Diagnostic Approach for ACL Injuries

Clinical Examination

The American Academy of Orthopaedic Surgeons recommends the Lachman test as the most sensitive clinical examination for ACL rupture, with optimal timing 4-5 days post-injury, achieving 84% sensitivity and 96% specificity 1, 2
The Lachman test should be performed with the knee flexed at 20-30 degrees to assess anterior tibial translation relative to the femur 1

The American College of Radiology suggests MRI as the imaging gold standard with the highest positive predictive value (92.5%) and overall diagnostic accuracy (92-98.3%) for confirming ACL tears 2, 3
MRI identifies associated injuries (meniscal tears, bone contusions, posterolateral corner injuries) that affect surgical planning, and can predict concomitant injuries, such as lateral bone contusions correlating with high-grade pivot-shift and lateral meniscal lesions 3, 4
MRI technical considerations include similar high sensitivities and specificities for ACL tears using both 1.5T and 3T protocols, with routine 3T not significantly improving accuracy over 1.5T 3, 4

CT scanning has a limited role, with 87.5-100% sensitivity and 100% specificity for ACL tears, but is inferior to MRI for soft tissue evaluation, and may be used for suspected occult fractures or bony avulsion injuries 3, 5
Ultrasound has moderate accuracy, with 90% sensitivity and 97% specificity for ACL injuries, but is operator-dependent and provides less comprehensive evaluation 6, 2

Initial clinical assessment should include the Lachman test, ideally 4-5 days post-injury, followed by knee radiographs to exclude fracture if the Lachman test is positive or equivocal 1, 2, 5
With negative radiographs and suspected ACL injury, proceed directly to MRI for definitive diagnosis, considering special patient populations, such as those under 40 years with knee effusion >10 mm on lateral radiograph 3, 4