The shoulder is a complex joint with high mobility but a lack of stability. The anatomy of the joint is explained here:
The Shoulder Girdle
The anatomy of the shoulder girdle consists of several joints, or “articulations”, which connect the upper limb to the rest of the skeleton and provide a large range of movement. You may also see this referred to as the “pectoral girdle” in some textbooks.
The three bones which form the shoulder girdle are the clavicle, the scapula and the humerus.
The Scapula (or shoulder blade)
This bone is quite complex and is an attachment site for numerous muscles which support movement and stabilisation of the shoulder. It overlies the 2nd – 7th ribs, is tilted forwards by an angle of 30°, and is encased by 17 muscles which provide control and stabilisation against the thoracic wall (the ribcage). This is sometimes referred to as the “Scapulothoracic Joint” although it is not technically an actual joint.
The scapula has a shallow fossa (socket) on its lateral side into which the head of the humerus fits to form the glenohumeral joint.
The Clavicle (or collar bone)
The clavicle is an S-shaped bone and is the main connection between the upper arm and the rest of the axial skeleton. The clavicle is also an important site for muscle attachments including:
- Pectoralis Major
The clavicle meets the scapula at the top of the shoulder where it connects to the acromion process, forming the acromioclavicular joint.
The Humerus (upper arm bone)
The humerus is the upper arm bone. The head of the humerus is the ball of the ball and socket joint at the shoulder. The head fits into the glenoid fossa of the scapula.
The most important aspect of the shoulder is the large range of movement that it permits, which is central to many activities of daily living.
There are three main joints in the shoulder girdle, these are:
- Glenohumeral Joint (GHJ)
- Acromioclavicular Joint (ACJ)
- Sternoclavicular Joint (SCJ)
It is also important to consider another “joint” which is important in shoulder movement:
- Scapulothoracic Joint
The Glenohumeral Joint (shoulder ball and socket joint)
Ask someone to point at the shoulder joint, and the chances are they will point at the Glenohumeral Joint (GHJ). The Glenohumeral Joint is a ball and socket joint which provides a large proportion of the movement at the shoulder girdle.
The head of the humerus articulates (moves) with the glenoid fossa of the scapula – hence the name. The head of the humerus is, however, quite large in comparison to the fossa, resulting in only one third to one half of the head being in contact with the fossa at any one time. The humerus is further supported by the glenoid labrum – a ring of fibrous cartilage which extends the fossa slightly making it wider and deeper (almost like if you have a deeper bowl, you can fit more in it!).
Both articulating surfaces are covered with articular cartilage which is a hard, shiny cartilage which protects the bone underneath.
The Acromioclavicular Joint
The Acromioclavicular Joint (ACJ) is formed by the lateral end of the clavicle articulating with the medial aspect of the anterior acromium.
The ACJ is important in transmitting forces through the upper limb and shoulder to the axial skeleton. The ACJ has minimal mobility due to its supporting ligaments:
- Acromioclavicular Ligament which is composed of strong superior (top) and inferior (bottom) ligaments, and weak anterior (front) and posterior (back) ligaments restricting anterior-posterior (forwards and backwards) movement of the clavicle on the acromion
- Coracoclavicular Ligament is composed of the Conoid and Trapezoid ligaments. It forms a strong heavy band to prevent vertical movement.
The Sternoclavicular Joint
The Sternoclavicular Joint occurs at the sternal end of the clavicle, the cartilage of the first rib, and the upper and lateral parts of the manubrium sterni (the upper part of the sternum, or breastbone).
It is the only joint that truly links the upper extremity to the axial skeleton, via the clavicles. The Sternoclavicular Joint functions in all movements of the upper limbs, and is particularly important in throwing and thrusting movements.
The Scapulothoracic Joint
This joint relies entirely on the surrounding musculature for its control. The main muscles which control this joint are:
- Serratus Anterior which holds the medial (inside) angle of the scapula against the chest wall.
- Trapezius which rotates and elevates the scapula with elevation (lifting up) of the upper arm.
Note that during elevation the Glenohumeral Joint rotates 2° for every 1° of scapulothoracic rotation (most of us don't need to worry about this fact but it can be handy for clinicians to know).