What is the significance of the empty‑can (thumb‑down) test in assessing the supraspinatus muscle and tendon?

Empty Can Test for Supraspinatus Assessment

Primary Clinical Significance

The empty can test (thumb-down position) is a validated diagnostic maneuver for detecting supraspinatus tendon tears, with muscle weakness—not pain—serving as the most accurate indicator of a true tear. 1, 2

Diagnostic Performance

Test Interpretation Criteria

• Muscle weakness is the gold standard for interpreting a positive test, achieving 70-75% accuracy for detecting full-thickness supraspinatus tears 1, 2
• Pain alone is a poor indicator of tendon tears and should not be used as the primary criterion for a positive test 1, 3
• The test demonstrates significant correlation with arthroscopic findings when weakness is used as the criterion (P = 0.004) 2

Sensitivity and Specificity Patterns

• The test is sensitive to both full-thickness and partial-thickness tears but specific only for full-thickness tears 3
• When partial-thickness tears are included in the "tear" category, sensitivity increases but specificity decreases 3
• The test shows higher diagnostic precision for tears larger than 1 cm in size 2

Comparison with Full Can Test

Relative Performance

• The empty can and full can tests show equivalent diagnostic accuracy (70% vs 75%, not statistically significant) when muscle weakness is the criterion 1, 2
• The full can test achieves a slightly better area under the curve (AUC) and correlation with intraoperative findings (P = 0.001 vs P = 0.004) 2
• The empty can test provokes more pain (50% vs 43% of patients), which may limit patient cooperation but does not improve diagnostic accuracy 1

Clinical Recommendation

• The full can test may be more beneficial in clinical practice due to less pain provocation while maintaining equivalent diagnostic accuracy 1

Muscle Activation Patterns

Important Caveat About Selectivity

• The empty can test does NOT selectively isolate the supraspinatus muscle, contrary to traditional teaching 4
• Electromyographic studies demonstrate simultaneous activation of the infraspinatus and middle deltoid during the maneuver 4, 5
• The supraspinatus and subscapularis show greater activation during the empty can test compared to the full can test, particularly at 90° elevation in the scapular and coronal planes 5

Optimizing Diagnostic Accuracy

Combination Testing Strategy

• No single clinical test can distinguish between partial-thickness and full-thickness tears 2
• Combining at least 3 tests significantly improves diagnostic value: the empty can, full can, and zero-degree abduction tests together achieve the best AUC (0.795) and correlation with arthroscopic findings (P = 0.02) 2
• Physical maneuvers that simulate tendon loading and reproduce the patient's pain support the diagnosis of tendinopathy 6

Clinical Context and Limitations

When Imaging is Needed

• Palpation and physical examination are limited by the deep anatomic location of the supraspinatus tendon 6
• Further imaging (ultrasound or MRI) should be reserved for unclear diagnoses after thorough examination, recalcitrant pain despite 3-6 months of conservative management, or preoperative evaluation 6, 7
• MRI demonstrates 95% sensitivity and specificity for full-thickness rotator cuff tears 6
• Ultrasound shows 91% sensitivity and 88% specificity for complete supraspinatus tears 7

Common Pitfalls to Avoid

• Do not interpret pain alone as indicating a tear—this leads to false positives 1, 3, 2
• Do not rely on the empty can test to selectively assess only the supraspinatus, as multiple muscles are activated 4, 5
• Do not use a single test in isolation—combine multiple maneuvers to improve diagnostic accuracy 2
• Recognize that the test cannot differentiate tear thickness, which requires imaging 3, 2