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The Wrist Joint

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 itself is positioned between the forearm and the hand and consists of eight small bones known as the carpal bones which are arranged in two rows and situated in between the ends of the radius and ulna and the metacarpal bones. The metacarpals run from the furthest row of carpal bones down towards the knuckles to join the finger bones. As the metacarpals are narrow and run almost parallel to each other this gives them the ability to flatten themselves out to make the hand wide or to curl themselves up to aid grasping, a very useful ability.

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.

The movements of the carpal bones can be in unison in small amounts as they move together to allow a movement to occur. As the hands move small amplitudes of movement occur between the individual carpal bones and the carpal rows. The metacarpals are able to rotate around their long axes which allows the palm to be curled into a cupped position. As the palm moulds round to assist gripping it also allows the fingers to align so that they can effectively grip at the correct angle. Any loss of the accessory movements of the carpals and metacarpals can reduce the ability of the hand to function adequately.

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 fall on the outstretched hand (FOOSH) is the most typical reason for the wrist to be extended forcibly and a Colles fracture is a common result where the break is located in the last inch of the radius and ulna near the wrist. Older women are most likely to suffer from this fracture and although most attention is concentrated on the fracture there is often a significant soft tissue injury of the wrist bones as well. The fracture will heal in five or six weeks but pain, weakness and functional difficulty may persist for much longer, related to some extent to the loss of individual movements between the carpal bones.

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