Week 9 - The Shoulder Joint Complex - Joints and Muscles

Which joints form the pectoral girdle?

Sternoclavicular (SC) joint and acromioclavicular (AC) joint; together with glenohumeral joint form the shoulder joint complex.

Describe the sternoclavicular joint surfaces.

Medial/ sternal end of clavicle, sternal notch of manubrium, part of first costal cartilage. Classified as a synovial saddle joint.

Describe the SC joint capsule and ligaments.

Fibrous capsule surrounds joint margins; thicker anteriorly and posteriorly; lined by synovial membrane. Reinforced by anterior and posterior SC ligaments, interclavicular ligament, and costoclavicular ligament.

What is the function of the SC joint fibrocartilaginous disc?

Attaches from superior clavicle to first costal cartilage; splits cavity into 2 compartments; increases congruency; absorbs forces along clavicle.

What movements occur at the SC joint?

Rolling and gliding of clavicle anterior/posterior and vertical planes; small rotation along clavicle axis; important for scapular protraction, retraction, elevation, depression.

Describe the acromioclavicular joint surfaces.

Lateral/acromial end of clavicle, oval facet on medial acromion; synovial plane joint.

Describe the AC joint capsule and ligaments.

Capsule surrounds all sides, lined with synovial membrane; fibrocartilaginous disc partially separates cavity, mainly superiorly. Ligaments: acromioclavicular ligament (superior), coracoclavicular ligaments (trapezoid and conoid, inferior).

Function of AC joint ligaments?

Acromioclavicular ligament reinforces superior aspect. Trapezoid ligament (anterolateral) and conoid ligament (posteromedial) attach clavicle to coracoid process; prevent vertical translation; resist rotation.

What movements occur at the AC joint?

Small gliding movements anterior/posterior and vertical; some rotation along clavicle axis; supports scapular rotation, protraction, retraction, elevation, and depression.

What injuries commonly affect the SC and AC joints?

SC joint is strong and rarely dislocated; shoulder falls usually fracture clavicle or dislocate AC joint.

What forms the glenohumeral joint?

The hemispherical head of the humerus and the glenoid fossa form the glenohumeral (shoulder) joint.

What is the classification of the glenohumeral joint?

Synovial ball-and-socket joint.

What is the primary functional characteristic of the glenohumeral joint?

It is the most mobile joint in the body, adapted for movement.

What movement does the glenohumeral joint allow?

It allows movement in all directions, including circumduction.

Describe the stability of the glenohumeral joint.

• Sacrifices stability for mobility.

• Dislocations are common.

• Relies on surrounding rotator cuff muscles for compression and stability.

Describe the articular surfaces of the glenohumeral joint.

• Relatively incongruent surfaces.

• Rounded humeral head articulates with flat, slightly concave glenoid fossa.

• Only ⅓ of the humeral head contacts the fossa at any time.

• Humeral head covered with hyaline cartilage.

What is the glenoid labrum and what is its function?

A ring of fibrocartilage around glenoid fossa that deepens the cavity and increases joint congruency.

Where does the fibrous capsule attach?

Medially: glenoid neck
Laterally: anatomical neck of humerus
Inferiorly: 1 cm below anatomical neck to medial humeral shaft

Describe the fibrous capsule of the glenohumeral joint.

• Loose and weak

• Tightens when arm is elevated

• Loose inferior capsule allows overhead elevation

What does the synovial membrane line?

The entire fibrous capsule.

What are the two openings in the joint capsule?

• Superior opening for long head of biceps tendon

  • Tendon is intracapsular and extrasynovial

• Subscapular opening

  • Communicates with subscapular bursa

What are the three glenohumeral ligaments and what do they do?

Three anterior capsule thickenings:

• Superior glenohumeral ligament

• Middle glenohumeral ligament

• Inferior glenohumeral ligament
  Function:
  Run from margins of glenoid to lesser tubercle/anatomical neck; reinforce anterior joint.
  Relaxed during medial rotation.

What is the transverse humeral ligament?

• Thickening of superior joint capsule

• Covers both sides of intertubercular groove

• Turns groove into a canal

• Holds long head of biceps tendon in place

What is the coracohumeral ligament?

• Runs from coracoid process → greater tubercle

• Supports arm weight against gravity

• Prevents inferior dislocation of humerus

What is the coracoacromial ligament?

• Triangular ligament from coracoid → acromion

• Forms the coracoacromial arch

What is the coracoacromial arch and what does it do?

