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Shoulder joint

Bones, ligaments, muscles and movements of the shoulder joint.

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You’re feeling all set for your anatomy exam. You've absolutely nailed the shoulder joint. You know everything. What it looks like from the front, from the back, from the - uh oh. Didn't look at it from the side? It is a bit of an odd angle to study anatomy from, but it gives us such a great perspective of the joint and really helps us with the interaction and relationships of muscles from the anterior and posterior sides.

In this tutorial, we're going to explore the shoulder joint from a lateral view, so when you get those questions on your next exam, you'll nail those, too.

Before we get started, here's a quick overview of what we'll cover in this tutorial. 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 relieve pressure and friction within the joint before moving on to the clinical notes.

Ready for it? Here we go!

First up, a quick review of the shoulder joint itself. The shoulder joint, also called the glenohumeral joint, is in the proximal part of the upper limb. It is a ball-and-socket-type synovial joint, which means it has a wide range of movements including flexion and extension, abduction and adduction, medial and lateral rotation, and circumduction. It is the most mobile joint in the body. So, which two bones actually make up this joint?

Well, the first bone we'll look at is the humerus. The feature of the humerus that articulates as part of the glenohumeral joint is this smooth articular surface here – the head of the humerus. Other features of the humerus are related to structures that help to stabilize the joint – these two raised areas on the anterior aspect of the proximal humerus, the lesser and greater tubercles. Between them, you'll find the intertubercular groove or sulcus. While we're here, I'd also like to point out the anatomical neck of the humerus. All of these features are attachment sites for structures and we'll learn more about these a little bit later.

Next up, we have the scapula, colloquially known as the shoulder blade. This triangular flat bone has a few key structures that are particularly relevant to the shoulder joint. We'll look at them from the anterior aspect, which you’re probably used to, and then from the more unusual lateral view, which is our main focus of the tutorial.

The surface of the scapula that articulates in the shoulder joint is the glenoid cavity and is on the lateral aspect of the scapula. We'll see this again later on. At the superior end of the glenoid cavity is a bony process called the supraglenoid tubercle. At the inferior end is the infraglenoid tubercle. Both of these tubercles are attachment sites for muscles that support the shoulder joint.

This bony projection that is superior to the shoulder joint is the acromion process. Try palpating it yourself. Find the long hard ridge on the back of your shoulder and follow it laterally and superiorly. When you come to the end, just above your shoulder joint, you've found your acromion. And, finally, this bony feature that projects anteriorly is the coracoid process. We'll see the importance of these processes in joint stabilization later on.

The image that we're going to be looking at a lot today is with the humerus removed from the shoulder joint. Because of this, we can see into the joint itself and get a better idea of how the ligaments and the muscles interact with the joint. So in this image, we see the scapula and the surrounding muscles isolated from the rest of the body and are looking at it 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 those same five features of the scapula that we just identified. Highlighted in green now is the glenoid cavity – the articular surface of the scapula for the glenohumeral joint. At the superior aspect is the supraglenoid tubercle and inferiorly is the infraglenoid tubercle.

The large bony process highlighted now is the one we palpated superior and posterior to the shoulder joint. This is the acromion. It's important to note that the acromion doesn't usually contribute to the glenohumeral joint, but it does articulate with the clavicle to help form the shoulder girdle.

And the final feature we’ll look at is highlighted now – the coracoid process. This more anterior projection of the scapula does not articulate with any other bones, but is an important attachment site for ligaments that help to stabilize the shoulder joint.

Alright, so that's the basics covered. Let's get into the joint cavity itself and see what we can find there.

Alright, so I know this is the third time we're covering this already, but I promise you, it's the very last time.

Okay, so again, we can see the glenoid cavity highlighted in green, and there are just a few extra things I want to point out. To understand the shoulder joint, it's best to look at it from an anterior aspect. Remember, we said that the shoulder joint is a ball-and-socket joint? Well, the glenoid cavity, also called 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 cavity. So as you can imagine, that results in a relatively small contact area between the two at any given time. This is beneficial, though, for us in terms of mobility of the joint, but it means stability is compromised.

There are, however, some extra 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. The first is this structure highlighted now called the glenoid labrum. The glenoid labrum is a fibrocartilaginous ring attaching around the outer margin of the glenoid fossa. This helps to deepen the glenoid fossa giving the socket part of the joint a bit more of a functional socket shape.

Surrounding the glenohumeral joint is the joint capsule, which we can now see highlighted in green. Like all synovial joints, the joint capsule made from an outer fibrous layer and an inner synovial membrane. The inner synovial membrane secrete synovial fluid which fills the joint capsule lubricating it. It's easy to forget looking at the lateral aspect that the structures in question extends beyond the plane of the glenoid cavity. The fibrous part of the joint capsule actually engulfs the whole humeral head and extends far 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 osseofibrous acetabulum.

