EM@3AM: Knee Dislocation

Author: Rachel Bridwell, MD (@rebridwell, EM Resident Physician, SAUSHEC / San Antonio, TX) // Edited by: Brit Long, MD (@long_brit, EM Attending Physician, San Antonio, TX)  and Alex Koyfman, MD (@EMHighAK, EM Attending Physician, UTSW / Parkland Memorial Hospital)

Welcome to EM@3AM, an emDOCs series designed to foster your working knowledge by providing an expedited review of clinical basics. We’ll keep it short, while you keep that EM brain sharp.

A 21-year-old male is brought to the ED by EMS after an MVC at 60 mph. The patient was a restrained, and airbags deployed with the car traveling at 50 mph. Another passenger reports that the patient hit his head on the seat in front of him. At the scene, the patient complained of severe right lower extremity pain. EMS splinted the leg in an air splint.

Triage vital signs (VS): BP 130/80, HR 108, T 98.1, RR 19, SpO2 99% on room air, GCS 15. The patient appears in pain. You cannot find significant injuries on your exam, other than the right leg. Upon removal of the right lower extremity splint, the patient has a clear deformity. You cannot detect a dorsalis pedis or posterior tibialis pulse on the right.

What’s the next step in your evaluation and treatment?

Answer: Tibiofemoral knee dislocations1-15


  • 2 types of knee dislocations
    • Patellofemoral
    • Tibiofemoral
  • High energy trauma
    • Low energy mechanism in the obese e.g. stepping off a curb1
  • Obesity is an independent risk factor; nearly 50% of knee dislocations occur in BMI <40%1
  • Many dislocations spontaneously reduce prior to evaluation



  • Requires complete disruption of multiple ligaments (ACL, PCL, MCL, LCL)
  • Popliteal artery tethered posteriorly; high risk of vascular injury
    • 40% of patients will sustain a vascular injury2
  • Peroneal nerve runs in close proximity at fibular neck
    • 20% of tibiofemoral dislocations will have peroneal nerve damage, which can present with foot drop2


Clinical Presentation:

  • Classification: open or closed
    • 30-50% anterior: hyperextension damaging posterior structures, with femur posterior to tibia3
      • 39% of anterior dislocations have popliteal artery injury4
    • 25% posterior: dashboard injury where axial load on flexed knee displaces tibia posterior to femur3
      • 44% of posterior dislocations have popliteal artery injury4
    • 13% lateral: varus force to proximal tibia
      • Highest risk of peroneal nerve damage3
      • Lowest risk of popliteal injury at 6%4
    • Medial: most rare, valgus force to proximal tibia, most commonly the body rotates the opposite direction of the planted foot3
      • 35% of medial dislocations have popliteal artery injury4
    • Also consider if multidirectional instability, report of knee buckling with evidence of foot drop



  • Assess ABCs with potential for other injuries in a potential polytrauma patient if high energy mechanism
  • Inspect knee joint as well as hip and ankle
    • Place towel under femur to flexion knee 20 degrees, which will assist your exam
    • Check for multidirectional instability, as spontaneous reduction is common
    • Compare to contralateral side
      • In obese patients with low mechanism of injury and spontaneous reduction, lifting both legs off the bed by the heel may show hyperextension of affected side
        • Up to 50% with spontaneous reduction prior to presentation
      • If dashboard or floorboard injury, tibial or talar fractures may be present
    • Vascular assessment
      • Start with vascular assessment and assessing pulses
        • Dorsalis pedis and posterior tibialis
          • 23% of knee dislocations have popliteal artery injury5,6
        • If hard findings of vascular injury, immediate operative repair required7
          • Hard findings: pulse deficits, distal ischemia, active hemorrhage, rapidly expanding hematoma
        • If soft signs, CT angiography
          • Soft signs: cooler, decreased cap refill,
        • Check ABIs
          • >0.9—100% accurate in excluding vascular injury8
          • <0.9—100% sensitivity and specificity for vascular injury8
        • Neuro: Focal neurologic deficits in L4-S1 distribution
      • Imaging: XR and CT angiography
        • XR: can show dislocation along with secondary avulsions or osteochondral defects9
          • Segond sign—lateral tibial condyle avulsion fracture9
          • 53% fractures have concomitant dislocations6
        • CT angiography:
          • Popliteal injury can present similar to compartment syndrome, though does not improve with fasciotomy 10
            • Intimal tears common, but not all of them will require operative repair; anticoagulation is recommended
          • MRA: shows both vascular as well as ligamentous injury, though temporally and monetarily costly
            • Time delay risk outweighs benefit, increasing time to surgical management and therefore increase in risk for amputation11



  • Reduction: immediate reduction important, especially with neurovascular deficit present, as >8 hours has been associated with increased rates of amputation12
  • Longitudinal traction to tibia13
    • Anterior: lift distal femur and posteriorly push tibia
    • Posterior: Lift tibia anteriorly and put pressure on distal tibia
    • Rotational: Rotate tibia towards natural position
    • TKA dislocations: while rare, more commonly posteriorly, associate neurovascular injury with difficult reduction due to vertical post, consult ortho14,15
  • Check pulses and ABIs after reduction



