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Difficulty: MediumOrdering Imaging Studies

During a local basketball tournament, a 24-year-old male athlete lands awkwardly on his left leg. He feels a distinct "pop" followed by immediate pain and rapid joint swelling. Two days later, he presents to an ambulatory clinic. He is unable to bear full weight due to a feeling that the knee will "give out." His medical history is unremarkable, and vital signs are within normal limits. Physical examination reveals a large left knee effusion. Active and passive flexion are limited to 45 degrees secondary to the swelling. The Lachman test demonstrates increased anterior translation of the tibia relative to the femur without a firm endpoint. Posterior drawer and varus/valgus stress testing are unremarkable. Which of the following is the most appropriate initial imaging study for this patient?

  1. Plain radiography of the kneeAnswer
  2. B
    Magnetic resonance imaging of the knee without contrast
  3. C
    Computed tomography of the knee without contrast
  4. D
    Musculoskeletal ultrasonography of the knee
  5. E
    Radionuclide bone scintigraphy

Answer

Plain radiography of the knee
The patient's presentation is classic for an anterior cruciate ligament (ACL) tear. However, the standard of care for acute knee trauma requires plain radiography as the initial imaging modality. This is explicitly supported by the Ottawa Knee Rules, which mandate knee radiographs for patients with acute injury who are unable to flex the knee to 90 degrees. Plain films are necessary to identify associated bony injuries, such as a Segond fracture (lateral capsular avulsion) or a tibial spine avulsion fracture, before proceeding to definitive soft-tissue evaluation.

Step-by-Step Solution

1
Analyze the clinical presentation.
The patient has an acute knee injury with a 'pop', rapid hemarthrosis, and a positive Lachman test, strongly suggesting an anterior cruciate ligament (ACL) tear.
Identifying the suspected primary pathology guides the definitive imaging required, but acute trauma protocols must still be followed.
2
Evaluate the need for initial structural assessment based on clinical decision rules.
The patient's inability to flex the knee past 45 degrees strictly satisfies the Ottawa Knee Rules for obtaining imaging.
Evidence-based guidelines dictate when imaging is necessary to avoid missing occult fractures.
3
Determine the correct sequence of diagnostic testing.
Despite high suspicion for a soft-tissue injury requiring advanced imaging, plain radiography must be ordered first.
Basic radiographs are required to rule out associated fractures (e.g., tibial spine avulsion, Segond fracture, or patellar fracture) before obtaining highly sensitive, expensive imaging.

Key Concept

In the evaluation of acute knee trauma with suspected internal derangement, standard cost-effective imaging (plain radiography) must be obtained initially to rule out fractures before proceeding to advanced imaging modalities.
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Ordering Imaging Studies

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