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.

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.

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.

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, 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.

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