Topic Overview

Muscles of the Scapular Region

Muscles of the Scapular Region

1. Deltoid

• Origin → lateral ⅓ of clavicle, acromion, spine of scapula.

• Insertion → deltoid tuberosity of humerus.

• Nerve supply → Axillary nerve (C5, C6).

• Action →

  • Anterior fibers → flex, medially rotate arm.

  • Middle fibers → abduct arm (15°–90°).

  • Posterior fibers → extend, laterally rotate arm.

• Clinical → axillary nerve injury → deltoid paralysis, loss of abduction beyond 15°, regimental badge anesthesia.

2. Supraspinatus

• Origin → supraspinous fossa of scapula.

• Insertion → superior facet of greater tubercle of humerus.

• Nerve supply → Suprascapular nerve (C5, C6).

• Action → initiates abduction of arm (first 15°).

• Clinical → supraspinatus tendon commonly involved in rotator cuff tears → painful arc syndrome.

3. Infraspinatus

• Origin → infraspinous fossa of scapula.

• Insertion → middle facet of greater tubercle of humerus.

• Nerve supply → Suprascapular nerve (C5, C6).

• Action → lateral rotation of arm.

• Clinical → injury leads to weakness of external rotation.

4. Teres Minor

• Origin → lateral border of scapula (upper ⅔).

• Insertion → inferior facet of greater tubercle of humerus.

• Nerve supply → Axillary nerve (C5, C6).

• Action → lateral rotation of arm, helps stabilize shoulder joint.

• Clinical → affected in axillary nerve injury.

5. Teres Major

• Origin → inferior angle and lower part of lateral border of scapula.

• Insertion → medial lip of intertubercular sulcus of humerus.

• Nerve supply → Lower subscapular nerve (C5, C6).

• Action → extension, adduction, medial rotation of arm.

• Clinical → contributes to posterior axillary fold (with latissimus dorsi).

6. Subscapularis

• Origin → subscapular fossa of scapula.

• Insertion → lesser tubercle of humerus.

• Nerve supply → Upper and lower subscapular nerves (C5, C6, C7).

• Action → medial rotation and adduction of arm.

• Clinical → part of rotator cuff; tendon commonly involved in cuff tears.

Functional Grouping

• Rotator Cuff Muscles (SITS):

  • Supraspinatus → abduction (0–15°).

  • Infraspinatus → lateral rotation.

  • Teres minor → lateral rotation.

  • Subscapularis → medial rotation, adduction.

  • Function: stabilize humeral head in glenoid cavity.

• Other Scapular Muscles:

  • Deltoid → major abductor (15°–90°).

  • Teres major → adduction and medial rotation.

Deltoid

Origin

• Anterior fibers → lateral ⅓ of clavicle.

• Middle fibers → acromion process.

• Posterior fibers → spine of scapula.

Insertion

• Deltoid tuberosity of humerus.

Nerve Supply

• Axillary nerve (C5, C6).

Actions

• Anterior fibers → flex and medially rotate arm.

• Middle fibers → abduct arm (chief abductor from 15°–90°).

• Posterior fibers → extend and laterally rotate arm.

• Collectively → give rounded contour of shoulder.

Clinical Anatomy

• Intramuscular injection: given in middle of deltoid (safe zone → avoid axillary nerve and posterior circumflex humeral vessels).

• Axillary nerve injury (fracture of surgical neck of humerus, shoulder dislocation) → paralysis of deltoid → loss of abduction beyond 15°, flattening of shoulder, loss of sensation over “regimental badge” area.

• Deltoid wasting: seen in axillary nerve palsy.

Dissection of Deltoid

Step 1: Exposure

• Make vertical skin incision over lateral aspect of shoulder.

• Reflect skin and superficial fascia to expose deltoid.

Step 2: Identification

• Identify its three parts (anterior, middle, posterior fibers).

• Note its origin from clavicle, acromion, and spine of scapula.

Step 3: Reflection

• Cut muscle fibers near origin and reflect laterally.

• Observe underlying subacromial bursa, supraspinatus tendon, and shoulder joint capsule.

Step 4: Nerve and Vessel Identification

• Deep surface of deltoid → identify axillary nerve and posterior circumflex humeral artery passing through quadrangular space.

Step 5: Clinical Demonstration

• Test deltoid by asking subject to abduct arm against resistance from 15°–90°.

• Demonstrate injection site (2–3 cm below acromion, lateral aspect of muscle belly).