Best Test for Detecting ACL Injury
The Lachman test is the most sensitive clinical examination for detecting ACL rupture, while MRI is the imaging gold standard with the highest positive predictive value (92.5%) and overall diagnostic accuracy (92-98.3%) for confirming ACL tears. 1, 2, 3
Clinical Examination: The Lachman Test
The Lachman test should be your first-line clinical assessment, performed with the knee flexed at 20-30 degrees to assess anterior tibial translation relative to the femur. 1
Key Performance Characteristics:
- Optimal timing: Perform 4-5 days post-injury when possible, achieving 84% sensitivity and 96% specificity 4
- Acute examination caveat: Initial acute examination may be limited by joint effusion or hemarthrosis, significantly affecting test performance 4
- The American Academy of Orthopaedic Surgeons specifically recommends the Lachman test as the most sensitive clinical examination for ACL rupture 1
Imaging: MRI as the Diagnostic Gold Standard
MRI is the imaging test with the best positive predictive value (92.5%) and should be obtained when clinical tests are equivocal, discordant, or when surgical planning is needed. 4, 2
MRI Diagnostic Performance:
- Sensitivity: 93-98.57% 4, 5, 2
- Specificity: 75-100% 6, 4, 5, 2
- Positive Predictive Value: 92.5% 2
- Overall Accuracy: 92-98.3% 5, 2, 3
MRI Technical Considerations:
- Both 1.5T and 3T MRI protocols show similar high sensitivities and specificities for ACL tears, with routine 3T not significantly improving accuracy over 1.5T 6
- Direct MRI signs with highest diagnostic value: ACL discontinuity, thickening and edema, and abnormal fiber orientation 5
- Indirect MRI signs: Posterior cruciate ligament buckling, bone marrow contusions, and anterior tibial displacement provide additional diagnostic support 6
Clinical Utility Beyond Diagnosis:
- MRI identifies associated injuries (meniscal tears, bone contusions, posterolateral corner injuries) that affect surgical planning 6
- MRI can predict concomitant injuries: lateral bone contusions correlate with high-grade pivot-shift and lateral meniscal lesions 6
- Early MRI (within 6 weeks) detects more anterolateral ligament injuries than delayed imaging 6
Alternative Imaging Modalities
CT Scanning:
- Limited role: 87.5-100% sensitivity and 100% specificity for ACL tears, but inferior to MRI for soft tissue evaluation 6
- May be used for suspected occult fractures or bony avulsion injuries, but MRI remains superior for ligamentous assessment 6
Ultrasound:
- Moderate accuracy: 90% sensitivity and 97% specificity for ACL injuries 7, 4
- Can serve as an alternative when immediate MRI is unavailable, but is operator-dependent and provides less comprehensive evaluation 7
Algorithmic Approach
Initial clinical assessment: Perform Lachman test, ideally 4-5 days post-injury if acute examination is limited 1, 4
If Lachman test is positive or equivocal: Obtain knee radiographs first to exclude fracture, especially if focal tenderness or inability to bear weight (Ottawa rules) 6
With negative radiographs and suspected ACL injury: Proceed directly to MRI for definitive diagnosis 6
Special consideration: In patients <40 years with knee effusion >10 mm on lateral radiograph, strongly consider MRI to decrease delayed diagnosis and improve outcomes 6
Important Caveats
- Partial tears: MRI accuracy is somewhat lower for partial versus complete tears (94.74% vs 92.86%), but remains clinically useful 5
- Chronic tears: MRI assessment of chronic ACL tears is less accurate than acute tears, though still valuable 8
- Avoid diagnostic arthroscopy: With modern MRI accuracy of 92-98%, diagnostic arthroscopy is unnecessary when MRI and clinical findings are concordant; proceed directly to therapeutic arthroscopy if surgery is indicated 3