What imaging is indicated after a audible pop is heard in the knee, suggesting a potential ligamentous or meniscal injury?

From the Guidelines

Imaging after a potential ligamentous or meniscal injury in the knee is indicated with MRI.

Initial Imaging

• The first imaging study should be radiographs in the setting of significant acute trauma to the knee, as stated by the American College of Radiology 1.
• Knee radiographs are usually appropriate as the initial imaging study of patients 5 years of age or older for the evaluation of a fall or acute twisting trauma to the knee when at least one of the following is present: focal tenderness, effusion, inability to bear weight 1.

Indications for MRI

• MRI knee without IV contrast is usually appropriate as the next imaging study, after radiographs did not show fracture, of adults or skeletally mature children, for the evaluation of suspected occult knee fractures or internal derangement after a fall or acute twisting trauma to the knee 1.
• MRI has many distinct advantages for the evaluation of the injured knee in the setting of negative radiographs, allowing earlier surgical intervention by obtaining a more accurate diagnosis 1.
• MRI can diagnose the patterns and severity of bone marrow contusions that frequently have an association with the specific mechanisms of injury and can predict associated soft-tissue injuries 1.

Benefits of MRI

• MRI can change management from surgical to conservative in up to 48% of patients presenting with a locked knee, which is usually an indication for arthroscopic procedure 1.
• MRI facilitates diagnosis of the anterolateral ligament injuries that have frequent association with ACL injuries and anterolateral knee instability 1.
• MRI is proven helpful in detection and characterization of posterolateral corner injuries, which can be associated with the ACL ruptures and, if missed, may lead to considerable morbidity 1.

From the Research

Imaging Indications for Ligamentous or Meniscal Injuries

After an audible pop is heard in the knee, suggesting a potential ligamentous or meniscal injury, the following imaging modalities are indicated:

• Magnetic Resonance Imaging (MRI) is the gold standard for diagnosing soft tissue injuries of the knee, including ligamentous and meniscal injuries 2, 3, 4
• MRI is highly sensitive and specific in diagnosing anterior cruciate ligament (ACL) injuries, with a positive predictive value (PPV) approaching 100% 2
• MRI is also useful in diagnosing posterior cruciate ligament (PCL) injuries, medial meniscus (MM) injuries, and lateral meniscus (LM) injuries, although the sensitivity and specificity may vary 2
• Radiographs may be used to show evidence of bony injury, but are not sufficient for diagnosing ligamentous or meniscal injuries 3
• Ultrasound imaging may be used as a complementary modality, particularly for evaluating meniscal injuries, with a sensitivity of 88% and specificity of 86% when combined with clinical examination 5

Characteristics of Meniscal Injuries on Imaging

Meniscal injuries may be characterized by:

• Full-thickness or partial-thickness delamination
• Meniscal cyst oedema
• Articular space stenosis
• The presence of at least two of these characteristics is associated with a high sensitivity and specificity for diagnosing meniscal injuries 5

Clinical Relevance

Accurate diagnosis of ligamentous and meniscal injuries is crucial for guiding treatment and management decisions, including the need for arthroscopic surgery or conservative management 2, 3