The shoulder is a complex structure comprised of several separate but complementary joints that include both the shoulder girdle and glenohumeral joint. Normal movement in this region requires coordinated efforts of the ball-and-socket glenohumeral joint, gliding motions of the acromioclavicular and sternoclavicular joints, and relative movements of the scapulothoracic joint.
Proper joint alignment and adequate spacing, optimal tension in key soft tissues, and coordinated contraction of dynamic stabilizers all contribute to optimal shoulder function. This article focuses on the specific contribution of the acromioclavicular joint to the function of the shoulder.

Anatomy

Seated at the front of the shoulder between the sternum and head of the humerus is the clavicle, this slightly S-shaped bone serves as a brace, levering the anterior shoulder laterally and away from the sternum and providing a significant attachment point for several large muscles of the neck, chest, and upper extremity. The acromion process is the curved lateral prominence of the spine of the scapula that articulates with the lateral portion of the clavicle. At their junction, these two bony landmarks form the point of the shoulder known as the acromioclavicular joint.
The acromioclavicular joint is a gliding synovial joint, but with very limited mobility. A network of strong ligaments maintains a strong union between the clavicle and the acromion. The coracoclavicular ligaments join the coracoid process of the scapula to the underside of the clavicle near the acromioclavicular joint. The two portions, the medial fan-shaped conoid ligament and the lateral quadrilateral-shaped trapezoid ligament, provide horizontal stability and limit scapular rotation. They maintain vertical tension on the lateral portion of the clavicle, preventing it from rising superiorly at the acromioclavicular joint.
The acromioclavicular ligament directly joins the acromion and lateral clavicle, preventing horizontal separation between the two. It also helps limit axial rotation of the clavicle during elevation of the upper extremity, an essential component of normal shoulder mechanics. The sternoclavicular ligament also limits clavicular motion, but at the medial portion rather than lateral. Finally, the coracoacromial ligament lies between the acromion and coracoid processes, forming a fibrous sling that helps stabilize the humeral head during overhead motions.

Mechanics

The primary movements of the scapulothoracic joint—elevation, depression, protraction, retraction, upward rotation, and downward rotation—are the result of a combination of movement at the acromioclavicular and sternoclavicular joints. The scapula moves freely, gliding across the posterior thorax with the acromioclavicular joint, serving as the primary point of suspension of the upper extremity from the trunk.
Several large, powerful prime-mover muscles attach to the clavicle and affect the function of the acromioclavicular joint. These include the pectoralis major anteriorly and inferiorly, the trapezius posteriorly and superiorly, the sternocleidomastoid medially, and the deltoid laterally. Additionally, the subclavius and pectoralis minor both strongly impact positioning of the acromioclavicular joint and the overall function of the scapulothoracic joint (see images above).
Dynamic positioning of the clavicle throughout the range of movement of the glenohumeral joint significantly increases the range and movement combinations possible at the upper extremity. Upward and downward rotation of the clavicle dramatically enhances the amount of shoulder flexion and extension possible. Anterior and posterior gliding of both the acromioclavicular and sternoclavicular joints, with resultant scapular protraction and retraction, broadens shoulder horizontal adduction and abduction. Superior and inferior gliding provides scapular upward rotation and downward rotation, offering greater range of motion in both shoulder abduction and adduction.

Pathology and Purpose of Soft-Tissue Intervention

Improper acromioclavicular mobility is a common cause of shoulder pain and dysfunction. Excessive mobility due to congenital joint laxity, chronic conditions, or specific injury may lead to joint instability, osteoarthritis, chronic subluxation or dislocation, impingement syndrome, or neurovascular compromise conditions such as thoracic outlet syndrome.
Conversely, limited mobility due to muscular imbalance, hypertonicity, excessive myofascial density or adhesions, scar tissue from previous injury, chronic dysfunctional posture, or repetitive stress is equally problematic. This is more readily addressed through soft-tissue manipulation and should include assessment and restoration of mobility at both the sternoclavicular and acromioclavicular joints, as well as associated glenohumeral joint motions.