Workings Of 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 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.
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.
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.
Using the hands very heavily such as in gripping and holding heavy objects, hauling ropes or operating heavy machinery can adversely affect wrist function. The longitudinal forces which are generated across the wrist are very high as the hand grasping power is applied, compressing the carpal bones between the forearm and the metacarpals. The carpal bones can then suffer a reduction in the accessory movements possible between them. If the wrist is forcibly extended this may dislodge the lunate bone, one of the wrist bones, forwards and cause pain.
The commonest reason for the wrist to be extended forcibly is a FOOSH or a fall on the outstretched hand, which can result in a Colles fracture which involves the last inch of the radius and ulna near the wrist. The fracture, commonest in older females, is the most obvious part of the overall injury which results also in wrist sprain and soft tissue injury. Five to six weeks will be enough to heal the fracture but there may be weakness, pain and difficulty with use in the hand for a longer period, partly related to a upset in the inter carpal movements.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about physiotherapy, physiotherapy, physiotherapists in Manchester, 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.