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M01.03.019 Shoulder Region: The Intrinsic Muscles of the Shoulder
The intrinsic muscles of the shoulder play a crucial role in the movement and stabilization of the upper limb. These muscles originate from the scapula and/or clavicle and attach to the humerus. Understanding their attachments, innervations, and actions is essential for students preparing for the USMLE Step 1.
Intrinsic Muscles Overview
The intrinsic muscles of the shoulder are collectively known as the scapulohumeral group. There are six muscles in this group, which include:
- Deltoid
- Teres Major
- Rotator Cuff Muscles (Supraspinatus, Infraspinatus, Subscapularis, and Teres Minor)
Table: Intrinsic Muscles of the Shoulder
| Muscle | Origin | Insertion | Action | Innervation |
|---|---|---|---|---|
| Deltoid | Lateral third of clavicle, acromion, spine of scapula | Deltoid tuberosity of humerus | Anterior: flexion and medial rotation Posterior: extension and lateral rotation Middle: abduction of arm | Axillary nerve |
| Teres Major | Posterior surface of the inferior angle of scapula | Medial lip of intertubercular groove of humerus | Adducts, extends, and medially rotates arm | Lower subscapular nerve |
| Supraspinatus | Supraspinous fossa of scapula | Greater tubercle of humerus | Abducts arm (0-15°), assists deltoid in 15-90° abduction | Suprascapular nerve |
| Infraspinatus | Infraspinous fossa of scapula | Greater tubercle of humerus | Laterally rotates arm | Suprascapular nerve |
| Subscapularis | Subscapular fossa (costal surface of scapula) | Lesser tubercle of humerus | Medially rotates arm | Upper and lower subscapular nerves |
| Teres Minor | Posterior surface of scapula, adjacent to lateral border | Greater tubercle of humerus | Laterally rotates arm | Axillary nerve |
Key Muscles of the Shoulder
1. Deltoid
- Shape: Inverted triangle
- Divisions:
- Anterior fibers: Flexion and medial rotation of the arm.
- Posterior fibers: Extension and lateral rotation of the arm.
- Middle fibers: Primary abductor of the arm (from 15° to 90°).
- Innervation: Axillary nerve.
2. Teres Major
- Action: Adducts, extends, and medially rotates the arm.
- Innervation: Lower subscapular nerve.
3. Rotator Cuff Muscles
The rotator cuff muscles collectively provide stability to the shoulder joint. Their tone helps keep the humeral head in the glenoid fossa.
- Supraspinatus: Assists in the first 15° of arm abduction.
- Infraspinatus: Laterally rotates the arm.
- Subscapularis: Medially rotates the arm.
- Teres Minor: Laterally rotates the arm.
Clinical Relevance: Rotator Cuff Tendonitis
Overview
- Cause: Inflammation of the rotator cuff tendons, often due to repetitive shoulder use.
- Most Commonly Affected: Supraspinatus tendon.
- Mechanism: The supraspinatus tendon rubs against the coraco-acromial arch during abduction, leading to tendon inflammation and degeneration.
Treatment
- Conservative: Rest, analgesia, physiotherapy.
- Severe Cases: Steroid injections, surgery.
Points to Remember
- The scapulohumeral group consists of six muscles: Deltoid, Teres Major, and the Rotator Cuff Muscles.
- The Deltoid is the primary abductor of the arm after the first 15° of abduction.
- Teres Major is innervated by the lower subscapular nerve, whereas the Deltoid is innervated by the axillary nerve.
- The rotator cuff muscles provide stability to the glenohumeral joint.
- Rotator cuff tendonitis is a common condition, especially in athletes or individuals with repetitive shoulder motions.