A 55-year-old male labourer presents with progressive shoulder pain and weakness of abduction for 3 months. Clinical examination reveals positive drop arm test and inability to initiate abduction. Which rotator cuff muscle is most commonly affected in this presentation?

Explanation

## Rotator Cuff Tears: Most Common Site **Key Point:** Supraspinatus is the most commonly torn rotator cuff muscle, accounting for 50–60% of all rotator cuff tears. ### Why Supraspinatus? **High-Yield:** The supraspinatus is vulnerable due to: 1. **Anatomical factors:** Passes through the narrow subacromial space, predisposing to impingement 2. **Mechanical stress:** Bears the greatest load during abduction and forward flexion 3. **Vascular watershed:** Has a relatively hypovascular zone near its insertion on the greater tuberosity 4. **Age-related degeneration:** Degenerative tears are most common in patients >50 years ### Clinical Correlation **Clinical Pearl:** The drop arm test (inability to maintain arm in 90° abduction against gravity) is highly specific for supraspinatus tear. Loss of abduction initiation is the hallmark finding. ### Frequency of Rotator Cuff Tears | Muscle | Frequency | Typical Presentation | |--------|-----------|---------------------| | Supraspinatus | 50–60% | Loss of abduction, drop arm test positive | | Infraspinatus | 20–30% | Loss of external rotation, weakness | | Subscapularis | 10–15% | Loss of internal rotation, lift-off test positive | | Teres minor | <5% | Rare in isolation; usually with infraspinatus | **Mnemonic:** **SITS** = Supraspinatus, Infraspinatus, Teres minor, Subscapularis — *Supraspinatus is the most commonly torn.* ### Pathophysiology Degenerative rotator cuff tears in this age group result from: - Chronic subacromial impingement - Repetitive microtrauma (occupational in this case) - Age-related collagen degeneration - Reduced blood supply at the critical zone