Shoulder and axilla

The upper limb bones and shoulder girdle form the superior appendicular skeleton. Four joints (glenohumeral, sternoclavicular, acromioclavicular, and scapulothoracic) and two bones (the clavicle and scapula) form the shoulder girdle. The glenohumeral joint (i.e., shoulder joint) is one of the most flexible joints in the body and relies on a complex system of muscles, including the rotator cuff, to keep it in shape and enable mobility. Branches of the subclavian and axillary arteries supply the entire upper extremity. Veins of the upper extremity drain into the axillary vein, and lymphatics of the upper extremity drain into the axillary lymph node groups. The branches of the brachial plexus supply the motor and sensory innervation of the upper extremity. The axilla is a three-dimensional pyramid-shaped area at the junction of the arm and thorax, inferior to the shoulder joint, through which important vessels and nerves pass from the trunk to the arm.

For more information, see “Neurovasculature of the upper limbs,” “Upper arm and elbow,” and “Forearm, wrist, and hand.”

• Definition: a bony ring that connects the upper limb to the axial skeleton
• Bones ^[1]
  • Clavicle
  • Scapula
• Joints ; ^[2]
  • True anatomical joints (movement occurs between two directly articulating skeletal structures)
    • Acromioclavicular joint
    • Sternoclavicular joint
    • Glenohumeral joint
  • Physiologic joint: scapulothoracic joint

Clavicle ^[1][2]

• Definition: an S-shaped bone that connects the scapula to the sternum, located above and anterior to the first rib
• Joints
  • Sternal end (medial): articulates with the manubrium of the sternum to form the sternoclavicular joint
  • Acromial end (lateral): articulates with the acromion of the scapula to form the acromioclavicular joint
• Muscle attachments
  • Lateral one-third: trapezius and deltoid muscles
  • Medial two-thirds: sternocleidomastoid, pectoralis major, and subclavius muscles
• Surface markings
  • Subclavian groove
    • A groove on the inferior surface of the middle third of the clavicle
    • The subclavius muscle attaches here.
  • Impression for costoclavicular ligament (medial)
    • A rough depressed oval area on the medial third of the clavicle
    • The costoclavicular ligament attaches here, binding the first rib to the clavicle and limiting shoulder elevation.
  • Tuberosity for coracoclavicular ligament (lateral)
    • Formed by the conoid tubercle and the trapezoid line
    • The site for attachment of the two parts of the coracoclavicular ligament (the conoid and trapezoid ligaments)
      • Conoid tubercle: conoid ligament insertion point
      • Trapezoid line: trapezoid ligament insertion point

Scapula ^[1][2]

• Definition: a flat and triangular-shaped bone that overlies the 2^nd–7^thposterior ribs.
• Borders: medial, lateral/axillary, and superior
• Angles: superior, inferior, and lateral
• Surfaces
  • Posterior surface
    • Spine of scapula
      • Horizontal bony ridge that divides the posterior scapular surface into the supraspinous fossa and infraspinous fossa
      • The deltoid muscle attaches to the deltoid tubercle on the scapular spine.
    • Acromion
      • Lateral continuation of the scapular spine that projects above the glenoid fossa
      • Articulates with the acromial end of the clavicle
    • Supraspinous fossa
      • Concave depression above the scapular spine
      • The supraspinatus muscle attaches here.
    • Infraspinous fossa
      • Concave depression below the scapular spine
      • The infraspinatus muscle attaches here.
  • Costal (anterior) surface
    • Subscapular fossa
      • Concave depression on the body of the scapula
      • The subscapularis muscle attaches here.
    • Coracoid process
      • Anteriorly projecting bony hook that arises from the superior border of the scapula
      • Lies below the lateral end of the clavicle, in close proximity to the brachial plexus and axillary vessels
      • The pectoralis minor, coracobrachialis, and short head of the biceps brachii muscles attach here.
    • Suprascapular notch
      • A deep groove between the coracoid process and the superior scapular border
      • The superior transverse ligament converts the notch into the suprascapular foramen, through which the suprascapular nerve passes.
    • Glenoid fossa
      • A shallow, concave, anterolaterally directed oval fossa located on the superolateral border of the scapula
      • Articulates with the head of the humerus to form the glenohumeral joint
      • The supraglenoid tubercle, where the glenohumeral ligament attaches, is located just above the glenoid fossa.

