What special tests should be performed on a patient with a knee injury presenting with worsening pain on extension?

Special Tests for Knee Injury with Worsening Pain on Extension

Begin with the Lachman test as your primary physical examination maneuver, as it is the most accurate clinical test for detecting ACL injury with over 80% sensitivity when performed properly. 1, 2

Initial Physical Examination Sequence

Step 1: Examine the Uninjured Knee First

• Always assess the contralateral knee first to establish a baseline for comparison 2
• This provides a reference point for normal range of motion, stability, and end-feel 2

Step 2: Primary ACL Assessment - The Lachman Test

The Lachman test should be your first-line special test because:

• It demonstrates the highest diagnostic accuracy for ACL tears among physical examination maneuvers 1, 3
• When performed properly, it achieves >80% sensitivity for ACL injury 1
• It is more sensitive than the anterior drawer test 3
• Technique: With the knee flexed at 20-30 degrees, stabilize the femur with one hand while applying anterior force to the proximal tibia with the other hand 2

Important caveat: The Lachman test has a predictive value of only 47% for a positive result but 70% for a negative result, meaning it is more reliable for ruling out ACL injury than confirming it 4

Step 3: Secondary ACL Tests

If the Lachman test is equivocal or you need additional confirmation:

• Anterior Drawer Test: Perform with knee flexed at 90 degrees, applying anterior force to the tibia 2, 3

  • Less sensitive than the Lachman test but provides complementary information 3

• Pivot Shift Test: Useful for evaluating ACL integrity, though more difficult to perform 2, 5

  • Requires more technical skill and is less commonly used in primary care 5

• Lever Sign Test: A relatively newer test, but interpret results with caution as evidence for its reliability is limited 3

Step 4: Posterior Cruciate Ligament Assessment

• Posterior Drawer Test: Apply posterior force to the tibia with knee flexed at 90 degrees 2
• Tibial Sag Test: Observe for posterior displacement of the tibia when the knee is flexed 2

Step 5: Collateral Ligament Testing

• Valgus Stress Test: Assess medial collateral ligament by applying lateral force to the knee 2
• Varus Stress Test: Assess lateral collateral ligament by applying medial force to the knee 2

Step 6: Meniscal Injury Assessment

For patients with pain on extension, meniscal pathology should be considered:

• McMurray's Test: Rotate the tibia while extending the knee from flexion 2
• Apley's Grind Test: Apply compression while rotating the tibia with patient prone 2
• Bounce Test: Assess for mechanical blocks to full extension 2

Critical Clinical Pearls

When Physical Examination is Limited

• Acute swelling and pain often limit examination accuracy immediately after injury 1
• If possible, examine the knee immediately post-injury before significant swelling develops 1
• A complete knee examination performed properly is >80% sensitive for ACL injury 1

Interpretation Hierarchy

Use this algorithm for decision-making:

1. Positive Lachman test → High suspicion for ACL injury, proceed to imaging 1, 3
2. Negative Lachman test → ACL injury less likely (70% predictive value), but consider other pathology 4
3. Equivocal findings → Combine multiple tests; if 2+ tests positive, ACL injury highly likely 3
4. All physical tests negative but symptoms persist → MRI indicated after 4-6 weeks of conservative treatment 6

Common Pitfalls to Avoid

• Do not rely solely on the anterior drawer test, as it is less sensitive than the Lachman test 3
• Do not skip examining the uninjured knee first, as this compromises your ability to detect subtle differences 2
• Do not interpret the lever sign test as definitive, as its clinical utility remains uncertain 3
• Do not order MRI without performing physical examination first, as clinical examination is highly specific and guides appropriate imaging 3

Imaging Follow-Up Based on Examination Findings

Immediate Imaging Indications

• Plain radiographs first (AP, lateral, tangential patellar views) are mandatory for all patients ≥5 years with knee injury 6
• Radiographs must be obtained before MRI in all cases 6

MRI Indications After Physical Examination

Order MRI without contrast when: 7, 8, 6

• Physical examination suggests ACL injury (positive Lachman or multiple positive tests) 1, 3
• Radiographs show osseous fragments, loose bodies, or signs of prior injury 7, 8
• Symptoms persist after 4-6 weeks of conservative treatment despite normal radiographs 6

Key point: MRI is more sensitive than clinical examination (all ACL tears diagnosed by physical examination are confirmed by MRI, but MRI detects additional injuries) 3