Shoulder Complex

The shoulder complex comprises the glenohumeral joint, sternoclavicular joint, acromioclavicular joint, and the scapulothoracic articulation, and connects the upper limb to the trunk. This group of joints consists of the clavicle, scapula, and humerus bones, multiple muscles and supporting ligaments, cartilage, and bursae. The muscles ensure the mobility and stability of the shoulder and upper limb and are divided into 3 groups: anterior axioappendicular, posterior axioappendicular, and scapulohumeral muscles.

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Bones

Clavicle

  • The only horizontal long bone in the body
  • S-shaped, connects the upper limb with the axial skeleton
  • Along with the scapula, forms the pectoral girdle
  • Landmarks and articulations:
    • Medially articulates with the sternum to form the sternoclavicular joint
    • Laterally articulates with the scapula to form the acromioclavicular joint
Clavicle Model

Inferior view of the right clavicle

Image by BioDigital, edited by Lecturio

Scapula

  • Flat, triangular-shaped, with 3 angles, 3 borders, and an anterior and posterior surface
  • Along with the clavicle, it forms the shoulder or pectoral girdle, connecting the upper limb to the trunk.
  • Landmarks and articulations:
    • Glenoid cavity: lateral, articulates with the head of the humerus to form the glenohumeral joint
    • Acromion: continuation of the scapular spine, articulates with the lateral end of the clavicle to form the acromioclavicular joint
    • Coracoid process: anterior, hook-shaped, place of attachment for multiple muscles and stabilizing ligaments

Humerus

  • Bone of the upper arm
  • The proximal end comprises the head, the neck, the greater and lesser tubercles, and the shaft.
  • Landmarks and articulations:
    • Head of the humerus: spherical, articulates with the glenoid cavity
    • Tubercles: greater is lateral, lesser is anterior. The supraspinatus, infraspinatus, and teres minor rotator cuff muscles attach to the greater tubercle, while the subscapularis attaches to the lesser tubercle.

Joints

Acromioclavicular (AC) joint

  • Type of joint: plane synovial joint between the acromion and the clavicle
  • Function: allows for motion between the clavicle and scapula during arm movement
  • Ligaments: named according to their anatomical insertions
    • Acromioclavicular ligament: reinforces the superior part of the joint capsule and resists anteroposterior displacement at the AC joint
    • Coracoclavicular ligament: consists of the trapezoid and conoid ligaments and resists vertical and rotational displacement of the clavicle relative to the scapula

Glenohumeral joint

  • Type of joint: synovial ball-and-socket joint between the head of the humerus and the glenoid cavity of the scapula
  • Function: most mobile joint in the body with multiple degrees of motion: flexion and extension, abduction and adduction, medial and lateral rotation, and circumduction of the shoulder
  • Ligaments and supporting structures:
    • Fibrous capsule: extends from the anatomical neck of the humerus to the rim of the glenoid 
    • Glenoid labrum: fibrocartilaginous, meniscus-like structure that deepens the glenoid cavity
    • Ligaments: named according to their anatomical insertions
      • Coracoacromial ligament: resists superior displacement of the humeral head from the glenoid cavity 
      • Coracohumeral ligament: resists superior and inferior displacement and reinforces the anterior portion of the joint capsule
      • Glenohumeral ligaments: consists of superior, middle, and inferior bands and stabilizes the joint during adduction, abduction, and external rotation, depending on the position of the shoulder
      • Transverse humeral ligament: maintains the long head of the biceps tendon within the bicipital groove
    • Subacromial bursa:
      • Synovial cavity located inferior to the acromion and coracoid and superior to the supraspinatus/infraspinatus tendons
      • The function of the bursa is to decrease friction and facilitate movement between these structures during movement of the shoulder.
Superficial ligaments and bursa glenohumeral joint

Anterior view of the superficial layer of ligaments and bursa of the right glenohumeral joint

