Video: Shoulder joint

Our shoulders are pretty important in everyday life. We use them to reach things on a high shelf, cook, type, walk, run, climb, or even dance. Our shoulder joints are highly mobile and, therefore, are useful in lots of movements. Keeping our shoulders in check and under control are important stabilizers which surround the joint. In this tutorial, we'll explore the components of the shoulder joint from an anterior, posterior, and lateral perspective. We'll start by reminding ourselves what the shoulder joint is including the bones involved and some of their features. We'll then look at the joint cavity itself and identify a few specific features of it. Next, we'll discover some ligaments associated with the joint and then move into muscles that strengthen and move the joint as well. Finally, we'll look at a few bursae that help to relieve pressure and friction within the joint before moving on to the clinical notes.

Ready for it? Here we go.

First up, let's take a look at the shoulder joint as a whole. The shoulder joint, also known as the glenohumeral joint, is located at the proximal part of the upper limb and is a ball-and-socket joint. It is formed by the articulation between the large, rounded humeral head and the much smaller, shallow, concave glenoid fossa. This mismatched articulation leads to a highly dynamic, but resultantly, unstable joint.

Movements of this ball-and-socket glenohumeral joint include flexion and extension, abduction and adduction, medial and lateral rotation, and circumduction. It is the most mobile joint in the body. Let's take a closer look at the bones which make up this joint and its basic structure.

The first bone we'll look at is the humerus which is divided into a proximal end, body, and distal end. Relevant to the glenohumeral joint is the proximal end of the humerus which consists of a head and anatomical neck. The large, rounded head of the humerus articulates with the glenoid fossa of the scapula to form the ball-and-socket glenohumeral joint.

There are some bony features and landmarks of the proximal humerus which are related to structures that help stabilize the joint. These two raised areas on the anterior aspect of the proximal humerus are the lesser and greater tubercles. Between them, you'll find the intertubercular groove or sulcus. These features function as attachment sites for stabilizing structures of the glenohumeral joint which we'll learn about a little later on.

Next up, we have the scapula, colloquially known as the shoulder blade. The scapula has three angles, two surfaces, and three borders. Today we will mainly focus on the lateral angle of the scapula as it contains a shallow cavity known as the glenoid fossa. As we know, the glenoid fossa articulates with the humeral head to form the glenohumeral joint. Superior to the glenoid fossa is a bony process known as the supraglenoid tubercle. Similarly, at its inferior end, is the infraglenoid tubercle. Both of these tubercles are attachment sites for muscles that support the shoulder joint.

Protruding from the superior border of the scapula are two bony processes known as the acromion and coracoid process. These processes are not directly involved in the formation of the glenohumeral joint but are important contributors to shoulder joint stability.

The acromion is located at the superior aspect of the scapula which we can see from this anterior view of the shoulder girdle. It articulates with the clavicle to form one of the joints of the shoulder girdle – the acromioclavicular joint. The coracoid process similarly arises from the superior border of the scapula. This more anterior projection of the scapula does not articulate with any other bones but is an important attachment site for ligaments that help stabilize the shoulder joint. We'll see the importance of these processes in joint stabilization later on.

We're now going to take a look at some of the features of the glenoid cavity of the glenohumeral joint from a lateral perspective. Here we can see the scapula and the surrounding muscles isolated from the rest of the body from a lateral view. To the right of the image is the anterior aspect and to the left of the image is the posterior aspect. From this lateral view, we can see the same five features of the scapula that we just identified. Highlighted in green now is the glenoid fossa. Here we can see the supraglenoid tubercle and inferiorly is the infraglenoid tubercle. The large bony process highlighted now is the acromion and the final feature that we looked at was the coracoid process which we can see here.

All right, that's the basics covered so let's get on to the joint cavity itself and see what we can find there.

We'll begin with taking a closer look at the glenoid fossa or cavity. Remember we said the shoulder joint is a ball-and-socket joint? Well, the glenoid fossa forms the socket part of the joint. As you can see from its very shallow curve, although it is the socket portion, it isn't a very good one. In addition, the surface area of the humeral head is approximately four times larger than that of the glenoid fossa. So as you can imagine, that results in a relatively small contact area between the two at any given time. This is beneficial for us, though, in terms of the mobility of the joint but it means that stability is compromised.

