The shoulder girdle is a sophisticated mechanism that acts as the fulcrum (or pivot point) 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 comprised of many bones, joints, bursae, muscles, tendons, and ligaments, and neurovasculature.
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). It is integral to shoulder motion by moving around the posterior aspect of the thorax. In combination, the shoulder girdle allows for the most movement of all of the body’s joints.
The clavicle, through the medial sterno-clavicular joint and the lateral acromioclavicular joints, acts as a load transmitting bony structure between the upper limb and the axial skeleton.
The humerus has the long shaft of the upper arm and ends with the humeral head, which articulates with the glenoid. There is a groove for the long head of the biceps muscle and tuberosities (bumps) for the attachment of the rotator cuff muscles.
These bones are linked by ligaments to form joints, upon which framework the tendons and muscles facilitate movement.
- Glenohumeral joint (GHJ) (main ball and socket joint of the shoulder girdle)
- Scapulothoracic joint (STJ) ( a plane of motion between the shoulder blade and chest wall)
- Acromioclacular joint (ACJ) (between the collarbone and the shoulderblade)
- Sternoclavicular joint (SCJ) (between the breastbone and the collarbone)
- Humeral head coracoclavicular ligament articulation.
These bones and joints depend on linkages provided by ligaments which are tough fibrous flexible bands.
Some of the important ligaments are:
- Coraco-clavicular ligaments (CCL) (suspending the scapula from the collarbone)
- Acromio-clavicular ligaments (connecting the collarbone to the tip of the shoulder blade)
- Coraco-acromial ligaments (CAL) (forms an arch over the ball of the humerus)
- Glenohumeral ligaments (GHL) (three in number – superior, middle and inferior – connecting the ball and socket, preventing dislocation)
- Transverse humeral ligament
- Levator scapulae
The above three muscles begin at the base of the skull, and connect the scapula and clavicle to the trunk of the body.
- Pectoralis major
- Pectoralis minor
- Latissimus dorsi
- Teres major
The above five, connect to the proximal end of the humerus and secure it to the body.
- Teres Minor
Finally, the above four muscles are the muscles of the rotator cuff. They connect the scapula to the humerus, as well as provide support for the glenohumeral joint.
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)
*Above, you can see a diagram of the entire anatomy of the shoulder; the innervation of the shoulder, as well as the bones structure, etc.