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.
Human hand function is a highly complex process as the thumb, fingers and hand are placed in a precise posture to suit the task being performed, with the wrist performing a pivotal role. The major, less precise, arm positioning is provided by the shoulder and shoulder blade, the body to hand distance is controlled by the elbow, the wrist angle is set by the forearm and the last adjustments of hand position are performed by the wrist. The movements become more precise the closer the joint becomes to the wrist.
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 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.
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.
