Radius and Ulna

This is a quick tutorial on the radius and ulnar bones of the forearm.   The radius bone is this bone here and it lies laterally in the anatomical position. The ulnar is this bone which lies medially in the forearm. These two bones articulate distally and proximally with each other and proximally, they also articulate with the humerus.

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At the humerus, they articulate with the condyle.   The ulna articulates with the trochlea and the radius articulates with the capitulum. It’s not that clear on this model here, but I’ll switch over to another diagram and show you the articular surface of the distal humerus.

We’re looking at an anterior view of the left humerus. You can see this articular surface here. This is the condyle of the humerus. You’ve got a trochlea, which articulates with the ulnar bone and a capitulum, which articulates with the radius.

The elbow joint is a hinge synovial joint. You only get flexion and extension at this joint.

The radius bone is this bone here, this lateral bone on the forearm. I’ll just highlight it. On the radius, you’ve got this bit here, which is called the head. Just inferior to that, you’ve got the neck. I’ll just zoom in a bit. This round bit at the top you’ve got is called the head. Inferior to that, you’ve got the neck. And then you’ve got this tuberosity here. This is called the radial tuberosity where the biceps inserts. And then you’ve got the shaft of the radius. And right at the end, you’ve got this little tubercle, the styloid tubercle, the styloid process of the radius.

I mentioned before, the other joints that you have in the forearm, you’ve got radioulnar joints. You’ve got a proximal radioulnar joint and a distal radioulnar joint. This is where you’ve got pronation and supination of the forearm.

At this proximal radioulnar joint, the radial head articulates with the ulnar at a little notch in the unlar called the radial notch. And here, the radius spins around so you get pronation and supination.

And then you’ve got the same sort of thing distally at the distal radioulnar joint. Here, you’ve got the ulnar head. The ulnar head articulates with the ulnar notch on the radius, so you get pronation and supination here.

Just again, you’ve got the radial head, which articulates with the ulna at the radial notch on the ulna. And distally, you’ve got the ulnar head articulating with the ulnar notch with the radius. These joints give you pronation and supination in the forearm.

Just a quick point on this model, the radial tuberosity, which is shown here as this little bump is shown slightly more laterally than it is in life. It is a bit more medial and anterior on the radius than is shown in this diagram, this model.

Another thing to note is that at this proximal radioulnar joint, you’ve got a ligament called the annular ligament. I’m just seeing you a diagram. You can see this annular ligament. We’re looking at a posterior view here of the left arm.   The olecranon process on the ulna, the ulnar bone – medial, the radius, lateral.   You’ve got this annular ligament at the proximal radioulnar joint, which surround the head of the radius.

What this does is it stabilizes the joint and it allows the radius to rotate against the radial notch on the ulna and also at the capitulum on the humerus.

This annular ligament actually blends with the fibrous membrane of the joint capsule. It’s called ‘annular’ because ‘annulus’ in Latin means ring.   It forms like a ring around the head of the radius.

Sometimes the radius can actually get forced out of this annular ligament, so you get radial head subluxation. This is called nursemaid’s elbow because if you forcefully pull on the arm of a young child as a nursemaid might do, you can sublux the radius from this ligament. It will slip out from the annular ligament. This is more common in young children under the age of five.

Moving on to the ulnar bone, this bone articulates at the trochlea of the humerus and also, it’s got its proximal and distal articulations with the radius.   Just rotating around posteriorly, you can see the olecranon. This is that posterior protuberance in the ulna, the olecranon process. And like I mentioned before, you’ve got this articulation with the radial head.   There’s a notch here on the ulna called the radial notch, which articulates with the head of the radius. Likewise, at the distal articulation, you’ve got the ulnar notch on the radius.   You’ve got the olecranon, the radial notch and you’ve got the trochlear process, which is this, this trochlear notch, which is this hollowing which articulates with the trochlea of the humerus. We’ve also got the coronoid process, which I’ll show you in another diagram.

Again here looking at the left side of the body, so an anterior view of the left radius and ulna. You’ve got the trochlear notch and you’ve got this coronoid process here, olecranon up here, the radial notch laterally, which articulates with the head of the radius. Then you’ve got the shaft and the ulnar head.

The head of the ulna is at the opposite side of the head of the radius.   We’ve got the head of the radius proximally and the head of the ulna is distal.   The head of the radius articulates with the radial notch on the ulna, head of the ulna articulates with the ulnar notch with the radius. And you’ve also got this little styloid process medially.   Just like you’ve got the styloid process on the radius, you’ve got the styloid process of the ulna.

Just looking here at an image I stole from Wikipedia, you’ve got the coronoid process. You’ve got the radial notch on the ulna articulating with the radial head. You can see this more clearly than the 3D model. There’s this clear facet for articulation with the radial head. And then you’ve got the trochlear notch, which articulates with the trochlea of the humerus and the olecranon process, which forms that it which sticks out of the elbow.

Looking at the radius, just a real-life image, you’ve got the radial tuberosity. You’ve got the radial neck and this head here.

Just too quickly mention about the pronators and supinators, you’ve got two supinators. You’ve got the biceps, which attaches to the radial tuberosity and you’ve got the supinator muscle at the back, which originates on the ulna and it wraps around the proximal radius.   You’ve got two supinators, the biceps brachii and the supinator. And we’ve got pronators, you’ve got the pronator teres proximally and the pronator quadratus distally. These obviously pronate the forearm.

That’s the radius and ulna.