rotator cuff

**Components (SITS), with nerve, action, and key points** - **Supraspinatus**: suprascapular nerve (C5–C6). Initiates abduction (first ~15°). Most commonly injured; tendon passes under the acromion and is prone to impingement. Has a relatively hypovascular "critical zone" near its insertion. - **Infraspinatus**: suprascapular nerve. External rotation. - **Teres minor**: axillary nerve. External rotation (and weak adduction). - **Subscapularis**: upper and lower subscapular nerves. Internal rotation. It is the only cuff muscle inserting on the lesser tubercle. The other three insert on the greater tubercle. - The deltoid (axillary nerve) does abduction from about 15° to 90°. The trapezius and serratus anterior (long thoracic nerve) rotate the scapula for abduction beyond 90°.

**Function** The cuff dynamically stabilizes the shallow glenohumeral joint, centering the humeral head in the glenoid during arm movement. It resists the upward pull of the deltoid.

**Pathophysiology and clinical correlates** - **Impingement syndrome (subacromial)**: Repetitive overhead activity (swimmers, throwers, painters) compresses the supraspinatus tendon and subacromial bursa under the coracoacromial arch. Presents with painful arc (~60–120° of abduction). Positive **Neer** and **Hawkins-Kennedy** tests. - **Tendinopathy and tears**: Degeneration with age (hypovascularity, microtrauma) or acute trauma (fall on outstretched arm). Supraspinatus tears are most common. - **Empty can (Jobe) test**: Pain or weakness with resisted abduction at 90° with the thumb pointing down, suggesting supraspinatus injury. - **Drop arm test**: With a full-thickness supraspinatus tear, the patient cannot hold the arm abducted at 90° or lower it slowly. - **Infraspinatus/teres minor**: Weak external rotation. The lag sign and the Hornblower sign suggest a teres minor tear. - **Subscapularis**: Weak internal rotation. Lift-off test and belly-press test are positive. - **Imaging**: MRI is the best test for a suspected tear. X-ray may show a high-riding humeral head, which suggests a chronic massive tear, or subacromial spurs. Ultrasound is an alternative. - **Management**: Most partial tears and impingement are managed with NSAIDs, physical therapy, and sometimes subacromial corticosteroid injection. Surgical repair is for acute tears in young active patients, full-thickness tears, or failed conservative therapy.

**High-yield distinctions** - Rotator cuff injury versus **adhesive capsulitis** (frozen shoulder): in adhesive capsulitis, both active and passive range of motion are reduced, classically in diabetics. In cuff tears, passive ROM is usually preserved. - **Anterior shoulder dislocation** can injure the axillary nerve (deltoid weakness, lateral shoulder sensory loss), and **posterior humeral head fractures** can damage the cuff. - Mnemonic: **SITS** for the muscles. **"Lazy Lady Tea Sip"**-style mnemonics also exist for the lesser/greater tubercle insertions. The supraspinatus is the "**Just Don't Move**" muscle for the first 15° of abduction (abduction initiation, "**Jobe**" test). - Biceps long head tendon pain often coexists with cuff pathology (Speed and Yergason tests).