Today’s Morning Report comes courtesy of Dr. Melendez:
Posterior Knee Dislocation
- Approximately 30 percent rate of amputation because the popliteal artery is interrupted and sometimes goes unrecognized
- In some instances, a knee will dislocate and relocate prior to medical evaluation. For this reason, it is quite important to evaluate distal pulses for deficit on all patients whom present with knee trauma.
- If a knee dislocation is suspected, all patients should receive x-ray as well as vascular evaluation by way of testing the ankle-brachial index (ABI).
- Get vascular and ortho on board
- Reduction of knee is an emergent procedure and will often improve the vascular flow to the distal lower extremity. In many cases, minimal procedural sedation is necessary as the peroneal nerve is affected as well. Patients will have minimal to no sensation prior to reduction.
- ABI: Briefly, the ankle-brachial index compares the Doppler pressure of an arm to a leg to screen for lower limb ischemia. This straightforward measurement is performed by recording the highest Doppler sound of the brachial pulse and comparing it to the highest Doppler sound of the posterior tibial or dorsalis pedis artery. The ankle Doppler pressure is then divided by the brachial Doppler pressure to calculate the index. Indexes less than 0.9 indicate an abnormal result and should prompt further vascular imaging/assessment.
- Duplex/ultrasonography: This is a reliable, noninvasive, low-risk, low-cost option. Duplex ultrasonography appears to be an excellent modality for vascular injury assessment. Fry et al reported 100% sensitivity and 97% specificity for clinically significant arterial injury. This modality only incurs about 10% of the cost of arteriography with little to no risk profile.
- CT angiography: CT angiography is another reliable alternative to arteriography without the risk of direct arterial injury. It does require additional contrast beyond that used for chest/abdomen/pelvis body CTs that are often also indicated in these types of trauma cases; thus, it may have added risk of nephropathy or contrast reactions over arteriography, which uses less contrast. Sens 95-100%, Spec 98-100%
- Direct arteriography: This is still the criterion standard but carries risk of arterial injury from direct catheterization of the artery while also requiring specialist involvement to perform (ie interventional radiologist or vascular surgeon).
- When vascular occlusion is found on CT angiography, the patient is taken to the operating room for an open bypass. Endovascular repair is not favorable because the place of occlusion is typically at the crease of the knee and there are often complications because of occlusion months to years post-op.
How to reduce a posterior dislocation?
- Longitudinal traction often works
- After reduction, splint the lower extremity in approximately 20 degrees of flexion to avoid post-reduction re-dislocation, apply ice, and keep the knee elevated.
- Post-reduction radiographs should be obtained, preferably before further ligamentous stressing/assessment.
- On occasion, external fixation will be required after vascular repair to keep all vasculature patent.
Thanks Dr. Melendez!
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The views expressed on this blog are the author's own and do not reflect the views of their employer. Please read our full disclaimer here. Any references to clinical cases refer to patients treated at a virtual hospital, Janus General Hospital.
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Jay Khadpe MD
Editor in Chief of "The Original Kings of County"
Assistant Professor of Emergency Medicine
Assistant Residency Director
SUNY Downstate / Kings County Hospital
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