Shoulder Anatomy

The three bones which form the shoulder girdle are the clavicle, the scapula and the humerus.

The human shoulder is the most mobile joint in the body. This mobility provides the upper extremity with tremendous range of motion such as adduction, abduction, flexion, extension, internal rotation, external rotation, and 360° circumduction in the sagittal plane. Furthermore, the shoulder allows for scapular protraction, retraction, elevation, and depression. This wide range of motion also makes the shoulder joint unstable. This instability is compensated for by rotator cuff muscles, tendons, ligaments, and the glenoid labrum.

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
  • Trapezius
  • Sternoclaedomastoid
  • Sternohyoid
  • Subclavius

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)