What diagnostic tool is used to determine if a Magnetic Resonance Imaging (MRI) of the knee is needed?

From the Guidelines

The Ottawa Knee Rules are the primary diagnostic tool used to determine if an MRI of the knee is needed following injury. These clinical decision rules help healthcare providers assess whether imaging is necessary by evaluating specific criteria: age 55 years or older, tenderness at the head of the fibula, isolated tenderness of the patella, inability to flex the knee to 90 degrees, inability to bear weight both immediately after the injury and during examination (four steps), or inability to transfer weight twice onto each lower limb. If any of these criteria are met, further imaging including MRI may be warranted.

Key Considerations

• Healthcare providers may also use the patient's history, physical examination findings, and the mechanism of injury to guide their decision 1.
• MRI is particularly valuable for evaluating soft tissue injuries such as meniscal tears, ligament injuries, and cartilage damage that cannot be visualized on X-rays.
• According to the American College of Radiology, radiographs of the knee are usually appropriate for the initial imaging of chronic knee pain in patients greater than or equal to 5 years of age 1.
• The use of MRI knee without IV contrast may be appropriate in patients with chronic knee pain when initial knee radiographs are normal or demonstrate a joint effusion 1.

Diagnostic Approach

• The diagnostic approach should prioritize the use of clinical decision rules, such as the Ottawa Knee Rules, to determine the need for imaging.
• If imaging is necessary, radiographs of the knee should be the initial imaging modality, followed by MRI if necessary.
• The decision to use MRI should be based on the patient's clinical presentation, the results of the initial radiographs, and the presence of specific criteria that suggest the need for further imaging 1.

From the Research

Diagnostic Tools for Determining the Need for an MRI of the Knee

• The Ottawa Knee Rules are a validated clinical decision rule used to determine the need for radiography in patients with acute knee injury 2, 3, 4.
• The rules consist of five criteria: injury due to trauma and age >55 years, tenderness at the head of the fibula or the patella, inability to bear weight for 4 steps, or inability to flex the knee to 90 degrees 2, 3, 4.
• A careful physical examination is sufficient to decide whether to refer patients with potential meniscal and ligament injuries 2, 5.
• The physical examination is moderately sensitive and specific for meniscal and ligamentous injuries, with the Lachman test being more sensitive and specific for ligamentous tears than the drawer sign 2.
• MRI is more sensitive for ligamentous and meniscal damage but less specific than the physical examination 2, 6.
• The initial evaluation of acute knee pain should include plain radiography, which is a quick and cost-effective way to identify a wide range of problems, including fracture, degenerative changes, osteochondral defects, and effusions 6.
• If the history and physical examination point to damage of the cartilage, the menisci, and the cruciate and collateral ligaments, then MRI is useful for evaluating these structures 6.

Decision-Making Process

• The Ottawa Knee Rules can be used to decide whether patients with acute blunt knee trauma should undergo radiography 3, 4.
• If the rules are negative, the probability of fracture is estimated to be 0% (95% CI 0% to 0.5%) 4.
• Advanced imaging studies, such as MRI, can affirm the clinical findings and are useful in evaluating meniscal and ligamentous damage 2, 6, 5.