What are the likely causes of a patient's inability to abduct laterally at the shoulder joint without a history of trauma, accompanied by pain upon attempting abduction, with other motions being executed with ease?

Likely Causes of Isolated Painful Shoulder Abduction Without Trauma

The most likely diagnosis is rotator cuff tendinopathy, specifically affecting the supraspinatus tendon, which is the primary pathology causing painful limited abduction in non-traumatic shoulder presentations. 1, 2

Primary Differential Diagnoses

Rotator Cuff Tendinopathy/Impingement (Most Likely)

This is the leading diagnosis when a patient presents with painful abduction without trauma, particularly if other shoulder movements remain intact. 1, 2

• The supraspinatus tendon is most vulnerable to impingement during abduction between 70-120 degrees due to its passage through the narrow subacromial space beneath the coracoacromial arch 1
• Repeated impingement of the coracoacromial arch onto the supraspinatus tendon, combined with hypovascularity in the region proximal to its insertion, creates the pathologic mechanism 1
• Pain during abduction with preserved ease of other motions (flexion, extension, internal/external rotation at side) is the classic presentation pattern 1, 2
• Physical examination findings to confirm: focal weakness during abduction with external or internal rotation, positive Hawkins' test (92% sensitive), and positive Neer's test (88% sensitive) 1, 2

Secondary Impingement Syndrome

• This occurs when weakness in the rotator cuff muscles combined with ligamentous laxity prevents the humeral head from staying centered in the glenoid fossa during arm motion 1
• The humeral head migrates superiorly due to improper rotator cuff functioning, resulting in supraspinatus tendon impingement 1
• Pain is typically located in the anterior or anterolateral aspect of the shoulder and is associated with overhead activities 1

Adhesive Capsulitis (Frozen Shoulder)

• While this typically presents with global restriction of motion, early-stage adhesive capsulitis can present with predominantly painful abduction 1
• The key distinguishing feature would be progressive loss of passive range of motion, particularly external rotation and abduction 1
• Scapulothoracic motion can be affected, with altered scapular positioning during attempted abduction 3

Less Common But Important Considerations

Subacromial Bursitis

• Inflammation of the subacromial bursa can cause isolated painful abduction without affecting other shoulder movements 1
• Often coexists with rotator cuff tendinopathy as part of the impingement syndrome spectrum 1

Calcific Tendinitis

• Calcium deposits in the rotator cuff tendons (most commonly supraspinatus) can cause acute painful limitation of abduction 2
• This would be visible on plain radiographs if present 2

Cervical Radiculopathy (Referred Pain)

• C5 nerve root compression can refer pain to the shoulder and cause weakness of shoulder abduction 2, 4
• However, this typically presents with additional neurological symptoms (numbness, tingling, radiation down the arm) and would not spare other shoulder movements 2

Critical Diagnostic Approach

Physical Examination Maneuvers

Perform these specific tests to differentiate the cause: 1, 2

• Hawkins' test: Forcible internal rotation with arm passively flexed forward at 90 degrees—pain indicates impingement (92% sensitive) 1
• Neer's test: Pain with full forward flexion between 70-120 degrees indicates impingement (88% sensitive) 1
• Empty can test: Abduction in scapular plane with internal rotation—tests supraspinatus function 2, 5
• Assess passive range of motion: If passive abduction is also limited and painful, consider adhesive capsulitis; if passive motion is preserved but active abduction is painful/weak, rotator cuff pathology is more likely 1, 2

Age-Specific Considerations

• Patients over 35-40 years: Rotator cuff tendinopathy and degenerative changes are the predominant causes 2
• Younger patients: Consider secondary impingement from glenohumeral instability or labral pathology 1, 2

Common Pitfalls to Avoid

• Do not assume absence of trauma means absence of significant pathology—rotator cuff tears can occur from chronic repetitive microtrauma without a discrete injury event 1, 2
• Avoid overhead pulleys during treatment—these encourage uncontrolled abduction and can worsen rotator cuff impingement 1
• Do not overlook scapular dyskinesis—poor scapular coordination contributes significantly to rotator cuff injury and impingement, as the scapula normally rotates upward and tilts posteriorly during arm elevation 1, 5
• Assess for "full can" versus "empty can" positioning—scapular internal rotation and anterior tilt are greater with internal rotation positioning, which increases impingement risk 5

Recommended Initial Management

For rotator cuff tendinopathy (the most likely diagnosis): 1

• Complete rest from aggravating activities until asymptomatic 1
• Improve range of motion through stretching and mobilization techniques, focusing especially on external rotation and abduction to prevent frozen shoulder 1
• Rotator cuff and scapular stabilizer strengthening once pain-free motion is achieved 1
• Consider modalities: ice, heat, soft tissue massage 1
• NSAIDs for acute pain management 1
• Corticosteroid injections (intra-articular triamcinolone) are more effective than NSAIDs in the acute phase but do not change long-term outcomes 1