Which clinical finding best distinguishes a supraspinatus rotator cuff tear from an infraspinatus tear in a 52-year-old man presenting with shoulder pain and weakness?

Explanation

## Distinguishing Supraspinatus from Infraspinatus Tears ### Clinical Presentation Comparison | Feature | Supraspinatus Tear | Infraspinatus Tear | |---------|-------------------|-------------------| | **Primary action lost** | Initiation of abduction (0–15°) | External rotation (especially at 90° abduction) | | **Drop arm test** | Positive (cannot maintain abduction) | Negative or weakly positive | | **Lag sign** | Supraspinatus lag sign (arm drifts into adduction) | Infraspinatus lag sign (arm drifts into internal rotation) | | **Weakness pattern** | Abduction weakness (0–15°) | External rotation weakness | | **Atrophy location** | Supraspinous fossa | Infraspinous fossa | ### Key Discriminator **Key Point:** The infraspinatus lag sign (also called external rotation lag sign) is the best clinical discriminator for infraspinatus-specific pathology. The patient cannot maintain external rotation at 90° shoulder abduction when the examiner releases the wrist, causing the arm to drift into internal rotation. **Clinical Pearl:** Supraspinatus tears present with a positive drop arm test (inability to lower the arm smoothly from 90° abduction), whereas infraspinatus tears present with preserved abduction but weakness of external rotation. The lag sign tests the specific rotator cuff muscle in isolation. **High-Yield:** The infraspinatus lag sign is performed by: 1. Abducting the shoulder to 90° 2. Externally rotating the shoulder to 45° 3. Releasing the wrist and observing for drift into internal rotation (positive sign) ### Why This Matters Infraspinatus tears are often missed because patients can still abduct the shoulder (supraspinatus is intact). The lag sign specifically tests external rotation, which is the primary function of the infraspinatus. [cite:Netter's Anatomy for Orthopaedic Surgery, Rotator Cuff Chapter]