FLEXOR AND EXTENSOR TENDONS OF THE HAND
As the flexor and extensor tendons pass from the wrist to the hand, clinical zones have been described that help physicians articulate more precisely the significant anatomic differences that exist in each zone that affect finger function after injury (see Plates 4-5 and 4-6). As the extensor digitorum tendons diverge over the dorsum of the hand, they are interconnected by inter-tendinous connections. These prominently interconnect the tendons for the third, fourth, and fifth digits and severely limit the independent action of these digits, especially the fourth digit. Independent extensor action is retained for the index finger. The convergence of the tendon of the extensor pollicis longus muscle toward the tendons of the abductor pollicis longus and extensor pollicis brevis muscles defines a hollow known as the anatomic snuffbox (see Plate 4-14). In the floor of this hollow, the radial artery passes toward the dorsum of the hand and gives off its dorsal carpal branch.
FLEXOR TENDONS AND ACCESSORIES
Digital synovial sheaths, after a gap in the midpalm, pick up over the heads of the metacarpals and continue over the pairs of tendons to the base of the distal phalanges of the second to fourth digits. Except for about 5 mm of their proximal ends, these synovial sheaths (and the tendons) are contained within the fibrous sheaths of the digits of the hand. The fibrous sheaths of the digits are strong coverings of the flexor tendons, which extend from the heads of the metacarpals to the base of the distal phalanges and serve to prevent “bow-stringing” of the tendon away from the bones during flexion. They attach along the borders of the proximal and middle phalanges, the capsules of the interphalangeal joints, and the palmar surface of the distal phalanx. They form strong semicylindrical sheaths that, with the bones, produce fibro-osseous tunnels through which the flexor tendons pass to their insertions. Over the shafts of the proximal and middle phalanges, the sheaths exhibit thick accumulations of transversely running fibers (sometimes called annular ligaments, or pulleys), whereas opposite the joints, an obliquely crisscrossing arrangement is characteristic (cruciate ligaments). These latter portions of the fibrous sheaths are thin and do not interfere with flexion at the joints. Proximally, the digital slips of the palmar aponeurosis attach to the fibrous digital sheaths.
The tendons of the flexor digitorum profundus muscle insert on the bases of the distal phalanges of digits 2 to 5, while the tendons of the flexor digitorum superficialis muscle end on the shafts of the middle phalanges of these digits. It is thus necessary for the tendons of the flexor digitorum profundus muscle to pass those of the flexor digitorum superficialis muscle, and this is accomplished by a splitting of the tendon of the superficialis to allow that of the profundus to pass distalward. The division of the flexor digitorum superficialis tendon takes place over the proximal phalanx, and the two halves separate and roll in under the flexor digitorum profundus tendon to reach the bone of the middle phalanx, their fibers crisscrossing as they attach to that phalanx.
The vincula tendinum spring from the internal surface of the digital sheaths of these muscles. They are folds of synovial membrane strengthened by some fibrous tissue, which conduct blood vessels to the tendons. The smaller vinculum breve is at the distal end of the sheath; the vincula longa are narrow strands that reach the tendons more proximally.
The lumbrical muscles are four small, cylindrical muscles associated with the tendons of the flexor digitorum profundus muscle. The two lateral muscles arise distal to the flexor retinaculum from the radial sides and palmar surfaces of the flexor digitorum profundus muscle destined for the second and third digits. These are supplied by the median nerve. The two medial muscles arise from the contiguous sides of the tendons for the third and fourth and the fourth and fifth digits. These are innervated by the deep branch of the ulnar nerve. Each lumbrical tendon passes distalward on the palmar side of the deep transverse metacarpal ligament and then shifts toward the dorsum. It inserts, at the level of the proximal phalanx, into the radial border of the expansion of the extensor digitorum muscle.
EXTENSOR MECHANISM OF FINGERS
The four tendons of the extensor digitorum muscle of the forearm pass across the metacarpophalangeal joints, become flattened and closely attached to the joint capsules, and substitute as dorsal ligaments for these capsules. At the metacarpophalangeal joint and over the proximal two phalanges, an extensor expansion is formed for each tendon by the participation of the tendons of the lumbrical and interosseous muscles of the hand. Opposite the metacarpophalangeal joints, a band of fibers passes from each side of the digital extensor tendon anteriorly on both sides of the joint and attaches to the palmar ligament of the joint. This proximal spreading of the extensor expansion appears like a hood of fibers over the metacarpophalangeal joint.
Over the dorsum of the proximal phalanx, the digital extensor tendon divides into three slips. Of these, the central, broader slip passes directly forward and inserts on the dorsum of the middle phalanx. The diverging bundles on either side, the lateral bands, receive and combine with the broadening tendon of a lumbrical muscle on the radial side of the digit, and with interosseous tendons on both sides of the digit. These tendons unite into a common band that proceeds distalward, the bands of the two sides forming a triangular aponeurosis over the distal end of the middle phalanx. The apex of this aponeurosis attaches to the base of the distal phalanx.
Muscle Actions in Digital Movement
Certain forearm muscles participate in movements of the digits. The tendons of the flexor digitorum superficialis and flexor digitorum profundus muscles emerge from the wrist at the distal border of the flexor retinaculum and enter the central compartment of the palm (see Plate 4-9). Here, they fan out toward their respective digits, arranged in pairs, superficial and deep. They are invested by the ulnar bursa through the upper part of the palm, except that the extension of the bursa along the tendons for the fifth digit continues to the base of its distal phalanx.
The flexor digitorum superficialis muscle is a flexor of the proximal interphalangeal and metacarpophalangeal joints of the medial four fingers and is the principal flexor of the wrist. The flexor digitorum profundus muscle primarily flexes the terminal phalanx but, continuing to act, also flexes the middle and proximal phalanges. This muscle flexes the digits in slow action, the flexor digitorum superficialis muscle being recruited for speed and against resistance. The extensor digitorum muscle, assisted by the extensors of the index and fifth fingers, is the extensor of the fingers. Interconnecting tendinous bands between the tendons of the third to fifth digits prevent completely independent extension of these digits, but the index finger can be moved quite separately.
The interosseous and lumbrical muscles of the hand are essential for full extension of the digits. The interosseous muscles act most effectively when there is combined metacarpophalangeal flexion and interphalangeal extension, principally producing interphalangeal extension. The lumbrical muscles are silent during total flexion but are very active in extension of the proximal or distal interphalangeal joints and also when these joints are being maintained in extension during metacarpophalangeal flexion.