Articulations within the hand

Introduction

Just as the foot has evolved as an organ of support and locomotion so the hand has developed into an instrument of manipulation endowed with fine sensory discrimination. It is often hard to accept that the hand and wrist and the foot and ankle have similar building blocks in terms of bone and muscular constituents, patterns of innervation and blood supply. Undoubtedly, the refinements that have occurred in the hand followed its release from the burden of supporting and propelling the body. The extent to which the hand is used indicates its importance in everyday life. We use hands to grip and manipulate; they enable us to dress, eat, play instruments and games. The hand has to be capable of applying large gripping forces between the fingers and thumb while performing precision movements. However, its sensory functions must not be overlooked; it relays information regarding texture and surface contour, warns against extremes of hot and cold, and prevents collisions, especially when we cannot use sight. All of these motor and sensory functions require considerable representation in the motor and sensory cortices of the brain. As the hand developed, so the cerebral cortex enlarged to obtain maximum benefit for the new freely mobile, sensitive structure.

Much of the motor functioning of the hand is due to its ability to grip objects. Yet prehension can be observed in many animals from the pincers of the crab to the hand of the great ape. It is the concomitant development of hand and brain forming an interacting functional pair that has led to human dominance in the animal kingdom. Because of the uses to which the hand is put, it is particularly disabling when part or all of it is injured. It is especially vulnerable because it is usually unprotected. As well as diseases of the joints, lesions of the peripheral and central nervous system and infections, accidental amputations, burns, lacerations and penetrating wounds all serve to disable the hand.

In many respects the arrangement of the bones and their intervening joints are simpler in the hand than the foot, principally because the carpus is limited to the wrist, whereas the tarsus forms the hindfoot. The metacarpals articulate with the wrist region via the carpometacarpal joints, of which the first is different from the remainder, and with each other via the intermetacarpal joints. The head of each metacarpal articulates with a proximal phalanx at the metacarpophalangeal joint(a). Adjacent phalanges articulate via interphalangeal joints(a). However, because the thumb only has two phalanges it has only one interphalangeal joint, whereas the fingers all have three phalanges and two interphalangeal joints.

Care has to be taken when using the terms fingers and digits as confusion can often arise. There are four fingers and a thumb, or five digits. If finger and thumb is the preferred terminology then, to avoid confusion, use of an appropriate prefix is advised, that is index, middle, ring and little.

The axis of the hand runs along the middle finger(third digit), and is in line with the long axis of the forearm(b). Certain movements of the digits are made with reference to this axis. In describing movements of the thumb, remember that it is rotated through 90° with respect to the remaining digits.

However fine the movements produced in the hand, they must be controlled from a stable base, so that the origins of intrinsic muscles of the hand remain fixed by the musculature of the forearm which is brought into play. The origins of the muscles in the forearm in turn require fixation at the elbow by muscles of the arm, and these in turn require fixation at their origin at the shoulder and pectoral girdle. Even writing therefore involves use of the shoulder muscles as well as the fingers and thumb.