A 58-year-old male carpenter from Delhi presents with a 3-month history of progressive right shoulder pain and weakness. He reports difficulty lifting objects overhead and pain when reaching behind his back. On examination, he has positive "drop arm" sign and "lag sign." Passive range of motion is preserved. MRI shows a full-thickness tear of the supraspinatus tendon with mild retraction. What is the most likely anatomical consequence of this injury?

Explanation

## Supraspinatus Tear: Anatomical and Clinical Consequences ### Anatomy of the Supraspinatus **Key Point:** The supraspinatus is the primary initiator of shoulder abduction, particularly in the first 15 degrees (0–15°). After 15°, the deltoid takes over as the main abductor. ### Mechanism of the Drop Arm Sign When the supraspinatus is torn: 1. The patient cannot initiate abduction smoothly 2. The deltoid alone cannot overcome inertia and gravity in the initial arc 3. The arm "drops" when the patient attempts to lower it in a controlled manner from 90° abduction 4. This is a highly specific sign for full-thickness supraspinatus tear ### Why Passive ROM Is Preserved **Clinical Pearl:** Preservation of passive range of motion distinguishes a rotator cuff tear from a frozen shoulder (adhesive capsulitis). The tear does not restrict movement; it prevents active initiation. ### Role of Each Rotator Cuff Muscle | Muscle | Primary Action | Nerve | Loss of Function | |--------|---|---|---| | Supraspinatus | Abduction 0–15° | Suprascapular | Drop arm sign, weak abduction | | Infraspinatus | External rotation | Suprascapular | Lag sign (external rotation lag) | | Teres minor | External rotation | Axillary | Lag sign | | Subscapularis | Internal rotation, adduction | Upper/lower subscapular | Loss of internal rotation, lift-off sign | ### The Drop Arm Sign and Lag Sign **High-Yield:** Both signs are present in this case: - **Drop arm sign** = inability to lower arm in a controlled manner from 90° abduction (supraspinatus) - **Lag sign** = external rotation lag (infraspinatus/teres minor involvement) This suggests a large tear involving supraspinatus ± infraspinatus. ### Why the First 15 Degrees Matter The supraspinatus acts as a "starter motor" for abduction. After 15°, the deltoid (innervated by axillary nerve) becomes the dominant abductor. Loss of supraspinatus function means: - Patient cannot initiate abduction from rest - Gravity and inertia cannot be overcome in the critical 0–15° range - The arm drops when attempting controlled descent **Mnemonic:** **SITS** = Supraspinatus (abduction 0–15°), Infraspinatus (external rotation), Teres minor (external rotation), Subscapularis (internal rotation). ### MRI Findings Full-thickness tear with mild retraction confirms complete disruption of the tendon. The absence of severe retraction suggests the tear is not chronic (which would show significant muscle atrophy and retraction).