The Human Wrist

The wrist-hand complex is a highly complicated tool which allows for the precise use of the hand and its very important role in human function, with the wrist a vital link in this process. The shoulder and scapula allow crude arm positioning, the elbow allows the distance from the body to be varied, the forearm sets the angle at which the wrist will be positioned and the wrist finishes off the last detail of hand positioning. As the joints get closer to the hand the smaller and more precise their movements.

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

This tight grouping of carpal bones endows the wrist with a large range of movement of 360 degrees in a conical shape facing forward. They are able to make individual and group movements to improve the precise positioning of the hand, fingers and thumb. Even though the arrangement is a little untidy the two rows of bones do line up with more or less two bones at the end of each metacarpal separating this from the forearm. The large number of in-line joints created with this arrangement allows a high degree of adaptability and precision of movement.

In the human hand the most specialised and most useful part is the thumb. Apes do not have the “opposable thumb” which humans possess and which allows us to perform the highly controlled manual activities we need to. Unlike the metacarpals of the palm, which all lie in one plane, the metacarpal of the thumb lies away from this plane and is rotatable across the palm of the hand, allowing the thumb to grip against the fingers. The joint between the thumb metacarpal and its carpal bone is unusual in structure and confers much specialised movement.

The carpal bones typically move in small motions which are reflected throughout the wrist, in other words they often move all together to accomplish a movement. There are small amounts of motion between all the carpal bones as the hand is moved, and with the ability of the metacarpals to rotate in regard to each other, this allows a cupping posture of the hand. Cupping the hand moulds the palm so that objects can be gripped and brings the fingers round to an appropriate angle to hold something. If the metacarpals lose the small accessory movements which occur between them this can affect the use of the wrist and so the ability of the hand.

The heavy use of the hands in manual work such as lifting and moving large objects, manipulating heavy machines and pulling ropes can damage the function of the wrist. High mechanical forces are generated when the hand grips something hard, squeezing the wrist bones between the metacarpal bones and the radius and ulna of the forearm. This can allow a reduction of the accessory movements between the individual wrist bones. The lunate bone can be moved from its position with painful consequences if the wrist is extended with force.

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.

Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Oxford. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.