Painful Arc Syndrome (Subacromial Impingement Syndrome)

1. ANATOMY - The Subacromial Space

Anatomy of the subacromial space showing the supraspinatus and subacromial bursa - Tintinalli's Emergency Medicine

• The acromion process (especially its anterior inferior surface)
• The coracoacromial ligament
• The coracoid process
• The acromioclavicular (AC) joint below

• The four rotator cuff tendons (supraspinatus, infraspinatus, teres minor, subscapularis)
• The long head of biceps tendon
• The subacromial/subdeltoid bursa

2. ETIOLOGY

Extrinsic (Primary) Causes - Structural narrowing of the subacromial space

Intrinsic (Secondary) Causes - Rotator cuff dysfunction

Risk factors

• Age >40 years (degenerative change)
• Dominant arm affected
• Workers' compensation populations (poor outcomes)
• Overhead sport athletes at any age

3. PATHOGENESIS - Neer's Three Stage Model

Stage I - Edema and Hemorrhage (Age <25 years)

• Reversible edema and hemorrhage around the rotator cuff tendons and bursa
• Caused by excessive overhead use (e.g., competitive swimming, pitching)
• Reversible with rest and activity modification
• Symptoms: dull ache over anterolateral shoulder, worse with activity, improved by rest

Stage II - Fibrosis and Tendinitis (Age 25-40 years)

• Repeated mechanical trauma leads to tendinitis of the rotator cuff
• Fibrosis and thickening of the tendons and bursa develop
• Chronic and recurrent course, often lasting weeks to months
• Night pain becomes prominent (due to relaxation of supporting muscle tone)
• Partially reversible; may require more aggressive treatment

Stage III - Rotator Cuff Tears and Bony Changes (Age >40 years)

• Continued overuse leads to:
  • Partial or full-thickness rotator cuff tears (typically supraspinatus first)
  • Rupture of the long head of biceps tendon
  • Subacromial bone spurs (osteophytes on anterior acromion)
• Progressive and largely irreversible without surgery
• Significant disability and functional limitation

4. SIGNS AND SYMPTOMS

Symptoms

• Located over the anterior to lateral shoulder
• Frequently radiates to the lateral mid-humerus (not below the elbow - differentiates from cervical radiculopathy)
• Night pain - deep, aching, worsened by lying on the affected shoulder or sleeping with arms overhead
• Aggravated by overhead activities - brushing hair, reaching into cupboards, throwing
• Associated weakness and stiffness (usually secondary to pain)

Signs

Fig. 15.3 - In painful arc syndrome, midrange abduction (60°-120°) is painful; the extremes are painless - S. Das Manual on Clinical Surgery

Arcs of painful abduction: subacromial (60°-120°) vs. acromioclavicular (120°-180°) - Rosen's Emergency Medicine

The painful arc 60°-120° corresponds to the moment the inflamed supraspinatus/bursa passes under the coracoacromial arch

• Subacromial painful arc: 60°-120° - Pathology in supraspinatus tendon/subacromial bursa
• AC joint painful arc: 120°-180° - Pathology in acromioclavicular joint

• Tenderness just below the acromion process when the arm is adducted
• The tender spot disappears under the acromion when the arm is abducted (the tendon slips underneath the acromion) - this is a characteristic finding of painful arc syndrome specifically

• Disuse atrophy of shoulder musculature in chronic stages
• Crepitus during range of motion (from fibrosis/scarring trapped under acromion)
• Normal or near-normal passive ROM (differentiates from frozen shoulder which has restricted passive ROM)
• Mild to moderate weakness of abduction and external rotation (supraspinatus, infraspinatus) - secondary to pain
• A "catching" sensation if scar tissue is trapped beneath the acromion

5. INVESTIGATIONS AND EVALUATION

Clinical Examination Tests

Neer Impingement Sign

Neer impingement sign: Examiner raises the affected arm in forced forward elevation while stabilizing the scapula, causing the greater tuberosity to impinge against the acromion - Campbell's Operative Orthopaedics

• Examiner prevents scapular rotation with one hand, then raises the patient's straightened arm in forward elevation
• Positive if pain occurs between 70°-120° (greater tuberosity impinges under the acromion)
• Sensitivity: 68%, Specificity: 68.7%

Hawkins-Kennedy Test

Hawkins-Kennedy test: Shoulder at 90° abduction + 90° elbow flexion, then internal rotation - Tintinalli's Emergency Medicine

• Shoulder at 90° abduction, 90° elbow flexion; examiner internally rotates the shoulder
• This drives the supraspinatus tendon against the coracoacromial ligament
• Sensitivity: 71.5-80%, Specificity: 60-66%

Empty Can Test (Jobe Test)

Empty can position: Arm abducted 90°, forward flexed 30°, thumb pointing down - Tintinalli's Emergency Medicine

• Arm at 90° abduction, 30° forward flexion, thumb pointing down ("empty can" position)
• Pain or weakness against resistance suggests supraspinatus injury
• Sensitivity: 44%, Specificity: 89.5%

Clinical Test Accuracy Table (Campbell's Operative Orthopaedics 2026)

