A 21-year-old man was brought in by ambulance after a mechanical slip and fall at a concert amphitheater. He fell on a staircase directly on his left knee. He has no significant past medical history except morbid obesity.
He was noted to have a left posterior knee dislocation. He admitted to marijuana and alcohol use, but was clinically sober on evaluation and had no other signs of trauma. His dorsalis pedis pulse was absent in the left foot. He was sedated shortly after arrival, and his knee dislocation was reduced. The physician was still unable to palpate a pulse in his left foot, and x-ray confirmed reduction of the joint. Significant swelling was noted to his lower leg. Compartment pressures were measured because of the firmness of the lower extremity compartments, paresthesia, pain, pallor, and lack of pulses to his foot. His anterior compartment was found to have a pressure of 50 mm Hg.
Unfortunately, the hospital was without OR capability, and the patient was transferred to another hospital for fasciotomy and vascular evaluation. He arrived at the second facility at 2 a.m., five hours after his injury, and a CT angiography was performed demonstrating complete lack of opacification of the left popliteal artery downstream from the femoral condyles with no vascular runoff to the foot.
Find the diagnosis and case discussion on p. 28.
Diagnosis: Posterior Knee Dislocation Complication
Posterior knee dislocations result from high-energy trauma, such as hitting the flexed knee on a dashboard during a motor vehicle crash or from low-energy trauma such as a fall from standing. Our patient possessed the major risk factor for a posterior knee dislocation due to a fall: obesity. (J Trauma Acute Care Surg 2014;76:715.) Typically, three of the four knee ligaments are disrupted during a posterior knee dislocation. Besides musculoskeletal injuries, associated injuries can occur to the vasculatures and nerves. Vasculature injuries are noted to occur in approximately 40-50 percent of anterior and posterior knee dislocations. The popliteal artery is tethered in the popliteal fossa, and is at high risk for injury. The most common nerve injury is to the common peroneal nerve with a rate approaching 25 percent.
Knee dislocations are classified by either the Kennedy or the Schenck system. The Kennedy system describes the direction of the displacement of the tibia (anterior, posterior, lateral). Anterior dislocation is the most common. Posterior knee dislocations represent approximately 25 percent of dislocations, and have the highest rate of complete tear to the popliteal artery. Lateral knee dislocations occur in 13 percent of dislocations, and pose the highest risk to the peroneal nerve. Posterolateral dislocations require surgical reduction, and closed reduction should not be attempted. The Schenck system describes the injury based on the ligamentous injury.
The physical exam for a patient who has suffered a posterior knee dislocation can be humbling. Approximately 50 percent of dislocations spontaneously reduce prior to ED arrival. If a patient arrives with an obvious dislocation, there is no need to wait for an x-ray confirmation (especially if the limb is pulseless). Emergent OR exploration is necessary if the pulse is absent or diminished after the reduction. Unfortunately, 86 percent of these patients will undergo amputation if the limb has been without pulses and perfusion for eight hours. (Surg Clin North Am 2002;82:67.)
Emergency physicians should be able to assess a patient's mechanism of injury and know the possible associated injuries. Remember, x-rays may be normal when the patient presents, but a good physical exam including range of motion, pulses, and neurologic exam of the patient is pertinent to elucidate associated injuries that could (but shouldn't) be missed. Emergent surgery is necessary if the exam, ankle-brachial index, or CTA shows a vascular injury. Fasciotomies may be necessary due to swelling and acute injury to the lower leg, but vascular repair is the definitive management. (J Emerg Med 2012;42:271.) This case highlights the importance of ED assessment, diagnosis, and admission to an OR within six to eight hours from time of injury.
This patient was taken promptly to the OR where he underwent a four-compartment fasciotomy, saphenous vein graft to the femoral artery, and external knee fixator placement. Unfortunately, he lost the pulse in his foot six days into his hospital stay, and after exploration in the OR, the graft was deemed a failure. He then underwent an above-the-knee amputation.
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