Made of:

• Acromion

• Coracoid process

• Coracoacromial ligament
  Function:
  Reinforces superior joint and prevents superior dislocation of humerus.

What is a bursa?

A sac-like structure filled with synovial fluid to reduce friction.

What is the subacromial (subdeltoid) bursa and what does it do?

• Located between deltoid and coracoacromial arch

• Lies over supraspinatus tendon and joint capsule

• Allows supraspinatus to glide smoothly during abduction/elevation

Where is the subscapular bursa and what is its function?

• Between tendon of subscapularis and neck of scapula

• Communicates with joint capsule

• Helps subscapularis tendon glide

What nerves supply the glenohumeral joint?

Articular branches of:

• Suprascapular nerve

• Axillary nerve

• Lateral pectoral nerve

What arteries supply the glenohumeral joint?

• Suprascapular artery

• Anterior circumflex humeral artery

• Posterior circumflex humeral artery

What joints make up the shoulder joint complex?

The glenohumeral joint, scapula joints, and pectoral girdle joints.

What controls shoulder complex movements?

Muscles that:

• Move/stabilize scapula

• Insert onto humerus to move/stabilize GH joint

What are the 3 functional groups of shoulder muscles?

• Thoracoscapular

• Scapulohumeral

• Thoracohumeral

What are the posterior thoracoscapular muscles?

4 muscles:

• Trapezius

• Levator scapulae

• Rhomboid minor

• Rhomboid major

What are the anterior thoracoscapular muscles?

2 muscles:

• Serratus anterior

• Pectoralis minor

What do thoracoscapular muscles do?

Act on the scapula — move or stabilize it.

Trapezius — origin

• Behind mastoid process (shared with SCM)

• Ligamentum nuchae

• Spinous processes of 12 thoracic vertebrae

Trapezius — insertion

Upper fibers: lateral 1/3 clavicle
Middle fibers: spine of scapula
Lower fibers: tubercle of scapular spine near medial end 

Trapezius — actions

Upper: elevate scapula
Middle: retract scapula
Lower: rotate scapula

Trapezius — nerve supply

• Motor: Spinal accessory nerve (CN XI)

• Proprioception: Anterior rami C3, C4

Levator scapulae — origin and insertion

Origin: Transverse processes C1–C4 (four tendinous slips)
Insertion: Superior angle of scapula

Levator scapulae — actions

Elevates scapula; rotates inferior angle medially → tilts glenoid downward

Levator scapulae — nerve supply

Anterior rami C3, C4 + C5 from dorsal scapular nerve

Rhomboid minor — origin & insertion

Origin: C7–T1 spinous processes + nuchal ligament
Insertion: Medial scapular border at spine level

Rhomboid major — origin & insertion

Origin: T2–T5 spinous processes
Insertion: Medial scapular border from spine → inferior angle

Rhomboids — actions

Retract scapula; rotate inferior border medially (glenoid faces down)
Most active during lowering the arm.

Rhomboids — nerve supply

Dorsal scapular nerve (runs under both muscles parallel to medial border)

Rhomboids — arterial supply

Dorsal scapular artery (runs deep with the nerve, joins suprascapular & subscapular arteries)

Pectoralis minor — origin & insertion

Origin: Ribs 3–5 near costal cartilages
Insertion: Medial upper surface of coracoid processOrigin: Ribs 3–5 near costal cartilages
Insertion: Medial upper surface of coracoid process

Pectoralis minor — action

Draws scapula forward and inferiorly (used when stretching/reaching)

Pectoralis minor — nerve supply

Medial pectoral nerve (usually pierces the muscle)

Why is pectoralis minor an important landmark?

The brachial plexus, axillary artery, and axillary vein lie directly underneath it.

Serratus anterior — origin & insertion

Origin: 8 slips from lateral ribs 1–8
Insertion: Entire anterior medial border of scapula

Serratus anterior — actions

• Powerful scapular protractor

• Rotates glenoid upward

• Holds scapula against thoracic wall

Serratus anterior — nerve supply

Long thoracic nerve (C5–C7) — runs on outer surface of muscle

What happens if the long thoracic nerve is damaged?

Serratus anterior paralysis → winged scapula → impaired shoulder elevation.

Arterial supply to pectoralis minor

From axillary artery branches:

• Superior thoracic artery

• Pectoral branch of thoracoacromial artery

• Lateral thoracic artery

Arterial supply to serratus anterior

Superior thoracic artery
Lateral thoracic artery

What do all thoracoscapular muscles have in common?

