55 year old female presenting with left knee pain after a fall. Patient appears intoxicated and states she tripped and fell. No preceding symptoms or head trauma. Pain is 5/10. She has not been able to ambulate since the incident. On exam the patient is neurovascularly intact. During your exam patient attempts to lift her leg off the stretcher and you observe this finding:

The patient rests her leg back on the stretcher with immediate reduction. What is the most appropriate imaging study to obtain at this time?

A. Xray of the femur, knee, and tibia/fibula

B. MRA of the knee

C. Ultrasound with PVR

D. CTA of the extremity

Answer: D. CTA of the extremity

The most common type of knee dislocation (named for direction of the tibia) is anterior (40%) and second most common is posterior (33%). Less commonly seen are lateral (18%) and medial (4%) dislocations. Typically dislocations occur during high-velocity mechanisms such as motor vehicle collisions. They are associated with significant damage to the ligaments, nerve, and vascular structures. Damage to the popliteal artery is particularly concerning because it can occur quickly and lead to limb ischemia in one third of cases. Management includes prompt reduction of the limb by applying traction then splinting the leg in twenty degrees of flexion. Knee immobilizers can be used for quick stabilization while obtaining splinting supplies. CTA is the best imaging modality to assess for damage to the popliteal artery. Both orthopedic surgery and vascular surgery are typically involved in surgical intervention. These patients must be admitted for neurovascular checks. 

Bjoernsen, L. P., & Ebinger, A. (2016). Section 22: Injuries to Bone and Joints. In Tintinalli's Emergency Medicine: A Comprehensive Study Guide (8th ed., pp. 1863-1864). New York, NY: McGraw-Hill Education.

Duprey, K & Lin, M. " Posterior Knee Dislocation." WestJEM. 2018. <https://westjem.com/articles/posterior-knee-dislocation.html>. 12 Aug 2020. 

Faust, J & Westafar, L. "Episode 22- The Knee." FOAMCAST. 12 Jan 2015. <https://foamcast.org/tag/knee-dislocation-in-the-ed/>. 12 Aug 2020.