The Wrist Joint
The ability to position the fingers and thumb in precise postures is vital for the highly coordinated use of the hand and the wrist has a significant role to play in this function. The shoulder blade and the shoulder perform the gross positioning of the arm, the elbow places the hand at varying distances from the body, the forearm dictates the angle of the wrist and the wrist performs the final positioning of the hand. The closer to the hand the body parts come the more precise and fine the movement becomes.
The wrist joint is placed between the hand and the forearm bones and is made up of a group of eight small bones which occur in two rows with articulations on one side with the radius and ulna and on the other side with the metacarpals. The metacarpals, the long bones in the palm of the hand, run from the further row of carpal bones down to the knuckles where they join the fingers. The metacarpals are arranged in an almost parallel pattern and are long and slim bones, giving them the ability to rotate around each other to some degree and improve grip.The Human Wrist Joint
The neat, close group of carpal bones allows the wrist to perform a conical range of movement facing forwards, with a full 360 degree rotation possible. The bones can move as a group or to some degree individually to permit fine control of the thumb, fingers and hand. The rows are somewhat irregular but on average there are two bones in line with each metacarpal between it and the forearm. This pattern creates a series of joints in line with each other and permits a great variety of individual movements to translate into precise and varied positioning.
The manoeuvrability of the thumb is one of the most amazing parts of the function of the hand. The "opposable thumb" that humans possess and which apes do not is one of the defining characteristics of precision movement and control. The metacarpal of the thumb on the outside of the hand is not inline with all the others but rotated inwards, having the ability to rotate further inwards to allow the end of the thumb to participate in grasping with one of the fingers. The thumb has a very specialised joint at the junction of the metacarpal and carpal, allowing the specialised movement.
As an overall movement of the wrist occurs to achieve a specific objective, the carpal bones move both separately and together to facilitate this. The carpal bones exhibit small levels of movement in between each other and between the rows of bones. To achieve the very functional cupping position of the hand there is a rotational ability of the metacarpals in respect of each other. The curving of the palm which assists grasping also brings the fingers round with the rotation of the metacarpals and allows the fingers to operate at functional angles. The hand can lose some of its functional ability if the accessory movements of the bones are lost.
Wrist function can be adversely affected by heavy work with the hands such as grasping and pulling heavy objects, pulling ropes and using vibrating machinery. When the hand is grasping something firmly the longitudinal forces this generates are very great as the carpal bones are compressed between the metacarpals and the forearm bones. This can cause a reduction in the essential accessory movements of the carpal bones. Forced extension of the wrist may wedge one of the carpal bones, the lunate, slightly forwards which causes pain and disability.
A forced extension movement is most commonly caused by a fall on the outstretched hand (FOOSH), which if severe enough can cause a fracture of the end of the radius and ulna, a so-called Colles fracture. The major injury, typically seen in older women, is the fracture but the fall also sprains the wrist, causing significant soft tissue injury to the carpal region. The bones typically heal well in five to six weeks but the hand may be painful, weak and difficult to use for much longer, secondary to the disruption of the subtle relationship between the carpal bones.