If you suspect an Anterior Cruciate Ligament (ACL), Posterior Cruciate Ligament (PCL), Lateral Collateral Ligament (LCL), or Medial Collateral Ligament (MCL) injury, would a Computed Tomography (CT) scan show this?

CT Cannot Adequately Visualize Knee Ligament Injuries

CT is inappropriate for diagnosing ACL, PCL, LCL, or MCL injuries because it is inferior to MRI for soft tissue evaluation, despite having high specificity for ACL tears (87.5-100% sensitivity, 100% specificity), CT remains inadequate for comprehensive ligament assessment. 1

Why CT Fails for Ligament Evaluation

CT's fundamental limitation is poor soft tissue contrast. While CT excels at detecting bony injuries, it cannot adequately visualize the ligamentous structures of the knee:

• CT is appropriate only for suspected occult fractures or bony avulsion injuries, not for routine ligament assessment 1
• Even in spine trauma where CT is the gold standard for fractures, it remains "limited in detection of cord injury and compressive epidural or subdural processes" and soft tissue structures 2
• CT's role in knee trauma is restricted to identifying associated bony pathology, such as lateral tibial plateau depression >11 mm, which predicts higher risk of lateral meniscus tear 3, 1

The Correct Imaging Algorithm

Start with radiographs, then proceed directly to MRI:

1. Obtain knee radiographs first to exclude fractures or bony avulsion injuries, particularly in patients under 40 years with knee effusion 1, 4

   • Radiographs serve as the mandatory screening tool before advanced imaging 1
   • In patients under 40 with acute knee injury, knee effusion >10 mm on lateral radiograph should prompt MRI consideration 3

2. MRI is the definitive imaging modality with 92.5% positive predictive value and 92-98.3% overall diagnostic accuracy for ACL tears 1, 4

   • Both 1.5T and 3T MRI protocols provide similar high sensitivities and specificities, so routine 3T imaging does not significantly improve accuracy 1, 4
   • MRI should be performed within 6 weeks of acute trauma for optimal visualization of certain injuries like anterolateral ligament tears 3, 1

Critical Advantages of MRI That CT Cannot Provide

MRI identifies concomitant injuries that profoundly impact management:

• MRI detects associated injuries in 19.7% of ACL cases, including posterolateral corner injuries that, if missed, lead to considerable morbidity 1
• In one study of 254 ACL tears, associated injuries were extremely common: MCL injuries (41.3%), medial meniscus tears (57.4%), lateral meniscus tears (25.2%), bone bruising (92.9%), and impaction fractures (45.7%) 5
• Lateral bone contusions on MRI correlate with high-grade pivot-shift and lateral meniscal lesions, predicting injury patterns 1
• MRI changes management from surgical to conservative in up to 48% of patients presenting with locked knee 3, 1

Alternative Imaging: Also Inadequate

Ultrasound is the recommended first-line alternative when MRI is contraindicated, achieving 90% sensitivity and 97% specificity for ACL injuries, but it is operator-dependent and cannot adequately evaluate portions of menisci, articular surfaces, or deep intra-articular structures 1, 4

Common Pitfalls to Avoid

• Do not rely on CT for soft tissue ligament evaluation, even though it has high specificity for ACL tears—MRI remains superior for comprehensive assessment 1
• Do not skip radiographs before MRI, as they exclude fractures and guide appropriate imaging selection 1, 4
• Avoid delayed MRI imaging beyond 6 weeks when anterolateral ligament injury is suspected, as injuries may become less visible 1
• Do not assume isolated ligament injury—MRI's ability to detect bone contusions predicts development of focal osteoarthritis 1 year after trauma 1