Sternoclavicular joint ^[1][2]

• Definition
  • A saddle joint between the medial end of the clavicle and the manubrium sternum
  • The only anatomical (true) joint connecting the shoulder girdle to the axial skeleton
• Ligaments
  • Anterior sternoclavicular ligament and posterior sternoclavicular ligament
    • Between the manubrium and the sternal end of the clavicle
    • Strengthens the joint capsule anteriorly and posteriorly and prevents displacement of the clavicle
  • Interclavicular ligament
    • Unites the sternal ends of both clavicles
    • Strengthens the joint capsule superiorly
  • Costoclavicular ligament
    • Unites the first rib to the sternal end of the clavicle
    • Limits elevation of the shoulder girdle
• Movements
  • Elevation and depression
  • Protraction and retraction
  • Circumduction

Acromioclavicular joint (AC joint) ^[1][2]

• Definition: : a synovial plane joint between the lateral end of the clavicle and the acromion of the scapula
• Ligaments
  • Acromioclavicular ligament
    • Extends between the acromion and the clavicle
    • Strengthens the superior aspect of the joint capsule
    • Resists anteroposterior AC joint displacement
  • Coracoclavicular ligament
    • Extends between the coracoid process of the scapula and the lateral end of the clavicle
    • Divided into the conoid ligament (medial) and trapezoid ligament (lateral)
    • Resists rotational and vertical AC joint displacement
• Movements
  • No muscles act specifically at the AC joint.
  • Anteroposterior and rotational movement is possible with movement of the scapulothoracic joint.

Scapulothoracic joint ^[1][2]

• Type of joint
  • A physiological rather than an anatomical joint
  • The term scapulothoracic joint refers to the movement of the scapula relative to the associated muscles and the thoracic wall.
• Movements of the scapula (scapulothoracic joint)
  • Elevation: trapezius (upper part), levator scapulae, and the rhomboid muscles
  • Depression: gravity, trapezius (lower fibers), pectoralis minor, and serratus anterior
  • Protraction (scapular abduction): serratus anterior, pectoralis major, and pectoralis minor
  • Retraction (scapular adduction): trapezius (middle part), latissimus dorsi, and the rhomboid muscles
  • Lateral (upward) rotation: serratus anterior, trapezius (upper and lower part)
  • Medial (downward) rotation: latissimus dorsi, levator scapulae, pectoralis minor, and rhomboid muscles

Shoulder joint (glenohumeral joint) ^[1][2]

• Type of joint: ball-and-socket joint between the head of the humerus and the glenoid cavity of the scapula
• Structures
  • Glenoid labrum: a fibrocartilaginous rim surrounding the glenoid fossa
  • Joint capsule
    • Fibrous layer
      • Envelops the entire joint
      • Medial attachment: margin of glenoid fossa
      • Lateral attachment: anatomical neck of the humerus
      • Reinforced superiorly by the rotator cuff muscles and the long head of the biceps brachii
      • Weakest inferiorly
    • Inner layer: synovial membrane
  • Coracoacromial arch
    • Forms a protective arch over the glenohumeral joint
    • Composed of the following structures:
      • Acromion
      • Coracoid process
      • Coracoacromial ligament
  • Ligaments
    • Coracoacromial ligament
      • Extends from the coracoid process to the acromion
      • Together with the coracoid process and the acromion, it forms the coracoacromial arch, which prevents superior displacement of the humeral head from the glenoid fossa.
    • Coracohumeral ligament
      • Extends from the coracoid process to the greater tubercle of the humerus
      • Strengthens the anterior part of the joint capsule
    • Glenohumeral ligament
      • Extends from the glenoid labrum at the supraglenoid tubercle of the scapula to the anatomical neck of the humerus
      • Strengthens the anterior part of the joint capsule
    • Transverse humeral ligament
      • Extends from the greater tubercle of the humerus to the lesser tubercle of the humerus
      • Bridges the bicipital groove and holds the biceps tendon in place
  • Bursae
    • Subacromial bursa
      • Synovial fluid-filled sac
      • Lies between the acromion, coracoacromial ligament, and deltoid muscle superiorly, and between the shoulder joint capsule and supraspinatus tendon inferiorly
      • Does not communicate with the shoulder joint cavity
      • Facilitates smooth gliding movement of the supraspinatus tendon and deltoid muscle
    • Other bursae are the subscapular, subcoracoid, subscapular recess, coracoclavicular and supraacromial bursae.
• Movements of the shoulder joint
  • Flexion and extension
  • Medial and lateral rotation
  • Abduction and adduction
  • For further details, see “Orthopedic shoulder examination.”

The head of the humerus is approximately four times larger than the glenoid fossa. The relative size of the humerus allows for a greater range of motion but also increases joint instability.

Overview ^[2]

The muscles of the proximal upper limb can be distinguished according to their attachments:

• Scapulohumeral muscles
• Axioappendicular muscles
  • Anterior axioappendicular muscles
  • Posterior axioappendicular muscles

The movements of the scapula and glenohumeral joint are also supported by the muscles of the arm, such as the triceps and biceps brachii muscles (see “Upper arm and elbow”).

