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Bones, ligaments and nerve supply of the elbow joint.
Did you ever think about how many times a day you bend your elbows? You probably haven't, but why would you? Unless you hit your funny bone. Aw! If you do stop and think, you'll realize you probably bend your elbows hundreds of times a day. From the moment you wake up and throw your alarm against the wall to brushing your teeth, you're bending our elbows. Working on a computer all day? Those elbows look bent to me, sir! With so much usage, the elbow is definitely a hard-working joint, which brings us to today's tutorial’s topic – the elbow joint.
So, let's have a closer look exactly how we're going to tackle this joint. We'll start by looking at the three bones that make up the elbow joint – the humerus, the radius, and the ulna; in particular, their bony landmarks relevant to the elbow joint. We'll learn all about the type of joint that is formed at the elbow, about how it works, and we'll even dive inside the joint by looking at it in sagittal section. We'll finish up the gross anatomy by looking at how the joint is supported with the help of various ligaments, and before we wrap up for the day, we'll take a peek at some clinical notes regarding the elbow joint.
So let's begin with the bones, which come together to form the elbow joint. Starting with the largest one – the humerus. Today, we're going to only focus on its distal end, and you can see its anterior view on the left and its posterior view on the right. Let's look at some bony landmarks.
So you'll first notice that the humerus bone flares out distally along these two expansions, which are the medial and the lateral epicondyle over here. The sharpish sides of the bone extending upwards from the epicondyles are called the medial and the lateral supracondylar bridges. On the anterior surface of the distal end of the humerus, you'll notice two depressions. The medial one is called the coronoid fossa and the lateral one right next to it is the radial fossa. Just distal to the fossae, you'll see the distal articular surface, which is divided into two parts.
This rounded little guy here, which you can see on both the anterior and posterior surfaces of the humerus, is the capitulum and it articulates with the proximal articular surface of the radius. This pulley-shaped part here is called the trochlea and it articulates with the proximal ulna. On the posterior surface of the bony part of the humerus, we can see another fossa – the olecranon fossa. Altogether, the epicondyles; the radial, coronoid and olecranon fossae; the capitulum, and the trochlea form the condyle of the humerus, which I’ve outlined for you here.
Let's move on to the bones of the forearm. First up, we have the ulna. Only its proximal end articulates at the elbow joint, so we'll only focus on this area right here. The ulna is the larger, medial, and longer of the two forearm bones. Due to its uniquely-shaped articular surface, it is considered to be the stabilizing bone of the forearm. Let's have a look at some of its bony landmarks.
The bony features important to the elbow joint are found on the enlarged proximal end of the ulna. This region contains the ulnar articular surface. Here on the anterior surface, you'll find the trochlea notch, which – oh, what a surprise – articulates with the trochlea we've already seen on the distal part of the humerus.
Staying anteriorly, right at the edge of the articular process, we have this little projection called the coronoid process. Looking at the posterior surface of the ulna, we can see this much larger projection called the olecranon, which sits in the olecranon fossa of the humerus when the elbow is extended.
Don’t worry if you're struggling to picture how the joint works at the moment, we'll be taking a much closer look at all the articulations in a tick.
Just lateral to the coronoid process, we can see the radial notch, which articulates with the head of the radius. Speaking of, why don't we move on to our next bone of interest. The radius is the smaller, shorter, lateral bone of the forearm. Again, we're only interested in the proximal end in this tutorial, so let's just zoom in a little.
First of all, even though we've zoomed in, you can still see a small portion of the shaft here. A little higher up, we find a slightly constricted part called the neck of the radius. On the medial aspect of the anterior surface between the neck and the shaft of the radius, we find the radial tuberosity. But perhaps the most important feature we need to know about in relation to the elbow joint is its little head on the proximal end. It's a concave, circular structure which articulates with the capitulum of the humerus superiorly and with the radial notch on the ulna medially.
Right, so we've covered the basics now. Let's move on to the main event – the elbow joint itself.
Firstly, the elbow joint is a synovial joint or diarthrosis. So even though you might know what this means, let's do a little recap. A synovial joint is a freely-moving specialized joint. Its bony elements are usually surrounded by an articular capsule which is in turn lined by a synovial membrane, and we'll see what that term means in just a moment.
For the moment, let's see how the elbow joint fits into the classification of synovial joint by having a look at it in sagittal section. First, let's see what bony elements we have here. So we have the humerus, and one of its articular parts, the trochlea. Articulating with it, we have the ulna which you can recognize by its hook-like proximal part and we have the coronoid process here and the olecranon of the ulna here. And you can easily work out where the coronoid and the olecranon fossae would be on the humerus.
Let's now zoom in on the joint and look at its synovial structures that allow for its smooth movement.
The first synovial structure we can see is articular cartilage, also known as hyaline cartilage, which lines the articulating or contact surfaces of the elbow joint. In this particular joint, we have two primary articular cartilages. One is the articular cartilage of the trochlea which we know is found at the distal end of the humerus, and the other is the articular cartilage of the trochlear notch. Between them, you'll find a small synovial cavity which is filled with synovial fluid. The fluid helps to lubricate the joint and allows it to move freely.
