SC, AC, Glenohumeral Joints

What bones/cartilage form the sternoclavicular joint?

• Clavicle

• Manubrium of the sternum

• 1st costal cartilage.

What type of joint is the sternoclavicular joint?

An atypical saddle-shaped synovial joint.

What typical features of a synovial joint does the sternoclavicular joint have?

• Joint capsule

• Inner synovium

• Synovial fluid

What is the unusual feature of the sternoclavicular joint?

It contains a fibrocartilaginous articular disc within the joint capsule

What type of cartilage is the articular disc of the SC joint made from?

Fibrocartilage, rather than hyaline cartilage.

What ligaments reinforce the sternoclavicular joint?

• Anterior sternoclavicular ligament

• Posterior sternoclavicular ligament

• Interclavicular ligament

• Costoclavicular ligament

Why does the clavicle tend to fracture rather than dislocate at its sternal end?

The SC joint is reinforced by exceptionally strong ligaments, making dislocation less likely.

What is the function of the fibrocartilaginous disc in the SC joint?

It acts as a cushion, helping distribute forces transmitted from the clavicle across the manubrium.

Are sternoclavicular joint dislocations common?

No. They are generally rare because the joint is exceptionally well reinforced

Which type of SC joint dislocation is more common?

Anterior dislocation is more common than posterior dislocation

What bones form the acromioclavicular (AC) joint?

The lateral clavicle and the acromion of the scapula.

What type of joint is the AC joint?

An atypical plane synovial joint.

What typical synovial joint features does the AC joint have?

• Joint capsule

• Inner synovium

• Synovial fluid

What are the atypical features of the AC joint?

• A fibrocartilaginous wedge-shaped disc in the superior part of the joint capsule

• Articular surfaces lined with fibrocartilage instead of hyaline cartilage.

What ligament forms the capsule of the AC joint?

The acromioclavicular ligament.

What ligament is the major stabiliser of the AC joint?

The coracoclavicular ligament.

What are the two components of the coracoclavicular ligament?

• Conoid ligament

• Trapezoid ligament

What is the function of the coracoclavicular ligaments?

They tether the scapula superiorly to the clavicle.

What does the AC ligament counteract?

Rotary forces caused by the weight of the arm.

What is an AC joint dislocation commonly called?

Shoulder separation

What can cause an AC joint dislocation?

A large blunt force applied to the superior aspect of the shoulder, pushing the scapula and humerus inferiorly.

What is the difference between an AC joint dislocation and a glenohumeral dislocation?

• AC dislocation → shoulder separation

• Glenohumeral dislocation → humeral head leaves the glenoid cavity

What movements can occur at the glenohumeral joint?

• Flexion

• Extension

• Abduction

• Adduction

• Internal (medial) rotation

• External (lateral) rotation

• Circumduction.

What type of joint is the glenohumeral joint?

A ball-and-socket synovial joint.

What is special about the mobility of the glenohumeral joint?

It is the most mobile joint in the body.

What is the major disadvantage of the glenohumeral joint’s high mobility?

It sacrifices stability for mobility, making it relatively unstable

Why is the glenohumeral joint relatively unstable?

Because:

• The glenoid fossa is shallow

• The head of the humerus is large

• The joint capsule is relatively lax and loose

This allows a huge range of movement but reduces bony stability.

Approximately what is the size relationship between the humeral head and glenoid fossa?

The lecture gives approximately a 1:4 ratio between the size of the glenoid fossa and the head of the humerus.

What analogy does the lecture use to describe the glenohumeral joint?

A golf ball sitting on a tee

What is the glenoid labrum?

A fibrocartilaginous ridge surrounding the glenoid cavity.

What is the function of the glenoid labrum?

It deepens the glenoid cavity, helping reduce the risk of dislocation

How is the glenohumeral joint passively reinforced?

• The glenoid labrum

• Joint ligaments

How is the glenohumeral joint actively reinforced?

By the rotator cuff muscles

What are the three glenohumeral ligaments?

• Superior glenohumeral ligament

• Middle glenohumeral ligament

• Inferior glenohumeral ligament.

What other ligament reinforces the glenohumeral joint?

The coracohumeral ligament.

What is the function of the coracoacromial ligament?

It helps prevent superior displacement of the humerus

What is the function of the transverse humeral ligament?

It forms a roof over the bicipital/intertubercular groove and holds the long head of biceps brachii in the groove.

What is the function of the coracoacromial arch?

It helps prevent superior dislocation/displacement of the humerus

What is the rotator cuff?

A group of four muscles that originate from the scapula and insert onto the humerus, helping actively stabilise the glenohumeral joint

What does SITS stand for?

S — Supraspinatus
I — Infraspinatus
T — Teres minor
S — Subscapularis.

What is the main stabilising function of the rotator cuff?

The muscles actively hold the head of the humerus within the glenoid fossa

What can happen if the rotator cuff is overloaded with force?

The rotator cuff tendons can tear, causing a rotator cuff injury