Scapulohumeral (intrinsic shoulder) muscles ^[1][2]

• Six muscles originate from the scapula and insert into the humerus:
  • Rotator cuff muscles
    • The rotator cuff muscles are responsible for providing the glenohumeral joint with stability and mobility.
    • The tendons of the rotator cuff muscles surround the head of the humerus from all sides and hold the humeral head to the articular surface of the scapula.
    • The deltoid muscle rests on the rotator cuff.
    • The contour of the shoulder is formed mainly by the deltoid muscle.
    • See “Examination of the rotator cuff.”
  • Teres major muscle: forms the inferior border of the quadrangular space

“roTISSerie:” Teres minor, Infraspinatus, Supraspinatus, and Subscapularis are the muscles of the rotator cuff.

Anterior axioappendicular muscles ^[1][2]

• Subclavius muscle: depresses the shoulder and lateral end of the clavicle
• Pectoralis major muscle
  • Shoulder adduction, internal rotation, and anteversion
  • Draws scapula anteriorly and inferiorly
  • Accessory muscle of respiration
• Pectoralis minor muscle
  • Stabilizes scapula (draws it anteriorly and inferiorly against the thoracic wall)
  • Accessory muscle of respiration
• Serratus anterior muscle
  • Scapula: protraction
  • Inferior angle of the scapula: upward rotation
  • Maintains position of the inferomedial scapula against the thoracic wall
  • Abduction of the arm (> 100°)
  • Accessory muscle of respiration
• See “Upper arm and elbow.”

SALT: The Serratus Anterior muscle is innervated by the Long Thoracic nerve.

Posterior axioappendicular (extrinsic shoulder) muscles ^[1][2]

• Superficial extrinsic shoulder muscles
  • Trapezius muscle
    • Descending part
      • Scapula elevation and upward rotation
      • Overhead abduction of the arm (> 90°)
    • Transverse part: scapula adduction (i.e., pulls the scapula medially)
    • Ascending part: scapula depression
  • Latissimus dorsi muscle
    • Adducts, extends, and internally rotates the arm
    • Accessory muscle of respiration
• Deep extrinsic shoulder muscles
  • Levator scapulae muscle
    • Scapula elevation (shrugging)
    • In conjunction with the rhomboid muscles: returns the elevated arm to its neutral position
  • Rhomboid major muscle and rhomboid minor muscle
    • Scapula elevation and retraction
    • Lateral angle of the scapula: caudal rotation

See “Superficial back muscles.”

Overview ^[1][2]

• Definition
  • A three-dimensional, pyramid-shaped area at the junction of the arm and thorax
  • Inferior to the shoulder joint, through which important vessels and nerves pass from the trunk to the arm
• Boundaries
  • Apex
    • Also called the cervicoaxillary canal or axillary inlet
    • Bounded by the lateral border of the 1^strib, the superior border of the scapula, and the clavicle
    • Vessels and lymphatics pass through this opening to and from the arm.
  • Anterior wall: pectoralis major and pectoralis minor muscles
  • Posterior wall: latissimus dorsi, teres major, and subscapularis muscles, and the scapula
  • Lateral wall: coracobrachialis muscle and the humerus
  • Medial wall: serratus anterior, the 1^st–4^thribs, and the intercostal muscles
  • Base
    • Formed by skin, subcutaneous tissue, and axillary fascia
    • Also known as the axillary fossa or armpit
    • Bounded externally by the anterior and posterior axillary folds
• Contents
  • Axillary artery and its branches
  • Axillary vein and its tributaries
  • Axillary lymph node groups and lymphatic vessels
  • Brachial plexus
  • Axillary fat and connective tissue

The anterior axillary fold is formed from the pectoralis major muscle. The posterior axillary fold is formed from the latissimus dorsi and teres major muscles.

Axillary spaces ^[1][2]

• Definition: anatomical spaces (2 triangular, 1 quadrangular) bound by the teres minor muscle (cranially) and the teres major muscle (caudally), through which structures leave the axilla

Medial triangular space (upper triangular space)

• Shape: a triangular space with a cranial, caudal, and lateral border and apex inferomedially (i.e., no medial border)
• Borders
  • Cranial: teres minor muscle
  • Caudal: teres major muscle
  • Lateral: triceps brachii muscle (long head)
• Structures within: circumflex scapular artery and vein

Triangular interval (lower triangular space)

• Shape: a triangular space with a cranial, lateral, and medial border and apex inferiorly (i.e., no caudal border)
• Borders
  • Cranial: teres major muscle
  • Lateral: humerus
  • Medial: triceps brachii muscle (long head)
• Structures within
  • Deep brachial artery
  • Radial nerve

Quadrangular space

• Shape: a quadrangular space that allows access to the posterior arm and shoulder area
• Borders
  • Cranial: teres minor muscle
  • Caudal: teres major muscle
  • Medial: triceps brachii (long head)
  • Lateral: humerus
• Structures within
  • Posterior humeral circumflex artery and vein
  • Axillary nerve

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 peace sign with both hands and superimpose them at ∼ 90°. Three spaces are created. These represent the axillary spaces.

