MUSCLES OF THE
HAND
The interosseous muscles occupy
the intermetacarpal intervals and are of two types: dorsal and palmar. Each
intermetacarpal space contains one palmar and one dorsal interosseous muscle.
The four dorsal interosseous muscles are abductors of the digits and are
bipennate; the three palmar interosseous muscles are adductors and are
unipennate. The plane of reference for abduction and adduction of the fingers
is the midplane of the third digit. This is evident on simultaneously spreading
and then approximating the extended digits. The placement of these muscles
follows from the above considerations of actions and reference plane for
abduction and adduction.
A dorsal interosseous muscle lies
on either side of the third metacarpal, since any movement of the third digit
away from its plane of reference is abduction. The other two dorsal
interosseous muscles occupy the space between the first second metacarpals for
the first dorsal muscle and between the fourth and fifth metacarpals for the
fourth dorsal muscle. These latter two muscles abduct the second and fourth
digits. The bipennate dorsal interosseous muscles arise by two heads from the
adjacent sides of the metacarpals between which they lie.
The first dorsal interosseous muscle is considerably larger than the others;
the radial artery also passes into the palm between its heads. Dorsal
perforating arteries pass between the heads of the other muscles.
The smaller palmar interosseous
muscles adduct the same digit from whose metacarpal bone they arise and
thus take origin from the palmar surfaces of the second, fourth, and fifth
metacarpals. The tendons of both the dorsal and the palmar interosseous muscles pass
dorsal to the deep transverse metacarpal ligaments between the heads of the
metacarpals, and they have two insertions. The first insertion is to the base
of the proximal phalanx; it is concerned with the abduction-adduction function.
The second insertion is into the extensor expansion of the tendon of the
extensor digitorum muscle; it produces flexion at the metacarpophalangeal
joints and extension of the middle and distal phalanges at
the interphalangeal joints. All the interosseous muscles are innervated by the
deep branch of the ulnar nerve.
Free movement of the thumb is most
important in the more precise activities of the hand. The flexor pollicis
longus muscle flexes the thumb, and the extensor pollicis longus and
extensor pollicis brevis muscles extend it. The abductor pollicis
longus muscle is an accessory flexor of the wrist; it abducts and extends
its metacarpal. The short muscles of the thumb provide flexion, abduction,
adduction, and opposition. Abduction of the thumb carries it anteriorly out of
the plane of the palm because of the rotated position of the first metacarpal,
which directs its palmar surface medially. The abductor pollicis brevis muscle
also assists in flexion. The opponens pollicis muscle acts solely on the
metacarpal of the thumb, drawing the digit across the palm and rotating it
medially.
The components of opposition are
abduction, flexion, and medial rotation, the tip of the thumb
reaching contact with the pads of the other slightly flexed digits. In firm
grasp, the flexor pollicis brevis muscle is especially active. The
motor, or recurrent, branch of the median nerve innervates the three muscles
involved. The adductor pollicis muscle adducts the thumb. The abductor digiti
minimi and the flexor digiti minimi brevis muscles produce their
characteristic movements. The opponens digiti minimi muscle rotates the
fifth metacarpal medially and deepens the hollow of the hand.
The intrinsic muscles of the hand
are palmar and are therefore innervated by either the median or the ulnar nerve.
Specific sets of muscles of the thumb and little finger, respectively, occupy
the thenar and hypothenar compartments.
Each compartment contains an abductor,
an opponens, and a flexor muscle for its specific digit
(abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, abductor
digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi
muscles). In each compartment, the positions and attachments of these muscles
are similar.
The flexor retinaculum and the bones to which it attaches (the scaphoid and
trapezium radially and the hamate and pisiform on the ulnar side) provide the
sites of origin for these muscles. The insertions of comparable muscles on the
two sides are also the same: the base of the proximal phalanx for the abductor
and flexor muscles and the shaft of the metacarpal for the opponens muscles.
The central compartment contains
four slender lumbrical muscles associated with the flexor digitorum
profundus tendon. The interosseous muscles located in the intervals
between the metacarpals occupy, with the adductor pollicis muscle, a
deeply placed interosseousadductor compartment that is bound by the dorsal and
palmar interosseous fasciae. To complete these generalizations, the rule of
nervous innervation may also be stated: the median nerve supplies the
abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and the
most radial two lumbrical muscles; the ulnar nerve supplies all the
other intrinsic muscles of the hand.
The adductor pollicis muscle has
two heads of origin, separated by a gap through which the radial artery enters
the palm. The oblique head arises from the capitate and from the bases of the
second and third meta-carpals. The transverse head arises from the palmar ridge
(shaft) of the third metacarpal. The two heads insert together by a tendon that
ends in the ulnar side of the base of the proximal phalanx of the thumb.
This tendon usually contains a
sesamoid that together with the sesamoid in the tendon of the flexor pollicis
brevis muscle forms a pair of small sesamoids on either side of the tendon of
the flexor pollicis longus muscle. The adductor pollicis muscle overlies the
interosseous muscles on the radial side of the third
metacarpal. This muscle is supplied by the deep branch of the ulnar nerve.
SPACES, BURSAE, AND TENDON
SHEATHS OF THE HAND
Friction between tendons and
compartments or bony surfaces is reduced by synovial sheaths. A sheath is formed
like a double-walled tube: the delicate inner wall is closely applied to the
tendon, and the outer wall is the lining of the compartment in which the tendon
lies. The layers are continuous with each other at the ends
of the tube (as elsewhere); their facing surfaces are smooth and separated by a
small amount of synovial fluid.
In the hand, the flexor tendons
have significant excursion, which causes friction between the tendons and the
carpal ligament and in each finger against the fibro-osseous pulley system
during gripping activities. Accordingly, the tendons to the thumb and fingers
are protected and lubricated for optimal movement.
Variations in the anatomy of the
synovial sheaths (bursae) exist and have an impact on the pattern of
presentation of infections when they occur (see Plates 4-37 and 4-38).
Similarly, several potential spaces exist in the palm and can become sites of
infection. The thenar space exists just anterior to the adductor pollicis
muscle. The midpalmar space exists posterior (deep) to the central compartment
that c ntains the long flexor tendons and lumbrical muscles.