Image by BioDigital, edited by Lecturio

Sternoclavicular (SC) joint

  • Type of joint: synovial saddle joint between the sternum and medial clavicle
  • Primary skeletal attachment between the axial skeleton and upper limb
  • Function: allows for motion of the clavicle in multiple planes
  • Ligaments:
    • Sternoclavicular ligament: primarily provides anterior/posterior stabilization and resists superior displacement
    • Costoclavicular ligament: anchors clavicle to the first rib and resists clavicular elevation
    • Interclavicular ligament: strengthens the capsule and resists downward displacement of the clavicle
Anterior view of the sternoclavicular joint

Anterior view of the sternoclavicular joint and the first sternocostal joint

Image by BioDigital, edited by Lecturio

Scapulothoracic articulation

  • Not a true synovial joint
  • Function:
    • Articulation between the concave anterior scapula and convex surface of the posterior thorax
    • Movement of the scapula on the thorax at this articulation requires movement at the AC joint and SC joint.
Scapulothoracic pseudojoint

Posterior view of the scapular region, highlighting the scapulothoracic pseudo-joint

Image by BioDigital, edited by Lecturio

Anterior Axioappendicular Muscles

The anterior axioappendicular muscles function to move the pectoral girdle, stabilize the clavicle, and move the upper arm.

Table: Origin, insertion, innervation, and function of the anterior axioappendicular muscles
MuscleOriginInsertionNerve supplyFunction
Pectoralis major
  • Clavicular head: anterior surface of medial clavicle
  • Sternocostal head: sternum and superior 6 costal cartilages
Lateral lip of intertubercular sulcusLateral and medial pectoral nerves (C5, C6: clavicular head; C7, C8: sternocostal head)
  • Adducts and medially rotates shoulder joint
  • Pulls scapula anteriorly and inferiorly
Pectoralis minorRibs 3–5Coracoid process of scapulaMedial pectoral nerves (C8, T1)Stabilizes scapula and pulls anteriorly and inferiorly
SubclaviusRib 1 and sternum junctionMiddle 3rd of clavicleSubclavian nerve (C5)Stabilizes and depresses clavicle
Serratus anteriorExternal surface of ribs 1–8Medial border of scapulaLong thoracic nerve (C6, C7)Protracts scapula; holds scapula against posterior thoracic wall

Posterior Axioappendicular Muscles

The posterior axioappendicular muscles stabilize and move the scapula and consist of a superficial and a deep layer.

Superficial or extrinsic layer

Table: Origin, insertion, innervation, and function of the superficial or extrinsic layer of the posterior axioappendicular muscles
MuscleOriginInsertionNerve supplyFunction
Trapezius
  • Descending part: superior nuchal line and external occipital protuberance
  • Transverse part: nuchal ligament
  • Ascending part: C7–T12 spinous processes
Lateral 3rd of clavicle and spine of scapulaSpinal accessory nerve (CN XI) and C3, C4 spinal nerves for proprioception
  • Descending part: elevates scapula
  • Transverse part: retracts scapula
  • Ascending part: depresses scapula
  • Descending and ascending: rotates scapula superiorly
Latissimus dorsiT6–12 spinous processes, thoracolumbar fascia and iliac crestFloor of intertubercular grooveThoracodorsal nerve (C6, C7)Extends, adducts, and medially rotates shoulder joint

Deep or intrinsic layer

Table: Origin, insertion, innervation, and function of the deep or intrinsic layer of the posterior axioappendicular muscles
MuscleOriginInsertionNerve supplyFunction
Rhomboid
  • Minor: nuchal ligament and C7–T1 spinous processes
  • Major: T2–T5 spinous processes
  • Minor: medial border of scapula (superior)
  • Major: medial border of scapula (inferior)
Dorsal scapula (C5)
  • Retracts scapula
  • Rotates scapula inferiorly (depressing glenoid cavity)
Levator scapulaeTransverse process of C1–C4Superior angle of scapulaDorsal scapula (C5)
  • Elevates scapula
  • Rotates scapula inferiorly (depressing glenoid cavity)

Scapulohumeral Muscles

The scapulohumeral muscles stabilize the glenohumeral joint by connecting the humerus to the scapula, including the rotator cuff muscles.