Arterial supply ^[1][2]

Axillary artery ^[1][2]

• Origin: continuation of the subclavian artery at the outer border of the 1^strib
• Course
  • In the axilla, it lies lateral to the axillary vein.
  • Continues as the brachial artery at the outer border of the teres major
  • Divided into 3 parts by the overlying pectoralis minor muscle
    • 1^st part: proximal to the pectoralis minor
    • 2^nd part: inferior to the pectoralis minor
    • 3^rd part: distal to the pectoralis minor

Scapular anastomosis ^[2]

• Definition: an anastomosis between the branches of the first part of the subclavian artery and the third part of the axillary artery that provides collateral circulation to the arm in case of occlusion or injury to the axillary or subclavian arteries

Venous drainage ^[1][2]

Axillary vein

• Origin: formed at the outer border of the teres major muscle by the union of the basilic vein and the accompanying veins of the brachial artery
• Course
  • Runs with the axillary artery
  • Divided into 3 sections that correspond to the parts of the axillary artery (the distal end is the first part)
  • Continues as the subclavian vein at the lateral border of the 1^strib
• Tributaries
  • Generally correspond to axillary artery tributaries
  • Exceptions
    • There is no common tributary for the veins that correspond to the branches of the thoracoacromial trunk.
    • Directly or indirectly receives the thoracoepigastric veins
• Drains: upper limb, axilla, lateral chest wall

Lymphatic drainage ^[1][2]

• Tributaries:
  • Lymphatics from the upper limb and upper torso drain into the five axillary lymph node groups.
  • Efferent vessels from the apical lymph node group exit via the apex of the axilla and unite to form the subclavian lymphatic trunk, and some vessels also drain into the clavicular nodes.
  • See “Axillary lymph nodes.”

• Description: a network of nerves derived from the C5–T1 spinal nerves that give rise to the peripheral nerves of the upper limb and shoulder
• Roots
  • Anterior rami of C5–T1 spinal nerves (i.e., five roots)
    • Dorsal scapular nerve: arises directly from C5 and innervates the rhomboid major, rhomboid minor, and levator scapulae muscles
    • Long thoracic nerve: arises from roots C5, C6, and C7 and innervates the serratus anterior muscle
    • The first intercostal nerve arises from T1.
• Trunks
  • The roots form three trunks that are located in the neck.
    • C5 and C6 form the upper trunk of the brachial plexus: The suprascapular nerve (innervates the infraspinatus and supraspinatus muscles) and nerve to subclavius (innervates the subclavius muscle) originate here.
    • C7 forms the middle trunk of the brachial plexus.
    • C8 and T1 form the lower trunk of the brachial plexus.
• Divisions: All three trunks separate into an anterior and posterior division that enters into the axillary fossa.
  • All three posterior divisions innervate the extensor compartment of the arm and forearm (i.e., the posterior compartment).
  • All three anterior divisions innervate the flexor compartment of the arm and forearm (i.e., the anterior compartment).
• Cords
  • Only the cords and proximal portions of their branches lie in the axilla.
  • There are three cords, which are named according to their position in relation to the axillary artery.
    • The anterior divisions of upper and middle trunks form the lateral cord of the brachial plexus.
    • The anterior division of the lower trunks forms the medial cord of the brachial plexus.
    • The posterior divisions of all the trunks form the posterior cord of the brachial plexus.
• Functional significance
  • Motor innervation: all upper limb and shoulder muscles (except for the trapezius, which is supplied by the spinal accessory nerve)
  • Sensory innervation: the axilla and upper limb (except for the medial portion of the arm, which is supplied by the intercostobrachial nerve)

Branches from the medial and lateral cords form the median nerve.

“Rugby Teams Drink Cool Beer!” (Roots, Trunks, Divisions, Cords, Branches is the order of the brachial plexus divisions)

References:^[2]

• Thoracic outlet syndrome
• Bones and joints
  • Clavicle fracture
  • Shoulder dislocation
  • Acromioclavicular joint injury
• Soft tissue lesions of the shoulder
  • Subacromial impingement syndrome
  • Rotator cuff tendinitis
  • Rotator cuff tear
  • Subacromial bursitis
  • Subclavian steal syndrome
  • Frozen shoulder (adhesive capsulitis)
  • Biceps tendinitis
• Nerve injuries in the upper body
  • Suprascapular nerve injury
  • Erb palsy
  • Klumpke palsy
  • Axillary nerve injury
  • Long thoracic nerve injury (Winged scapula)
  • Pancoast tumor