• Attach axial skeleton → scapula

• Move & stabilize scapula

• Each has its own distinct nerve supply

What are the scapulohumeral muscles and where are they located?

Muscles located in the posterior shoulder, attaching from the scapula to the humerus.

List all scapulohumeral muscles.

Deltoid, Teres Major, Teres Minor, Supraspinatus, Infraspinatus, Subscapularis.

Function of the scapulohumeral muscles (collectively).

To stabilize or move the glenohumeral joint.

Deltoid – location.

Covers the tip of the shoulder front and back.

Deltoid – origin (overall).

Arises from a continuous U-shaped bony strip:

• Lateral third of clavicle (anterior)

• Acromion (middle)

• Spine of scapula (posterior)

Deltoid – anterior fibres origin.

Lateral third of clavicle.

Deltoid – middle fibres origin.

Acromion.

Deltoid – posterior fibres origin.

Spine of scapula.

Deltoid – insertion.

All 3 parts converge to insert on the deltoid tuberosity.

Deltoid – action of all fibres.

Abduction of the humerus; initiation depends on supraspinatus (first 15°), then deltoid acts strongly.

Deltoid – individual parts' ability.

All 3 parts can act independently.

Deltoid – anterior fibres specific action.

Flexion of the shoulder; considered thoracohumeral because this action parallels clavicular head of pec major.

Deltoid – nerve and blood supply.

Axillary nerve; posterior circumflex humeral artery (both enter via quadrangular space).

Teres major – origin.

Posterior aspect of inferior angle of scapula.

Teres major – insertion.

Anterior humerus at the medial lip of intertubercular groove.

Teres major – actions.

Adduction, extension, medial rotation of the humerus.

Teres major – function.Teres major – function.

Stabilizes the glenohumeral joint during elevation; upper border is pulled into inferior capsule; keeps humeral head positioned in glenoid.

What are the rotator cuff (SITS) muscles?

Supraspinatus, Infraspinatus, Teres Minor, Subscapularis.

Rotator cuff stabilizing function.

Tendons blend with joint capsule anteriorly, superiorly, posteriorly; line of pull compresses humeral head into glenoid for dynamic stability.

Supraspinatus – origin.

Supraspinous fossa above spine of scapula.

Supraspinatus – insertion.

Summit of the greater tubercle; tendon reinforces superior glenohumeral capsule.

Supraspinatus – actions.

Initiates first 15° abduction; dynamic stability; depresses humeral head.

Function of supraspinatus (clinical).

Tear = loss of ability to initiate abduction + loss of humeral head depression.

Infraspinatus – origin and insertion.

Origin: Infraspinous fossa.
Insertion: Greater tubercle below supraspinatus.

Infraspinatus – capsule reinforcement.

Reinforces posterior glenohumeral capsule.

Infraspinatus – nerve supply.

Suprascapular nerve.

Infraspinatus – action.

Lateral rotation; compresses humeral head.

Teres minor – location and difficulty.

Located below infraspinatus; fibres can be hard to distinguish—differentiated by nerve supply.

Teres minor – origin and insertion.

Origin: Middle lateral border of scapula.
Insertion: Inferior part of greater tubercle.

Teres minor – nerve supply.

Axillary nerve.

Teres minor – action.

Assists lateral rotation; provides compressive forces to humeral head.

Subscapularis – origin and insertion.

Origin: Subscapular fossa (anterior scapula).
Insertion: Lesser tubercle of humerus.

Subscapularis – action.

Medial rotation, assists adduction, stabilizes humerus.

Suprascapular nerve pathway and supply.

Enters via suprascapular foramen (under superior transverse ligament); supplies supraspinatus, infraspinatus, and gives articular branches.

Suprascapular artery pathway.

Travels with nerve but passes above superior transverse ligament; supplies supraspinatus and contributes to scapular vascular network.

Axillary nerve & posterior circumflex humeral artery entry.

Enter through quadrangular space.

Axillary nerve – muscular supply.

Deltoid, Teres Minor; articular supply to shoulder.

Posterior circumflex humeral artery – supply.

Neck of humerus, shoulder joint, deltoid, teres major & minor, long head of triceps.

Upper and lower subscapular nerves – supply.

Upper: upper subscapularis.

Lower: lower subscapularis + teres major.

What are thoracohumeral muscles?

Muscles attaching from thorax to humerus.