What's the diagnosis? By Dr. Angela Ugorets

A 26 yo male presents to the ED with severe left leg pain after he slid into home plate during a softball game.  He was unable to get up and walk afterward.  He has an obvious left lower extremity deformity.  An x-ray is shown.  What's the diagnosis?  Scroll down for answer.






Answer:  Anterior knee dislocation

  • Usually the result of high force injury like MVC ("dashboard knee"), can also occur during sports (hyperextension) or with minor trauma in the morbidly obese
  • Feared complicatirons are popliteal artery injury (~1/3 of dislocations) and peroneal nerve injury
    • popliteal artery is tethered in the popliteal fossa behind the knee
  • Management
    • Pain control and neurovascular exam - check and document dorsalis pedis and posterior tibial pulses
    • X-ray (at least 2 view)
    • Reduction (splint in 20 degrees of flexion)
    • ABI - ankle brachial index*
    • Most specialists will also recommend CTA of leg
    • Orthopedics consultation (+/- vascular depending on result of ABI and CTA)
    • Admission for serial neurovascular checks 
  • Pitfall: some patients may come to the ED after dislocation and spontaeous reduction, exam will be an unstable knee; do not miss an occult popliteal artery injury, patient requires x-ray, ABI, splinting, CTA, and orthopedics evaluation




Over-inflate the BP cuff on the arm, place the doppler over the brachial artery, deflate the cuff slowly until you hear the doppler sound of the brachial artery pulse. This is the systolic pressure.

Again, use the blood pressure cuff on the ankle of the affected leg to measure the systolic pressure of BOTH the DP and PT arteries separately.
How: Again, over-inflate the BP cuff, place the doppler over the DP artery, deflate slowly until you hear the doppler sound of the pulse. This is the systolic pressure. Repeat this with the doppler over the PT artery.
Divide the highest of the two ankle arterial pressures you obtained (either DP or PT, whichever was higher) by the highest of the two brachial pressures you obtained (ether left or right arm, whichever was higher).
Highest ankle pressure on affected leg/highest brachial pressure on either arm = ABI
ABI > 0.9 is normal.
ABI <0.9 is abnormal.
Suk, Michael, et al. “Knee Dislocation.” Orthobullets, 18 Feb. 2020, www.orthobullets.com/trauma/1043/knee-dislocation.
Tintinalli, Judith E., et al. Tintinalli's Emergency Medicine: a Comprehensive Study Guide. McGraw-Hill, 2016.
Verghese, Abraham. “Ankle Brachial Index.” Stanford Medicine 25, 2020, stanfordmedicine25.stanford.edu/the25/ankle-brachial-index.html.