You might be wondering how this fluid doesn't just run out of the joint. Well, I'm glad you should ask. Synovial fluid is contained by a synovial membrane which seals off the synovial cavity in the joint. The membrane, in turn, is protected by a fibrous layer of tissue covering the whole joints capsule and this capsule is called the articular capsule of the elbow joint. It's quite an interesting structure, and yep, we'll be coming back to it a little bit later on in the tutorial.
So now we've seen what structures make up the elbow joint, but what things surround it? Let's have a look.
We've already seen the articular capsule, which is one of the protective structures of the elbow joint. Another pair of protective structures are these little fat pads between the joint and surrounding muscles. There's an anterior and a posterior pad and these serve as a reserve of cells for tendon and ligament repair and cells involved in inflammatory responses.
Another structure doing a similar job are synovial bursae. These are little sacs lined by synovial membrane and filled with synovial fluid. They are found close to synovial joints to create a cushioning between the joint and surrounding tendons or muscles. This one here is the olecranon bursa and this is the subtendinous bursa of the triceps brachii. It helps to reduce friction between the tendon of the triceps brachii and the joint.
In addition to it being a synovial joint, the elbow joint has a very specific type of movement that is only found in a couple of other joints in the body. And can you think of what this movement might be? Did I hear you say a hinge movement? Because if you did, you'd be right!
The elbow joint is indeed a hinge-type synovial joint. That means it only allows movement in one plane – in this case, flexion and extension of the elbow. Structurally, the elbow joint is also classified as a composite joint, which refers to the fact that it's made up of more than one joint.
So, remember how you talked about what a talented and hardworking joint the elbow was at the beginning of the tutorial? Well, that's because the elbow joint is not just any joint. There are also three smaller joints contained within it that help it be the multitasker that it is, and these are the humeroulnar joint, the humeroradial joint, and the proximal radioulnar joint. And let's quickly go through some of these joints now.
So we're looking at an anterior image of the elbow joint and between the trochlea of the humerus and the trochlear notch of the ulna, we have the humeroulnar joint right about here. And like the elbow joint itself, this is a hinge joint.
Moving on laterally, we find the humeroradial joint and the shape of the capitulum and the concave radial head form a ball-and-socket joint. If you need help visualizing this, try picturing a mortar and pestle hard at work grinding up some basil. Can you see how it moves in all sorts of ways? Well, in order to stabilize it, the body has to come up with an elegant solution.
In this image, you can see the annular ligament, highlighted in green, and it's like a little sling around the edge of the radial head. It attaches anteriorly and posteriorly to the edges of the radial notch on the ulna anchoring the radius to the ulna and ensuring that they move in unison. So although the humeroradial joint has the articular surface of a ball-and-socket joint due to the presence of the annular ligament, it moves more like a pivot joint.
Alright, so we're onto our last subjoint. The proximal radioulnar joint is not part of the hinge joint, which controls the flexion and extension at the elbow. Instead, it is one of the joints involved in the pronation and supination of the forearm. However, it shares its synovial capsule with the humeroulnar and humeroradial joints, so it's considered part of the elbow joint. The proximal radioulnar joint is a pivot joint.
Okay, so we've learned about how the elbow operates, but what really enables this movement are the muscles. There is a whopping total of 17 muscles crossing the elbow; however, there are only a few chief muscles that extend and flex the forearm. So let's talk about flexion first.
The two chief flexors that we're talking about today are the brachialis and biceps brachii. We'll start with the biceps brachii muscle, which is a large flexor of the elbow and it achieves it by passing anteriorly to the elbow joint. Deep to it, we find the brachialis muscle, which also passes in front of the elbow joint, and both of these muscles are innervated by the musculocutaneous nerve.
Unlike the biceps brachii and the brachialis which are mostly contained in the arm, the brachioradialis is mostly a muscle of the forearm. In general, it assists the chief flexors in slower flexion in the presence of resistance.
So, let's move on to the extension of the forearm. The extension is weakly assisted by the anconeus muscle, which is this little guy here. But the chief extensor is the triceps brachii, which is this muscle over here.
Okay, so, now that we've talked about the muscles of the elbow joint, let's have a chat about the arteries.
The blood supply of the elbow joint mainly derives from an anastomosis of the collateral and recurrent branches of four main arteries – the brachial artery which is the largest artery in the arm, the deep brachial artery, a branch of the radial artery, and branches of the ulnar artery.
And, of course, if we talk about the vascular supply, we must also talk about the innervation of the elbow joint.
The elbow joint is innervated by auricular branches of all the major nerves of the upper limb nerves except the axillary nerve – the musculocutaneous nerve, the median nerve, the radial nerve, and the ulnar nerves.
Okay, so, so far we've covered the joints within the elbow joint, its movement, muscles that make it happen, and even the innervation and blood supply to the joint. So, you might ask, what can possibly be left for me to learn? Well, if you're thinking ligaments, you're absolutely right.