Table: Origin, insertion, innervation, and function of the scapulohumeral muscles
MuscleOriginInsertionNerve supplyFunction
Deltoid
  • Clavicular head: lateral 3rd of clavicle
  • Acromial head: acromion
  • Spinal head: spine of scapula
Deltoid tuberosity of humerusAxillary nerve (C5)
  • Clavicular head: flexes and medially rotates shoulder joint
  • Acromial head: abducts shoulder
  • Spinal head: extends and laterally rotates shoulder joint
Teres majorLateral border of scapula, inferior portionMedial lip of intertubercular grooveLower subscapular nerve (C6)Adducts and medially rotates shoulder joint
CoracobrachialisCoracoid process of scapulaShaft of humerus, anteromedialMusculocutaneous nerve (C5, C6, C7)Flexes, adducts, and medially rotates the shoulder joint

Rotator cuff muscles

Rotator cuff muscles stabilize the shoulder joint and prevent detachment of the head of the humerus from the glenoid cavity via “concavity compression,” especially during abduction of the arm.

Mnemonic

To recall the rotator cuff muscles, remember “SITS”:

S: Supraspinatus

I: Infraspinatus

T: Teres minor

S: Subscapularis

Table: Origin, insertion, innervation, and function of the rotator cuff muscles
MuscleOriginInsertionNerve supplyFunction
SupraspinatusSupraspinous fossaSuperior facet of greater tubercle of humerusSuprascapular nerve (C5)Initiates and assists deltoid in abduction
InfraspinatusInfraspinous fossaMiddle facet of greater tubercle of humerusSuprascapular nerve (C5)Laterally rotates the shoulder joint, holds head of humerus in glenoid cavity
Teres minorLateral border of scapula, middle portionInferior facet of greater tubercle of humerusAxillary nerve (C6)Laterally rotates the shoulder joint, holds head of humerus in glenoid cavity
SubscapularisSubscapular fossaLesser tubercle of humerusUpper and lower subscapular nerves (C6)Medially rotates and adducts shoulder joint, holds head of humerus in glenoid cavity
Rotator cuff muscles

Posterior view of the scapular region and rotator cuff muscles (subscapularis missing)

Image by BioDigital, edited by Lecturio

Clinical Relevance

The following conditions are common conditions associated with the shoulder:

  • Shoulder dislocation: the separation of the head of the humerus from the glenoid cavity. The glenohumeral joint is the most commonly dislocated joint. The vast majority of these dislocations are anterior-inferior (95%). Presents as severe shoulder pain and restricted range of motion.
  • Rotator cuff pathology: consists of a tear of 1 or more of the tendons of the 4 rotator cuff muscles. The supraspinatus is the most commonly torn rotator cuff tendon. May be associated with trauma or repeated activities. Presents with severe pain or discomfort, loss of strength, and restricted range of motion. These injuries may require surgical repair.
  • SLAP (Superior Labral tear from Anterior to Posterior) lesion: consists of injury to the glenoid labrum. Presents as dull pain, discomfort, decreased range of motion and strength, joint instability, and change in velocity in overhead athletes.
  • Subacromial bursitis: inflammation of the bursa located between the acromion and the deltoid muscle superficially and the supraspinatus muscle. Often caused by repetitive overhead motion. Presents as pain, swelling, loss of strength, and stiffness of the shoulder. Can progress to impingement syndrome.
  • Impingement syndrome: a spectrum of clinical manifestations caused by the compression of tissues as they pass through the subacromial space. Impingement syndrome can progress to a rotator cuff tear. Caused by many etiologies, including repetitive overhead motions or any condition that further narrows the subacromial space. Impingement syndrome presents as shoulder pain with overhead motions (“painful arc”), restriction of range of motion, loss of strength, and pain when sleeping on the shoulder.
  • Adhesive capsulitis: also known as “frozen shoulder,” the painful loss of motion of the shoulder due to inflammation and fibrosis of the fibrous capsule; may be associated with diabetes, thyroid disease, trauma, or prolonged immobilization.

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