What is the likely diagnosis and treatment for a 2-week history of pain at the top of the right shoulder, possibly due to hyperextension, with symptoms including pain on abduction, external rotation, and a painful arch, limited abduction, and radiating pain to the lower arm when extended beyond 90 degrees, with no pain at rest?

Likely Diagnosis: Subacromial Impingement Syndrome with Possible Rotator Cuff Tendinopathy

Based on your clinical presentation—painful arc during abduction, pain with external rotation, anterior shoulder tenderness, and radiation to the lower arm beyond 90 degrees—you most likely have subacromial impingement syndrome, potentially with underlying rotator cuff dysfunction. 1, 2

Clinical Reasoning

Your symptom pattern is classic for impingement:

• Painful arc between 70-120 degrees of abduction is the hallmark clinical sign of impingement syndrome 3
• Anterior shoulder pain point suggests compression of the rotator cuff and subacromial bursa against the anterolateral acromion 1
• Pain with external rotation and limited abduction indicates rotator cuff involvement, particularly the supraspinatus tendon 4
• Pain radiating down the arm with overhead motion is typical of subacromial pathology 1
• No pain at rest suggests mechanical rather than inflammatory etiology 2

The hyperextension mechanism you describe could have caused acute trauma leading to rotator cuff strain or partial tear, which then progressed to secondary impingement 3, 2.

Diagnostic Workup

Initial Imaging

• Start with standard shoulder radiographs: anteroposterior views in internal and external rotation PLUS an axillary or scapular Y view 5, 6
• Look for bony excrescences on the anteroinferior acromion, greater tuberosity flattening, or sclerosis—these are radiographic signs of chronic impingement 7

Advanced Imaging if Radiographs Non-Contributory

• MRI without contrast or ultrasound are equivalent first-line studies for suspected rotator cuff pathology 6
• MRI is superior for characterizing soft tissue injuries including rotator cuff tears, while ultrasound is cost-effective and appropriate with experienced operators 2, 6

Physical Examination Findings to Confirm

• Focal weakness with decreased range of motion during abduction with external rotation 4
• Positive impingement signs (pain with forward flexion beyond 90 degrees) 1
• Assess for scapular dyskinesis which contributes to rotator cuff injury 4, 6

Treatment Algorithm

Conservative Management (First-Line for 6-12 Weeks)

Complete rest from aggravating activities until asymptomatic 4

Pain control:

• Acetaminophen or ibuprofen if no contraindications 4, 8
• Ice packs for acute inflammation 8

Physical therapy program focusing on:

• Gentle stretching and mobilization, emphasizing external rotation and abduction 4
• Rotator cuff strengthening (supraspinatus, external rotators) 4
• Scapular stabilizer strengthening 4
• Gradually increase active range of motion while restoring joint alignment 4

Subacromial corticosteroid injection can be used if pain is related to bursa or rotator cuff inflammation and conservative measures fail after 4-6 weeks 4, 8

Surgical Consideration

Surgery should be considered early if:

• You are young and sustained traumatic injury (which you did with hyperextension) 2
• Conservative management fails after 3-6 months 3, 8
• Complete rotator cuff tear is identified on imaging 2

Arthroscopic subacromial decompression is the preferred surgical approach if indicated 2

Timeline for Recovery

• 1-3 months for rehabilitation depending on severity 4
• Return to full activity only after achieving pain-free motion and strength 4
• Recovery is faster with smaller rotator cuff involvement; longer with larger tears 3

Critical Pitfalls to Avoid

• Do NOT attempt aggressive stretching early—this can worsen inflammation 4
• Ensure proper imaging views are obtained—AP views alone miss pathology; axillary or Y views are essential 5
• Do not delay imaging if symptoms persist beyond 2-4 weeks of conservative treatment 2
• In traumatic cases like yours, consider early MRI to rule out significant rotator cuff tear that would benefit from early surgical repair 2
• Address scapular dyskinesis—failure to correct abnormal scapular mechanics perpetuates impingement 4, 6