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.
