiple cases of occult arterial injury and subsequent distal ischemia in extremities with good pulses on serial exam.3,9–10
Reduction should be achieved in a neutral position; the knee joint should not be extended because this movement stretches the popliteal artery
A number of orthopedists, however, have questioned the routine use of angiography to assess vasculature post knee reduction.11–12 Angiography has a small but definite morbidity, and injuries identified on angiography in patients with normal pulses post-reduction are frequently insignificant and do not require repair. In fact, the use of angiography in patients with normal pulses is relatively low yield, and four to six percent of these patients have identifiable injuries, the great majority of which do not require repair. Nonetheless, the data from Frykberg's oft-cited study has been criticized, and individual approaches to vascular assessment often reflect personal experience.11 Some authors have championed the use of serial Doppler studies to evaluate vascular integrity, but this approach has met with limited acceptance.13
In every case, some evaluation for vascular injury must be performed and vascular status must be meticulously monitored during the post-reduction period. There can be no doubt that the sequelae of a missed vascular injury are devastating. Any emergency physician who in consultation chooses not to utilize angiography should clearly document his rationale, and document thorough and frequent neurovascular exams while the patient is in the ED.
In summary, there is no question that a knee dislocation always threatens the integrity of the vascular structures of the lower extremity, and can result in the loss of a limb. First, the dislocated joint must be rapidly reduced. Second, if pulses do not return, immediate operative intervention is required. Third, if post-reduction pulses are diminished, angiography is the study of choice. Finally, if the pulses return and the foot is warm and well perfused with good motor function and sensation, then frequent meticulous vascular examination with or without angiography is indicated. Remember: A knee dislocation is always a threat for vascular injury.
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