The wrist is a biomechanically complex joint allowing for movement in extension (up), flexion (down), radial deviation (towards the thumb), ulnar deviation (towards the small finger) and minimal degrees of rotation.
The wrist’s primary role is to position the hand in space. This accurate positioning allows the hand to perform a vast array of activities ranging from skillful manipulation of fine objects to heavy objects.
The wrist consists of three main layers:
- The capsule and ligaments
- Soft tissues
These three layers are covered by the skin. The first layer, or bone layer, begins at the metacarpal bases distally (finger side), and extends towards the very end of the forearm bones proximally (elbow side).
The bones comprising the wrist include the 5 metacarpal bases, the distal ends of the radius and ulna (forearm bones) and 8 carpal bones (wrist bones) which are organized into 2 rows.
From distal (finger side) to proximal (elbow side), these carpal bones form the following joints:
- Five carpo-metacarpal(CMC) joints
- Distal radio-ulnar joint (DRUJ)
The first CMC joint is at the base of the thumb. It articulates with the trapezium, a carpal wrist bone shaped like a horses™ saddle. The trapezium permits a wide range of motion, including thumb rotation of 120 degrees which distinguishes humans from other mammals. The ability to oppose the thumb is responsible for 40 % of the function generated by the hand.
The CMC joints of the 2nd and 3rd digits have minimal motion. The minimal motion is secondary to short, taut ligaments surrounding the joint at the 2nd (index) and 3rd (middle) metacarpals bases and the distal end of the carpal bones. This unit serves as the stable foundation around which the thumb, ring and small finger metacarpals move.
The CMC joints of the ring and small fingers are formed by 4th and 5th metacarpals, capitate and hamate wrist bones. These joints are more mobile. Together with the 1st metacarpal, they create a gutter or cup configuration, in the palm of the hand which facilitates the gripping of instruments and small objects.
The Carpal Bones
The Carpal (Wrist) Bones are comprised of 8 bones. These bones function as a unit by virtue of having limited motion between each other. The carpal bones are almost entirely covered by cartilage. They receive their limited blood supply through small vessel branches entering the bones through little tunnels where the ligaments attach.
Only 7 of the 8 carpal bones play a significant functional role in wrist stability and mobility.
The 7 Important Carpal Bones
Starting at the distal (finger) side
The proximal row starting on the thumb side
The Pisiform is the eighth bone but does not play a functional role. However, it can be a source of wrist pain when sustaining a fracture or as a consequence of degenerative arthritis in the pisotriquetral articulation.
The radius and ulna bones are just proximal to the carpal bones. Their articular surfaces form a cup that complements the carpal bones. Together they form the radiocarpal joint. The radiocarpal joint serves to support the proximal carpal row in a functional ball and socket mechanism.
The radius bone has a much larger articular surface to cradle the carpus and transfers about 80 % of the wrist load. The radius and ulna have a second joint providing forearm rotation, called the Distal Radio Ulnar Joint (DRUJ).
The DRUJ provides a cushion between the carpus and the distal ulna. For full normal forearm rotation to occur, the DRUJ is complemented at the elbow by a joint called the proximal radio-ulnar joint (PRUJ).
The second layer of the wrist is composed of ligaments, as well as a complex intermingling of collagen fibers joining the metacarpals to the carpals and to the radius and ulna, creating stable and balanced wrist motion during our daily activities. The wrist ligaments are taut structures embedded in a thin capsule. They function to reinforce key areas of the wrist.
Between the carpal bones, a deeper layer of ligaments, called the intra-carpal ligaments, allow a restricted amount of motion and is the reason the carpal bones function as a unit.
The shape and position of the carpal bones is critical for proper function. When injury occurs, it is of vital importance to re-establish the bone anatomic shapes if normal function is to be achieved. The third layer of the wrist is formed by the tendons, muscles, vessels and nerves.
Tendons of the Wrist
- Flexors – Palmar side
- Extensors – Back hand side
There are 9 flexor tendons that travel through a fibro-osseous tunnel at the wrist called the carpal canal. It is formed by the carpal bones and the transverse carpal ligament. There are 3 tendons that flex the wrist but only 1 of these 3, the flexor carpi radialis, travels in an independent wrist compartment in the carpal canal.
Radial Artery Ulnar Artery
The radial and ulnar arteries travel across the wrist and are the primary vessels supplying blood to the hand. An arcade of smaller branches that interconnect the radial and ulnar arteries are safety pathways to allow the continuation of blood flow if the radial and ulnar artery becomes occluded (obstructed). The blood returns to the heart through the more prominent veins in the back of the hand and wrist.
Major Nerves of the Wrist
- Median: travels together with the 9 flexor tendons in the carpal tunnel
- Ulnar: travels with the ulnar artery through Guyon’s canal and the radial nerve in the back of the forearm
Terminal smaller branches provide sensation to the superficial and deeper levels within the wrist.