Shoulder Anatomy

The shoulder joint is the second largest and most mobile joint in the human body and can be easily understood if divided into three layers.

Anatomy

The shoulder joint is the most mobile joint in the human body with the unique capability to swivel within a vast range of motion. The shoulder consists of three main layers:

  • Bones
  • Connective Tissue
  • Muscles

The bones are the deepest foundation. There are two bones constructing the shoulder:

  • Humerus (upper arm bone)
  • Scapula (shoulder blade)

The junctions of these bones form three joints which together make up the shoulder:

  • Glenohumeral (GH)
  • Sternoclavicular (SC)
  • Acromioclavicular (AC)

The glenohumeral joint is the main component of the shoulder. The humerus bone top is rounded and is called the humeral head. The humeral head fits into the saucer shaped socket of the scapula called the glenoid. This fitting is padded by a layer of cartilage (the blue area in the image below), allowing for smooth and pain-free shoulder motion.

Clavicle (collarbone)

The stenoclavicular joint is located at the end of the clavicle and aids in the range of motion of the shoulder, particularly in forward actions. The acromioclavicular joint is the connection between the top of the shoulder and the clavicle and allows the arm to be raised above the head. The second layer of the shoulder, connective tissue, prevents the bones from separating during movement, allowing the different bones to function as a unit. There are three types of connective tissue in the shoulder:

  • Collagen tissue fibers
  • Ligaments
  • Labrum

Collagen tissue fibers attach to the shoulder bones on both sides and form the joint capsule. Within the joint capsule are dense cordlike collagen bundles that unite to form ligaments. Ligaments act as reinforcement in strategic locations between bones to enhance the stability of a joint by resisting the forces created when we perform our routine daily activities.

In the image below you see from left to right: a joint capsule, a cross section of the joint capsule showing ligaments, and a cross section of the glenohumeral joint.

The labrum is a rubbery fibro-cartilage tissue encircling the rim of the glenoid. The labrum is an anchor point for the ligaments of the shoulder capsule and the long head of the biceps tendon. This creates a suction seal mechanism when the humeral head is in contact with the glenoid. This sealing mechanism adds another joint stabilizing effect to the shoulder.

The third layer in the shoulder, the muscles, provides movement. The most important shoulder muscle group are the rotator cuff muscles:

  • Supraspinatus
  • Infraspinatus
  • Teres minor
  • Subscapularis

The major function of the rotator cuff muscles is to provide dynamic stability, and motion and strength to the arm. The four muscles begin on the scapulas (shoulder blade) flat surface and attach to the humerus (arm bone).

This connection forms four tendons that interdigitate with the muscles to form a protective mechanism. This mechanism prevents rotator cuff tendon tears from quickly spreading once they occur. The interdigitation also allows the muscles to work together as a unit, carefully balancing the shoulder joint against the forces of the deltoid muscle to create precise shoulder movement.

The deltoid muscle, biceps muscle and other muscles of the shoulder are called scapular stabilizers. They provide balance to the shoulder blade. Together, the muscles of the third shoulder layer create a dynamic balanced force which allows for normal function.