Review Question - QID 215736

This patient has suffered an anterior knee dislocation, which after reduction warrants an ankle-brachial index (ABI) evaluation for potential vascular injury. ABIs are performed by taking the systolic pressure at the ankle divided by the brachial systolic pressure with normal values ranging from 0.9-1.1.

Knee dislocations are rare injuries with potentially devastating neurologic and vascular complications. Typically the result of high energy mechanisms, a missed arterial injury can be a threat to both life and limb. The appropriate management of knee dislocations includes prompt identification of the injury, reduction, and a thorough neurovascular assessment, including ABIs. Calculating ABIs allows for the identification of subtle vascular injuries and is a screening tool to determine whether further workup is warranted, including angiography and vascular consultation.

Weinberg et al. found increased BMI and open injuries to be significant patient factors predictive of vascular injury after knee dislocations. The authors stress the importance of an ABI workup as the combination of a normal palpable pulse exam and an ABI ≥ 0.9 has 100% sensitivity for ruling out a vascular injury.

Medina et al. systematically reviewed the literature regarding knee dislocations. They reported a vascular injury frequency of 18% and a nerve injury frequency of 25%. Of the vascular injuries, 80% required repair and 12% necessitated amputation. Due to the profound morbidity, the authors conclude that clinicians must be especially vigilant in their evaluation.

Figure A is a lateral radiograph depicting an anterior knee dislocation. Illustration A depicts the correct steps in obtaining ABIs.

Incorrect Answers
Answers 2,3,4,5: ABIs are performed by taking the systolic pressure at the ankle (either posterior tibialis or dorsalis pedis) and dividing it by the brachial systolic pressure with normal values ranging from 0.9-1.1.