A ball-and-socket joint whose socket is only about one third the size of its ball
Shoulder joint — the glenoid cavity is small and shallow.
Anatomy · Joints & cartilage
Shoulder joint
The most mobile joint in the body
Upper limb · pectoral girdle
The shoulder (glenohumeral) joint is a ball-and-socket joint between the rounded head of the humerus and the small, shallow glenoid cavity of the scapula. Its huge range of movement is bought at the price of stability, so it is held in place mainly by the muscles that cross it, above all the rotator cuff, and it is the joint most often dislocated.
How to spot them
- ▸A large humeral head sits against a glenoid cavity only about a third of its size
- ▸A rim of fibrocartilage, the glenoid labrum, slightly deepens the socket
- ▸The capsule is loose and thin, with little ligament support, so muscle tendons do the stabilising
- ▸Roofed over by the acromion and coracoid process, with the subacromial bursa between roof and cuff
What to know
The joint with the widest range of movement in the body, and the one most often dislocated
Shoulder joint — mobility traded for stability.
A fibrocartilage lip deepens its socket, while four muscle tendons fuse with its capsule to hold the head in place
Shoulder joint — labrum and rotator cuff.
After an anterior dislocation the axillary nerve, which wraps the surgical neck of the humerus, is the one at risk
Shoulder joint — sensation over the 'regimental badge' patch of the upper arm can be lost.
A condition of pain and stiffness as the capsule thickens and tightens, called adhesive capsulitis
Shoulder joint — frozen shoulder.
Key ideas
- Stability comes from muscles, not from bone shape: the rotator cuff tendons blend with the capsule and press the head into the socket.
- It is the joint of a whole girdle: raising the arm above about 90 degrees also needs the scapula to rotate on the chest wall.
Quick-fire facts
- •It is a multiaxial ball-and-socket (spheroidal) synovial joint: flexion, extension, abduction, adduction, medial and lateral rotation and circumduction.
- •The glenoid cavity is only about one third the size of the humeral head.
- •The glenoid labrum is a rim of fibrocartilage that deepens the socket slightly.
- •The commonest dislocation is anterior (forwards and downwards), usually quoted at around 95 per cent of shoulder dislocations.
- •The axillary nerve, running round the surgical neck of the humerus, can be damaged in dislocation or fracture.
- •Frozen shoulder (adhesive capsulitis) causes pain and progressive stiffness and usually eases over many months to a couple of years.
- •The subacromial bursa lies between the acromion and the supraspinatus tendon and is involved in impingement.
- •The tendon of the long head of biceps runs through the joint capsule: inside the joint but outside its synovial cavity.
Drill it — 7 cards
1 / 7new card