## Infraspinatus Anatomy and Clinical Correlation **Key Point:** The infraspinatus is one of the four rotator cuff muscles, responsible for external rotation and posterior stabilization of the glenohumeral joint. Understanding its precise anatomical attachments is critical for interpreting imaging and predicting functional deficits. ### Infraspinatus Anatomy | Feature | Detail | |---------|--------| | **Origin** | Infraspinous fossa of the scapula (posterior surface) | | **Insertion** | Middle facet of the greater tubercle of the humerus | | **Innervation** | Suprascapular nerve (C5–C6) | | **Primary Action** | External rotation of the shoulder | | **Secondary Actions** | Posterior stabilization, assists in adduction | ### Correct Statements **High-Yield:** - Innervation by suprascapular nerve is **correct** — the suprascapular nerve arises from the upper trunk of the brachial plexus and supplies both supraspinatus and infraspinatus. - Origin from infraspinous fossa is **correct** — this is the anatomical definition of the muscle. - Primary external rotator is **correct** — infraspinatus is the main external rotator, especially at 90° abduction. ### The Exception: Insertion Site **Warning:** Infraspinatus inserts on the **middle facet** of the greater tubercle, NOT the superior facet. The three facets of the greater tubercle are assigned as follows: | Facet | Muscle | |-------|--------| | Superior | Supraspinatus | | Middle | Infraspinatus | | Inferior | Teres minor | This distinction is clinically important because partial rotator cuff tears can spare one muscle while damaging another, and the insertion site determines the mechanical advantage and lever arm of each muscle. **Clinical Pearl:** In this patient's case, a full-thickness infraspinatus tear would cause weakness of external rotation (positive lag sign or external rotation lag sign) and loss of the posterior shoulder contour on inspection.
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