There are, however, structures that support the shoulder joint and help to give it some extra stability. In fact, the shoulder joint relies more on the surrounding muscles and ligaments for its stability than it does on its bony elements. Some of these structures, such as the muscles and ligaments, aid in structurally limiting movements at the joint while others function to increase the depth of the joint and thereby contributes to concavity compression. Concavity compression or compression of the humeral head into the concave glenoid fossa aids in increasing stability within the joint against translating forces.

The first stabilizing structure of the glenoid fossa we will explore today is the glenoid labrum. The glenoid labrum is a fibrocartilaginous ring attaching around the outer margin of the glenoid fossa. It helps to deepen the glenoid fossa giving the socket part of the joint a bit more of a functional socket shape and subsequently contributes to compression of the humeral head within the glenoid fossa.

Surrounding the glenohumeral joint is the joint capsule. Like all synovial joints, the joint capsule is made of an outer fibrous layer and an inner synovial membrane. The inner synovial membrane secretes synovial fluid which fills the joint capsule, thereby, lubricating it. It arises from the periphery of the glenoid fossa with partial attachment to the glenoid labrum. It's easy to forget, looking at the lateral aspect, that the structures in question extends beyond the plane of the glenoid fossa. The fibrous part of the joint capsule actually engulfs the whole humeral head and extends beyond the margin of the articular surface to the anatomical neck of the humerus. The glenoid cavity, together with the capsule, form a roughly spherical space around the humeral head called the osteofibrous acetabulum. The glenohumeral joint's capsule acts as a weak stabilizer. Anteriorly and inferiorly, the joint capsule is strengthened by some ligaments which we'll look at next as well as other supportive ligaments.

The first ligament we'll look at is the coracohumeral ligament and you'll notice that its name is quite indicative of its attachments. We can see it running from the posterior aspect of this bony feature of the scapula that we identified earlier, the coracoid process, and the cut end here which will continue to insert on the humerus. We can observe this attachment if we switch to the anterior view of the joint. The ligament flares out onto an anterior and a posterior band, attaching to the margin of the lesser and greater tubercles, respectively. The coracohumeral ligament reinforces the articular capsule of the glenohumeral joint and is important in preventing the superior and inferior displacement of the humeral head during lateral rotation.

Next up, we're looking at a set of three glenohumeral ligaments starting with the superior glenohumeral ligament, which is also visible from an anterior view. It spans the anterosuperior aspect of the joint from the supraglenoid tubercle of the scapula to the medial ridge of the intertubercular groove of the humerus. This ligament works with the coracohumeral ligament to stabilize the humeral head and prevent it from moving out of the glenoid cavity, particularly, during shoulder adduction. It also strengthens the articular capsule particularly at its anterior aspect.

Moving inferiorly, we come across the middle glenohumeral ligament. The middle glenohumeral ligament is also located along the anterior aspect of the joint. It arises from the anterior glenoid margin, inferior to the superior glenohumeral ligament, and extends laterally to insert onto the lesser tubercle of the humerus. The middle glenohumeral ligament limits external rotation of the humeral head by tensing and pulling the bone. This way, it prevents the humeral head from slipping out of the glenoid cavity anteriorly.

The last of the glenohumeral ligaments is the inferior glenohumeral ligament. As I'm sure you've guessed, it's the ligament that is running across the inferior aspect of the joint. The inferior glenohumeral ligament originates from the anterior, middle, and inferior glenoid margin. We can see part of it from this anterior view as well. Laterally, the inferior glenohumeral ligament extends to attach onto the medial aspect of the anatomical neck of the humerus.

The anterosuperior and posterior margins of the inferior glenohumeral ligament are thickened forming anterior and posterior bands. Between the anterior and posterior bands, the articular capsule is thin and loose, forming a recess known as the auxiliary pouch. The anterior band of the inferior glenohumeral ligament will apply pulling forces on the humeral head during external rotation while the posterior band will get tensed during internal rotation thus preventing anterior movement of the humeral head from the glenoid cavity. Both bands will become tense when we abduct the arm over 90 degrees.

A little note here is that all the ligaments we've discussed so far blend with the capsule and are not clearly distinguishable.

