Now let’s look at the wrist joint. Though we often speak of it as one joint, there are really two joints here, very close together. They’re called the radiocarpal joint, and the mid-carpal joint. To understand them let’s look at the bones. We’ll look at them this way up.
Eight small carpal bones form the carpus. Distal to the carpus are the metacarpal bones, numbered one, two, three, four and five.
The carpal bones are in two rows, a proximal and a distal. The bones in each row are attached closely to one another. The four bones of the proximal row are the scaphoid, the lunate, the triquetral, and the pisiform, which sits by itself on the triquetral. The scaphoid, the lunate and part of the triquetral articulate with the distal end of the radius, to form the radio-carpal joint.
The distal surface of the proximal row forms a deeply concave notch, which the bones of the distal row fit into. The bones of the distal row are the trapezium, the trapezoid, the capitate, and the hamate. The capitate and part of the hamate project proximally.
The bases of the five metacarpals articulate with the distal row of carpal bones. The first, the one for the thumb, articulates by itself with the trapezium. The other four articulate in a row, here. The distal row of carpal bones articulates with the proximal row here, to form the midcarpal joint. The projecting capitate and hamate fit into the notch in the proximal row.
When flexion and extension occur at the wrist, the movement happens partly at the radiocarpal joint ,and partly at the midcarpal joint. When radial deviation and ulnar deviation occur, the action happens mainly at the radio-carpal joint.
Here’s the wrist joint, or rather joints, with much of the capsule removed, and the two collateral ligaments, here, and here, intact. Here’s the radiocarpal joint, here’s the midcarpal joint.
The radial collateral ligament goes from the radial styloid to the scaphoid and its neighbor, the trapezium. The ulnar collateral ligament goes from the ulnar styloid, to the triquetral and pisiform bones.
Here’s the wrist joint with the joint capsule intact. The joint capsule is thick and strong all the way round the joint. On the extensor aspect, the capsule forms the broad dorsal radiocarpal ligament. On the flexor aspect it forms the palmar radiocarpal ligament.
Unlike the elbow, which is held together partly by the interlocking shape of the bones, the wrist is held together entirely by the strength of its ligaments. The two collateral ligaments hold the bones together in radial abduction and ulnar abduction, and the radio-carpal ligaments hold them together in flexion and extension. The strength of the radio-carpal ligaments also ensures that, when the radius rotates, the hand goes with it.