Now let’s look at this unique joint, where two quite different things happen. The humerus articulates with the forearm bones to form the elbow joint, and the forearm bones articulate with each other to form the proximal radio-ulnar joint.
Here’s the joint with its loose capsule removed and its ligaments intact. Here’s the front of the joint in extension, and here’s the back of the joint in flexion.
The key structure to understand is this remarkable ligament, which not only holds the radial side of the elbow together, but also holds the rotating head of the radius in place against the ulna. It has two parts. This part is the radial collateral ligament, this part is the anular ligament. We’ll take the humerus out of the picture for a minute, to get a look at the proximal radio-ulnar joint.
Here’s the trochlear notch of the ulna, here’s the head of the radius seen end on. The anular ligament, together with the radial notch of the ulna, provides a perfectly fitting socket for the head of the radius to rotate in.
Here’s the anular ligament with the radial head removed. It’s attached to the edges of the radial notch of the ulna. It’s shaped like a shallow cup, wider here than here, to fit the radial head not just round here, but also under here. So the radial head, while it’s free to rotate, is otherwise totally trapped.
Now let’s go back to the intact elbow joint, and see how it’s held together by its two collateral ligaments. The radial one arises from the lateral epicondyle. It fans out, and becomes continuous with the anular ligament.
The two parts of this complex ligament hold the humerus and the radial head securely together. What we see here isn’t the edge of the ligament, it’s the cut edge of the tendon of origin of a muscle, the supinator, which arises from the ligament. We’ll see this shortly.
Here’s the ulnar collateral ligament. It arises from the medial epicondyle, and fans out in a triangle. It’s attached to the ulna all along the medial side of the trochlear notch.
To complete our picture of the elbow joint, here it is with its its capsule intact. It’s thin and baggy in front, and also behind, to allow a full range of movement. There’s also a very flexible sleeve of joint capsule here, between the anular ligament and the neck of the radius.
The elbow joint is stable, that means it stays together, for two reasons: partly because of the strength of the ligaments, which we’ve seen, and partly because of the shape of the bones. The humerus and the ulna interlock closely and deeply. Their surfaces are curved in two planes, from front to back, and from side to side.