Intrinsic muscles of the shoulder

Question 1

Extrinsic and intrinsic muscles of the shoulder

Answer 1

The muscles of the shoulder are associated with movements of the upper limb. They produce the characteristic shape of the shoulder, and can be divided into two groups:

Extrinsic – originate from the torso, and attach to the bones of the shoulder (clavicle, scapula or humerus).
Intrinsic – originate from the scapula and/or clavicle, and attach to the humerus.

he intrinsic muscles (also known as the scapulohumeral group) originate from the scapula and/or clavicle, and attach to the humerus.

There are six muscles in this group – the deltoid, teres major, and the four rotator cuff muscles (supraspinatus, infraspinatus, subscapularis and teres minor).

Question 2

Deltoid muscle

Answer 2

Origins
Lateral 1/3 of Clavicle (clavicular part), Acromion (acromial part), Spine of Scapula (spinal part)

Mnemonic: ‘Deltoid helps you carry SACS’

Insertion
Deltoid tuberosity of humerus
Innervation
Axillary nerve (C5, C6)
Blood supply
Deltoid and acromial branches of thoracoacromial artery, subscapular artery, anterior and posterior circumflex humeral arteries, deltoid branch of profunda brachii
Function
Clavicular part: flexion and internal rotation of the arm,
Acromial part: abduction of the arm beyond the initial 15°
Spinal part: extension and external rotation of the arm.

Question 3

Teres major

Answer 3

Origins
Inferior angle and lower part of the lateral border of the scapula
Insertions
Intertubercular sulcus (medial lip) of the humerus
Innervation
Lower subscapular nerve from the posterior cord of the brachial plexus
Function
Adduction and medial rotation of the humerus (arm)
The teres major muscle is vascularized by the thoracodorsal branch of the subscapular artery and the posterior circumflex humeral artery.

Question 4

Rotator cuff muscles:

Supraspinatus muscle

Answer 4

Origin
Supraspinous fossa of scapula
Insertion
Greater tubercle of humerus
Action
Shoulder joint: abduction of arm, stabilization of the humeral head in the glenoid cavity
Innervation
Suprascapular nerve (C5, C6)
Blood supply
Suprascapular artery

Supraspinatus lies deep to the trapezius muscle and superior to the spine of the scapula and infraspinatus muscle. The tendon of supraspinatus is separated from the coracoacromial ligament, the acromion and the deltoid muscle by the subacromial bursa.

Question 5

Rotator cuff muscles:

Infraspinatus muscle

Answer 5

Origin
Infraspinous fossa of scapula
Insertion
Greater tubercle of humerus
Action
Shoulder joint: Arm external rotation; 
Stabilizes humeral head in glenoid cavity
Innervation
Suprascapular nerve (C5, C6)
Blood supply
Suprascapular artery, circumflex scapular arteries

Question 6

Rotator cuff muscles:

Teres minor

Answer 6

Origin
Lateral border of scapula
Insertion
Greater tubercle of humerus
Action
Shoulder joint: Arm external rotation, arm adduction;
Stabilizes humeral head in glenoid cavity

Innervation
Axillary nerve (C5, C6)
Blood supply
Suprascapular artery, dorsal scapular artery

Question 7

Rotator cuff muscle:

Subscapularis muscle

Answer 7

Origin
Subscapular fossa of scapula
Insertion
Lesser tubercle of humerus
Action
Shoulder joint: Arm internal rotation
Stabilizes humeral head in glenoid cavity
Innervation
Upper and lower subscapular nerves (C5 - C6)
Blood supply
Suprascapular artery, axillary artery, subscapular artery

Question 8

Clinical relevance: rotator cuff tendonitis

Answer 8

Rotator cuff tendonitis refers to inflammation of the tendons of the rotator cuff muscles. This usually occurs secondary to repetitive use of the shoulder joint.

The muscle most commonly affected is the supraspinatus. During abduction, it ‘rubs’ against the coraco-acromial arch. Over time, this causes inflammation and degenerative changes in the tendon itself.

Conservative treatment of rotator cuff tendonitis involves rest, analgesia, and physiotherapy. In more severe cases, steroid injections and surgery can be considered.