Differential Diagnosis for Positive Empty Can Test
A positive empty can test primarily indicates supraspinatus pathology, but the key differential diagnosis is between rotator cuff tear and cervical spondylotic amyotrophy (CSA), which can only be reliably distinguished by the presence or absence of pain during the test. 1
Primary Diagnostic Considerations
Rotator Cuff Tear (Most Common)
- Full-thickness supraspinatus tear is the most likely diagnosis when the empty can test demonstrates both weakness AND pain provocation (96% positive predictive value) 1
- Partial-thickness tears also produce positive findings with both pain and weakness, though with lower specificity 2
- When combined with weakness in external rotation and impingement signs in patients ≥60 years, there is a 98% probability of rotator cuff tear 3
Cervical Spondylotic Amyotrophy (Critical to Exclude)
- CSA produces weakness without pain during the empty can test, making pain provocation the critical distinguishing feature 1
- Weakness alone has 0% specificity for rotator cuff tear because CSA patients demonstrate identical weakness patterns 1
- This represents a common diagnostic pitfall—muscle weakness during the test cannot differentiate between these conditions 1
Secondary Differential Diagnoses
Subacromial Impingement Syndrome
- Produces pain with overhead activities and positive impingement signs (Neer's test 88% sensitive, Hawkins' test 92% sensitive) 4
- May demonstrate supraspinatus tendon irritation without complete tear 4
- Often associated with scapular dyskinesis and rotator cuff weakness 4
Rotator Cuff Tendinopathy (Without Tear)
- Presents with localized tenderness and load-related pain during activity 5
- Pain may be present during the empty can test but typically with less severe weakness 5
- Ultrasound or MRI shows tendon thickening and degeneration without discontinuity 5, 6
Other Shoulder Pathology
- Subscapularis tendon tears can coexist and produce weakness patterns 5
- Long head biceps tendinopathy may cause anterior shoulder pain with positive impingement signs 5
- Glenohumeral joint pathology including capsular stiffness in chronic cases 5
Diagnostic Algorithm
Step 1: Assess Pain vs. Weakness Pattern
- Pain + Weakness → 96% specific for rotator cuff tear 1
- Weakness alone → Cannot distinguish rotator cuff tear from CSA; requires further evaluation 1
Step 2: Perform Confirmatory Tests
- External rotation weakness test 3
- Impingement signs (Neer's, Hawkins-Kennedy) 3
- If all three positive (or two positive + age ≥60): 98% probability of rotator cuff tear 3
Step 3: Consider Imaging Based on Clinical Findings
- Ultrasound (91% sensitivity, 88% specificity for full-thickness tears) for initial evaluation 6
- MRI without contrast (95% sensitivity and specificity) when diagnosis remains unclear or surgical planning needed 5, 6
- Plain radiographs initially to exclude bony pathology, though they cannot visualize tendon tears 5, 6
Critical Clinical Pitfalls
Do Not Rely on Weakness Alone
- The empty can test demonstrates muscle activity in supraspinatus, infraspinatus, AND middle deltoid—it does not isolate the supraspinatus 7
- Weakness has 77.8% sensitivity but 0% specificity for rotator cuff tears 1
Always Assess for Cervical Pathology
- Obtain cervical spine history and perform neurological examination when weakness is present without pain 1
- CSA patients may present with identical shoulder elevation difficulty but lack pain provocation 1
Recognize Age-Related Factors
- Patients ≥60 years with positive findings have significantly higher probability of rotator cuff tear 3
- Younger athletes more commonly have secondary impingement from rotator cuff weakness rather than structural tears 4