The joint capsule doesn't do all the work alone. 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 highlighted in green now. This is the coracohumeral ligament and you'll notice that its name is quite indicative of its attachments. We can just about 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 which is exactly what we can see if we switch to the anterior view of the joint.

From this anterior view, we can't see the attachment of the ligament on the coracoid process very well. That's because it's coming from the posterior aspect of it. If we flip back to the lateral view image, we can see that attachment point on the posterior side. We can, though, see the humeral attachment.

The ligament flares out into an anterior and posterior band attaching to the lesser tubercle and the greater tubercle, respectively. The coracohumeral ligament is important in preventing inferior displacement of the humerus.

Next up, we're looking at a set of three glenohumeral ligaments starting with the superior glenohumeral ligament. In the image on the right, we can also see it from an anterior view. It spans 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 help prevent inferior displacement of the humerus.

Moving inferiorly, we come across the middle glenohumeral ligament. This ligament is across the anterior aspect of the joint originating inferior to the superior glenohumeral ligament and inserting on the lesser tubercle of the humerus. The middle glenohumeral ligament adds to the anterior stability of the joint.

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. It originates mostly along the inferior edge of the glenoid margin. We can see part of it from this anterior view as well. Laterally, the inferior glenohumeral ligament mostly attaches onto the neck of the humerus. A little note here is that all the ligaments we've discussed so far blend with the capsule and are not clearly distinguishable.

This 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 coracoid process anteriorly to the larger 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.

You didn't think the ligaments do all the work to support the shoulder joints, 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, perhaps because of all the injuries we’re quite used to hearing about. And, of course, we're talking about the rotator cuff. The rotator cuff is a group of four muscles, which creates a compressive force around the humeral head ensuring its stability against the glenoid cavity in every position and helping to center it. The muscles that make up the rotator cuff are the supraspinatus, the infraspinatus, the teres minor, and the subscapularis. Let's take a closer look at them one by one.

First up is the muscle found on the superior aspect of the joint. This is the tendon of the supraspinatus – the first of our rotator cuff muscles. The supraspinatus muscle originates from the supraspinous fossa of the scapula and inserts on the greater tubercle of the humerus. It crosses the superior aspect of the shoulder joint where its tendon blends with the joint capsule. We’ll come across a structure in the next section called the bursa that helps to reduce friction between this muscle tendon and the surrounding structures.

Posteriorly, the joint capsule is strengthened by the infraspinatus muscle tendon, which we can see highlighted now. The infraspinatus 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.

Moving inferiorly now, 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.

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

Now, we're moving to muscles which are not part of the rotator cuff, but still contribute 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 many of 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 a lateral view of the shoulder joint, we can see the posterior to anterior trajectory of the muscle even at this cut level.

The most inferior aspect of the joint is where we finds the long head of the triceps brachii muscle. The long head of the triceps brachii originates from the infraglenoid tubercle, which is the bony feature of the scapula highlighted on the image on the left. It travels distally to blend with the other two heads of the triceps brachii and insert on the olecranon process of the ulna. The long head is the only head to cross the shoulder joint.

The final muscle that contributes closely to the stability of the glenohumeral joint is the long head of the biceps brachii, the 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. In the image on the left, we can see the tendon originating within the joint capsule. It then continues to travel 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.

Okay, so swapping from strengthening support, 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 also 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 abduction.

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 to 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 beneath 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 likely pass into the joint, too.

And there you have it, those structures of the glenohumeral joint as seen from a lateral view.

We mentioned a lot of structures today, many of which can be damaged in this highly mobile joint. We'll take a quick look at the clinical correlate 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 acromial 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 pain felt when the arm is abducted between sixty and a hundred and twenty degrees. At less than sixty degrees, there usually isn't much pain, and once the arm is abducted more than a hundred and twenty degrees, there's a great sense of relief. This is because the injured structures are compressed and aggravated within the sixty to a hundred and twenty degree range.

And there you have it. You're now ready to ace your spotter exam looking at the shoulder from a lateral view.

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 the articular surface of the scapula involved in the glenohumeral joint. The glenoid cavity 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 help 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. Superiorly, we found the tendon of the supraspinatus originating from the supraspinous fossa of the scapula. Inferior to that, on the posterior aspect of the joint, we saw the infraspinatus, which inserts onto the greater tubercle of the humerus.

Moving inferior again is where we found the teres minor, also inserting onto the greater tubercle of the humerus. Finally, on the anterior aspect, we found the subscapularis muscle, inserting onto the lesser tubercle of the humerus. The most inferior of the muscles we looked at was the teres major, travelling to the anterior aspect of the humerus to insert onto 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 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 while 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 the subacromial bursitis and painful arc syndrome that is experienced by those with an inflamed subacromial bursa.

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

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