Part 1: Introduction and anatomy

Understanding the shoulder’s anatomy can be crucial in diagnosing, treating and preventing injuries and building strength and power. Shoulder anatomy is a complex and intricate system of bones, joints, muscles, tendons, and ligaments that work together to allow a wide range of movement. It is considered a “freely movable” joint and has the greatest range of motion of any joint in the body, but stability is the trade-off for such mobility.

Joints

The shoulder comprises four joints; the acromioclavicular, glenohumeral, scapulothoracic, and sternoclavicular. The glenohumeral joint is a ball-and-socket joint made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone). The rounded end of the humerus fits into a shallow socket called the glenoid (located on the scapula) like a golf ball on a tee.
The shoulder has several critical structures to help stabilize the shoulder joint and control its movements. The rotator cuff is essential in controlling the arm’s movements and preventing the head of the humerus from dislocating from the glenoid. The rotator cuff comprises four muscles and their associated tendons surrounding the shoulder joint. These muscles originate from the scapula and attach to the humerus, and they help to keep the head of the humerus centred in the glenoid socket.

Rotator Cuff

The rotator cuff is a group of four muscles that help stabilize the shoulder joint and allow for smooth movement. The muscles of the rotator cuff are the supraspinatus, infraspinatus, teres minor, and subscapularis.

Supraspinatus

Origin: Supraspinous fossa of scapula

Insertions: Greater tubercle of humerus

Actions: Shoulder abduction (lifting arm away from body) of arm (0–15 degrees), and stabilization of the humeral head in the glenoid cavity

Infraspinatus

Origin: Infraspinous fossa of scapula

Insertion: Greater tubercle of humerus, Stabilization of humeral head in the glenoid cavity

Action: external rotation; Stabilizes humeral head in glenoid cavity

Subscapularis

Origin: Subscapular fossa of scapula

Insertions: Lesser tubercle of humerus

Action: Internal rotation, and Stabilizes humeral head in glenoid cavity

Teres Minor

Origin: Lateral border of scapula

Insertions: Greater tubercle of humerus

Action: Arm external rotation, arm adduction, and stabilizes humeral head in glenoid cavity

Ligaments and capsule

Underneath all the muscles discussed so far, the shoulder has several ligaments that help stabilize the joint and prevent dislocation. The most important of these is the glenohumeral ligament, which runs from the head of the humerus to the glenoid rim and helps control the joint’s movement. Several other ligaments help to stabilize the joint, including the coracohumeral ligament, the coracoacromial ligament, and the acromioclavicular ligament.

The bursae are tiny fluid-filled sacs near the shoulder joint that help reduce friction between bones and tendons. The tendons are strong, fibrous structures that attach muscle to bone and play a key role in transmitting the force generated by the muscles to the bones and joints.

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