The final joint-supporting ligament we'll look at is quite different from the ligaments we've seen so far in that it is completely separate from the joint capsule. This is the coracoacromial ligament. From this lateral view, we can see it running from the lateral border of the coracoid process anteriorly to the apex of the acromion process posteriorly. This anterior view of the joint allows us to see this ligament as well. In both views, we can see how this ligament provides a superior arch above the humerus. This arch forms a protective roof over the head of the humerus, thus, preventing superior displacement from its glenoid cavity.

You didn't think ligaments do all the work to support the shoulder joint, did you? We also have lots of help from a group of muscles surrounding the joint, so let's have a look at some of those.

Let's start with a group of muscles that you might already be familiar with – the rotator cuff. The rotator cuff is a group of four muscles which form a tendinous cuff around the glenohumeral joint. The muscles that make up the rotator cuff are the supraspinatus, the infraspinatus, the teres minor, and subscapularis. This muscle group, as we can see from this posterior view, creates a compressive force around the humeral head, ensuring its stability against the glenoid cavity in every position helping to center it. Let's take a closer look at each of the rotator cuff muscles.

First up is the muscle found on the superior aspect of the joint, the supraspinatus muscle. This muscle originates from the supraspinous fossa of the scapula and inserts onto the greater tubercle of the humerus. It crosses the superior aspect of the shoulder joint where its tendon blends with the joint capsule. It functions to stabilize the posterosuperior aspect of the glenohumeral joint and contributes to movements of the arm at the shoulder joint. We'll come across a structure in the next section called a bursa that helps reduce friction between this muscle tendon and surrounding structures.

Posteriorly, the joint capsule is strengthened by the infraspinatus muscle tendon which we can see highlighted now. As we can see from this posterior view, the infraspinatus muscle originates from the infraspinous fossa of the scapula and also inserts on the greater tubercle of the humerus. This tendon crosses the shoulder joint posteriorly, strengthening it from that aspect. It is also a powerful external rotator of the arm at the shoulder joint.

Moving inferiorly, we can see the teres minor muscle and tendon, also contributing to the strengthening of the shoulder joint capsule. This muscle originates from the lateral border of the scapula and also inserts on the greater tubercle of the humerus. This muscle strengthens and stabilizes the inferior portion of the glenohumeral joint while also eliciting external rotation of the arm at the shoulder joint on contraction.

The last of our rotator cuff muscles is also the only muscle on the anterior aspect of the glenohumeral joint. We're looking at the subscapularis muscle. From this anterior view of the scapula and humerus, we can see the subscapularis originating from the subscapular fossa and inserting onto the lesser tubercle of the humerus. This muscle strengthens and stabilizes the anterior portion of the glenohumeral joint. It also internally rotates and adducts the arm on contraction. Just like the supraspinatus, this muscle also has an associated bursa which we'll discover soon.

Now we're moving on to muscles which are not part of the rotator cuff but which still contributes to the stability of the shoulder joint.

Highlighted now is the teres major muscle. It crosses the joint much more inferiorly than the other muscles we've been looking at so it does not attach to or blend with the joint capsule as the others do. It originates from the inferior angle of the scapula, which we can see on this image of the posterior aspect of the joint, and inserts on the anterior aspect of the humerus, specifically, the medial lip of the intertubercular groove of the humerus. If we swap back to our lateral view of the shoulder joint, we can see the posterior to anterior trajectory of the muscle, even at this cut level. The teres major elicits movements of the arm at the shoulder and is thought to support the humeral head when the arm is in an elevated position.

The most inferior aspect of the joint is where we find the long head of the triceps brachii muscle. The long head of the triceps brachii originates from the infraglenoid tubercle, a bony feature of the scapula, and travels distally to blend with the other two heads of the triceps brachii, inserting onto the olecranon process of the ulna. The long head is the only head to cross the shoulder joint. When the arm is in an adducted position, the long head of the triceps brachii muscle acts to hold the head of the humerus within the glenoid cavity.

The final muscle that contributes closely to the stability of the glenohumeral joint is the long head of the biceps brachii, a tendon of which we can now see highlighted in green. The tendon of the long head of the biceps brachii originates from the supraglenoid tubercle of the scapula. We can see the tendon traveling within the joint capsule over the humeral head and into the intertubercular groove on the humerus. It then merges with the short head of the biceps brachii and inserts into the radial tuberosity as well as the deep forearm fascia via the bicipital aponeurosis. The long head of the biceps brachii mainly functions to flex the forearm but also contributes to the anterior stability of the glenohumeral joint.

