Anatomy of the Shoulder
The anatomy of the shoulder is very complex because of the multiple joints that it consists of, the dozens of muscles that act upon it, and the nerve and vascular supply that are closely correlated with it. If you are reading this article, chances are you have probably experienced shoulder pain at some point either in your athletic career or in everyday life. Injuries at the shoulder are very common for various reasons, which we will get into, but in this article, we will break down the anatomy of the shoulder joint and keep working our way throughout the body.
The main joint of the shoulder is glenohumeral (GH) joint or “shoulder girdle” as you may have heard it referred to. It is a ball-and-socket joint just like the hip, so it has three degrees of freedom, which makes it highly moveable. It moves in flexion/extension, abduction/adduction, medial/lateral rotation and circumduction, which is great for the various movements throughout our daily lives. The downside to having so much movement is the amount of instability that can comes along with it, which can be heightened if there is anatomical deformity or lack of development of the musculature that stabilizes the shoulder. The articulation at this joint consists of the head of the humerus and the glenoid cavity of the scapula, or shoulder blade. If you think about how a golf ball sits on a tee, that is basically consistent with how the GH joint is structured. We do receive a little extra deepening of the socket from the glenoid labrum, which is made up of fibrocartilage to allow for better articulation, provide smooth movement and act as a shock absorber for the joint. Due to the high amount of instability of the joint, it relies on static stabilization from the ligaments but heavily relies on dynamic stabilization from the rotator cuff muscles.
The ligaments of the GH joint include the coracohumeral ligament, transverse humeral ligament and coracoacromial ligament. The combination of these ligaments provide resistance against excessive translation of the joint superiorly and inferiorly. The transverse humeral ligament plays an important role in that it runs across the humerus and holds the tendon of the biceps brachii in place.
The rotator cuff is a group of four scapulohumeral muscles that aid in movement of the glenohumeral joint (shoulder joint) throughout much of its range of motion. They are considered scapulohumeral muscles because of their origin on the scapula and insertion on the humerus. The shoulder relies heavily on dynamic stabilization from muscles and tendons, which is offered primarily by the scapulohumeral muscles. The rotator cuff group offers vital stabilization and movement of the upper extremity. As said before, this group includes four muscles, all of which offer specific actions at the shoulder.
The four rotator cuff muscles or “SITS” muscles include: the supraspinatus, infraspinatus, teres minor and subscapularis, all of which form a “cuff” which helps to form the joint capsule at the shoulder. The supraspinatus is the most superior of the rotator cuff, sitting atop the scapula. This is the only rotator cuff muscle which does not directly help with rotation. Its main action is to initiate arm abduction (taking arm away from the body) by assisting the deltoid. The infraspinatus sits below the supraspinatus and spine of the scapula. Its individual action along with the teres minor is external rotation of the arm. This is commonly the muscle aggravated or injured during low bar back squats, pitching or overhead activities, just to name a few. Working inferiorly, the teres minor sits below the infraspinatus on the shoulder blade. This individual muscle, again works with the infraspinatus to externally rotate the arm. It also is responsible for arm adduction or bringing the arm towards the body. The majority of the muscle belly and tendon is covered by the deltoid muscle group. The final rotator cuff muscle, the subscapularis, arises from a different area of the scapula. It sits in the subscapular fossa on the anterior (front) side. Its main responsibility is internal rotation of the arm, while also assisting the teres minor in arm adduction. The other muscles that act on the GH joint include: the biceps brachii, triceps, the deltoid muscle group, pectoralis major/minor, and latissimus dorsi.
The second major joint of the shoulder is the acromioclavicular (AC) joint. It is a plane synovial joint where the lateral portion of the acromion process of the scapula meets with the clavicle, or collarbone. This articulation is completed with a articular disc to help with joint efficiency. The overall joint capsule is weak, but the fibers of the upper trapezius help provide it with dynamic stabilty. The static stability of the joint is supplied by the multitude of ligaments that support the joint, and when this joint is injured, it is usually a combination of these ligaments that are affected from a traumatic impact. The ligaments include the acromioclavicular ligament, the coracoclavicular ligament, conoid ligament and trapezoid ligament. The AC joint moves in correlation with the scapulothoracic joint, because when it moves, the AC joint moves with it.
The final joint we will talk about specifically is the sternoclavicular (SC) joint. It is a saddle synovial joint that functions like a ball-and-socket joint. Although the joint is considered a saddle joint, it allows the clavicle to raise up to 60 degrees with slight rotation. It can even be translated anteriorly and posteriorly, so in conjuction with the other joints, it allows a lot of movement in the upper limb and pectoral girdle. The stabilization of this joint is provided by the anterior and posterior sternoclavicular ligaments, interclavicular ligament and costoclavicular ligament. These ligaments provide stability of the joint, so dislocations of the clavicle are not common, however, when a traumatic force is sustained fractures are common. The final important thing to note about the importance of this joint is, it the only articulation between the upper limb and axial skeleton.
I would be missing an important aspect if I didn’t touch on this, so it will be the last thing we mention, and that is scapulothoracic rhythm. This movement is associated with all the other movements we have talked about, because without the scapula properly moving on the thoracic cavity, we would not be able to move efficiently. The initial 30 degrees of abduction can occur without scapular involvement, but after that point, in every additional 3 degrees of motion, 2 degrees come from the GH joint, and 1 degree comes from the scapulothoracic joint. If this rhythm is off, movement efficiency will be poor, pain will likely present and further injury can occur. The important movements of the scapulothoracic joint are elevation, depression, protraction, retraction and rotation. There are 17 muscles that attach to the scapula, and these all are associated with the quality of its movement in one way or another. If you have had any shoulder injuries, or would like to see specific structures, injuries or conditions discussed in future articles, please let us know and always remember to #HealByMoving.