Content Reviewers:Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.), Viviana Popa, MD
The wrist is a complex joint that serves as the bridge connecting the forearm and the hand together.
It consists of a main radiocarpal joint, between the radius and the proximal row of the carpal bones minus the pisiform, and the smaller intercarpal joints, which are small joints among the carpals.
Then we have the hand, which consists of the carpo-metacarpal joints, the intermetacarpal joints, the metacarpophalangeal joints, and the interphalangeal joints.
Try saying those three times fast!!
Ok, let’s start with the wrist joint.
First, there’s the radiocarpal joint which is a condyloid type of synovial joint consisting of the distal radius and scaphoid, the lunate, and the triquetrum carpal bones.
Interestingly enough, the ulna doesn’t participate in the radiocarpal joint.
It is the articulating surface of the distal radius and the articular disc of the distal radio-ulnar joint which articulate with the proximal row of carpal bones to form the radiocarpal joint.
The articulating surfaces of the radiocarpal joint are surrounded by the tough joint capsule and synovial membrane, extending from the distal ends of the radius and ulna, to the scaphoid, the lunate, and the triquetrum.
The joint capsule of the radiocarpal joint is reinforced by a couple of ligaments, both on the dorsal and the palmar sides of the joint.
The palmar ligaments extend from the distal radius to the two rows of the carpal bones.
These ligaments strengthen the joint and make it possible for the hand and the radius to move as one unit during supination of the forearm, or turning the palm upwards.
To remember this, think about cupping both your hands and bringing them together to hold a bowl of ‘soup’, which mimics the movement of supination.
On the posterior side of the radiocarpal joint there are the dorsal radiocarpal ligaments, which also extend from the distal radius to the carpal bones.
These ligaments stabilize the wrist joint, and ensure that the hand follows the radius during pronation of the forearm, or the turning of the palm downwards.
Other ligaments that strengthen the radiocarpal joint are the ulnar collateral ligament on the medial side, which extends from the ulnar styloid process to the triquetrum; and the radial collateral ligament on the lateral side, extending from the radial styloid process to the scaphoid bone.
Now, a couple of muscles act on the radiocarpal joint.
They’re easy to remember, because they all attach to the carpals distally, and they have “carpi” in their names.
So there’s the flexor carpi radialis and the flexor carpi ulnaris, which flex the wrist; the extensor carpi radialis longus, the extensor carpi radialis brevis, and the extensor carpi ulnaris which extend the wrist; the abductor pollicis longus, the flexor carpi radialis, the extensor carpi radialis longus, and the extensor carpi radialis brevis which abduct or radially deviate the wrist; and then finally, the extensor carpi ulnaris and the flexor carpi ulnaris muscles which adduct or ulnar deviate the wrist.
Now, the radiocarpal joint receives its blood supply from the dorsal and palmar carpal arches, originating from the radial and ulnar arteries.
Regarding innervation, remember Hilton’s law, which says the nerves that innervate the muscles crossing over a particular joint also innervate that joint.
So according to Hilton’s law, the radiocarpal joint is innervated by branches of the median, ulnar, and radial nerve.
Now let’s look at the intercarpal joints, which are the small joints between the carpal bones.
They can be put into four groups to make things simple.
There are joints between the carpal bones of the proximal row; the joints between the carpal bones of the distal row; the midcarpal joint between the proximal and distal rows of carpal bones; and the pisotriquetral joint located between the pisiform and the palmar surface of the triquetrum.
All of these joints are the plane type of synovial joint, limited to the gliding movements between the carpals.
Though this sounds like a minor role, these joints are important because their combined little movements augment the range of motion of the radiocarpal joint.
Sometimes less is more!
Now, after the distal row of carpal bones, there are the long metacarpals, which form carpometacarpal joints with the carpals, and intermetacarpal joints among themselves.
The first carpometacarpal joint is responsible for the movements of the thumb, and consists of the trapezium and the first metacarpal bone, which is the only saddle type of synovial joint of the carpometacarpal joints.
The other 4 carpometacarpal joints are plane type synovial joints, formed by the articulations between metacarpals 2 through 5 and the distal row of the carpal bones..
The thumb’s carpometacarpal joint is a much more mobile joint, so it allows for flexion and extension; abduction and adduction; and circumduction.