infraspinatus

**Anatomy** - Origin: Infraspinous fossa of the scapula. - Insertion: Middle facet of the greater tubercle of the humerus. - Action: External (lateral) rotation of the arm at the glenohumeral joint. It also helps stabilize the humeral head in the glenoid fossa (dynamic stabilizer) and weakly adducts/extends the arm. - Innervation: Suprascapular nerve (C5-C6), from the upper trunk of the brachial plexus. The nerve passes through the suprascapular notch, under the superior transverse scapular ligament, to supply supraspinatus. It then travels around the spinoglenoid notch to reach infraspinatus. - Blood supply: Suprascapular artery (from the thyrocervical trunk) and circumflex scapular artery (from the subscapular artery). These two form an anastomosis around the scapula.

**Rotator cuff (SITS)** - Supraspinatus: initiates abduction (0-15°). Axillary nerve takes over abduction after that (deltoid). It is the most commonly injured rotator cuff muscle (Empty can/Jobe test; drop arm test). - Infraspinatus: external rotation. - Teres minor: external rotation. Innervated by the axillary nerve. - Subscapularis: internal rotation. Innervated by the upper and lower subscapular nerves (Lift-off test/Gerber). - Mnemonic: "SItS" with the rotators attaching to the greater tubercle: Supraspinatus, Infraspinatus, Teres minor (superior, middle and inferior facets: "SIT" from top to bottom). Subscapularis attaches to the lesser tubercle ("Lesser = Little/Lift-off").

**Clinical significance** 1. Rotator cuff tear/tendinopathy: Weakness of external rotation, a positive "lag sign" (external rotation lag sign), and a positive Patte test. The supraspinatus is most commonly torn, followed by infraspinatus in larger tears. 2. Suprascapular nerve entrapment: Nerve compression at the suprascapular notch causes supraspinatus and infraspinatus weakness and atrophy, with pain and weakness in abduction and external rotation. Compression at the spinoglenoid notch (for example by a ganglion cyst, or in overhead athletes such as volleyball players and baseball pitchers) affects infraspinatus only, with sparing of supraspinatus. 3. Upper trunk brachial plexus injury (Erb palsy): Involves C5-C6, causing the "waiter's tip" position with the arm adducted and internally rotated, and the forearm extended and pronated. Abduction (supraspinatus, deltoid), external rotation (infraspinatus, teres minor) and flexion (biceps) are lost. 4. Physical exam: Test external rotation against resistance with the elbow flexed to 90° and the arm at the side. Weakness or pain suggests infraspinatus pathology. The Hornblower sign is the inability to externally rotate with the arm abducted. It indicates teres minor (and infraspinatus) tears. 5. Imaging: MRI is the best test for rotator cuff tears. Infraspinatus atrophy can be seen with chronic tears or nerve injury (fatty infiltration).

**High-yield points** - Suprascapular nerve = supraspinatus + infraspinatus. Axillary nerve = deltoid + teres minor. Subscapular nerves = subscapularis (and teres major, via the lower subscapular nerve). - Lesion at the spinoglenoid notch spares supraspinatus. Lesion at the suprascapular notch affects both. - Shoulder dislocation (anterior-inferior is the most common) may injure the axillary nerve, and may also tear the rotator cuff in older patients.