What is the hook test for assessing distal biceps tendon integrity?

What is the Hook Test?

The hook test is a highly sensitive and specific physical examination maneuver where the examiner attempts to "hook" their index finger under the distal biceps tendon from the lateral side of the antecubital fossa while the patient actively supinates the forearm with the elbow flexed at 90 degrees.

How to Perform the Test

The technique is straightforward:

• Position the patient with their elbow flexed to 90 degrees
• Have the patient actively supinate their forearm (palm up position)
• From the lateral side of the antecubital fossa, attempt to hook your index finger under the biceps tendon
• The tendon should feel like a firm, cord-like structure

Interpretation

Positive Test (Abnormal)

• No cord-like structure can be hooked under your finger
• Indicates a complete distal biceps tendon rupture
• The tendon has avulsed from the radial tuberosity

Negative Test (Normal)

• You can easily hook your finger under an intact, taut tendon
• However, pain during the maneuver may indicate a partial tear even when the test is technically negative

Diagnostic Performance

The hook test's accuracy varies significantly based on clinical context:

For Acute Complete Ruptures (≤1 month)

The test performs best in this setting 1, 2:

• Sensitivity: 94-96% when combined with biceps crease interval measurement
• Specificity: 100% for complete tears
• When both the hook test AND biceps crease interval are positive, you can diagnose an acute complete rupture with certainty 2

For Chronic Complete Ruptures (>1 month)

Performance declines 3:

• Sensitivity drops to 78-83% overall
• Only 45% sensitive when the lacertus fibrosus remains intact (which can mask the rupture)
• Specificity remains high at 100%

Critical Limitation - Partial Tears

The hook test is unreliable for partial tears 3:

• Sensitivity only 30% for partial ruptures
• The test may be painful but technically negative
• Do not rely on the hook test alone to exclude pathology

Clinical Decision-Making Algorithm

When Hook Test is POSITIVE:

1. In acute injuries: Diagnose complete rupture - proceed directly to surgical consultation 2
2. In chronic injuries (>8 weeks): >75% probability will require graft reconstruction 3

When Hook Test is NEGATIVE:

1. Does NOT exclude rupture - particularly partial tears 4
2. Check for weakness on active supination (100% sensitive for chronic complete ruptures) 2
3. Palpate the tendon footprint for pain (100% sensitive for partial ruptures) 2
4. If clinical suspicion remains, obtain MRI (100% sensitivity and specificity) 5, 1

Common Pitfalls

Intact lacertus fibrosus: This structure can bridge the gap in complete ruptures, making the hook test falsely negative in 55% of cases where it's intact 3. The patient may retain some flexion strength, further obscuring the diagnosis.

Examiner experience matters: While the test shows substantial interrater reliability (Cohen kappa 0.71) 1, the original 100% sensitivity reported has not been replicated in real-world practice. The test performs best when done by experienced upper extremity surgeons.

Timing affects accuracy: The test is most reliable in acute injuries. As time passes and tissue retracts, the test becomes less sensitive 3.

Comparison to Imaging

MRI remains the gold standard 5, 1:

• 100% sensitivity and specificity for complete tears
• Superior for detecting partial tears (59% sensitivity vs 30% for hook test)
• The FABS (flexion-abduction-supination) view provides optimal visualization 5

Ultrasound is operator-dependent 5:

• 62.5% sensitivity for detecting tears overall
• Inferior to both MRI and clinical examination for complete ruptures

Bottom line: Use the hook test as part of your initial assessment, but never rely on it alone. A positive test in acute injuries is highly reliable for complete rupture. A negative test requires additional examination maneuvers and often imaging to exclude partial tears or complete ruptures with intact lacertus fibrosus 1, 2, 4.