The shoulder is a complex multi-planar joint. Due to this, the shoulder is subject to a variety of uses and a multitude of stresses. The most common injury in the throwing shoulder is impingement. This injury is the result of another trauma such as a muscle strain, muscle overuse, or a contusion. The condition will put a rapid stop to any arm activity that occurs above shoulder level.

The shoulder joint is comprised of two true joints and one pseudojoint, as well as several muscles and ligaments. The two true joints are the glenohumeral and acromioclavicular, and the pseudojoint is the scapulothoracic articulation. The glenohumeral joint (G-H) is the joint formed by the humerus (arm bone) and the scapula (shoulder blade). The acromioclavicular joint (A-C) is a strut formed by the attachment of the clavicle (collar bone) and the scapula. This attaches the scapula physically to the axial skeleton. The A-C joint forms a hood over the top of the G-H joint. It is under this hood in the subacromial space that the impingement occurs.

The scapulothoracic articulation is not a true joint due to the fact that two bones do not directly articulate. This joint is formed by the gliding of the scapula along the rib cage. It is anchored to the ribs by muscle rather than ligament, like most joints. Proper movement in this area is a major issue in the treatment of shoulder injuries. When the scapula does not glide along the rib cage properly, the stresses of the joint are improperly applied to the G-H joint resulting in an impingement.

The main muscles of the shoulder are the Deltoid, Biceps, Pectorals, and Rotator Cuff. The Rotator Cuff (RC) is a group of four muscles that act as secondary stabilizers for the shoulder. The RC muscles originate on the scapula and rib cage, and attach on the humerus, passing under the hood formed by the A-C joint, acromion and C-C ligament (the coracoclavicular arch). Their function is to rotate the humerus and to help stabilize the humerus, like guy wires on a telephone pole.

An impingement (figure 1) results from a change in the proper mechanics of the shoulder. The highly mobile nature of the shoulder is a result of the natural laxity of the ligaments that hold the G-H joint. Due to this the muscles around the shoulder, specifically the rotator cuff, must provide secondary structural support. An injury that affects the rotator cuff muscles is the most common cause of impingement.

The RC tendons become pinched or impinged upon by the humerus and the A-C joint, etc. This impingement is the result of the RC weakness which allows the head of the humerus to drift upward in glenoid fossa of the scapula pinching the RC tendons. This results in inflammation of the tendons which allows for easier impingement. It is a viscous cycle of impingement, inflammation, and weakness that is self sustaining. Until this cycle is broken with proper strengthening of the RC group and oral anti-inflammatory medication, it will continue.