The combined joints connecting the scapula (scapulothoracic joint), clavicle (sternoclavicular and acromioclavicular joints), and humerus (glenohumeral joint) form the shoulder complex and anchor the upper limb to the trunk. The only boney stability of the upper limb to the trunk is through the connection between the clavicle and the sternum. The remaining stability of the shoulder complex depends on muscles, and as a result, the shoulder complex has a wide range of motion.
To best understand the actions of the scapula, it is important to understand the scapulothoracic, acromioclavicular, and sternoclavicular joints (Figure 30-1A).
- Scapulothoracic joint. Formed by the articulation of the scapula with the thoracic wall through the scapular muscles, including the trapezius and the serratus anterior muscles. The scapulothoracic joint is not considered a true anatomic joint; as such, it is frequently referred to as a “pseudo joint” because it does not contain the typical joint characteristics (e.g., synovial fluid and cartilage).
- Acromioclavicular joint. A synovial joint formed by the articulations of the scapula (acromion) and the clavicle.
- Sternoclavicular joint. A synovial joint formed by the articulations between the clavicle and sternum.
A. Superior view of the scapulothoracic joint. B. Scapular actions. C, D. Glenohumeral joint actions.
The scapulothoracic, sternoclavicular, and acromioclavicular joints are interdependent. For example, if motion occurs at one joint (e.g., the scapula elevates), the movement will directly affect the other two joints. Therefore, the motions produced frequently involve more than a single joint. Although the scapular movements include the scapulothoracic, acromioclavicular, and sternoclavicular joints, we will refer only to the scapula in the following text.
The following terms describe the movements of the scapula (Figure 30-1B):
- Protraction. Anterior movement of the scapula on the thoracic wall (e.g., reaching in front of the body).
- Retraction. Posterior movement of the scapula on the thoracic wall (e.g., squeezing the shoulder blades together).
- Elevation. Raising the entire scapula in a superior direction without rotation.
- Depression. Lowering the entire scapula in an inferior direction without rotation.
- Upward rotation. Named according to the upward rotation and direction that the glenoid fossa faces.
- Downward rotation. Named according to the downward rotation and direction that the glenoid fossa faces.
Actions of the Glenohumeral Joint
The glenohumeral joint is a synovial, ball-and-socket joint. The “ball” is the head of the humerus, and the “socket” is the glenoid fossa of the scapula. The glenohumeral joint is considered to be the most mobile joint in the body and produces the following actions (Figure 30-1C and D):
- Flexion. Movement anterior in the sagittal plane.
- Extension. Movement posterior in the sagittal plane.