In the preceding article of this series, published in the September 1988 issue of MPPA, the author noted that the five fingers and their many joints (polyarticulated digital chains) are activated by two groups of muscles, the extrinsic and the intrinsic. The extrinsic muscles are proximal to the wrist and are located in the forearm. This provides the fingers with strength and dexterity, allowing them to remain slim and nimble without the encumbrance of heavy muscles. The extrinsic muscles of the fingers are elongated by tendons that cross the wrist and finger joints. Each of these joints must be stabilized by other muscles in order to permit the extrinsic tendons to selectively mobilize a single joint. The intrinsic muscles are located in the hand and need only short tendons. Although this dual system of muscles exists both for the thumb and the fingers, the balance of the respective forces for thumb and fingers is different, as will be demonstrated subsequently.
The hand muscles are activated by the three major nerves of the arm: the radial nerve, for hand and wrist extension; the median nerve, for most of the extrinsic flexors; and the ulnar nerve, for most of the intrinsic (entirely within the hand) muscles. The ulnar and median nerves provide sensation for the hand: the ulnar nerve supplies the area toward the little finger and the median nerve supplies the area toward the thumb on the palmar face of the hand.
MOVEMENTS OF THE THUMB
The thumb is the most important digit because of its mobility and force, and because of its privileged and indispensable relationship with the other fingers, allowing it to function in opposition to the other digits as well as to the palm.
The thumb ray is made up of the first metacarpal bone, a proximal phalanx, and a distal phalanx. The three joints of this bony articular chain have different sectors of mobility; the more proximal they are, the more mobility they have. The movements of the three joints of this functional unit are able partially to compensate for each other in flexion-adduction (moving inward) or the contrary, i.e., extension-abduction (moving away). However, bringing the thumb ray in front of the plane of the palm (anteposition) is a movement of the first metacarpal at the level of the carpometacarpal (wristhand) joint (Fig. 1).
Similar to the fingers, movements of the thumb are produced by both extrinsic and intrinsic muscles. However, it is important to note that for the thumb the total force developed by the intrinsic muscles is greater than the force of the extrinsic muscles. This is the reverse for the fingers. The long (extrinsic) muscles of the thumb are, anteriorly, the flexor pollicis longus, which is the most powerful muscle, and, posteriorly, three long muscles that stabilize the thumb like shrouds. This permits the opening of the thumb-finger pinch, the requisite prelude to opposition. After opposition, the muscles return the thumb to its initial position.
The four intrinsic muscles of the thumb, the thenar...