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Difficulty: MediumOrthopedic Surgery

During a collegiate rugby match, an 18-year-old athlete sustains a high-impact direct blow to the anterior right proximal tibia while the foot is firmly planted on the turf. The athlete falls to the ground in severe distress and is unable to bear weight. On examination in the emergency department 45 minutes later, there is a prominent step-off at the right knee joint, with the tibia displaced posteriorly relative to the femur. The overlying skin is intact. The patient reports severe pain but denies any numbness or tingling in the foot. Dorsalis pedis and posterior tibial pulses are palpable and symmetric to the uninjured leg. Capillary refill is less than 2 seconds. A bedside closed reduction is performed promptly under conscious sedation, successfully restoring normal anatomic alignment of the knee. Post-reduction, the distal pulses remain palpable and symmetric. Which of the following is the most appropriate next step in the management of this patient?

  1. Measure the ankle-brachial indexAnswer
  2. B
    Obtain magnetic resonance imaging of the right knee
  3. C
    Perform immediate surgical exploration of the popliteal artery
  4. D
    Apply a rigid circumferential long-leg cast
  5. E
    Administer prophylactic intravenous broad-spectrum antibiotics

Answer

Measure the ankle-brachial index
Knee dislocations involve extreme displacement of the tibia relative to the femur, which exerts tremendous sheer and traction forces on the tethered popliteal artery. Because up to one-third of knee dislocations are associated with popliteal artery injury, ruling out vascular compromise is the most critical next step. The presence of palpable distal pulses and brisk capillary refill are reassuring 'soft signs,' but they do not definitively exclude an intimal tear, which can progress to complete thrombosis and limb ischemia. Therefore, the standard of care is to measure the ankle-brachial index (ABI) in all patients with a knee dislocation and normal pulses. If the ABI≥0.9ABI \geq 0.9, the patient can be managed with serial neurovascular checks. If the ABI<0.9ABI < 0.9, or if distal pulses are asymmetric, computed tomography (CT) angiography is required.

Step-by-Step Solution

1
Identify the injury based on the mechanism and physical exam findings.
The patient suffered a posterior knee dislocation, which was successfully reduced.
Recognizing the injury pattern is crucial because knee dislocations are high-energy injuries tightly associated with specific neurovascular complications.
2
Assess the risk of associated complications.
The popliteal artery is firmly anchored proximally and distally to the knee joint, making it highly susceptible to stretch, intimal tear, or complete transection during a dislocation.
Vascular compromise is the most limb-threatening complication of a knee dislocation.
3
Determine the appropriate diagnostic algorithm for a patient with palpable pulses post-reduction.
Even with palpable pulses (which can be maintained by collateral circulation or partial tears), an ankle-brachial index (ABI) must be measured.
An ABI ABI≥0.9ABI \geq 0.9 can safely allow for serial observation, while an ABI<0.9ABI < 0.9 mandates computed tomography (CT) angiography to locate the vascular injury.

Key Concept

All knee dislocations require mandatory vascular assessment with an ankle-brachial index (ABI), even if distal pulses are palpable, to rule out occult popliteal artery injury.
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Orthopedic Surgery

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