The elbow and the proximal radio-ulnar joint are considered to be all one joint, because they’re enclosed in one continuous space. By contrast, the two joints that we’ll look at next, the distal radio-ulnar joint and the wrist joint are physically separate, even though they’re close together, so we’ll look at them separately.
To understand the distal radio-ulnar joint, let’s look at the distal ends of the radius and ulna. The head of the ulna has a rounded articular surface. This part articulates with the radius, this part articulates with a key structure that we’ll see shortly, the triangular fibrocartilage. The pointed tip of the ulna is called the ulnar styloid.
The broad distal end of the radius has two articular surfaces. This large one articulates with the proximal row of carpal bones. This small surface articulates with the ulna. This point is the radial styloid. Here’s the distal radio-ulnar joint with its capsule intact, and with its capsule removed.
Here’s the structure that holds it together, the triangular fibrocartilage. It’s also known as the articular disk. It’s attached to the radius here, and to the ulnar styloid here. As the distal end of the radius rides around the head of the ulna, the ulnar styloid provides the pivot point.