The Wrist

The wrist has a large, three hundred and sixty degree cone of movement facing forwards at the end of the arm, a consequence of the close arrangement of the small carpal bones. Group movements or individual ones between the bones can occur and this allows precise control of the hand, thumb and fingers. The rows are arranged irregularly but in general two of the bones line up with the end of each of the metacarpals. This allows the creation of a line of several joints leading to each finger which enables the separate and flexible movements of the hand.

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.

The thumb is the most manoeuvrable and astonishing part of the human hand. We possess an “opposable thumb” which is absent from apes and allows us to achieve the high levels of precision movements we require. On the outside of the hand the thumb’s metacarpal is not flat in the same plane as the others in the palm but is turned inwards, giving it the function of crossing the palm to allow the thumb to meet the ends of the fingers in gripping. Much of the specialised thumb movement comes from the junction of its carpal and metacarpal bones.

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.

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.

Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, Physiotherapists in Bournemouth, 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.

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