All About Knee Dislocations

One of only a handful of injuries that prompts the orthopedics resident to call the attending immediately at any hour of the day, a knee dislocation is a serious injury with high rates of neurovascular injury. Whether your hospital has orthopedics on call or not, it is prudent as an emergency physician to be knowledgeable on this injury and its management. Often misdiagnosed as compartment syndrome or missed altogether, knee dislocations are not always as straightforward to diagnose as one would think. I will talk about all this and more below, going more in depth on the thing that matters most in these patients – vascular injury.

Limb-threatening injury

• Delay of popliteal artery repair for more than 8 hours invariably leads to limb amputation

• Posterior knee dislocations (see picture) have the highest rates of neurovascular injury. By convention anterior versus posterior is decided based on the tibia’s position relatively to the femur.

Radiographic evidence of knee dislocation

Spontaneous reduction

• Spontaneous reductions occur in as many as 50% of knee dislocations

• Despite spontaneous reduction, patients are still at increased risk of neurovascular injury, so look for other clues

  • Hematoma leak into calf or thigh may be from joint capsule disruption due to dislocation

  • Usually 3+ ligaments (ACL, PCL, LCL, MCL) must rupture to allow for dislocation, so multiplanar instability must be assumed to be a dislocation that spontaneously resolved

  • Evidence of peroneal nerve injury (inability to dorsiflex foot / toes, numbness / tingling along dorsum of foot or shin)

Vascular injury

• No physical exam finding reliably rules out popliteal vascular damage

  • Warm skin reported in cases of complete popliteal artery occlusion

  • Good DP/PT pulses post reduction do not exclude vessel injury

    • May still have occult intimal tear > increased risk of thrombus in coming hours to days

• Ankle-Brachial Index

  • The gold standard for assessing arterial damage.

  • Get a doppler and find the dorsalis pedis pulse. Inflate a manual blood pressure cuff over the calf. Slowly deflate it until you start hearing a doppler pulse. Repeat this process with the posterior tibialis pressure. Use the higher of these two values in the numerator for the formula below.

  • Get a doppler and find the brachial artery. Inflate the blood pressure cuff over the bicep. Repeat this for both arms and use the higher of these two values in the denominator of the formula below.

• Normally the ankle should have slightly higher pressure than the arm

  • If ABI >0.9 admit for serial examinations, consult ortho

  • If ABI <0.9 obtain a CTA, consult ortho and vascular surgery

  • If there’s hard signs of vascular injury (expanding hematoma, bruit/thrill, or pulsatile bleeding) consult vascular surgery immediately

The reduction

• Should be the next step immediately after performing a thorough neurovascular exam in a suspected dislocation or multi-ligament injury

• Apply longitudinal traction, and in the case of a posterior dislocation, for example, apply anterior-facing tibial force while an assistant applies posterior-facing femur pressure

The “dimple sign”

• A sign of posterolateral knee dislocation

• Except for a few case reports, believed to be an irreducible dislocation

  • Attempts at reduction may cause new or worsening neurovascular injury with obsolete efforts at reduction

  • Consult orthopedics early

Radiographic evidence of posterolateral knee dislocation with corresponding dimple sign on physical exam

About the author

Bernhard Wolmarans is a first-year resident in the emergency medicine program at USF. He completed medical school at UF.

Post edited by Michael Weaver.