Normal Anatomy of the Shoulder Joint

The shoulder is the most flexible joint in the body, making it the most susceptible to instability and injury. It is a ‘ball-and-socket’ joint. A ‘ball’ at the top of the upper arm bone (humerus) fits neatly into a ‘socket’, called the glenoid, which is part of the shoulder blade (scapula).

The shoulder joint is made up of several bones and soft tissues. It has three bones, the humerus, scapula and collarbone (clavicle).

The humerus is the upper arm bone. It provides attachment to the muscles of the upper arm.

The scapula is a flat and roughly triangular bone that connects the upper arm bone with the collarbone. It provides attachment to the muscles of the back and neck. Glenoid is the smooth shallow depression at the end of the scapula that forms a socket in which the humerus head articulates to form the ball and socket joint. The coracoid process is the extension of the scapula around the shoulder joint, at the front portion of the scapula. The acromial process is the extension of the scapula around the shoulder joint at the back, which forms a roof (acromion). 

The collarbone or clavicle is a short S-shaped bone that connects the shoulder girdle to the body (trunk). It supports the shoulder in a functional position with the axial skeleton so that the arm has maximum range of movement. It also protects major underlying nerves and blood vessels as they pass from the neck to the axilla.

The soft tissues of the shoulder joint include:

• Rotator cuff: A group of 4 tendons makes up the rotator cuff. It holds the head of the humerus in the socket.
• Biceps tendon: The biceps tendon is a long cord-like structure that attaches the biceps muscle to the shoulder, and stabilizes the joint.
• Caraco clavicular ligament: This ligament connects the clavicle to the coracoid process of the scapula.
• Acromio clavicular ligament: It connects the clavicle to the acromion process.
• Glenoid labrum: It is a ring of fibrocartilage that surrounds the cavity of the scapula to stabilize the shoulder joint.
• Articular cartilage or capsule: It is a capsule that surrounds the shoulder joint and helps to keep the ball and socket normally aligned.