Shoulder, axilla, and brachial plexus

Abstract

Shoulder girdle (pectoral girdle)

The shoulder girdle is composed of the two bones (clavicle and scapula) and two joints (acromioclavicular and sternoclavicular joints) that connect the upper limb to the trunk.

Bones of the shoulder girdle

Clavicle

Scapula

  • Anatomy
    • The scapula is a flat and triangular bone
    • It has three borders (medial, lateral/axillary, and superior), three angles (superior, inferior, and lateral), and two surfaces
    • Posterior (dorsal) scapular surface landmarks
    • Costal (ventral) scapular surface landmarks:
      • Subscapular fossa: A concave depression on the ventral aspect of the body of the scapula.
      • Coracoid process: A bony hook that arises from the superior border of the scapula and lies below the lateral end of the clavicle
      • Suprascapular notch
        • A deep groove between the coracoid process and the superior scapular border.
        • The suprascapular nerve passes through this notch.
        • Ossification of the superior transverse ligament converts the notch into the suprascapular foramen.
    • The glenoid cavity is located on the lateral scapular border and is the socket of the glenohumeral joint!
  • Suprascapular nerve entrapment
  • Scapular anastomosis: An anastomosis between the branches of the first part of the subclavian artery and third part of the axillary artery that provides collateral circulation in case of occlusion of one of the involved branches.
    • Subscapular fossa: subscapular artery
    • Infraspinatus fossa: circumflex scapular artery, branch of the subscapular artery
    • Supraspinous fossa: suprascapular artery, branch of the thyrocervical trunk
    • Inferior angle: anastomosis between dorsal scapular artery and the subscapular artery
    • Lateral border of scapula: subscapular artery (branch of axillary artery)
    • Medial border of scapula: Dorsal scapular artery

Joints of the shoulder girdle

Sternoclavicular joint

Acromioclavicular joint (AC joint)

Scapulothoracic joint

Movements of the shoulder girdle

Shoulder joint (Glenohumoral joint)

Shoulder joint (Glenohumeral joint)

Bone structures associated with the glenohumeral joint differ in size. The head of the humerus is approximately four times larger than the joint socket of the scapula. The result is a greater range of motion but also joint instability!

Muscles of the shoulder joint

Rotator cuff

Muscle

Origin

All originate from the scapula

Insertion

All insert into the humerus

Function

All movements occur at the shoulder joint

Innervation
Supraspinatus
  • Abduction of the arm (from 0°–15°)
  • Suprascapular nerve
Infraspinatus
  • External rotation of the arm
Teres minor muscle
  • Axillary (lateral) border of the scapula
  • Adduction of the arm
  • External rotation of the arm
  • Axillary nerve

Subscapularis

  • Internal rotation of the arm
  • Subscapular nerves

Fascial slings of the pectoral girdle

  • Fascial slings are made up of several functionally antagonistic muscles.
  • There are four fascial slings in the shoulder region, which move the scapula in different directions (oblique sling, horizontal sling, anterior vertical sling, and posterior vertical sling).
  • The trapezius muscle is divided into three parts (anterior, transverse, and descending) that have different functions

Oblique sling

Name Anatomy Innervation Function Special characteristics

Rhomboid muscles

  • Dorsal scapular nerve
  • Scapula: elevation and retraction
  • Lateral angle of the scapula: caudal rotation

Serratus anterior

  • Origin: Ribs 1–9
  • Insertion
    • Superior angle of scapula (superior part)
    • Medial border of scapula (middle and inferior part)
    • Inferior angle of scapula (inferior part)
  • Long thoracic nerve
  • Scapula: protraction (i.e., pulls the scapulae forward as in a punching action)
  • Inferior angle of the scapula: cranial rotation
  • Maintains position of the medial border and inferior angle of the scapula in contact with the thoracic wall
  • Accessory muscles of respiration
  • Applied anatomy: winging of the scapula
    • Abnormally prominent medial border of the scapula
    • Caused by long thoracic nerve palsy (e.g., due to shoulder dislocation, certain repetitive arm movements as in tennis or archery)

Horizontal sling

Muscle Anatomy Innervation Function
Transverse part of the trapezius

The serratus anterior muscle is part of both the oblique and horizontal slings!

Anterior vertical sling

Muscle Anatomy Innervation Function
Pectoralis minor
  • Medial and lateral pectoral nerves
  • Scapula: depression and protraction
  • Lateral angle of scapula: downward rotation
  • Accessory muscle of respiration
Descending part of trapezius
  • Scapula: retraction (adduction) and depression
  • Overhead abduction of the arm

Posterior vertical sling

Name Origin Innervation Function
Levator scapulae
  • Origin
    • Transverse processes of C1–2
    • Posterior tubercle of the transverse process of C3–4
  • Insertion: superior angle of scapula
  • Dorsal scapular nerve

Ascending part of the trapezius

(upper fibers)

Muscles of the shoulder that insert into the arm

  • In addition to the antagonistic fascial slings, the shoulder is brought to motion by other muscles that insert into the arm.
  • The deltoid muscle primarily forms the contour of the shoulder.
Muscle Anatomy Innervation Function
Ventral muscles

Pectoralis major

(forms the anterior axillary fold)

