A patient with knee pain and is unable to bear weight
A 58-year-old woman presents to the emergency department with an acute right knee injury. She reports “stepping wrong” and twisting it with a painful “pop” of the knee. She has been unable to bear weight on the extremity since the injury. Her medical history is remarkable only for morbid obesity. No additional injuries are found on exam.
Portable X-ray images taken of the patient’s knee are shown below. Which of the following findings is the strongest indication to obtain a CT angiogram of the knee?
- Ankle-brachial index of 0.7
- Ankle-brachial index of 1.0
- Inability to extend at the knee
- Laxity with valgus stressing of the knee joint
- Painful “click” with passive manipulation of the knee
Correct answer: A. Ankle-brachial index of 0.7
A ratio of ankle systolic blood pressure to brachial systolic blood pressure of <0.9 suggests diminished arterial flow to the lower extremity and could indicate arterial injury. As many as 50% of knee dislocations will be spontaneously reduced prior to arrival in the ED. These radiographs show a knee effusion, but normal anatomical alignment of the bones; there is no evidence of fracture.
Although knee dislocations have historically been viewed as a “high-energy” mechanism of injury, such as suffering an MVC, it is increasingly described in the morbidly obese population given the force with which a simple buckling fall can stress the knee joint. Upon identifying concern for arterial injury, with essentially normal radiographs, arterial imaging should be undertaken — ideally with CT angiogram of the extremity. Below are radiographs demonstrating the knee dislocation prior to reduction.
Incorrect answer choices:
Ankle-brachial index of 1.0 (Choice B) is a normal finding (it denotes the same systolic blood pressure in the arm and the leg). This would be reassuring that sufficient arterial flow to the lower extremity, distal to the site of injury, is intact. If concern for knee dislocation persists, a CT angiogram could be undertaken regardless; alternatively, admission with serial vascular exam and serial ABI monitoring can be explored.
Inability to extend the knee (Choice C) is a very nonspecific finding that could indicate any painful knee condition, or could localize to an extensor function disruption. An isolated extensor function impairment suggests a patellar tendon rupture or complete patella fracture (which would be seen on X-ray, especially with a sunrise view), which would require immobilization in extension and orthopedic surgical management. An MRI would probably be obtained outpatient, but a CT scan would not be especially helpful for soft tissue (tendon) injury.
Laxity with valgus stressing of the knee joint (Choice D) suggests disruption of the MCL. This would be a more common injury secondary to painful twisting injury to the knee than knee dislocation and arterial injury. Although an MRI of the knee is indicated to better characterize such an injury, this is not needed emergently.
A knee with suspected intrinsic ligament injury (e.g., ACL, PCL, MCL) that is neurovascularly intact can be splinted and discharged from the ED for close orthopedic surgery follow-up.
Painful “click” with passive manipulation of the knee (Choice E) suggests a possible meniscus injury. This is often associated with an MCL injury and is often from a twisting mechanism of injury.
Two specific exam maneuvers used to specifically test the meniscus integrity is the Apley Test (patient prone, axial loading and rotation of the flexed knee) and the McMurray Test (patient supine, hip and knee in flexion to 90 degrees with compression of the knee joint while rotating of the lower leg). Like other injuries to the ligamentous structures of the knee, this can typically be discharged from the ED.