Here you can see the elbow joint in a flexed position from the medial and the lateral aspects, and we have the humerus, the radius, and this bone here is the ulna. And over here, we have the annular ligament. It's the sling-like ligament that straps the radius to the ulna protecting the radius from dislocation during movement at the elbow.
But let's move on now to talk about some ligaments that we haven't discussed yet, and why don't we start by looking at the medial aspect of the elbow where you'll find the ulnar collateral ligament.
The ulnar collateral ligament is a triangular ligament that's made up of three parts. The first part is this anterior band here. It's cord-like and is the strongest of the three bands. It attaches to the anterior aspect of the medial epicondyle of the humerus by its slightly narrower apex and to the proximal tubercle on the medial margin of the coronoid of the ulna by its wider base.
We then have the posterior band, which is this fan-like structure here, and it is attached by its apex to the posterior aspect of the medial epicondyle of the humerus and by its base to the medial margin of the olecranon of the ulna.
The last inferior band, sometimes also known as the oblique band, essentially connects the bases of the other two bands creating a socket for the trochlea in which to sit. It extends between the coronoid process and the olecranon of the ulna.
So we're now moving on to the lateral view of the right elbow joint to have a look at the radial collateral ligament. And it essentially lies opposite to the ulnar collateral ligament and attaches to the lateral epicondyle of the humerus and inferiorly blends with the annular ligament. Some fibers actually cross over the annular ligament and insert into the proximal aspect of the supinator crest of the ulna.
Okay, so, let's now have a look at the fibrous articular capsule of the elbow joint, which we can see from an anterior view of the joint. This capsule blends well with the ulnar collateral ligament medially and the radial collateral ligament laterally, and we've already seen its inner layer called the synovial capsule when we looked at the sagittal section of the elbow joint. The fibrous capsule is kind of like its tougher outer layer.
Proximally on the humerus, it attaches along the margin of the coronoid and radial fossae and on to the anterior surface of the medial epicondyle. Distally, it reaches as low down as the edge of the coronoid process and the annular ligament.
Let's have a look at the posterior aspect of the elbow joint now, and here the joint capsule would attach around the margin of the olecranon fossa and the posterior surface of the medial epicondyle of the humerus proximally. Distally, it attaches along the superior and lateral aspects of the olecranon of the ulna medially, and laterally, it continues over the proximal radioulnar joint, and here I've highlighted the area covered by the fibrous capsule posteriorly.
And I just want to show you the capsule side on as well. So, here we have its medial and lateral views, and you can see how it blends with the ulnar collateral ligament medially and the radial collateral ligament laterally.
Okay, guys, so we've had a very comprehensive roundup of all the things related to the elbow joint, so let's see what happens when those parts don't work as they should in our clinical notes.
A very common injury occurring at the elbow joint is a small radial head fracture, which usually occurs due to a fall on an outstretched arm, and you can see it right about here on this x-ray. In this image, you can see a little sail sign here and these darkened areas I've indicated for you are actually showing fat pads normally found adjacent to the elbow joint, which become pushed up due to the fracture. In the long run, this sort of injury can result in a reduced range of motion and development of osteoarthritis, which is a degenerative joint disease.
Other injuries incurred at the elbow joint are anterior dislocation of the radius, which often occurs due to rupture of the annular ligament, and a much rarer terrible triad of the elbow, which is a common phrase used to describe a combined fracture of the coronoid process of the ulna and the head of the radius coupled with a posterior dislocation of the proximal radioulnar joint.
Okay, so we’ve finished our tutorial. Why don't we summarize what we've learned today?
So we started by looking at the three bones which come together to form the elbow joint and we looked at the distal end of the humerus paying particular attention to its oddly-shaped articular surfaces; the trochlea, which articulates with the trochlear notch of the ulna; and the capitulum, which articulates with the head of the radius. We then moved on to inspect the ulna and the radius and any important landmarks on their proximal ends.
We then looked at the sagittal section of the elbow joint to discover the structures found in the synovial joint at the elbow including the articular surfaces, the synovial cavity, and synovial fluid, which is contained by the articular capsule which encloses the joint.
We moved on to learn that the elbow joint is a composite hinge joint which, in fact, comprises of three separate joints – the humeroulnar joint, the humeroradial joint, and the radioulnar joint. Once we knew about the movements at the elbow joint, we learned about the muscles which make these movements happen. We learned that the flexion is mainly carried out by the biceps brachii and brachialis and assisted by the brachioradialis muscle. The chief extensor is the triceps brachii muscle with weak assistance by the anconeus muscle.
We then had a little look at the innervation and the vascular supply to the elbow joint. Finally, we had another look at the annular ligament anchoring the radius to the ulna, the ulnar collateral ligament on the medial aspect, the radial collateral ligament on the lateral aspect, and a fibrous articular capsule enclosing the joint.
We finished up with some clinical notes related to the elbow joint and we looked at the fracture and dislocation of the radius and a chain of events called the terrible triad.
Okay, that's it folks. So we've made it to the end of this tutorial. Thanks for watching, see you next time, and happy studying!