The shoulder girdle is a sophisticated mechanism that acts as the fulcrum for the upper limb, and its smooth function, strength and stability are vital in order to reliably place the hand in space to undertake everyday tasks.
It is composed of 3 bones:
- Scapula (shoulder blade)
- Clavicle (collarbone)
- Humerus (arm bone)
The scapula is a large flat bone which has a body (the flat part), a spine (the ridge at the back of the shoulder), an acromion (tip of the shoulder blade) and a coracoid process (a projection from the front of the blade that serves as an attachment point for muscles and ligaments)
These bones are linked by ligaments to form joints, upon which framework the tendons and muscles facilitate movement.
The joints are:
- Sternoclavicular (between the breastbone and the collarbone)
- Acromioclavicular (between the collarbone and the shoulderblade)
- Glenohumeral (the main ball and socket joint)
- Scapulothoracic (not a joint in the usual sense, but a plane of motion between the shoulder blade and chest wall)
View of the left shoulder from in front, showing the ball and socket, with the labrum (rim) serving to deepen the socket and provide a site for attachement of the ligaments and joint capsule.
These bones and joints depend on linkages provided by ligaments which are tough fibrous flexible bands. Some of the important ligaments are:
- Coraco-clavicular (suspending the scapula from the collarbone)
- Acromio-clavicular (connecting the collarbone to the tip of the shoulder blade)
- Coraco-acromial (forms an arch over the ball of the humerus)
- Glenohumeral (three in number – superior, middle and inferior- connecting the ball and socket )
- Joint capsule (a sheet of ligament tissue wrapping around the ball, and thickened in parts to form the glenohumeral ligaments)
View of the left shoulder from the front, showing the ligaments of the shoulder and acromioclavicular joints
The shoulder blade serves as the origin of several important muscles that attach to the humerus. Tendons are the short bands or cords of tough connective tissue that connect muscles to bones. The tendons from the muscles arising from the scapula converge on the ball, together forming the ‘rotator cuff’.
The main functional components of the rotator cuff are:
- Subscapularis (at the front, rotates the ball inwards)
- Supraspinatus (at the top, elevates the arm)
- Infraspinatus (at the back, rotates the ball outwards)
- Teres minor (rotates the ball outwards when the arm is elevated)
In addition, the biceps muscle has two attachments from the scapula. One (the “short head’) arises from the coracoid process outside the joint, while the other (the “long head’) arises from the top of the socket and travels across the ball, through a groove and into the arm to join together to the main biceps muscle.
View of the left shoulder from the front, showing the long head of the biceps tendon emerging from the joint and traversing the subscapularis tendon.
View of the left shoulder from the side, showing the supraspinatus emerging from under the acromion to attach to the highest part of the ball (proximal humerus).
View of the right shoulder from behind, showing the infraspinatus and teres minor tendons inserting on the back of the ball (humeral head).
Where the tendons pass over or under bony prominences a special membranous sac called a bursa exists to lubricate the motion. Numerous bursae have been identified around the shoulder, of which the subacromial (or subdeltoid) bursa is the largest and the most often implicated in shoulder problems. When irritated or injured, the bursa may produce fluid and the walls of the sac may thicken and contribute to impingement.
View of the left shoulder from the front, showing the large subacromial or subdeltoid bursa lying between the acromion and humeral head.
The shoulder is also richly supplied with nerves and blood vessels. Nerves carry sensory signals from the joint tissues to the brain and also convey motor signals from the brain via the spinal cord to the muscles. The most important nerves are:
- Axillary (supplying the deltoid muscle)
- Suprascapular (supplying the supraspinatus and infraspinatus)
- Musculocutaneous (supplying the biceps and brachialis muscles)
- Accessory (supplying the trapezius muscle)
View of the right shoulder from the back, showing the network of nerves around the shoulder, in particular the axillary nerve emerging from under the lower border of the teres minor muscle.
Our thanks to Primal Pictures for permission to use images from their CD-ROM The Interactive Shoulder.