  • Medial and lateral pectoral nerves
  • Adduction, internal rotation, anteversion
  • Accessory muscle of respiration
Coracobrachialis
  • Anteversion, adduction, internal rotation
Lateral muscle Deltoid
  • Axillary nerve
  • Clavicular part: anteversion, internal rotation, adduction
  • Acromial part: abduction
  • Spinal part: retroversion, external rotation, adduction
Dorsal muscles

Latissimus dorsi muscle

(forms the posterior axillary fold)

  • Thoracodorsal nerve
  • Adduction, internal rotation, retroversion
  • Also called the “coughing muscle”
  • Accessory muscles of respiration
Teres major muscle
  • Thoracodorsal and/or subscapular nerve
  • Adduction, internal rotation, retroversion

Other muscles involved in shoulder motion

Name Anatomy Function Innervation Special characteristics
Subclavius muscle
  • Depresses the shoulder and lateral end of the clavicle
  • Subclavian nerve
Biceps brachii
  • Long head: shoulder abduction
  • Short head: shoulder adduction
  • Both: internal rotation of the shoulder
  • None
Triceps brachii
  • Origin: infraglenoid tubercle
  • Insertion: olecranon
  • Radial nerve
  • None

References:[1]

The axilla

Axillary fossa

The anterior axillary fold is formed from the pectoralis major muscle and the posterior axillary fold from the latissimus dorsi muscle!

Axillary contents

Axillary artery

Axillary vein

Axillary lymph node groups

  • Anterior group (pectoral): drain mammary and pectoral regions
  • Posterior group (subscapular): drain the upper back and posterior neck
  • Lateral group (humeral): drain the upper limb
  • Central group (lies in axillary fat): receives lymphatics from the anterior, posterior, and lateral groups
  • Apical group (lies behind the pectoralis minor): receives lymphatics from the central group and upper outer quadrant of the breast

Brachial plexus

  • Description: A network of nerves that are derived from C5–T1 spinal nerves spinal nerves and give rise to the peripheral nerves of the upper limb and shoulder.
  • Roots: anterior rami of C5–T1 spinal nerves
    • Dorsal scapular nerve arises directly from C5
    • Long thoracic nerve arises from roots C5, C6, and C7
    • First intercostal nerve arises from T1
  • Trunks: the roots form 3 trunks that are located in the neck
    • C5 and C6 → upper trunk
      • The suprascapular nerve and nerve to subclavius arise from the upper trunk (C5, C6)
    • C7 → middle trunk
    • C7 and C8 → lower trunk
  • Divisions: all 3 trunks divide into anterior and posterior divisions that enter into the axillary fossa
    • All anterior divisions: innervate the flexor compartment of the arm and forearm (i.e., the anterior compartment)
    • All posterior divisions: innervate the extensor compartment of the arm and forearm (i.e., the posterior compartment)
  • Cords:
    • Only the cords and proximal portions of their branches lie in the axillary fossa.
    • There are 3 cords, which are named according to their position in relation to the axillary artery.
      • Anterior divisions of upper and middle trunks → lateral cord
      • Anterior division of the middle trunk → medial cord
      • Posterior divisions of all trunks → posterior cord
  • Branches
  • Functional significance
    • Provides the motor innervation of all upper limb and shoulder muscles except for the trapezius, which is supplied by the spinal accessory nerve.
    • Provides the sensory innervation of the axilla and the upper limb except for medial portion of the arm, which is supplied by the intercostobrachial nerve.
  • Clinical significance: see peripheral nerve injuries

The main branch of the lateral cord is the musculocutaneous nerve
The main branch of the medial cord is the ulnar nerve
The main branches of the posterior cord is the radial nerve and axillary nerves
Branches from the medial and lateral cords form the median nerve

Axillary spaces

  • Anatomical spaces (2 triangular; 1 quadrilateral) bound by the teres minor muscle (cranially) and the teres major muscle (caudally) through which axillary contents leave the axilla.

Medial axillary space

A triangular space and with a cranial, caudal, and lateral border and apex facing medially (no medial border)

Lateral axillary space

A quadrangular space

Triangular interval

A triangular space with a cranial, lateral, and medial border and apex facing caudally (no caudal border)

A simple trick for remembering the structures at the borders of the axillary spaces is to simulate them with the middle and index fingers of both hands. Form a victory sign with both hands and superimpose them at ∼ 90°. Three spaces are created that simulate the axillary spaces.

Clinical significance

  • 1. Nawa S. Scapular winging secondary to apparent long thoracic nerve palsy in a young female swimmer. J Brachial Plex Peripher Nerve Inj. 2015; 10(1): pp. e57–e61. doi: 10.1055/s-0035-1567806.
  • Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences; 2015.
  • Waldman SD. Atlas of Uncommon Pain Syndromes. Elsevier Health Sciences; 2013.
  • Murphy A, Hoskins A. Axillary Vein. https://radiopaedia.org/articles/axillary-vein-1. Updated January 1, 2018. Accessed November 18, 2018.
  • Pacifici S. Axillary Nodes. https://radiopaedia.org/articles/axillary-nodes. Updated January 1, 2018. Accessed November 18, 2018.
last updated 11/21/2018
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