I.    Overview

The function of the shoulder is to place the hand in space. There are six directions of movement in the shoulder. The shoulder complex consists of three "joints" which allow for increased function, strength and flexibility. It is a mobile and strong joint and the only connection between the arm and the trunk.

II.    Shoulder Joint Complex

The shoulder is unique because it is a "complex" comprised of four different joints. A joint is the coming together of two bones, allowing one to move over another. The shoulder complex is made up of three joints and a fourth pseudo-joint, or soft tissue articulation. The adjacent soft tissue structures allow for increased movement and stability.

 

Osteology (bones) of the shoulder

The glenohumeral joint is the ball and socket, which consists of the humerus and scapula bones. It has the most range of motion of any joint in the human body. The acromioclavicular joint consists of the clavicle bone and the part of the scapula called the acromion. The sternoclavicular joint is where the clavicle bone and the sternum (the breast plate in the middle of the chest) connect. The scapulothoracic articulation or scapulothoracic "joint" is the scapula bone and its soft tissue connection to the ribs of the upper back.

 

Scapula (the "wing" bone)

This has several different sections: the body, the spine, the glenoid (the "socket" in the glenohumeral joint) and the acromion (the top part of the bone that projects out).

 

Clavicle (the collarbone)

This bone is a tubular, twisting bone that connects the shoulder joint complex to the chest. If looked at from above, the scapula and clavicle form a triangular anchor that provides stability to permit the arm to move freely in space.

 

Humerus (the upper arm bone)

This bone connects the scapula to the forearm bones (radius and ulna bone). At the top of the humerus is the "head" of the bone (the "ball" of the glenohumeral joint).

Articular cartilage is a specialized soft tissue that covers the bone ends. It is a delicate structure and has no blood supply or nerves. In a healthy joint, it is smooth, spongy, and slippery, and allows for the bones to glide easily over one another during motion.



III.    Muscles

A muscle connects to a bone via a tendon, which is a rope-like structure. A muscle crosses the joint to attach to the bone which allows for movement.

 

The rotator cuff is the name of a group of four muscles: supraspinatus, infraspinatus, teres minor and subscapularis (SITS muscles). These muscles provide the following motions, respectively: abduction, external rotation (two muscles), and internal rotation. The rotator cuff muscle group is primarily responsible for dynamic stability in the glenohumeral joint. It also has a secondary function of depressing the head of the humerus in the glenoid as the arm is elevated over shoulder height (supraspinatus, infraspinatus, and teres minor).

 

The deltoid is the muscle that is the "cap" over the shoulder. It has three heads (attachments to bone): anterior, middle and posterior. The directions of motion for the three heads, respectively, are forward, side, and backward.

 

The biceps sits on the front of the humerus bone. It has two (bi) heads (ceps) to the muscle. The muscle runs from the scapula bone to one of the forearm bones (the radius). The biceps muscle assists in elbow bending (flexion), forearm supination, and shoulder forward elevation (flexion).

 

The triceps sits directly opposite the biceps. It has three (tri) heads (ceps) to the muscle. All three attach to the base of one of the forearm bones (the ulna). The triceps assists in elbow straightening (extension) and in moving the shoulder backwards (extension).

 

There are two pectoralis or chest muscles: the pectoralis major and the pectoralis minor. These muscles assist in lifting the arm forward (flexion), across the body (adduction), and turning it inward (internal rotation). They also help stabilize the scapula by moving it forward and down along the rib cage.

 

The levator scapula connects the scapula to the cervical vertebrae. This muscle elevates and rotates the scapula by tilting the glenoid cavity downward.

 

The platysmus sits on the front of the neck. It is a very superficial and thin muscle. This muscle assists in facial expression and movement of the mouth, and helps stabilize the front of the neck.

 

The trapezius muscle covers the upper and middle back. It connects from the base of the skull to the scapula, and down to the lower vertebrae. It consists of three parts: upper, middle, and lower trapezius. The directions of movement of the scapula respectively are: scapula elevation, retraction, rotation and depression.

 

The rhomboid muscles sit between the thoracic vertebrae and the scapula. These muscles help stabilize the scapula to the skeleton, and assist in moving the shoulder backwards towards the midline.

IV.    Ligaments

Ligaments are like small ropes, or rubber bands that connect bone to bone. Ligaments are static stabilizers in that they stabilize joints based on their connection. They prevent excessive motion in a joint.

There are three major glenohumeral joint ligaments: the superior, middle and inferior. These stabilize the shoulder at different positions by keeping the "ball on the socket".

 

The labrum is a circumferential fibrocartilage thickening of the capsule that covers the edge of the glenoid cavity. It deepens the socket, which allows for increased stability and congruency during shoulder motion.