The Shoulder Joint Simplified

The shoulder joint is a synovial ball-and-socket joint, which is fundamentally quite simple. However, with the huge range of motion at the shoulder joint, combined with its use in almost every task involving the upper body, requires a fairly robust external structure via many different soft tissue structures. This turns the shoulder from a simple joint to very complex joint with lots of interdependent anatomical structures.

Due to the complexity of this joint, combined with the fact that it's responsible for a huge range of motion,​ shoulder joint injuries are quite common. This vulnerability is increased when participating in contact sports where you may be combining impacts from opposing players, boards, or falling on the field/ground. Additionally, non-contact sports that involve a lot of repetitive shoulder movements, especially over the head, such as baseball, also see an increased prevalence of shoulder injuries.

If you have a history of shoulder injuries and are looking to prevent injuries in sports, check out our reviews of the best shoulder braces, which are further broken down into the best shoulder braces for football. Otherwise, please feel free to browse our simplified guide to the shoulder joint below.

Shoulder Movements

We won't spend too much time on this. The main reason we wanted to briefly show the different shoulder movements is for reference for information that can be found further along in this article. Essentially, the shoulder's ball-and-socket arrangement provides it with a wealth of possible movements, including flexion, extension, abduction, adduction, lateral rotation, medial rotation, and circumduction.

Bones of the Shoulder

At its most basic level, the shoulder joint is an articulation between two bones: the humerus (upper arm) and the scapula (shoulder blade), and is formally termed the glenohumeral joint. The collar bone is often considered to be part of the shoulder as well, as it located in the same region, but it doesn't contribute directly to the articulation of the glenohumeral joint itself. Like other synovial joints, the shoulder joint includes cartilage on the articulating surfaces as well as a synovial membrane.

Even though this is a ball-and-socket joint, the glenoid cavity, which receives the head of the humerus, isn't really that deep. In fact, at any time, only about 30% of the surface of the humeral head is in contact with the glenoid cavity. As you may be able to imagine, this arrangement alone doesn't really lend itself to superior stability, so we need to include other support mechanisms that are external to the joint itself.

Ligaments and Bursa of the Shoulder

Given that the shoulder joint is responsible for a huge range of motion, it is important to provide further stability in addition to the form-fitting ball-and-socket joint. This will come in the form of anatomical structures external to the joint itself, namely, the ligaments.

The fibrous joint capsule encapsulates the head of the humerus and glenoid fossa, and this provides a strong stabilizing force during shoulder movements, primarily abduction (raising the arm to the side). The joint capsule is also reinforced on the front by three strong bands of glenohumeral ligaments: the superior, middle, and inferior glenohumeral ligaments. In a neutral position, these ligaments don't really add too much stability to the joint capsule itself, but they quickly become taut when the shoulder goes into lateral rotation.

There are numerous other ligaments around the shoulder as well, including the coracohumeral ligament and coracoacromial ligaments. These names sound confusing, but if you remember that the names of ligaments often contain the structures that they attach to, you can have a better idea of where they might be found. For example, the coracohumeral ligament runs from the coracoid process of the scapula to the humerus. This ligament in particular is very strong and helps support the weight of the arm against gravity as the arm hangs by the side. The coracoacromial ligament has a wide base on the coracoid process and narrows as it runs to the acromion, where it inserts over a smaller region. This ligament reinforces the top of the joint, preventing superior displacement of the humerus from the glenoid.

Bursa, on the other hand, are way different than ligaments. These are fluid filled sacks that don't really do anything for stability, but they reduce friction between structures that move over top of each other, allowing for proper movement overall. These bursa are commonly "injured" via overuse or infection. Fortunately, the treatment ranges but is rather simple, from rest and ice, to the draining of excess fluid, the latter of which can sometimes just be a short procedure at the doctor's office.

We won't get into all the different bursae here, as there are lots of them and they all serve the same purpose: reduce friction. In the image above, the bursae are the green structures, but in lots of anatomy textbooks and online resources, they are often depicted in purple as well.

Muscles of the Shoulder Joint

We could spend at least a few articles discussing the muscles that act on the shoulder joint, but for the sake of this article, we will try and simply this as much as possible, including only the main muscles we think are worth mentioning here.

Rotator Cuff Muscles

You may have heard of the "rotator cuff" before. This isn't one single structure, but instead, a combination of four muscles that cross the glenohumeral joint to enhance stability while also providing a means of movement. To make them easier to remember, the rotator cuff muscles are sometimes called the SITS muscles, where each letter represents the first letter of one of the rotator cuff muscles:

Supraspinatous
Infraspinatous
Teres Minor
Subscapularis

All of these muscles act as strong dynamic stabilizers of the shoulder joint, and together, are known as the rotator cuff. These muscle not serve to blend with and reinforce the fibrous joint capsule, but they also have their own functional movements as well.

The supraspinatous reinforces the superior aspect of the joint capsule, which helps to keep the head of the humerus in the glenoid cavity during shoulder abduction. Given its line of pull, it's also responsible for the first 15 degrees of shoulder abduction, after which point the deltoid muscle largely takes over. People with injuries to the supraspinatous or the nerves supplying it often need to hike their shoulder girdle and bend their trunk sideways to initiate shoulder abduction.

The infraspinatous reinforces the posterior aspect of the joint capsule. Aside from providing stability mainly for the head of the humerus, it can also rotate the shoulder in a lateral direction. Teres minor sits just below infraspinatous and it is often difficult to distinguish the two muscles. They have almost the exact same function, but are differentiated by their nerve supply.

Subscapularis differs from the other rotator cuff muscles, as it arises on the anterior aspect of the scapula. The tendon of this muscle reinforces the front of the shoulder joint, and in addition to acting as a strong stabilizer, it is also responsible for medial rotation of the shoulder, and also assists in adduction.

Deltoid and Trapezius

These two muscles sit on top of the rotator cuff muscles and are some of the most easily identifiable muscle around the shoulder. The deltoid can be considered the main shoulder muscle, and you can often distinguish it on people even when they are wearing a shirt. The deltoid is a powerful muscle that actually has three parts, each with its own primary action. The anterior part is responsible for shoulder flexion, the middle part is responsible for shoulder abduction, and the posterior part is responsible for extension and lateral rotation.

The trapezius muscles, also known as the "traps", also has three groups of fibers. The upper group is responsible for elevating the scapula, the middle group is responsible for retracting the scapula (e.g. standing to attention position), and the lower group is responsible for rotating the scapula upwards. These muscle fibers all help support the scapula, but don't provide direct stability to the glenohumeral joint itself.

There are lots of other muscles in the shoulder area, but we will stop the discussion here for now. The muscle we listed above are all important in the normal functioning of the shoulder joint. Some are primarily meant for support and stabilization, while others are highly capable of providing strong movements of the shoulder joint.

Summary

We hope this brief guide to shoulder anatomy has helped you understand the basic components in a little more detail. ​In any case, you can probably see that this "simple" breakdown is still quite complex, and given the amount of moving parts involved in the shoulder at once, it's not exactly surprising that the shoulder is commonly injured, especially in contact sports or sports involving lots of arm/overhead movements.

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