  • Consult vascular surgery
    • If ABI<0.9, obtain CT angiography
    • If ABI>0.9, admit for serial ABIs and compartment checks, may need CT angiography
      • If normal distal pulses, well perfused limb, and serial ABI>0.9, can consider 24 hours of vascular checks without angiography



  • Occurs in both high energy traumas as well as low energy mechanism in the obese
  • Lift extended leg to assess for hyperextension in a spontaneously reduced knee
  • Distal pulses can be present even if vascular damage as occurred
  • If hard signs of vascular damage present, immediately precede to the OR
  • Admit for serial evaluations with high risk of compartment syndrome

A 17-year-old football player presents after sustaining a knee injury during his game earlier in the evening. He has no medical problems and is not on any medications. He states he was running with the ball when he felt a “pop.” Immediately after he had severe pain in his right knee, was unable to walk initially, and now has a noticeable limp. On exam, the patient has intact strength with knee extension and flexion. He has intact range of motion with exacerbation of symptoms during range of motion. There is no laxity noted with valgus or varus testing, nor is there appreciable joint line tenderness. With the patient’s knee flexed to 30 degrees, there is increased translation of the proximal tibial anteriorly compared to posterior translation. What is the most likely diagnosis?

A) Anterior cruciate ligament tear

B) Lateral collateral ligament tear

C) Meniscus tear

D) Patellar tendon rupture



Answer: A

The anterior cruciate ligament (ACL) is an important ligament for stabilizing the knee and is frequently injured by athletes and trauma victims. The primary function of the ACL is to control anterior translation of the tibia. It also prevents tibial rotation and some varus or valgus strain. The ACL is the most commonly injured ligament in the knee and the majority of tears occur from noncontact athletic injuries. Women and girls are at greater risk for ACL injury compared to men and boys. Other risk factors include quadriceps-dominant deceleration, valgus knee angulation during pivoting, deceleration or landing from great heights, effects of estrogen, decreased intercondylar notch width, and discrepancies in Q-angle and bone length. Patients may present with a recent history of direct trauma to the affected knee or there may be no history of trauma. Noncontact ACL tears predominate with a recent history of sudden deceleration, changes in directions, pivots, or lateral bending. Contact injuries are typically seen when the knee is forced into hyperextension or valgus stress. Patients often describe the feeling of a “pop” and have the sensation of instability. Acute swelling is typically seen with a knee effusion, usually secondary to hemarthrosis due to the ACL tear. Three physical exam maneuvers help test the ACL stability: the Lachman test, pivot shift, and the anterior drawer. The Lachman test is performed with the knee flexed to 30 degrees, followed by stabilization of the distal femur with one hand while pulling the proximal tibia anteriorly with the other hand. The pivot shift test is performed by having the clinician hold the lower leg with one hand and internally rotating the tibia, while applying a valgus stress to the knee with the other hand. Then, while maintaining these forces, the clinician flexes the knee, and in ACL-deficient patients, this causes a reduction of a subluxed tibia resulting in a “clunk.” This test is highly specific for ACL rupture, but is very difficult to perform in an awake patient due to patient guarding and pain. The anterior drawer test is performed with the patient lying supine and the knees flexed to 90 degrees. The proximal tibia is grabbed with both hands and pulled anteriorly. The clinician will often sit on the patient’s feet to provide better stability. Plain X-rays are often performed in the acute setting to evaluate for fractures and are typically normal. In some cases an avulsion fracture (Segond fracture) can be noticed on plain X-rays of the knee near the lateral tibial plateau. MRI is the test most often used to help diagnose a suspected ACL tear, as this helps confirm the diagnosis and evaluate for any other knee soft tissue injury. Management acutely focuses on rest, ice, compression, and elevation. Crutches should be given to remain nonweight bearing and nonsteroidal anti-inflammatory drugs (NSAIDs) used for pain relief. Follow-up with an orthopedic surgeon should occur to determine if the patient is an appropriate candidate for surgical correction.

A lateral collateral ligament (LCL) tear (B) is among the least common knee injuries and usually occurs with extreme varus stress (e.g., knee being struck medially). The LCL is more commonly injured along with other structures, and not in isolation. Patients with an LCL rupture demonstrate laxity of the knee with varus stress testing. The patient above does not have this on physical exam, therefore, LCL injury is much less likely. A meniscus tear (C) is a common concomitant injury, however, it produces joint line tenderness on exam, and it alone should not result in anterior translocation laxity. A patellar tendon rupture (D) is more common than a quadriceps tendon rupture and occurs generally in the setting of sudden, strong contraction of the quadriceps muscle. Landing from a high jump or making a sudden change in direction are common inciting events. A sudden “pop” sensation, followed by swelling and difficulty in bearing weight, is common. In partial tears, patients will have limited knee extension and in complete tears, knee extension will be absent. An elevated patella on lateral knee X-ray suggests rupture. The patient above has intact knee extension, so patellar tendon rupture is less likely.

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