Imaging

• First-line imaging; usually normal in early stages
• May show:
  • Hooked (Type III) acromion on outlet view
  • Subacromial spur on anterior acromion
  • Sclerosis and subchondral cyst formation of the greater tuberosity
  • Calcification above the greater tuberosity (calcific tendinitis)
  • Superior migration of the humeral head (suggests chronic large rotator cuff tear - normal AHI is >7mm)
  • AC joint osteophytes

• Dynamic, real-time assessment of the rotator cuff
• Shows: thickening of supraspinatus tendon, subacromial bursa fluid/thickening, partial or full-thickness tendon tears
• Operator-dependent but cost-effective
• First-line modality in many centers for soft tissue evaluation

• Best delineates extent of tendon disease: tendinopathy vs. partial tear vs. full-thickness tear
• Shows subacromial bursal thickening/fluid
• Demonstrates fatty atrophy of cuff muscles in chronic disease
• Not necessary in acute/early presentation; reserved for surgical planning or when conservative treatment fails

• Useful for bony anatomy assessment (acromion morphology, spurring)
• CT arthrography accurately identifies full-thickness tears
• Less commonly used than MRI for primary evaluation

• Not diagnostic but used to rule out systemic causes
• FBC, ESR, CRP, uric acid, rheumatoid factor - if inflammatory arthropathy, infection, or gout suspected

6. TREATMENT

Conservative (Non-Operative) Management - First Line

1. Relative rest and activity modification
   • Avoid overhead activities and the painful arc of elevation
   • A brief sling may help, but complete immobilization should be avoided (risk of adhesive capsulitis)
2. Analgesics and Anti-inflammatories
   • NSAIDs for 7-21 days (mainstay)
   • Paracetamol/acetaminophen for milder pain
   • Short-term opioid analgesics if severe (3 days only)
3. Cryotherapy
   • Ice applied to affected shoulder 10-15 minutes, 3-4 times daily
   • Analgesia and reduction of inflammation/edema
4. Range-of-motion exercises (3-4 times daily)
   • Pendulum swings: patient slightly bent at waist, arm hanging freely, gentle clockwise and counterclockwise arcs - progressing in size as symptoms allow
   • Wall walking: stand sideways arm's length from wall, walk fingers upward to pain tolerance

1. Physical Therapy (supervised)
   • Rotator cuff strengthening (particularly supraspinatus, infraspinatus)
   • Scapular stabilization exercises
   • Postural correction and biomechanical retraining
   • Evidence supports exercise therapy as first-line for all patients
2. Subacromial Corticosteroid Injection

Technique of administering a subacromial bursa injection - Bailey and Love's Surgery

• Indicated when pain persists despite rest, NSAIDs, and PT
• Reduces pain and allows better engagement with physical therapy
• Approach: lateral (most common) or posterior route below the acromion
• Agent: methylprednisolone or triamcinolone + local anesthetic (lidocaine)
• Adverse effects: local skin atrophy, tendon weakening, infection risk; direct tendon injection can cause necrosis/rupture
• Limit to 2-3 injections; diminishing returns thereafter

1. Platelet-Rich Plasma (PRP) / Prolotherapy:
   • Emerging evidence; not yet standard of care

Surgical Management - After 4-6 Months of Failed Conservative Treatment

• Most common surgical procedure
• Arthroscopic approach is now standard (vs. open)
• Involves:
  • Bursectomy - removal of thickened subacromial bursa
  • Acromioplasty - resection of the anterior acromion to create a flat Type I profile
  • Coracoacromial ligament release
  • If needed: rotator cuff repair, distal clavicle resection (for AC joint OA)

• Surgery restores a flat acromion and creates more room for rotator cuff gliding
• However, evidence does not clearly show surgery improves long-term outcomes versus physical therapy alone
• Poor prognostic factors: workers' compensation claims, chronic symptoms, significant fatty muscle atrophy

• Failure of minimum 4-6 months of non-operative treatment
• Stage III disease with full-thickness rotator cuff tear often requiring repair
• Exception: massive irreparable tears may benefit from debridement with preservation of the coracoacromial arch (to prevent anterosuperior humeral migration)

Treatment Summary Algorithm

Painful Arc Syndrome
        |
        v
Conservative (0-6 months)
  - Rest, NSAIDs, ice
  - ROM exercises
  - Physical therapy (rotator cuff + scapular strengthening)
        |
  Not improving at 6-8 weeks?
        |
        v
  Subacromial Corticosteroid Injection
  + Continued PT
        |
  Not improving at 3-6 months total?
        |
        v
  MRI / Advanced Imaging
  to characterize rotator cuff
        |
        v
  Surgical: Arthroscopic Acromioplasty
  +/- Rotator Cuff Repair

7. DIFFERENTIAL DIAGNOSIS

Key Teaching Points

1. Painful arc = subacromial pathology (60°-120°) - this is the defining clinical hallmark
2. The syndrome encompasses subacromial bursitis, supraspinatus tendinitis, and biceps tendon involvement - these are not separate diagnoses
3. Histologically, it is tendinopathy (collagen disorganization), not true inflammation
4. The combination of painful arc sign + Hawkins-Kennedy + Jobe test gives >95% likelihood of impingement if all three are positive
5. Conservative management works in 60-90% of patients when pursued for 3-6 months
6. Workers' compensation status is an independent predictor of poor surgical outcome