Knee Dislocation

This is the 501st PedEM Morsel! (While I missed the opportunity to buy myself a large cake and eat it all last week, I am willing to accept any gifts of cake you would like to send to me… it will help with the self-isolation.) As you are all aware, a common theme of the PedEMMorsels has always been the need to stay vigilant when caring for children. Kids certainly can present with severe conditions masked as benign entities. The other challenge is the fact that, fortunately, the severe conditions are usually more rare in children (so we may become complacent). For this 501st Morsel, let us look at a rare problem that if not appreciated early can lead to devastating outcomesKnee Dislocation:

Knee Dislocation: Basics

  • Knee Dislocations are rare in all ages, but true incidence in children is unknown. [Mayer, 2015; Boyce, 2015]
    • Estimated to account for 0.02% of all orthopaedic injuries.
    • Most common in young adults.
    • True incidence is likely underestimated, as they may spontaneous reduce before evaluation.
  • Knee Dislocations can be due to both High and Low energy mechanisms! [Mayer, 2015; Boyce, 2015]
    • High: MVCs, falls, crush injuries, pedestrian strike
    • Low: Sports, martial arts kicks, missteps (especially if morbidly obese), trampolines (another great reason to hate them)
  • Knee Dislocation = disruption of the tibiofemoral articulation [Mayer, 2015; Boyce, 2015]
    • Terminology may inappropriately be used for Patellar Dislocations (which are distinctly different!).
    • Can be classified by direction of dislocation or severity of ligamentous injury.
  • Knee Dislocation Classification by Direction of dislocation [Mayer, 2015; Boyce, 2015]
    • Anterior (most common)
      • Hyperextension mechanism
      • Both ACL and PCL often injured
    • Posterior (2nd most common)
      • Force directed posteriorly onto proximal tibia.
      • “Dashboard” injury from MVC
    • Lateral
    • Medial
    • Rotatory
    • Spontaneously reduced dislocations make this classification system more challenging to identify.
  • Knee Dislocation Classification by Severity of Ligamentous Injury [Mayer, 2015; Boyce, 2015]
    • Knee Dislocation (KD) I: single cruciate ligament injured
    • KD II: bi-cruciate injury
    • KD III: bi-cruciate and medial or lateral collateral ligaments injuries
    • KD IV: bi-cruciate and both medial and lateral collateral ligament injuries
    • KD V: fracture-dislocation
    • Can be further sub-classified whether there is presence of nerve (N) or vascular (C) compromise.
    • Obviously, requires understanding of the soft tissues involved, so likely classified after our evaluation in the ED.

Knee Dislocation: Evaluation

  • Assess as you would any traumatized patient!
    • ABCDEs first! Don’t get distracted by the ugly extremity.
    • While knee dislocation can occur via low-energy mechanisms, start by assuming the worse.
    • There is a high incidence of concomitant injuries to other regions. [Chowdhry, 2020]
  • Vulnerable Vasculature [Mayer, 2015; Boyce, 2015; Medina, 2014]
    • Popliteal artery is vulnerable as it courses through popliteal fossa.
      • Anterior dislocation can lead to traction of the vessel -> intimal damage
      • Posterior dislocation can transect the vessel.
      • Geniculate arterial branches provide collateral flow, but are not sufficient to compensate for injury to the popliteal artery.
    • Evaluation of circulatory status is paramount!
      • Quality of distal pulses is important.
      • Ankle-Brachial (or Ankle-Ankle) Index (ABI or AAI)
        • Often used to assess circulatory status for adults with chronic vascular disease or acute traumatic injuries to extremities.
        • If Systolic Pressure of Ankle / Systolic Pressure of Arm is < 0.9, then there is concern for possible vascular injury.
          • Place BP cuff above ankle and use doppler to detect arterial flow.
          • Inflate cuff until pulse disappears and then slow deflate cuff.
          • Systolic BP is the noted measurement when doppler pulse returns.
          • Repeat for other extremity.
          • Some literature supports using cutoff of 0.7, but not specifically for pediatric knee dislocations (so, right now I’d still use 0.9 myself).
  • Don’t Be NERVous, but don’t forget the Nerves either! [Mayer, 2015; Boyce, 2015]
    • The Sciatic nerve divides into the tibial nerve and the common peroneal nerve.
    • The common peroneal nerve is the most commonly injured during posterolateral dislocations.
    • Nerve injury is more likely related to over-stretching rather than transection.
    • Presence of nerve injury, increases risk for vascular injury.
    • Any symptoms of paresthesias or sensory deficits need to be taken seriously.
  • Compare the Compartments!
    • As with any extremity injury, consider compartment syndrome.
    • Compartment Syndrome may present differently in young children.

Knee Dislocation: Management

  • Know the vascular and neurologic status of the extremity. [Mayer, 2015; Boyce, 2015]
    • Hard signs of vascular injury require surgical intervention and intra-operative angiogram.
    • Hard Signs = Pulseless, Pallor, Coolness, Delayed Cap Refill, Pulsatile Hematoma
  • If dislocation still present, will need to reduce. [Mayer, 2015; Boyce, 2015]
    • Re-create the deformity, apply tractional force, and gently manipulate the tibia relative to the femur.
    • DO NOT forcefully hyperextend or posteriorly translate the tibia as this will further endanger the vessels and nerves.
    • Puckered skin may be a clue that the injury is irreducible (entrapped soft tissues) and needs to go to the OR.
  • Know the vascular and neurologic status after the reduction! [Mayer, 2015; Boyce, 2015]
    • If ABI < 0.9, you need to image!
    • CT with Angiogram is currently favored.
    • Duplex Sonography has also shown promising results.
    • MRI will be eventually needed to help determine soft-tissue injuries.
  • Place in knee immobilizer.
  • If ABI > 0.9, then admit for close observation and repeat vascular assessments.

Moral of the Morsel

  • It may be worse than you suspect! The dislocated knee may have spontaneously reduced. Evaluate all extremity injuries thoroughly!
  • It’s not just about the bones and their attachments! Blood vessels and nerves are at great risk in this region!
  • Pulses and Pressures Please!! Don’t be reassured by the warmth alone. Check the quality of the pulses and check comparison systolic pressures (ankle-ankle or ankle-brachial index)!


Sean M. Fox
Sean M. Fox

I enjoy taking care of patients and I finding it endlessly rewarding to help train others to do the same. I trained at the Combined Emergency Medicine and Pediatrics residency program at University of Maryland, where I had the tremendous fortune of learning from world renowned educators and clinicians. Now I have the unbelievable honor of working with an unbelievably gifted group of practitioners at Carolinas Medical Center. I strive every day to inspire my residents as much as they inspire me.

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