Shoulder

1. Muscles

There are 6 muscles which converge from the scapula onto the humerus: subscapularis, infraspinatus, supraspinatus, deltoid, teres major and teres minor.

Subscapularis

Description: broad, flat, triangular muscles of lower back. Covered by trapezius superiorly. Contributes to posterior wall of axilla. Lateral border of latissimus dorsi forms a boundary of the lumbar triangle.

Origin: indirect attachment via lumbodorsal fascia into spines of lower six thoracic vertebrae, lumbar vertebrae, lower 3 – 4 ribs and iliac crest

Insertion: spirals around teres major, inserts in floor of intertubcular sulcus of humerus

Action: primer mover of shoulder joint extension, powerful arm adductor, medially rotates arm at shoulder, depresses scapula

Relations:

Nerve supply: Subscapular nerves C₅ – C₇

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Infraspinatous

Description: A rotator cuff muscle. Covered by infraspinatus fascia (strong membrane which also covers teres minor, firmly attached to bone at margin of muscle).

Origin: infraspinous fossa

Insertion: greater tubercle of humerus (posterior to insertion of supraspinatus)

Action: stabilises joint by bracing humeral head against glenoid cavity, powerful lateral rotator of humerus

Relations: partially covered by deltoid and trapezius

Nerve supply: Supracapular nerves C₅ and C₆

Clinical relevance: fracture of the the scapula blade will result in a haematoma confined by the infraspinatus fascia.

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Supraspinatus

Description: A rotator cuff muscle. 

Origin: supraspinous fossa

Insertion: superior portion of greater tubercle of humerus

Action: stabilises joint by bracing humeral head against glenoid cavity, assists deltoid in abduction

Relations:

Nerve supply: Supracapular nerves C₅ and C₆

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Deltoid

Description: Thick, multipennate muscle forming rounded shoulder muscle mass

Origin: embraces insertion of trapezis, lateral third of clavicle, acromion and inferior lip of crest of scapular spine

Insertion: deltoid tuberosity of humerus

Action: 

• acromial fibres: prime mover of arm abduction
• anterior fibres: assist pectoralis major in flexing and medially rotating arm
• posterior fibres assist latissimus dorsi arm extension. Lateral rotator.

Relations:

Nerve supply: Axillary nerve C₅ and C₆

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Teres Major

Description: Thick, rounded muscle. Forms part of posterior axilla wall

Origin: posterior surface of scapula at inferior angle

Insertion: medial lip of intertubercular sulcus of humerus

Action: posteromedially extends, medially rotates, adducts humerus, synergist of latissimus dorsi

Relations: inferior to teres minor,

Nerve supply: Lower subscapular nerve C₅ and C₇

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Teres Minor

Description: Small, elongated muscle. Rotator cuff muscle

Origin: posterior surface of scapula at inferior angle

Insertion: medial lip of intertubercular sulcus of humerus

Action: posteromedially extends, medially rotates, adducts humerus, synergist of latissimus dorsi

Relations: inferior to infraspinatus

Nerve supply: Lower subscapular nerve C₅ and C₇

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2. Joints

Shoulder Joint

Description: A multi-axial ball-and-socket synovial joint. A ring of fibrocartilage called the glenoid labrum attaches to the margins of the glenoid cavity

 

Special regions

Axilla

Description: pyramidal region consisting of 4 sides with an open apex

Borders:

• Apex (axillary inlet) – lateral border of first rib, superior border of scapula, posterior border of clavicle.

• Lateral wall – intertubercular sulcus of humerus.

• Medial wall – serratus anterior and thoracic wall

• Anterior wall – pectoralis major, pectoralis minor and the subclavius muscles.

• Posterior wall – subscapularis, teres major and latissimus dorsi.

Contents:

• Blood vessels:
  • Axillary artery
  • Axillary vein

• Nerves – brachial plexus (spinal nerves that form the peripheral nerves of the upper limb).
• Lymphatics – axillary lymph nodes (pectoral, subscapular, lateral, central, infraclavicular groups)

• Muscles – Biceps brachii (short head) and coracobrachialis

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Axillary artery

Description: main arterial supply of upper limb, continues from third part of subclavian artery

Course:

• Commences at outer border of first rib
• Enters apex of axilla by passing over first digitation of serratus anterior
• At border of teres major it becomes the brachial arteries

Branches: The first part has one branch, the second part two, and the third part three branches.

• First part:
  1. superior thoracic artery – supplies both pectoral muscles
• Second part:
  1. thoracoacromial artery – follows upper border of pectoralis minor to pierce the clavipectoral fascia by 4 terminal branches (clavicular, deltoid, acromial and pectoral)
  2. lateral thoracic artery – follows lower border of pectoralis minor supplying pectoralis and serratus anterior muscles
• Third part:
  1. subscapular artery – largest branch of the axillary artery, runs down posterior axillary wall giving of dorsal branch (circumflex scapular artery)
  2. anterior circumflex humeral artery –
  3. posterior circumflex humeral artery –

Axillary vein

Description: large vein commencing at lower border of teres major, continues from basilic vein

Course: At the outer border of the first rib the axillary vein enters the root of the neck as the subclavian vein in front of scalenus anterior