Wiler, Jennifer L. MD, MBA
A45-year-old male pedestrian was brought to the emergency department after being struck on his left side by a car going at an unknown rate of speed. He complains of severe left knee pain.
Here is what you observe when you walk in the room. What conditions are you concerned about, and how would you evaluate the knee? See p. 30.
Knee dislocation is a rare (< 0.02% of musculoskeletal trauma) but limb-threatening injury that can be challenging to diagnose because patients frequently present with the knee relocated. (Phys Sportsmed 2010;38:101.) Blunt popliteal artery injury, a known complication of knee dislocation, carries the highest morbidity and amputation rates of all peripheral vascular injuries because it can result in complete lower limb ischemia. (J Vasc Surg 2004;40:61.) It is a condition that emergency physicians must be vigilant in identifying.
Knee dislocation is typically the result of direct blunt force trauma on the joint area as the result of a vehicle or vehicle-pedestrian crash, sports-related injury, or fall. (J Orthop Trauma 2003;17:198.) And as many as 30 percent of cases are complicated by an open joint. Described by the relationship of the tibia on the femur, most knee dislocations are anterior (31%) versus posterior (25%), lateral (13%), medial (3%), or rotary (4%, typically posterolateral). Anterior dislocations are caused by knee hyperextension (usually greater than 30 degrees), and are frequently associated with posterior and anterior cruciate ligament rupture (PCL and ACL respectively) and possibly the medial and lateral collateral ligaments.
Associated popliteal artery injury is also common. The popliteal artery is held in position proximally by the adductor hiatus (opening between the adductor magnus muscle and the femur, which allows passage of the femoral vessels) and distally by the tendinous arch of the soleus muscle. With anterior dislocations, stretching of the artery along this route is common, and can cause arterial injury to a relatively large segment with intimal separation and subsequent intraluminal thrombus formation. Posterior dislocations, however, are the most likely to have associated vascular injury (typically isolated popliteal artery avulsion or complete transaction) because of the significant forces required to overcome the strength of the leg extensor muscles, allowing the knee to dislocate posteriorly. These forces typically also rupture the ACL and PCL. (http://www.wheelessonline.com/.)
Low-velocity dislocations may only be associated with vascular injury in five percent and peroneal injury in 20 percent of cases. (Orthop Rev 1991;20:995.) Peroneal nerve injury has been reported in as many as 20 to 40 percent of knee dislocations, with approximately 50 percent of these palsies being permanent. (J Bone Joint Surg Br 2005;87:664.) Symptoms of peroneal injury include impaired dorsiflexion (foot drop) and sensation at the first webspace (deep peroneal nerve). In general, peroneal damage is associated with injury to both cruciates and at least one collateral ligament, but can be the result of ischemia. In patients with signs of peroneal injury, one should have a high index of suspicion for an associated vascular injury.
Patients with traumatic knee dislocation may present with the knee acutely dislocated, but commonly (50%) the knee is reduced prior to ED presentation. Significant localized swelling, pain, and immobility are expected. Evaluation for associated vascular or nerve injury is critical, with a pulse deficit appreciated in up to 84 percent of affected limbs. (http://bit.ly/kneedislocation.) The diagnosis of acute dislocation is confirmed by radiographs, which may demonstrate associated tibial, patellar, or femur fractures. Patients with acute dislocations and evidence of acute vascular compromise require emergent closed reduction. Reduction should not be delayed for radiographs if there is evidence of acute vascular compromise. If emergently reduced in the ED, a post-reduction splint at approximately 20 degrees of flexion, ice, elevation, and post-reduction radiographs are recommended.
Because knee dislocations require significant blunt force, evaluation for other traumatic injuries should also be performed following standardized trauma protocols. Physical examination to determine ligament integrity should be attempted but deferred until associated fractures are ruled out, preventing fracture fragment dislocation.
Patients with clinical evidence of vascular injury (expanding hematoma, absent or decreased distal pulses, pulsatile bleeding) require emergent operative repair where intraoperative arteriography can be performed. The indications for arteriography in a patient with questionable vascular injury, however, have not been validated. Routine direct arteriography was previously the gold standard for diagnosing vascular injury secondary to traumatic knee dislocation, but direct arterial puncture by a vascular surgeon or interventional radiologist, exposure to contrast dye, and radiation are required.
For patients with a low likelihood of vascular injury, the ankle-brachial index (ABI) (ratio of systolic blood pressure at ankle to arm) may be an appropriate initial screening tool. In one study of 38 patients with known knee dislocation, those with an ABI less than 0.90 underwent arteriography and those with an ABI of 0.90 or greater were immobilized and admitted for serial examination and delayed arterial duplex evaluation.
The authors state that no vascular injuries were missed, and that routine arteriography for all patients with knee dislocation is not supported. (J Trauma 2004;56:1261.) Another study of 133 knees found that serial physical examination was sufficient to rule out vascular injury. (J Bone Joint Surg Br 2005;87:664.) It should be noted, however, that the presence of a distal pulse post-reduction does not rule out clinically significant vascular injury. (J Trauma 1988;28:240.) If sufficient clinical suspicion for vascular injury exists or ABIs are less than 0.9, imaging with either CT angiography (Trauma 2006;60:502; discussion 506) or duplex ultrasonography (Arch Surg 1993;128:1368) is warranted in consultation with a vascular surgeon.
For patients with vascular injury, intraoperative repair varies based on the type of injury with some advocating for intraoperative intrarterial anticoagulation to maximize limb salvage. (Ann Surg 1997;225:518; discussion 527.) Despite current treatments, the risk of limb amputation in patients with traumatic popliteal arterial injury range from 12-21 percent. (J Vasc Surg 2006;44:94.)
All patients with confirmed or suspected knee dislocation require hospital admission. Delayed vascular complications (typically 48-72 hours) including intimal flap thromboses, arteriovenous fistulas, pseudoaneurysms, and compartment syndrome are limb-threatening conditions that require inpatient observation to rule out. Despite optimal care, patients with knee dislocation commonly have long-term pain and instability. (Phys Sportsmed 2010;38:101.)
In this case, the photographs are from different patients with similar auto-pedestrian mechanisms. Both patients had a posterior knee dislocation with arterial injury. The second patient had a CT angiogram that demonstrated occlusion of the left popliteal artery with no three-vessel runoff to the distal left lower extremity associated with extensive soft tissue swelling or hematoma within the left lower extremity in addition to a medial patellar multiple avulsion fractures. Both patients required operative repair of arterial injuries.
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Dr. Wiler is an assistant professor of emergency medicine and the medical director of reimbursement at the University of Colorado Denver School of Medicine and an adjunct assistant professor of emergency medicine at the Washington University School of Medicine in St. Louis.
© 2011 Lippincott Williams & Wilkins, Inc.