The Sternoclavicular And Acromioclavicular Joints: Completing The Shoulder Complex
Our shoulder complex has a tricky job of allowing movement in all directions while also maintaining sufficient stability. Many structures come together to form the larger shoulder complex and make this amazing balance of mobility and stability possible. It takes the coordination of all the structures at the shoulder complex to experience a full range of functional movement. We’ve already covered the glenohumeral joint, which is the true shoulder joint. There are two other joints that work with the glenohumeral joint to make up the shoulder complex, the sternoclavicular joint and the acromioclavicular joint.
In this article, we’ll take a look at those joints to complete our picture of the shoulder complex. Read on to learn where these joints are located, what muscles cross them, and what actions they do.
What structures make up the sternoclavicular and acromioclavicular joints?
Bones
The sternoclavicular joint includes two bones, the sternum (breast bone) and clavicle (collar bone). As the name suggests, the joint is where the sternum articulates with the clavicle. Specifically, it’s where the medial end of the clavicle meets the manubrium (superior end) of the sternum. Additionally, there is a connection that completes this joint surface between the costal cartilage of the first rib and the manubrium of the sternum. The sternoclavicular joint is where the appendicular skeleton (your arm) connects to the axial skeleton.
The acromioclavicular joint is the joint at the other end of the clavicle. It includes two bones, the lateral end of the clavicle and the acromion. The acromion is the bony shelf that juts off the front of the scapula (the shoulder blade). It hangs over the head of the humerus where it articulates with the glenoid fossa of the scapula to create the shoulder joint itself.
Joint type
The sternoclavicular joint is a multiaxial saddle-shaped gliding synovial joint, with a fibrocartilage disc. The acromioclavicular joint is a multiaxial plane-type gliding synovial joint.
Ligaments
There are four ligaments that cross the sternoclavicular joint. They are the anterior sternoclavicular ligament, posterior sternoclavicular ligament, costoclavicular ligament, and the interclavicular ligament. These strong ligaments are an important part of stability at the joint. This joint is a key connection for connecting the appendicular skeleton (arm) to the axial skeleton, but there aren’t any muscles that cross this joint. The ligaments then are essential to maintaining the necessary stability.
The acromioclavicular joint is stabilized by two pairs of ligaments. The first pair includes the conoid and trapezoid coracoclavicular ligaments. The second pair includes the superior and inferior acromioclavicular ligaments. As with the sternoclavicular joint ligaments, the ligaments at the acromioclavicular joint are essential for keeping this uniquely shaped joint connected and stable.
Muscles
No muscles directly cross the sternoclavicular or acromioclavicular joints and move them (like you might think of with a muscle like the biceps brachii). Instead, movement is caused indirectly. Six muscles attach to the clavicle. Those are the deltoid, pectoralis major, trapezius, sternocleidomastoid, and the subclavius muscle.
Muscle contractions by any of the muscles that attach to the clavicle can affect movement at the joints at either end of the clavicle. Movement at the sternoclavicular joint and the acromioclavicular joint are also intimately related to movements of the scapula. Movements of the scapula by muscles that don’t attach to the clavicle, like the rhomboids, serratus anterior, or rotator cuff group will still be reflected in movements at the sternoclavicular and acromioclavicular joints.
Movements at the sternoclavicular and acromioclavicular joints
As you’ve learned already, there are similarities in the structure of the sternoclavicular and acromioclavicular joints. However, the range of movement created at each is significantly different. The movements at the acromioclavicular joint would be measured in millimeters. At the sternoclavicular end of the clavicle, the joint surfaces don’t necessarily move far, but enough that the distal end of the clavicle can be moved by inches.
Both the sternoclavicular and acromioclavicular joints allow for elevation and depression movements. Both joints also do protraction and retraction movements. While they both also do a rotational movement, however, their directions of rotation are different. The sternoclavicular joint can do rotation about its own axis, or in the sagittal plane. The acromioclavicular joint, however, can rotate in a lateral and medial direction, or in the transverse plane.
What about the scapulothoracic joint?
There is another relationship that should be included to complete our picture of the shoulder complex. It’s sometimes referred to as the scapulothoracic joint. However, the scapulothoracic joint is not a true joint. It’s not an articulation between bones. The term scapulothoracic joint is a misnomer that is sometimes used to describe the movement of the scapula in the myofascial sling that connects the scapula to the posterior side of the ribcage.
The tension and movement at the scapulothoracic junction will certainly affect the position and movement of both the acromioclavicular and the sternoclavicular joints. So, the positional relationship between the scapula and the thorax is an important part of our whole shoulder complex. However, it’s important to understand that this connection is not a true joint, so the way that movement happens there is different than it is where two bones articulate.
Conclusion
The shoulder complex is, as its name suggests, a complicated interaction of structures. When all of these parts work together, we experience an amazing variety of available movement. Understanding the individual parts that make up the shoulder complex can help us understand its function as a whole. The sternoclavicular and acromioclavicular joints are key parts of what makes our shoulder complex work.
References
Chaitow, L. and J. DeLany. 2011. Chapter 13: Shoulder, arm, and hand. In Clinical Application of Neuromuscular Techniques. Volume 1 – The Upper Body (Second Edition). New York, NY: Churchill Livingstone Publishing.