Swapping from strengthening support to protective mechanisms within the joint, we'll now look at some bursae which act as cushions within the joint space.

Bursae are small sac-like structures normally found between joints and ligaments or tendons. The largest bursa associated with the glenohumeral joint that we can see from this angle is the subacromial bursa, now highlighted in green. These are the cut edges of the fluid-filled sac. You can see here that it lies deep to the coracoacromial ligament and the acromion process which is how it gets its name. Inferior to it, we find the joint capsule and the supraspinatus muscle tendon, therefore, the subacromial bursa creates padding between these structures.

If we look at the same structure from the anterior aspect, we can again see the subacromial bursa highlighted just inferior to the acromion process and the coracoacromial ligament. Laterally, the subacromial bursa extends to lie deep to the deltoid muscle so it has a portion called the subdeltoid bursa. The subdeltoid portion is the portion here. These bursae together help to reduce friction, protect the joint capsule, and prevent wear of the supraspinatus tendon, in particular during the abduction of the arm. It's important to note that this bursa does not normally communicate with the joint capsule so in case of an infection, it likely won't spread into the glenohumeral joint.

The other bursa we can see from this lateral view of the shoulder is on the anterior aspect. It is the subtendinous bursa of the subscapularis. Just as its name suggests, it sits underneath the tendon of the subscapularis muscle to reduce friction and prevent wear of the subscapularis tendon. Unlike the subacromial bursa we just looked at, the subtendinous bursa of the subscapularis normally does have an opening which connects it to the glenohumeral joint. This means that any infection arising in it will also likely pass to the joint.

And there you have it, all the structures of the glenohumeral joint. We mentioned a lot of structures today, many of which can be damaged in this highly mobile joint but first we'll take a quick look at a clinical correlation of one of these structures now.

The subacromial bursa, which we can see highlighted with the subdeltoid bursa in this image, can become inflamed causing subacromial bursitis. This inflammation can result from overuse or trauma. Since the subacromial bursa is between the tendon of the supraspinatus and the acromion process, it is often aggravated when the shoulder is abducted but subacromial bursitis can also cause pain when lying on the affected shoulder. Those with damage to structures under the acromion process such as the subacromial bursa and the tendon of the supraspinatus often exhibit painful arc syndrome which is a pain felt when the arm is abducted between 60 and 120 degrees. At less than 60 degrees, there usually isn't much pain and once the arm is abducted more than 120 degrees, there is a great sense of relief. This is because the injured structures are compressed and aggravated within the 60- to 120-degree range.

And there you have it. We've covered all the structures of the shoulder. But before I let you go, let's quickly review what we looked at today.

We began with features of the joint cavity itself, identifying the glenoid cavity of the scapula, which is deepened by the glenoid labrum of the scapula, a fibrocartilaginous ring. We then saw the glenohumeral capsule which surrounds the entire joint. Next, we looked at ligaments which helped to strengthen the capsule and stabilize the joint. We started with the most superior one, the coracohumeral ligament, running from the coracoid process to the humerus. Anteriorly, we identified the three glenohumeral ligaments – the superior glenohumeral ligament, the middle glenohumeral ligament, and the inferior glenohumeral ligament. The final ligament we identified was the coracoacromial ligament which forms a superior arch over the shoulder joint.

The next group of structures we identified from this lateral view of the shoulder joint were muscles. First we looked at the four muscles that make up the rotator cuff, namely, the supraspinatus, the infraspinatus, the teres minor, and the subscapularis muscles. The most inferior of the muscles we looked at was the teres major, traveling to the anterior aspect of the humerus to insert on the medial lip of the intertubercular groove. Originating just inferior to the glenoid cavity on the infraglenoid tubercle was the long head of the triceps brachii. And finally, superiorly we saw the long head of the biceps brachii with its tendon within the joint capsule originating from the supraglenoid tubercle.

The final group of structures we identified were a couple of fluid-filled cushions called bursae. First was the subacromial bursa which is found between the acromion process and the tendon of the supraspinatus. The other bursa, the subtendinous bursa of the subscapularis, is on the anterior aspect, found deep to the tendon of the subscapularis. Lastly, we looked at subacromial bursitis and painful arc syndrome that is experienced by those with an inflamed subacromial bursa.

And that brings us to the end of the tutorial. I hope you enjoyed it. Thanks for joining me and happy studying!